WATER AND WASTE WATER ENGINEERING

Domestic water Treatment
& Supply-Domestic
wastewater
Collection
water Treatment
and Supply
Module 1: Municipal
water Supply:
Sources and Quality
Lecture 1: Raw water Source and Quality
Module 2: water Quantity
and Intake
Details
Lecture
2: water Quantity
Estimation
Lecture
3: Intake,
Pumping and Conveyance
Module 3: Unit Processes in Municipal
water Treatment
Lecture 4: water Treatment
Philosophy
Lecture
5: Preliminary
Treatment:
Silt
Excluder
Design
Lecture 6: Sedimentation
Tank Design
Lecture 7: Coagulation
- Flocculation
Theory
Lecture 8: Rapid Mixing,
Coagulation
- Flocculation
Lecture 9: Coagulation
- Flocculation
Lecture 16: Filtration
Theory
Lecture 11: Rapid Sand Filtration
Lecture

12:

& Treatment-Domestic

Disinfection

Module 4: Municipal
water Treatment
Plant Design Details
Lecture 13: Treatment
Plant Siting
and Hydraulics
Module 5: water Storage Tanks and Distribution
Network
Lecture 14: water Storage Tanks and water Supply
Lecture 15: water Supply Network Design
Module 6: Rural water Supply
Lecture 16: water Treatment
and Supply for Rural

Areas

Raw water

be classified

Source:

The

various

sources

of

water

can

Network

into

two

categories:
1. Surface sources,
such as a. Ponds and lakes;
b. Streams and rivers;
c. Storage
reservoirs;
and d. Oceans, generally
not used for water supplies,
at present.
2. Sub-surface
sources or underground
sources,
such as
a. Springs;
b. Infiltration
wells
; and c. wells and Tube-wells.
water Quality:
The raw or treated
water is analysed
by testing
their
physical,
chemical
and bacteriological
characteristics:
Physical
Characteristics:
Turbidity;
Colour;
Taste and Odour;
Temperature;
Turbidity:
If a large amount of suspended solids
are present
in water,
it will
appear
turbid
in appearance.
The turbidity
depends upon fineness
and concentration
of
particles
present
in water.
turbidity
was determined
by measuring the depth of column
of liquid
required
to cause the image of a candle flame at the bottom to diffuse
into a
uniform glow. This was measured by Jackson candle turbidity
meter. The calibration
was
done based on suspensions
of silica
from Fuller'searth.
The depth of sample in the

tube was read against the part per million
(ppm) silica
scale with one ppm of suspended
silica
called one Jackson Turbidity
unit(JTU).
Beacause standards were prepared from
materials
was

found

difficult

in

to

nature

such as Fuller'searth,

These days turbidity
is measured by applying
of light
scattered
by the particles
at right
scattered
light
level
is proportional
to the

unit

of expression

water

is

16 to

consistency

in

standard

formulation

achieve.

is Nephelometric

Nephelometry,
a technique
to measure level
angles to the incident
light
beam. The
particle
concentration
in the sample. The

Turbidity

Unit

(NTU). The IS values for

drinking

25 NTU.

Colour:
Dissolved
organic
matter from decaying vegetation
or some inorganic
materials
may impart colour to the water.
The standard
unit of colour
is that which is
produced by one milligram
of platinum
cobalt
dissolved
in one litre
of distilled
water.
The

IS

value

for

treated

water

is

5 to

25 cobalt

units.

Taste and Odour: Most organic
and some inorganic
chemicals,
originating
from
municipal
or industrial
wastes,
contribute
taste
and odour to the water.
Taste and
odour can be expessed in terms of odour intensity
or threshold
values.
Temperature:
The increase
in temperature
decreases
palatability,
because at
elevated
temperatures
carbon dioxide
and some other volatile
gases are expelled.
The
ideal temperature
of water for drinking
purposes is 5 to 12 °C - above 25 °C, water is
not recommended for drinking.

Chemical

pH

Characteristics:

Acidity

Solids

Alkalinity

Hardness

Chlorides

Sulphates

Iron

Nitrates

pH
pH value denotes the acidic
or alkaline
condition
of water.
It is expressed on a scale
ranging from 6 to 14, which is the common logarithm
of the reciprocal
of the hydrogen
ion concentration.
The recommended pH range for treated
drinking
waters is 6.5 to 8.5.
Acidity
The acidity
of water is a measure of its capacity
to neutralise
bases. Acidity
of water
may be caused by the presence of uncombined carbon dioxide,
mineral
acids and salts
of
strong acids and weak bases. It is expressed as mg/L in terms of calcium carbonate.
Acidity
is nothing
but representation
of carbon dioxide
or carbonic
acids.
Carbon
dioxide
causes corrosion
in public
water supply systems.
Alkalinity
The alkalinity
of water is a measure of its capacity
to neutralise
acids.
It is
expressed as mg/L in terms of calcium carbonate.
The various
forms of alkalinity
are

(a) hydroxide alkalinity,
alkalinity,
(d) carbonate
which is useful
is an important

(b) carbonate alkalinity,
(c) hydroxide plus carbonate
plus bicarbonate alkalinity,
and (e) bicarbonate alkalinity,

mainly in water softening
and boiler
parameter
in evaluating
the optimum

feed water processes.
coagulant
dosage.

Alkalinity

Hardness

If water consumes excessive
soap to produce lather,
it is said to be hard. Hardness is
caused by divalent
metallic
cations.
The principal
hardness causing cations
are
calcium,
magnesium, strontium,
ferrous
and manganese ions. The major anions associated
with these cations
are sulphates,
carbonates,
bicarbonates,
chlorides
and nitrates.
The total
hardness of water is defined
as the sum of calcium and magnesium
concentrations,
both expressed as calcium carbonate,
in mg/L. Hardness are of two
types,
temporary
or carbonate
hardness and permanent or non carbonate
hardness.
Temporary hardness is one in which bicarbonate
and carbonate
ion can be precipitated
by
prolonged
boiling.
Non-carbonate
ions cannot be precipitated
or removed by boiling,
hence the term permanent hardness.
IS value for drinking
water is 366 mg/L as CaCO3.
Sulphates
Sulphates
occur in water due to leaching
from sulphate
mineral
and oxidation
of
sulphides.
Sulphates
are associated
generally
with calcium,
magnesium and sodium ions.
Sulphate
in drinking
water causes a laxative
effect
and leads to scale formation
in
boilers.
It also causes odour and corrosion
problems under aerobic
conditions.
Sulphate
should be less than 56 mg/L, for some industries.
Desirable
limit
for drinking
water
is 156 mg/L. May be extended upto 466 mg/L.
Chlorides

Chloride
ion may be present
in combination
with one or more of the cations
of calcium,
magnesium, iron and sodium. Chlorides
of these minerals
are present
in water because of
their
high solubility
in water.
Each human being consumes about six to eight grams of
sodium chloride
per day, a part of which is discharged
through
urine and night soil.
Thus, excessive
presence of chloride
in water indicates
sewage pollution.
IS value for
drinking
water is 256 to 1666 mg/L.
Iron

Iron is found on earth mainly as insoluble
ferric
oxide.
when it comes in contact
with
water,
it dissolves
to form ferrous
bicarbonate
under favourable
conditions.
This
ferrous
bicarbonate
is oxidised
into ferric
hydroxide,
which is a precipitate.
Under
anaerobic
conditions,
ferric
ion is reduced to soluble
ferrous
ion. Iron can impart bad
taste to the water,
causes discolouration
in clothes
and incrustations
in water mains.
IS value for drinking
water is 6.3 to 1.6 mg/L.
Solids

The sum total
solids
is the

of foreign
matter
present
in water is termed as total
solids.
matter that remains as residue
after
evaporation
of the sample

subsequent drying at a defined temperature
Total solids consist of volatile
(organic)

(163 to 165 °C).
and non-volatile

(inorganic

Total
and its

or fixed)

solids.
Further,
solids
are divided
into suspended and dissolved
solids.
Solids
that
can settle
by gravity
are settleable
solids.
The others
are non-settleable
solids.
IS
acceptable
limit
for total
solids
is 566 mg/L and tolerable
limit
is 3666 mg/L of
dissolved

limits.

Nitrates

Nitrates
in surface
waters occur by the leaching
of fertilizers
from soil
during
surface
run-off
and also nitrification
of organic
matter.
Presence of high
concentration
of nitrates
is an indication
of pollution.
Concentration
of nitrates
above 45 mg/L cause a disease methemoglobinemia.
IS value is 45 mg/L.
Bacteriological
Characteristics:
Bacterial
examination
of water is very important,
since it indicates
the degree of
pollution.
water polluted
by sewage contain
one or more species of disease producing
pathogenic
bacteria.
Pathogenic
organisms
cause water borne diseases,
and many non
pathogenic
bacteria
such as E.Coli,
a member of coliform
group, also live
in the
intestinal
tract
of human beings.
Coliform
itself
is not a harmful group but it has
more resistance
to adverse condition
than any other group. So, if it is ensured to
minimize
the number of coliforms,
the harmful
species will
be very less.
So, coliform
group serves as indicator
of contamination
of water with sewage and presence of
pathogens.
The methods to estimate
the bacterial
quality
of water are:
Standard Plate Count Test
Most Probable Number
Membrane Filter
Technique
Standard

In this

Plate

test,

Count

the

Test

bacteria

are made to

grow as colonies,

by innoculating

a known volume

of sample into a solidifiable
nutrient
medium (Nutrient Agar), which is poured in a
petridish.
After incubating
(35°C) for a specified
period (24 hours), the colonies of
bacteria (as spots) are counted. The bacterial
density is expressed as number of
colonies
Most

per 166 ml of

Probable

sample.

Number

Most probable
number is a number which represents
the bacterial
density
which is most
likely
to be present.
E.Coli
is used as indicator
of pollution.
E.Coli
ferment
lactose
with gas formation
with 48 hours incubation
at 35°C. Based on this
E.Coli
density
in a
sample is estimated
by multiple
tube fermentation
procedure,
which consists
of
identification
follows:

of

Five 16 ml (five

E.Coli

in

dilution

different

combination)

dilution

combination.

MPN value

tubes of a sample is tested

for

is

calculated

E.Coli.

If

as

out

of five
only one gives positive
test for E.Coli
and all others
negative.
From the
tables,
MPN value for one positive
and four negative
results
is read which is 2.2 in
present
case. The MPN value is expressed as 2.2 per 166 ml. These numbers are given by
Maccardy based on the laws of statistics.
Membrane Filter
Technique
In this
test a known volume of water sample is filtered
through
a membrane with opening
less than 6.5 microns.
The bacteria
present
in the sample will
be retained
upon the
filter
paper. The filter
paper is put in contact
of a suitable
nutrient
medium and kept
in an incubator
for 24 hours at 35°C. The bacteria
will
grow upon the nutrient
medium
and visible
colonies
are counted.
Each colony represents
one bacterium
of the original
sample. The bacterial
count is expressed as number of colonies
per 166 ml of sample
water Quantity
Estimation
The quantity
of water required
for municipal
uses for which the water supply scheme has
to be designed requires
following
data:

1. water consumption rate

(Per Capita Demand in litres

per day per head)

2. Population
to be served.
Quantity:
Per capita
demand x Population
water Consumption Rate
It is very difficult
to precisely
assess the quantity
of water demanded by the
since there are many variable
factors
affecting
water consumption.
The various
water demands, which a city may have, may be broken into following
classes:
water Consumption for Various
Purposes:
Types of

Consumption

Normal Range (lit/capita/day)
2 Industrial

and

Commercial

3 Public

Uses including

4 Losses

and

waste

45-156

Fire

Average % 1 Domestic Consumption65-366
Demand 45-456

Demand 26-96
62 25

135

45 16

36

166

35

public,
types of

Fire Fighting
Demand:
The per capita
fire
demand is very less on an average basis but the rate at which the
water is required
is very large.
The rate of fire
demand is sometimes traeted
as a
function
of population
and is worked out from following
empirical
formulae:
Authority
Formulae
American

Insurance

Association

Kuchling'sFormul
Freeman'sFormula
4

Ministry
Factors
a.
compared
b.
and

of Urban Development Manual Formula
affecting
per capita
demand:
Size of the city:
Per capita
demand for
to that for smaller
towns as big cities
Presence of industries.
c. Climatic

their

economic

big cities
is
have sewered
conditions.

generally
houses.
d.

large

as

Habits

of

e. Quality
of water:
If water is aesthetically
good, medically
safe, the
consumption
will
increase
as people will
not resort
to private
wells,
etc.
f. Pressure in the distribution
system.
g. Efficiency
of water works administration:
Leaks in water mains and services;
unauthorised
use of water can be kept to a minimum by surveys.
h.

Cost

of

metering
of meter

Fluctuations

in

Rate

Average Daily
Population)
If

this

and

water.

i. Policy
of
on the basis

the

people

status.

average

and charging
method: water tax is charged
reading and on the basis of certain
fixed
of

in two different
monthly rate.

ways:

Demand:

Per Capita Demand
demand is

supplied

= Quantity
at

all

the

Required in 12 Months/ (365 x

times,

it

will

not

be sufficient

to

meet

fluctuations.

Seasonal variation:
The demand peaks during
summer. Firebreak
outs are generally
more in summer, increasing
demand. So, there is seasonal variation
.
Daily variation
depends on the activity.
People draw out more water on Sundays and
Festival
days, thus increasing
demand on these days.
Hourly variations
are very important
as they have a wide range. During active
household
working
hours i.e.
from six to ten in the morning and four to eight
in the evening,
the
bulk of the daily
requirement
is taken.
During other hours the requirement
is
negligible.
Moreover,
if a fire
breaks out, a huge quantity
of water is required
to be
supplied
during
short duration,
necessitating
the need for a maximum rate of hourly
supply.
So, an adequate quantity
of water must be available
to meet the peak demand. To meet
all the fluctuations,
the supply pipes,
service
reservoirs
and distribution
pipes must
be properly
proportioned.
The water is supplied
by pumping directly
and the pumps and
distribution
system must be designed to meet the peak demand. The effect
of monthly
variation
influences
the design of storage
reservoirs
and the hourly variations
influences
the design of pumps and service
reservoirs.
As the population
decreases,
the
fluctuation

rate

increases.

Maximum daily
demand = 1.8 x average daily
demand
Maximum hourly
demand of maximum day i.e.
Peak demand
= 1.5 x average hourly
demand
- 1.5 x Maximum daily
demand/24

1.5 x (1.8 x average daily
- 2.7
2.7

demand)/24

x average daily
demand/24
x annual average hourly
demand

Design Periods & Population
Forecast:
The future
period for which a provision
made in the water supply scheme is known as the design period.
Design period is
estimated
based on the following:
*Useful
life
of the component, considering
obsolescence,
wear, tear,
etc.
*Expandability
aspect.
*Anticipated
rate of growth of population,
including
industrial,
commercial
developments
& migration-immigration.

is

*Available

resources.

*Performance

of the

system

during

initial

period.

Population
Forecasting
Methods:
The various
methods adopted for estimating
future
populations
are given below.
1. Arithmetic
Increase
Method:
This method is based on the assumption
that the
population
increases
at a constant
rate;
i.e.
dP/dt=constant=k;
Pt= Pe+kt. This method
is most applicable
to large and established
cities.
2.Geometric
Increase
Method:
This method is based on the assumption
that percentage
growth rate is constant
i.e.
dP/dt=kP;
lnP= lnPe+kt.
This method must be used with
caution,
for when applied
it may produce too large results
for rapidly
grown cities
in
comparatively
short time. This would apply to cities
with unlimited
scope of expansion.
As cities
grow large,
there is a tendency to decrease in the rate of growth.
Incremental

Increase

Method

Growth rate is assumed to be progressively
increasing
or decreasing,
depending upon
whether the average of the incremental
increases
in the past is positive
or negative.
The population
for a future
decade is worked out by adding the mean arithmetic
increase
to the last known population
as in the arithmetic
increase
method, and to this
is added
the average of incremental
increases,
once for first
decade, twice for second and so
on.

Decreasing
Rate of Growth Method
In this
method, the average decrease in the percentage
then subtracted
from the latest
percentage
increase
to
next

increase
get the

is worked
percentage

out, and is
increase
of

decade.

Simple Graphical
Method
In this
method, a graph is plotted
from the available
data, between time and
population.
The curve is then smoothly extended upto the desired
year. This method
gives very approximate
results
and should be used along with other forecasting
methods.

Comparative
Graphical
Method
In this
method, the cities
having conditions
and characteristics
whose future
population
is to be estimated
are selected.
It is
city
under consideration
will
develop,
as the selected
similar
the past.
Ratio

similar
to the city
then assumed that the
cities
have developed

in

Method

In this
method, the local
population
and the country'spopulation
for the last four to
five
decades is obtained
from the census records.
The ratios
of the local
population
to
national
population
are then worked out for these decades. A graph is then plotted
between time and these ratios,
and extended upto the design period to extrapolate
the
ratio
corresponding
to future
design year. This ratio
is then multiplied
by the
expected national
population
at the end of the design period,
so as to obtain
the
required
city'sfuture
population.
Drawbacks:

1. Depends on accuracy of national
population
2. Does not consider
the abnormal or special
shifts
from one city to another.
3. Logistic
Curve Method
4. The three factors
responsible
for changes

(i)

Births,

(ii)

Deaths and (iii)

estimate.
conditions

in

which

population

can lead

are

to

population

:

Migrations.

Logistic
curve method is based on the hypothesis
that when these varying
influences
do
not produce extraordinary
changes, the population
would probably
follow
the growth
curve characteristics
of living
things
within
limited
space and with limited
economic
opportunity.
The curve is S-shaped and is known as logistic
curve.
Intake

Structure

The basic function
of the intake
structure
the source over predetermined
pool levels

withdrawal

conduit

(normally

treatment
plant.
Factors Governing
Location
1. As far as possible,
the

called

intake

of Intake
site
should

is to help
and then to

conduit),

be near

the

in safely
discharge

withdrawing
this water

through which it

treatment

plant

flows

so that

water from
into the

up to water

the

cost

of

conveying water to the city
is less.
2. The intake
must be located
in the purer zone of the source to draw best quality
water from the source,
thereby
reducing
load on the treatment
plant.
3. The intake
must never be located
at the downstream or in the vicinity
of the point
of disposal
of wastewater.
4. The site should be such as to permit greater
withdrawal
of water,
if required
at a
future

date.

5. The intake
must be located
at a place from where it can draw water even during the
driest
period of the year.
6. The intake
site should remain easily
accessible
during floods
and should noy get
flooded.
Moreover,
the flood waters should not be concentrated
in the vicinity
of the
intake.

Design Considerations
1. sufficient
factor
of safety
against
external
forces
such as heavy currents,
floating
materials,
submerged bodies,
ice pressure,
etc.
2. should have sufficient
self weight so that it does not float
by upthrust
of water.
Types of Intake
Depending on the source of water,
the intake
works are classified
as follows:
Pumping
A pump is a device which converts
mechanical
energy into hydraulic
energy.
It lifts
water from a lower to a higher level
and delivers
it at high pressure.
Pumps are
employed in water supply projects
at various
stages for following
purposes:
. To lift

raw

water

from

wells.

. To deliver
treated
water to the consumer at
. To supply pressured
water for fire
hydrants.
. To boost up pressure
in water mains.

desired

pressure.

\lO\U'|-RU. To fill
elevated
overhead
. To back-wash
filters.

water

. To pump chemical
solutions,
Classification
of Pumps
Based on principle
of operation,

1.
2.
3.
4.

tanks.

needed for

water

pumps may be classified

Displacement pumps (reciprocating,
rotary)
Velocity
pumps (centrifugal,
turbine and jet
Buoyancy pumps (air lift
pumps)
Impulse pumps (hydraulic
rams)

Capacity
of Pumps
work done by the pump,
H.P.=EAQH/75
where, EA: specific
weight of water kg/m3,
head against
which pump has to work.

H= Hs + Hd + Hf + (losses
where,

Hs=suction

Efficiency

head,

treatment.

due to exit,
Hd = delivery

pumps)

Q= discharge

entrance,
head,

as follows:

of

pump, m3/s;

bends, valves,

and Hf = friction

and H= total

and so on)
loss.

of pump (E) = EAQH/Brake H.P.

Total brake horse power required
= EAQH/E
Provide even number of motors say 2,4,...
with their
total
capacity
being equal to the
total
BHP and provide
half of the motors required
as stand-by.
Conveyance
There are two stages in the transportation
of water:
1. Conveyance of water from the source to the treatment
plant.
2. Conveyance of treated
water from treatment
plant to the distribution
system.
In the first
stage water is transported
by gravity
or by pumping or by the combined
action
of both, depending upon the relative
elevations
of the treatment
plant and the
source of supply.
In the second stage water transmission
may be either
by pumping into an overhead tank
and then supplying
by gravity
or by pumping directly
into the water-main
for
distribution.

Free Flow System
In this
system, the surface
of water in the conveying
section
flows freely
due to
gravity.
In such a conduit
the hydraulic
gradient
line
coincide
with the water surface
and is parallel
to the bed of the conduit.
It is often
necessary to construct
very long
conveying
sections,
to suit the slope of the existing
ground. The sections
used for
free-flow
are: Canals, flumes,
grade aqueducts and grade tunnels.

Pressure System
In pressure
conduits,
which are closed conduits,
the water flows under pressure
above
the atmospheric
pressure.
The bed or invert
of the conduit
in pressure
flows is thus
independant
of the grade of the hydraulic
gradient
line and can, therefore,
follow
the
natural
available
ground surface
thus requiring
lesser
length of conduit.
The pressure
aqueducts may be in the form of closed pipes or closed aqueducts and tunnels
called
pressure
aqueducts or pressure
tunnels
designed for the pressure
likely
to come
on them. Due to their
circular
shapes, every pressure
conduit
is generally
termed as
a pressure
pipe. when a pressure
pipe drops beneath a valley,
stream, or some other
depression,
it is called
a depressed pipe or an inverted
siphon.
Depending upon the construction
material,
the pressure
pipes are of following
types:
Cast iron,
steel,
R.C.C, hume steel,
vitrified
clay,
asbestos
cement, wrought iron,
copper,
brass and lead,
plastic,
and glass reinforced
plastic
pipes.
Hydraulic
Design
The design of water supply conduits
depends on the resistance
to flow,
available
pressure
or head, and allowable
velocities
of flow.
Generally,
Hazen-william's formula
for pressure
conduits
and Manning's
formula
for freeflow
conduits
are used.
Hazen-william's
formula
U=6.85
C rH6.63S6.54

Manning's
formula
U=1/n
rH2/3S1/2
where, U= velocity,
m/s; rH= hydraulic
radius,m;
coefficient,
and n = Manning's
coefficient.
Darcy-weisbach
formula

S= slope,

C= Hazen-william's

hL=(fLU2)/(2gd)
The available
raw waters must be treated
and purified
before they can be supplied
to
the public
for their
domestic,
industrial
or any other uses. The extent
of treatment
required
to be given to the particular
water depends upon the characteristics
and
quality
of the available
water,
and also upon the quality
requirements
for the intended
use..

The layout

of

conventional

water

treatment

plant

is

as follows:

Depending upon the magnitude of treatment
required,
proper unit operations
are selected
and arranged
in the proper sequential
order for the purpose of modifying
the quality
of
raw water to meet the desired
standards.
Indian
Standards
for drinking
water are given
in

the

Indian

table

below.

Standards

for

drinking

water

Parameter
Desirable-Tolerable

If no alternative
Physical

Turbidity

source

(NTU unit)

objectionable

available,

limit

< 16 25 Colour

Un-objectionable

extended

(Hazen scale)

Chemical

pH 7.6-8.5

upto

< 16 56 Taste and Odour Un6.5-9.2

Total

Dissolved

Solids

mg/l

566-1566 3666 Total Hardness mg/l (as CaCO3) 266-366 666 Chlorides mg/l (as Cl) 266-256
1666 Sulphates mg/l (as S04) 156-266 466 Fluorides mg/l (as F ) 6.6-1.2 1.5 Nitrates
mg/l (as N03) 45 45 Calcium mg/l (as Ca)
75

The typical

functions

of

Functions

of

water

The types

of

treatment

each unit

Treatment

operations

are given

in

the

following

table:

Units

required

for

different

sources

are given

in

the

following

table:
Aeration

Aeration
removes odour and tastes
due to volatile
gases like
hydrogen sulphide
and due
to algae and related
organisms.
Aeration
also oxidise
iron and manganese, increases
dissolved
oxygen content
in water,
removes CO2 and reduces corrosion
and removes methane and other flammable gases.
Principle
of treatment
underlines
on the fact that volatile
gases in water escape into
atmosphere from the air-water
interface
and atmospheric
oxygen takes their
place in
water,
provided
the water body can expose itself
over a vast surface
to the atmosphere.
This process continues
until
an equilibrium
is reached depending on the partial
pressure
of each specific
gas in the atmosphere.
Types of Aerators
1. Gravity
aerators
2. Fountain
aerators
3. Diffused
aerators
4. Mechanical
aerators.

Gravity

Aerators

such that

(Cascades):

a large

area

In gravity

of water

is

aerators,

exposed

to

water is allowed to fall

atmosphere,

sometimes

aided

by gravity
by

turbulence.

Fountain Aerators
: These are also known as spray aerators
with special
nozzles to
produce a fine
spray.
Each nozzle is 2.5 to 4 cm diameter
discharging
about 18 to 36
l/h.
Nozzle spacing should be such that each m3 of water has aerator
area of 6.63 to
6.69

m2 for

one

hour.

Injection
or Diffused
Aerators
: It consists
of a tank with perforated
pipes,
tubes or
diffuser
plates,
fixed
at the bottom to release fine air bubbles from compressor unit.
The tank depth is kept as 3 to 4 m and tank width is within
1.5 times its depth.
If
depth is more, the diffusers
must be placed at 3 to 4 m depth below water surface.
Time
of aeration
is 16 to 36 min and 6.2 to 6.4 litres
of air is required
for 1 litre
of
water.

Mechanical
submerged

Aerators
or at the

Settling
Solid liquid

: Mixing
surface.

separation

paddles

process

as in flocculation

in which

a suspension

are

is

used.

separated

Paddles

into

may be either

two phases

Clarified
supernatant leaving the top of the sedimentation tank (overflow).
Concentrated sludge leaving the bottom of the sedimentation tank (underflow).
Purpose of Settling
To remove coarse dispersed
phase.
To remove precipitated
impurities

(biomass) after

activated

To remove coagulated
and flocculated
after
chemical treatment.
To settle

sludge process / tricking

impurities.
the sludge

filters.

Principle
of Settling
Suspended solids
present
in water having specific
gravity
greater
than that of water
tend to settle
down by gravity
as soon as the turbulence
is retarded
by offering
storage.
Basin in which the flow is retarded
is called
settling
tank.
Theoretical
average time for which the water is detained
in the settling
tank is called
the detention
period.
Types of Settling
Type I: Discrete
particle
settling
- Particles
settle
individually
without
interaction
with neighboring
particles.
Type II:
Flocculent
Particles
Flocculation
causes the particles
to increase
in mass
and

settle

at

a faster

rate.

Type III:
Hindered or Zone settling
The mass of particles
tends to settle
as a unit
with individual
particles
remaining
in fixed
positions
with respect
to each other.
Type IV: Compression
The concentration
of particles
is so high that sedimentation
only occur through
compaction
of the structure.
Type I Settling
Size, shape and specific
gravity
of the particles
do not change with time.
Settling
velocity
remains constant.
If a particle
is suspended in water,
it initially
has two forces
acting
upon it:

(1) force of gravity:
(2) the buoyant force

Fg="pgVp
quantified

If

particle

the

density

of the

by Archimedes as: Fb="gVp
differs

from

that

of

the

water,

a net

force

is

exerted

can

and the

particle

is

accelaratd

in the

direction

of

the

force:

Fnet=(" P-" )gVp
This net force becomes the driving
Once the motion has been initiated,
This force,
called
the drag force,

force.
a third
force is
is quantified
by:

created

due to

viscous

friction.

Fd=CDAp"v2/2
CD= drag coefficient.
Ap = projected
area of the particle.
Because the drag force acts in the opposite
Calculate
and check Reynolds number.
Calculate

direction

to th

CD.

Use general
formula.
Repeat from step 2 until
convergence.
Types of Settling
Tanks
Sedimentation
tanks
may function
either
intermittently
or continuously.The
intermittent
tanks also called
quiescent
type tanks are those which store water for a
certain
period and keep it in complete rest.
In a continuous
flow type tank, the flow
velocity
is only reduced and the water is not brought to complete rest as is done in an
intermittent
type.
Settling
basins may be either
long rectangular
or circular
in plan.
Long narrow
rectangular
tanks with horizontal
flow are generally
preferred
to the circular
tanks
with radial
or spiral
flow.
Long Rectangular
Settling
Basin
Long rectangular
basins are hydraulically
more stable,
and flow control
for large
volumes is easier with this
configuration.
A typical
long rectangular
tank have length
ranging from 2 to 4 times their
width.
The
bottom is slightly
sloped to facilitate
sludge scraping.
A slow moving mechanical
sludge scraper
continuously
pulls
the settled
material
into a sludge hopper from where
it is pumped out periodically.
A long rectangular
settling
tank can be divided
into four different
functional
zones:
Inlet
zone: Region in which the flow is uniformly
distributed
over the cross section
such that the flow through
settling
zone follows
horizontal
path.
Settling
zone: Settling
occurs under quiescent
conditions.
Outlet
zone: Clarified
effluent
is collected
and discharge
through outlet
weir.
Sludge zone: For collection
of sludge below settling
zone.
Inlet
and Outlet
Arrangement
Inlet
devices:
Inlets
shall
be designed to distribute
the water equally
and at uniform
velocities.

A baffle

should

be constructed

across

the

basin

close

to

the

inlet

and

should project
several
feet below the water surface
to dissipate
inlet
velocities
and
provide
uniform flow;
Outlet
Devices:
Outlet
weirs or submerged orifices
shall
be designed to maintain
velocities
suitable
for settling
in the basin and to minimize
short-circuiting.
weirs
shall
be adjustable,
and at least
equivalent
in length
to the perimeter
of the tank.
However, peripheral
weirs are not acceptable
as they tend to cause excessive
shortcircuiting.
weir

Overflow

Rates

Large weir overflow
rates result
in excessive
velocities
at the outlet.
These
velocities
extend backward into the settling
zone, causing particles
and flocs
to be
drawn into the outlet.
weir loadings
are generally
used upto 366 m3/d/m. It may be
necessary to provide
special
inboard weir designs as shown to lower the weir overflow
rates.

Inboard
Circular

weir

Arrangement

to

Increase

weir

Length

Basins

Circular
settling
basins have the same functional
zones as the long rectangular
basin,
but the flow regime is different.
when the flow enters at the center and is
baffled
to flow radially
towards the perimeter,
the horizontal
velocity
of the water is
continuously
decreasing
as the distance
from the center increases.
Thus, the particle
path in a circular
basin is a parabola
as opposed to the straight
line
path in the long
rectangular
tank.
Sludge
removal mechanisms in
circular
tanks are
simpler
and require
less

maintenance.

Settling
Solid liquid

separation

process

in which

a suspension

is

separated

into

two phases

Clarified
supernatant leaving the top of the sedimentation tank (overflow).
Concentrated sludge leaving the bottom of the sedimentation tank (underflow).
Purpose of Settling
To remove coarse dispersed
phase.
To remove coagulated
and flocculated
impurities.
To remove precipitated
impurities
after
chemical

To settle

the sludge (biomass)

after

activated

treatment.

sludge process / tricking

filters.

Principle
of Settling
Suspended solids
present
in water having specific
gravity
greater
than that of water
tend to settle
down by gravity
as soon as the turbulence
is retarded
by offering
storage.
Basin in which the flow is retarded
is called
settling
tank.
Theoretical
average time for which the water is detained
in the settling
tank is called
the detention
period.
Types of Settling
Type I: Discrete
particle
settling
- Particles
settle
individually
without
interaction
with neighboring
particles.
Type II:
Flocculent
Particles
Flocculation
causes the particles
to increase
in mass
and

settle

at

a faster

rate.

Type III:
Hindered or Zone settling
The mass of particles
tends to settle
as a unit
with individual
particles
remaining
in fixed
positions
with respect
to each other.
Type IV: Compression
The concentration
of particles
is so high that sedimentation
only occur through
compaction
of the structure.
Type I Settling
Size, shape and specific
gravity
of the particles
do not change with time.
Settling
velocity
remains constant.
If a particle
is suspended in water,
it initially
has two forces
acting
upon it:

(1) force of gravity:
(2) the buoyant force

Fg="pgVp
quantified

If the density
of the particle
and the particle
is accelaratd

can

by Archimedes as: Fb="gVp
differs
in the

from that of the water,
direction
of the force:

a net

force

is

exerted

Fnet=(" P-" )gVp
This net force becomes the driving
Once the motion has been initiated,
This force,
called
the drag force,

force.
a third
force is
is quantified
by:

created

due to

viscous

friction.

Fd=CDAp"v2/2
CD= drag coefficient.
Ap = projected
area of the particle.
Because the drag force acts in the opposite
direction
to the
increases
as the square of the velocity,
accelaration
occurs
a steady velocity
is reached at a point where the drag force

driving
force and
at a decreasing
rate until
equals the driving
force:

(" P-" )gVp = CDAp" v2/2
For spherical
particles,
Vp= d3/6 and Ap= d2/4

Thus, v2= 4g("p-")d
3

Expressions

for

CD

CD change with

characteristics

of

different

flow

regimes.

For laminar,

tr

Calculate

and check

Calculate

CD.

Reynolds

number.

Use general
formula.
Repeat from step 2 until
convergence.
Types of Settling
Tanks
Sedimentation
tanks
may function
either
intermittently
or continuously.The
intermittent
tanks also called
quiescent
type tanks are those which store water for a
certain
period and keep it in complete rest.
In a continuous
flow type tank, the flow
velocity
is only reduced and the water is not brought to complete rest as is done in an
intermittent
type.
Settling
basins may be either
long rectangular
or circular
in plan.
Long narrow

rectangular
tanks with horizontal
flow are generally
preferred
to the circular
tanks
with radial
or spiral
flow.
Long Rectangular
Settling
Basin
Long rectangular
basins are hydraulically
more stable,
and flow control
for large
volumes is easier with this
configuration.
A typical
long rectangular
tank have length
ranging from 2 to 4 times their
width.
The
bottom is slightly
sloped to facilitate
sludge scraping.
A slow moving mechanical
sludge scraper
continuously
pulls
the settled
material
into a sludge hopper from where
it is pumped out periodically.
A long rectangular
settling
tank can be divided
into four different
functional
zones:
Inlet
zone: Region in which the flow is uniformly
distributed
over the cross section
such that the flow through
settling
zone follows
horizontal
path.
Settling
zone: Settling
occurs under quiescent
conditions.
Outlet
zone: Clarified
effluent
is collected
and discharge
through outlet
weir.
Sludge zone: For collection
of sludge below settling
zone.
Inlet
and Outlet
Arrangement
Inlet
devices:
Inlets
shall
be designed to distribute
the water equally
and at uniform
velocities.

A baffle

should

be constructed

across

the

basin

close

to

the

inlet

and

should project
several
feet below the water surface
to dissipate
inlet
velocities
and
provide
uniform flow;
Outlet
Devices:
Outlet
weirs or submerged orifices
shall
be designed to maintain
velocities
suitable
for settling
in the basin and to minimize
short-circuiting.
weirs
shall
be adjustable,
and at least
equivalent
in length
to the perimeter
of the tank.
However, peripheral
weirs are not acceptable
as they tend to cause excessive
shortcircuiting.
weir

Overflow

Rates

Large weir overflow
rates result
in excessive
velocities
at the outlet.
These
velocities
extend backward into the settling
zone, causing particles
and flocs
to be
drawn into the outlet.
weir loadings
are generally
used upto 366 m3/d/m. It may be
necessary to provide
special
inboard weir designs as shown to lower the weir overflow
rates.

Inboard

weir

Circular

Arrangement

to

Increase

weir

Length

Basins

Circular
settling
basins have the same functional
zones as the long rectangular
basin,
but the flow regime is different.
when the flow enters at the center and is
baffled
to flow radially
towards the perimeter,
the horizontal
velocity
of the water is
continuously
decreasing
as the distance
from the center increases.
Thus, the particle
path in a circular
basin is a parabola
as opposed to the straight
line
path in the long
rectangular
tank.
Sludge
removal mechanisms in
circular
tanks are
simpler
and require
less
maintenance.

izontal
velocity
vh and vertical
settling
velocity
vt.
Assume that a settling
column is suspended in the flow of the settling
zone and that
the column travels
with the flow across the settling
zone. Consider the particle
in the
batch analysis
for type-1
settling
which was initially
at the surface
and settled
through the depth of the column 26, in the time te. If te also corresponds
to the time
required
for the column to be carried
horizontally
across the settling
zone, then the
particle
will
fall
into the sludge zone and be removed from the suspension
at the point
at which the column reaches the end of the settling
zone.
All particles
with vt>ve will
be removed from suspension
at some point along the
settling
zone.
Now consider
the particle
with settling
velocity
< ve. If the initial
depth of this
particle
was such that Zp/vt=t6,
this
particle
will
also be removed. Therefore,
the
removal of suspended particles
passing through the settling
zone will
be in proportion
to the ratio
of the individual
settling
velocities
to the settling
velocity
V6.
The time te corresponds
to the retention
time in the settling
zone. t= V = Lzew
Q

Also,t6=26

Q

V0
Therefore,

26 = LZGW and v6:

Q

v6

orv6=Q

Q

Lw

AS
Thus, the depth of the basin is not a factor
in determining
the size particle
that can
be removed completely
in the settling
zone. The determining
factor
is the quantity
Q/As, which has the units of velocity
and is referred
to as the overflow
rate q6. This
overflow
rate is the design factor
for settling
basins and corresponds
to the terminal
setting
velocity
of the particle
that is 166% removed.
Design Details
1. Detention
period:
for plain
sedimentation:
3 to 4 h, and for coagulated
sedimentation:

2 to

2.5

h.

2. Velocity of flow: Not greater than 36 cm/min (horizontal
flow).
3. Tank dimensions: L:B = 3 to 5:1. Generally L= 36 m (common) maximum 166 m. Breadth=
6 m to

16 m. Circular:

Diameter

not greater

than

66 m. generally

26 to 46 m.

4. Depth 2.5 to 5.6 m (3 m).
5. Surface
thoroughly
6. Slopes:
Settling
Particles

Overflow
Rate: For plain
sedimentation
12666 to 18666 L/d/m2
flocculated
water 24666 to 36666 L/d/m2 tank area.
Rectangular
1% towards inlet
and circular
8%.
Operations
falling
through

the

settling

basin

1)Vertical
component:
vt=("p-")gd2

have two components

of

tank

area;

for

velocity:

18
2) Horizontal

component: vh=Q/A

The path of the particle
is given by the vector
sum of horizontal
velocity
vh and
vertical
settling
velocity
vt.
Assume that a settling
column is suspended in the flow of the settling
zone and that
the column travels
with the flow across the settling
zone. Consider the particle
in the
batch analysis
for type-1
settling
which was initially
at the surface
and settled
through the depth of the column 26, in the time t6. If t6 also corresponds
to the time
required
for the column to be carried
horizontally
across the settling
zone, then the
particle
will
fall
into the sludge zone and be removed from the suspension
at the point
at which the column reaches the end of the settling
zone.
All particles
with vt>v6 will
be removed from suspension
at some point along the
settling
zone.
Now consider
the particle
with settling
velocity
< v6. If the initial
depth of this
particle
was such that Zp/vt=t6,
this
particle
will
also be removed. Therefore,
the
removal of suspended particles
passing through the settling
zone will
be in proportion
to the ratio
of the individual
settling
velocities
to the settling
velocity
v6.
The time t6 corresponds
to the retention
time in the settling
zone. t= V = LZ6w
or

v6=

Q
AS

Thus, the depth of the basin is not a factor
in determining
the size particle
that can
be removed completely
in the settling
zone. The determining
factor
is the quantity
Q/As, which has the units of velocity
and is referred
to as the overflow
rate q6. This
overflow
rate is the design factor
for settling
basins and corresponds
to the terminal
setting
velocity
of the particle
that is 166% removed.
Design Details
1. Detention
period:
for plain
sedimentation:
3 to 4 h, and for coagulated
sedimentation:

2 to

2.5

h.

2. Velocity of flow: Not greater than 36 cm/min (horizontal
flow).
3. Tank dimensions: L:B = 3 to 5:1. Generally L= 36 m (common) maximum 166 m. Breadth=
6 m to

16 m. Circular:

Diameter

not greater

than

66 m. generally

26 to 46 m.

4. Depth 2.5 to 5.6 m (3 m).
5. Surface Overflow
Rate: For plain
sedimentation
12666 to 18666 L/d/m2
thoroughly
flocculated
water 24666 to 36666 L/d/m2 tank area.
6. Slopes:
Rectangular
1% towards inlet
and circular
8%.
General Properties
of Colloids

tank

area;

for

2. Electrical
properties:
All colloidal
particles
are electrically
charged.
If
electrodes
from a D.C. source are placed in a colloidal
dispersion,
the particles
migrate
towards the pole of opposite
charge.
3. Colloidal
particles
are in constant
motion because of bombardment by molecules

dispersion medium. This motion is called
first
noticed it).

Brownian motion (named after

of

Robert Brown who

4. Tyndall
effect:
Colloidal
particles
have dimensioThese
are reversible
upon heating.
e.g. organics
in water.
5. Adsorption:
Colloids
have high surface
area and hence have a lot of active
surface
for adsorption
to occur.
The stability
of colloids
is mainly due to preferential
adsorption
of ions. There are two types of colloids:
i. Lyophobic
colloids:
that are solvent
hating.
These are irreversible
upon heating.
e.g. inorganic
colloids,
metal halides.
ii.
Lyophilic
colloids:
that are solvent
loving.
These are reversible
upon heating.
e.g. organics
in water.
Coagulation
and Flocculation
Colloidal
particles
are difficult
to separate
from water because they do not settle
by
gravity
and are so small that they pass through the pores of filtration
media.
To be removed, the individual
colloids
must aggregate
and grow in size.
The aggregation
of colloidal
particles
can be considered
as involving
two separate
and
distinct
steps:
1. Particle
transport
to effect
interparticle
collision.
2. Particle
destabilization
to permit attachment
when contact
occurs.
Transport
step is known as flocculation
whereas coagulationis
the overall
process
involving
destabilization
and transport.
Electrical
Double Layer
Although
individual
hydrophobic
colloids
have an electrical
charge,
a colloidal
dispersion
does not have a net electrical
charge. The diffuse
layer in a colloidal
dispersion
contains
a quantity
of counter
ions sufficient
to balance the electrical
charge on the particle.
The charge distribution
in the diffuse
layer of a negatively
charged colloid
can be represented
by the curve ABCD in the figure.
The ions involved
in this
electroneutrality
are arranged
in such a way as to constitute
what is
called
electrical
double layer.
Net repulsion

force,

which

may be considered

as energy

aggregation occurs. The magnitude of energy barrier
particle,
and (2) ionic composition of water.

barrier

must be overcome

before

depends on (1) charge on the

Destabilization
of Colloidal
Dispersion
Particle
destabilization
can be achieved by four mechanisms:
Change characteristics
of medium-Compression
of double layer.
Change characteristics
of colloid
particles-Adsorption
and charge
Provide bridges1. Enmeshment in a precipitate.
2. Adsorption
and interparticle
bridging.

neutralization.

(Double Layer Compression
Colloidal
opposite
of

systems could
to that of the

1:16:1666

as the

be destabilized
by the addition
of ions having a charge
colloid.
The coagulating
power of ions increased
in the

valence

of

the

ions

increased

in

the

ratio

from

1 to

2 to

3.

ratio
This

is called Schulze-Hardy rule.)
(Adsorption and Charge Neutralization
Some chemical
species are capable of being adsorbed at the surface
of colloidal
particles.
If the adsorbed species carry a charge opposite
to that of the colloids,
such adsorption
causes a reduction
of surface
potential
and a resulting
destabilization
of the colloidal
particle.
Reduction
of surface
charge by adsorption
is a much
different
mechanism than reduction
by double layer compression.
1. The sorbable
species are capable of destabilizing
colloids
at much lower dosage than
nonsorbable
"double layer compressing"
ions.
2. Destabilization
by adsorption
is stoichiometric.
Thus, the required
dosage of
coagulant
increases
as the concentration
of colloids
increases.

3. It

is

possible

to

overdose

restabilization
as a result
(Enmeshment in a Precipitate

a system

with

of a reversal

an adsorbable

species

and cause

of charge on the colloidal

particle.)

If certain
metal salts
are added to water or wastewater
in sufficient
amounts, rapid
formation
of precipitates
will
occur.
Colloids
may serve as condensation
nuclei
for
these precipitates
or may become enmeshed as the precipitates
settle.
Removal of
colloids
in this manner is frequently
referred
to as sweep-floc
coagulation.
Several
characteristics
that distinguish
sweep-floc
coagulation
from double layer compression
and adsorption
have been reported.
1. An inverse
relationship
exists
between the optimum coagulant
dosage and the
concentration
of colloids
to be removed. At low colloid
concentrations
a large excess
of coagulant
is required
to produce a large amount of precipitate
that will
enmesh the
relatively
few colloidal
particles
as it settles.
At high colloid
concentrations,
coagulation
will
occur at a lower chemical
dosage because the colloids
serve as nuclei
to enhance precipitate
formation.
2. Optimum coagulation
conditions
do not correspond
to a minimum zeta potential
but

depends on pH depending on solubility-pH
(Adsorption and Interparticle
Bridging
Many different
natural
proteineous
materials,
known to be effective

negative

relationship

for

that

coagulant.)

compounds such as starch,
cellulose,
polysaccharide
gums, and
as well as a wide variety
of synthetic
polymeric
compounds are
coagulating
agents.
Research has revealed
that both positive
and

polymers are capable of destabilizing

negatively

charged colloidal

particles.)

Flocculation

Flocculation

into

is

stimulation

compact, fast

by mechanical

settleable

particles

means to

agglomerate

(or flocs).

destabilised

Flocculation

particles

or gentle

agitation

results
from velocity
differences
or gradients
in the coagulated
water,
which
the fine moving, destabilized
particles
to come into contact
and become large,

settleable

flocs.

It

is a commonpractice

to provide

an initial

rapid

(or)

causes
readily

flash

mix

for the dispersal
of the coagulant
or other chemicals
into the water.
Slow mixing is
then done, during which the growth of the floc
takes place.
Rapid or Flash mixing is the process by which a coagulant
is rapidly
and uniformly
dispersed
through the mass of water.
This process usually
occurs in a small basin
immediately
preceding
or at the head of the coagulation
basin.
Generally,
the detention
period is 36 to 66 seconds and the head loss is 26 to 66 cms of water.
Here colloids
are

destabilised

and

the

Slow mixing brings the
during
rapid mixing.
Perikinetic

and

nucleus

contacts

Orthokinetic

The flocculation

process

for

the

between

floc

the

is

finely

formed.

divided

destabilised

matter

formed

Flocculation

can be broadly

classified

into

two types,

perikinetic

and

orthokinetic.

Perikinetic
particles)
colloidal
the

flocculation
refers to flocculation
due to Brownian motion of colloidal
particles

results

from

their

rapid

(contact
particles.

or collisions
of colloidal
The random motion of

and random bombardment

by the

molecules

of

fluid.

Orthokinetic
flocculation
resulting
from bulk fluid

refers
to contacts
or collisions
of colloidal
particles
motion,
such as stirring.
In systems of stirring,
the

velocity
of the fluid varies both spatially
(from point to point) and temporally
(from
time to time). The spatial
changes in velocity
are identified
by a velocity
gradient,
G. G is estimated as G=(P/aEV)1/2, where P=Power, V=channel volume, and ,5: Absolute
viscosity.
Mechanism

of

Flocculation

Gravitational
flocculation:
Baffle
type mixing basins are examples of gravitational
flocculation.
water flows by gravity
and baffles
are provided
in the basins which
induce the required
velocity
gradients
for achieving
floc formation.
Mechanical
flocculation:
Mechanical
flocculators
consists
of revolving
paddles with
horizontal
or vertical
shafts
or paddles suspended from horizontal
oscillating
beams,
moving up and down.
Coagulation
in water Treatment

Salts

of Al(III)

and Fe(III)

are commonly used as coagulants

in water and wastewater

treatment.

when a salt of Al(III)
and Fe(III)
is added to water, it dissociates
to yield trivalent
ions, which hydrate to form aquometal complexes Al(H20)63+ and Fe(H20)63+. These

complexes then pass through
a series
of hydrolytic
reactions
in which H20 molecules
in
the hydration
shell
are replaced
by OH- ions to form a variety
of soluble
species such

as Al(OH)2+ and Al(OH)2+.

These products

adsorb

the

very

strongly

onto

surface

are quite

effective

of most negative

as coagulants

as they

colloids.

Destabilization
using Al(III)
and Fe(III)
Salts
Al(III)
and Fe(III)
accomplish destabilization
by two mechanisms:
(1) Adsorption and charge neutralization.
(2) Enmeshment in a sweep floc.
Interrelations
between pH, coagulant
dosage, and colloid
concentration
determine
mechanism responsible
for coagulation.
Charge on hydrolysis
products
and precipitation
of metal hydroxides
are both controlled
by pH. The hydrolysis
products
possess a positive
charge at pH values below isoelectric
point of the metal hydroxide.
Negatively
charged species which predominate
above iso-electric
point,
are ineffective
for the destabilization
of negatively
charged
colloids.

Precipitation

of

The solubility

amorphous

metal

of Al(OH)3(s)

hydroxide

is

necessary

as the pH increases
or decreases from that value.
Thus,
establish
optimum conditions
for coagulation.
Alum and Ferric
Chloride
reacts with natural
alkalinity

Al2(SO4)3.14H20 + 6 HCO32 Al(0H)3(s)
FeCl3 + 3 HCO3Fe(OH)3(S) +3 CO2 + 3 ClJar

for

sweep-floc

and Fe(OH)3(s) is minimal at a particular

coagulation.

pH and increases

pH must be controlled
in water

to

as follows:

+ 6CO2 +14 H20 + 3 S042-

Test

The jar test is a common laboratory
procedure
used to determine
the optimum operating
conditions
for water or wastewater
treatment.
This method allows adjustments
in pH,
variations
in coagulant
or polymer dose, alternating
mixing speeds, or testing
of
different
coagulant
or polymer types,
on a small scale in order to predict
the
functioning
of a large scale treatment
operation.
Jar Testing
Apparatus
The jar testing
apparatus
consists
of six paddles which stir
the contents
of six 1
liter
containers.
One container
acts as a control
while the operating
conditions
can be
varied
among the remaining
five
containers.
A rpm gage at the top-center
of the device
allows for the uniform
control
of the mixing speed in all of the containers.
Jar

Test

Procedure

The jar test procedures
involves
the following
steps:
Fill
the jar testing
apparatus
containers
with sample water.
One container
will
be used
as a control
while the other 5 containers
can be adjusted
depending on what conditions
are being tested.
For example, the pH of the jars can be adjusted
or variations
of
coagulant
dosages can be added to determine
optimum operating
conditions.
Add the coagulant
to each container
and stir
at approximately
166 rpm for 1 minute.
The
rapid mix stage helps to disperse
the coagulant
throughout
each container.
Turn

off

the

mixers

and

allow

the

containers

to

settle

for

36 to

45 minutes.

Then

measure the final
turbidity
in each container.
Reduce the stirring
speed to 25 to 35 rpm and continue
mixing for 15 to 26 minutes.
This slower mixing speed helps promote floc formation
by enhancing
particle
collisions
which lead to larger
flocs.
Residual
turbidity
vs. coagulant
dose is then plotted
and optimal
conditions
are
determined.
The values that are obtained
through the experiment
are correlated
and
adjusted
in order to account for the actual
treatment
system.
Filtration

The resultant
water after
sedimentation
will
not be pure, and may contain
some very
fine suspended particles
and bacteria
in it.
To remove or to reduce the remaining
impurities
still
further,
the water is filtered
through the beds of fine granular
material,
such as sand, etc. The process of passing the water through the beds of such
granular
materials
is known as Filtration.
How Filters
There
are

work:
Filtration
four
basic
filtration

Mechanisms
mechanisms:

SEDIMENTATION : The mechanism of sedimentation
associate
settling
velocity
of the particle,
and

reach

the

INTERCEPTION :

is due to force of gravity
which causes it to cross the

and the
streamlines

collector.

Interception

of

particles

is

common for

large

particles.

If

a large

enough particle
follows
the streamline,
that lies very close to the media surface
it
will
hit the media grain and be captured.
BROWNIANDIFFUSION : Diffusion
towards media granules
occurs for very small particles,
such as viruses.
Particles
move randomly about within
the fluid,
due to thermal
gradients.
This mechanism is only important
for particles
with diameters
< 1 micron.
INERTIA : Attachment
by inertia
occurs when larger
particles
move fast enough to
travel
off their
streamlines
and bump into media grains.
Filter

Materials

Sand: Sand, either
fine or coarse,
is generally
used as filter
media. The size of the
sand is measured and expressed
by the term called
effective
size.
The effective
size,
i.e.
D16 may be defined
as the size of the sieve in mm through which ten percent of the
sample of sand by weight will
pass. The uniformity
in size or degree of variations
in
sizes of particles
is measured and expressed
by the term called
uniformity
coefficient.

The uniformity

coefficient,

size

in

mm through

size

of

the

which

i.e.

(D66/D16) may be defined

66 percent

of

the

sample of

as the ratio

sand will

pass,

of the sieve
to the

effective

sand.

Gravel:
The layers
of sand may be supported
on gravel,
which permits
the filtered
water
to move freely
to the under drains,
and allows the wash water to move uniformly
upwards.
Other materials:
Instead
of using sand, sometimes,
anthrafilt
is used as filter
media.
Anthrafilt
is made from anthracite,
which is a type of coal-stone
that burns without
smoke or flames.
It is cheaper and has been able to give a high rate of filtration.
Types of Filter
Slow sand filter:
They consist
of fine sand, supported
by gravel.
They capture
particles
near the surface
of the bed and are usually
cleaned by scraping
away the top
layer of sand that contains
the particles.
Rapid-sand
filter:
They consist
of larger
sand grains
supported
by gravel
and capture
particles
throughout
the bed. They are cleaned by backwashing water through the bed to
lift
out&#39;
the particles.
Multimedia
filters:
They consist
of two or more layers
of different
granular
materials,
with different
densities.
Usually,
anthracite
coal,
sand, and gravel
are used. The
different
layers
combined may provide
more versatile
collection
than a single
sand
layer.
Because of the differences
in densities,
the layers
stay neatly
separated,
even
after
backwashing.
Principles
of Slow Sand Filtration
In a slow sand filter
impurities
in the water are removed by a combination
of
processes:
sedimentation,
straining,
adsorption,
and chemical
and bacteriological
action.

During the first
few days, water is purified
mainly by mechanical
and physical-chemical
processes.
The resulting
accumulation
of sediment and organic
matter forms a thin layer
on the sand surface,
which remains permeable and retains
particles
even smaller
than
the spaces between the sand grains.

As this

layer

(referred

to as Schmutzdecke)

develops,

it

becomes living

quarters

of

vast numbers of micro-organisms
which break down organic
material
retained
from the
water,
converting
it into water,
carbon dioxide
and other oxides.
Most impurities,
including
bacteria
and viruses,
are removed from the raw water as it
passes through the filter
skin and the layer of filter
bed sand just
below. The
purification
mechanisms extend from the filter
skin to approx.
6.3-6.4
m below the
surface
of the filter
bed, gradually
decreasing
in activity
at lower levels
as the
water becomes purified
and contains
less organic
material.
when the micro-organisms
become well established,
the filter
will
work efficiently
and
produce high quality
effluent
which is virtually
free of disease carrying
organisms
and
biodegradable
organic
matter.
They are suitable
for treating
waters with low colors,
low turbidities
and low
bacterial

contents.

Sand Filters
vs. Rapid Sand Filters
Base material:
In SSF it varies
from 3 to 65 mm in size and 36 to
in RSF it varies
from 3 to 46 mm in size and its depth is slightly
to

75 cm in depth while
more, i.e.
about 66

96 cm.

Filter
sand: In SSF the effective
size ranges between 6.2
coefficient
between 1.8 to 2.5 or 3.6. In RSF the effective
6.55 and uniformity
coefficient
between 1.2 to 1.8.

to 6.4
size

mm and uniformity
ranges between 6.35

to

Rate of filtration:
In SSF it is small,
such as 166 to 266 L/h/sq.m.
of filter
area
while in RSF it is large,
such as 3666 to 6666 L/h/sq.m.
of filter
area.
Flexibility:
SSF are not flexible
for meeting variation
in demand whereas RSF are quite
flexible
for meeting reasonable
variations
in demand.
Post treatment
required:
Almost pure water is obtained
from SSF. However, water may be
disinfected
slightly
to make it completely
safe. Disinfection
is a must after
RSF.
Method of cleaning:
Scrapping
and removing of the top 1.5 to 3 cm thick
layer
is done
to clean SSF. To clean RSF, sand is agitated
and backwashed with or without
compressed
air.

Loss of head: In case of SSF approx.
16 cm is the initial
loss,
and 6.8 to 1.2m is the
final
limit
when cleaning
is required.
For RSF 6.3m is the initial
loss,
and 2.5 to
3.5m is the final
limit
when cleaning
is required.
Typical

Rapid Gravity

Isometric
Clean

view

water

of

Filter

Rapid

Flow Operation

Sand Filter

Headloss

Several equations
have been developed
porous medium. Carman-Kozeny equation

to describe
the flow of clean water through
used to calculate
head loss is as follows:

a

h= f (1-,A)Lvs2
dg = geometric
Ng = Reynolds

mean diameter
number

L = viscosity,

between

sieve

sizes

d1 and

d2

N-s/m2

Backwashing of Rapid Sand Filter
For a filter
to operate
efficiently,
it must be cleaned before the next filter
run. If
the water applied
to a filter
is of very good quality,
the filter
runs can be very
long. Some filters
can operate
longer than one week before needing to be backwashed.
However, this
is not recommended as long filter
runs can cause the filter
media to pack
down so that it is difficult
to expand the bed during the backwash.
Treated water from storage
is used for the backwash cycle.
This treated
water is
generally
taken from elevated
storage tanks or pumped in from the clear well.
The filter
backwash rate has to be great enough to expand and agitate
the filter
media
and suspend the floc
in the water for removal.
However, if the filter
backwash rate is
too high, media will
be washed from the filter
into the troughs
and out of the filter.
when is Backwashing Needed
The filter
should be backwashed when the following
conditions
have been met:
The head loss is so high that the filter
no longer produces water at the desired
rate;
and/or

Floc starts
to break
increases;
and/or
A filter
run reaches

through

the

filter

a given

hour of

and the

turbidity

in

the

filter

effluent

operation.

Operational
Troubles
in Rapid Gravity
Filters
Air Binding
:
when the filter
is newly commissioned,
the loss of head of water percolating
through
the filter
is generally
very small.
However, the loss of head goes on increasing
as
more and more impurities
get trapped
into it.
A stage is finally
reached when the frictional
resistance
offered
by the filter
media
exceeds

the

static

head

of

water

above

the

and

bed.

Most

of

this

resistance

is

offered

by the top 16 to 15 cm sand layer.
The bottom sand acts like
a vacuum, and water is
sucked through the filter
media rather
than getting
filtered
through
it.
The negative
pressure
so developed,
tends to release
the dissolved
air and other gases
present
in water.
The formation
of bubbles takes place which stick
to the sand grains.
This phenomenon is known as Air Binding
as the air binds the filter
and stops its
functioning.
To avoid such troubles,
the filters
are cleaned as soon as the head loss exceeds the
optimum allowable
value.
Formation

of

Mud Balls

:

The mud from the atmosphere usually
mat. During inadequate
washing this

accumulates
on the sand surface
to form a dense
mud may sink down into the sand bed and stick
to

the sand grains
and other arrested
impurities,
thereby
forming
mud balls.
Cracking
of Filters
:
The fine
sand contained
in the top layers
of the filter
bed shrinks
and causes the
development
of shrinkage
cracks in the sand bed. with the use of filter,
the loss of
head and, therefore,
pressure
on thesand bed goes on increasing,
which further
goes on
widening
these cracks.
Remedial Measures to Prevent Cracking
of Filters
andFormation
of Mud Balls
Breaking the top fine mud layer with rakes and washing off the particles.
washing the filter
with a solution
of caustic
soda.
Removing, cleaning
and replacing
the damaged filter
sand.
Standard design practice
of Rapid Sand filter:
Maximum length of lateral
= not less
than 66 times its diameter.
Spacing of holes = 6 mm holes at 7.5 cm c/c or 13 at 15
c/c. C.S area of lateral
= not less than 2 times area of perforations.
C.S area of
manifold
= 2 times total
area of laterals.
Maximum loss of head = 2 to 5 m. Spacing of
laterals
= 15 to 36 cm c/c. Pressure of wash water at perforations
= not greater
than
1.65 kg/cm2. Velocity
of flow in lateral
= 2 m/s. Velocity
of flow in manifold
= 2.25
m/s. Velocity
of flow in manifold
for washwater= 1.8 to 2.5 m/s. Velocity
of rising
washwater=

6.5

to

1.6

m/min.

Time of backwashing
= 16 to
board = 66 cm. Bottom slope

Amount

of

washwater

= 6.2

to

15 min. Head of water over
= 1 to 66 towards manifold.

the

6.4%

of

filter

total

filtered

= 1.5

to

water.

2.5 m. Free

Q = (1.71 x b x h3/2)
where Q is

in

m3/s,

b is

in

m, h is

in

m. L:B = 1.25

to

1.33:1

.

Disinfection

The filtered
water may normally
contain
some harmful
disease producing
bacteria
in it.
These bacteria
must be killed
in order to make the water safe for drinking.
The process
of killing
these bacteria
is known as Disinfection
or Sterilization.
Disinfection

Kinetics

when a single
unit of microorganisms
reduction
in microorganisms
follows
dN/dt=-kN

This

is exposed
a first-order

a single
reaction.

unit

of

disinfectant,

the

N=N6e-kt

equation

is

known as Chicks

Law:-

N = number of microorganism
k = disinfection
t = contact
time
Methods

to

of

(N6 is initial

number)

constant

Disinfection

1. Boiling:
The bacteria
present
in water can be destroyed
by boiling
it for a long
time.
However it is not practically
possible
to boil
huge amounts of water.
Moreover it
cannot take care of future
possible
contaminations.
2. Treatment
with Excess Lime: Lime is used in water treatment
plant for softening.
But
if excess lime is added to the water,
it can in addition,
kill
the bacteria
also.
Lime
when added raises
the pH value o water making it extremely
alkaline.
This extreme
alkalinity
has been found detrimental
to the survival
of bacteria.
This method needs
the removal of excess lime from the water before it can be supplied
to the general
public.
Treatment
like
recarbonation
for lime removal should be used after
disinfection.

3. Treament with Ozone: Ozone readily
breaks down into normal oxygen, and releases
nascent oxygen. The nascent oxygen is a powerful
oxidising
agent and removes the
organic
matter as well as the bacteria
from the water.
4. Chlorination:
The germicidal
action
of chlorine
is explained
by the recent theory
of Enzymatic hypothesis,
according
to which the chlorine
enters the cell walls of
bacteria
and kill
the enzymes which are essential
for the metabolic
processes of living
organisms.
Chlorine
Chemistry
Chlorine
is added to the water supply in two ways. It is most often added as a gas,

Cl2(g).

However, it

bleach.

Chlorine

also can be added as a salt,

gas dissolves

Cl2(g)
Once dissolved,

in water

Cl2(aq)

following

such as sodium hypochlorite

(NaOCl) or

Henry&#39;s
Law.

KH =6.2 x 19-2

the following
reaction occurs forming
Cl2(aq)+H2O HOCl + H+ + Cl-

hypochlorous

acid (HOCl):

Hypochlorous

acid is a weak acid that
HOCl OCl

All

forms of chlorine

+ H+

Ka

dissociates
= 3.2

to form hypochlorite

ion (OCl).

X 10-8

are measured as mg/L of C12 (Mw = 2 x 35.45 = 76.9 g/mol)

Hypochlorous
acid and hypochlorite
ion compose what is called
the free chlorine
residual.
These free chlorine
compounds can react with many organic
and inorganic
compounds to form chlorinated
compounds. If the products
of these reactions
posses
oxidizing
potential,
they are considered
the combined chlorine
residual.
A common
compound in drinking
water systems that reacts with chlorine
to form combined residual
is ammonia. Reactions
between ammonia and chlorine
form chloramines,
which is mainly

monochloramine (NH2Cl), although

some dichloramine

(NHCl2) and trichloramine

(NCl3)

also can form. Many drinking
water utilities
use monochloramine
as a disinfectant.
If
excess free chlorine
exits
once all ammonia nitrogen
has been converted
to
monochloramine,
chloramine
species are oxidized
through what is termed the breakpoint
reactions.
The overall
reactions
of free chlorine
and nitrogen
can be represented
by
two simplified
reactions
as follows:
Monochloramine
Formation
Reaction.
This reaction
occurs rapidly
when ammonia nitrogen
is combined with free chlorine
up to a molar ratio
of 1:1.
HOCl +NH3

Breakpoint
Reaction:
ratio,
monochloramine

NH2Cl

+ HOCl

when excess free chlorine
is removed as follows:

is

added beyond the

1:1

initial

molar

2NH2Cl + HOCl N2(g)+ 3H++ 3Cl-+ H20
The formation
between

of

chlorine

chloramines
dose

and

Free Chlorine,
Chlorine

and the
the

breakpoint

amount

and

Chloramine,

form

reaction
of

create

chlorine

as

and Ammonia Nitrogen

a unique

illustrated

relationship
below.

Reactions

Demand

Free chlorine
and chloramines
readily
react with a variety
compounds, including
substances,
and inorganic
substances
like
iron and manganese. The stoichiometry
chlorine
reactions
with organics
can be represented
as shown below:
HOCl:
1/10C5H702N
OCl:

+ HOCl4/10CO2

1/10C5H702N
NH2Cl:
1/10C5H702N

Chlorine
reduced
chlorine

+ OCl

4/10CO2

+ NH2Cl

+ 1/10HCO3+ 1/10HCO3-

+ 9/10H204/10CO2

demand can be increased
iron at corrosion
sites
and

iron

are

+ 1/10NH4++
+ 1/10NH4++
+ 1/10HCO3-

H+ + ClCl-

organic
of

+ 1/10H2O

+ 1/10H2O

+ 11/10NH4++

Cl-

by oxidation
reactions
with inorganics,
such as
at the pipe wall.
Possible
reactions
with all forms

of

as follows:

Treatment
Plant Layout and Siting
Plant layout
is the arrangement
of designed treatment
units
on the selected
site.
Siting
is the selection
of site for treatment
plant based on features
as
character,
topography,
and shoreline.
Site development
should take the advantage of the
existing
site topography.
The following
principles
are important
to consider:
1. A site on a side-hill
can facilitate
gravity
flow that will
reduce pumping
requirements
and locate
normal sequence of units without
excessive
excavation
or fill.
2. when landscaping
is utilized
it should reflect
the character
of the surrounding
area. Site development
should alter
existing
naturally
stabilized
site
contours
and
drainage
as little
as possible.
3. The developed site
should be compatible
with the existing
land uses and the
comprehensive
development
plan.
Treatment
Plant Hydraulics
Hydraulic
profile
is the graphical
representation
of the hydraulic
grade line through
the treatment
plant.
The head loss computations
are started
in the direction
of flow
using water surface
in the influent
of first
treatment
unit as the reference
level.
The total
available
head at the treatment
plant
is the difference
in water surface
elevations

in

the

influent

of

first

treatment

unit

and

that

in

the

effluent

of

last

treatment
unit.
If the total
available
head is less than the head loss through the
plant,
flow by gravity
cannot be achieved.
In such cases pumping is needed to raise
head so that flow by gravity
can occur.
There are many basic principles
that must be considered
when preparing
the hydraulic
profile
through the plant.
Some are listed
below:

the

1. The hydraulic
initial

profiles

are

prepared

at

peak and average

design

flows

and at minimum

flow.

2. The hydraulic
profile
is generally
prepared for all main paths of flow through the
plant.
3. The head loss through the treatment
plant is the sum of head losses in the treatment
units
and the connecting
piping
and appurtenances.
4. The head losses through the treatment
unit include
the following:
. Head
. Head

losses
losses

at
at

the
the

influent
effluent

structure.
structure.

OU&#39;|Q.O&#39
-hrbqncrm
. Head losses

through

. Miscellaneous

and

. The total
f following:

loss

. Head
. Head

due
due

loss
loss

the

unit.

free

fall

through

the

to
to

due to

contraction

. Head

due

friction.

to

allowance.

pipings,

channels

and appurtenances

is

the

and enlargement.

. Head loss due to bends, fittings,
gates,
valves,
and meters.
. Head required
over weir and other hydraulic
controls.
g. Free-fall
surface
allowance.
water Distribution
Systems
The purpose of distribution
system is to deliver
water to consumer with appropriate
quality,
quantity
and pressure.
Distribution
system is used to describe
collectively
the facilities
used to supply water from its source to the point of usage.
Requirements
of Good Distribution
System
1. water quality
should not get deteriorated
in the distribution
pipes.
2. It should be capable of supplying
water at all the intended
places with sufficient
pressure
head.
3. It should be capable of supplying
the requisite
amount of water during fire
fighting.
4. The layout
should be such that no consumer would be without
water supply,
during
repair
of any section
of the system.
5. All the distribution
pipes should be preferably
laid one metre away or above the
sewer

sum

entrance.
exit.

. Head loss
loss

surface

connecting

the

lines.

6. It should be fairly
water-tight
as to keep losses due
Layouts of Distribution
Network
The distribution
pipes are generally
laid below the road
layouts
generally
follow
the layouts
of roads. There are,
types of pipe networks;
any one of which either
singly
or
for a particular
place.
They are:
Dead End System
Grid Iron System
Ring System
Radial System
Dead End System:
It is suitable
for old towns and cities
having no definite

to

leakage

to

the

minimum.

pavements,
and as such their
in general,
four different
in combinations,
can be used

pattern

of

roads.

Advantages:
1. Relatively
cheap.
2. Determination
of discharges
and pressure
easier
due to less number of valves.
Disadvantages
1. Due to many dead ends, stagnation
of water occurs in pipes.
Grid Iron System:
It is suitable
for cities
with rectangular
layout,
where the water mains and branches
are laid
in rectangles.
Advantages:
1. water is kept in good circulation
due to the
2. In the cases of a breakdown in some section,

absence of dead ends.
water is available
from

some other

direction.

Disadvantages
1. Exact calculation

of

sizes

of

pipes

is

not

possible

due to

provision

of

valves

on

all

branches.

Ring System:
The supply main is laid all along the peripheral
roads and sub
mains branch out from the mains. Thus, this
system also follows
the grid iron system with the flow pattern
similar
in character
to that of dead end system. So, determination
of the size
of pipes is easy.

Advantages:
1.water
can
besupplied
toany
point
from
atleast
two
directions.

Distribution

Reservoirs

Distribution

reservoirs,

also

called

service

reservoirs,

are the

storage

reservoirs,

which store the treated water for supplying water during emergencies (such as during
fires,
repairs,
etc.) and also to help in absorbing the hourly fluctuations
in the
normal
water
Functions
of

demand.
Distribution

Reservoirs:

to absorb the hourly variations
in demand.
to maintain
constant
pressure
in the distribution
mains.
water stored can be supplied
during emergencies.
Location
and Height of Distribution
Reservoirs:
should be located
as close as possible
to the center of demand.
water level
in the reservoir
must be at a sufficient
elevation
to
at an adequate pressure.
Types of Reservoirs
1. Underground
reservoirs.
2. Small ground level
reservoirs.
3. Large ground level
reservoirs.
4.

Overhead

permit

gravity

flow

tanks.

Storage Capacity
of Distribution
Reservoirs
The total
storage
capacity
of a distribution
reservoir
is the summation of:
1. Balancing
Storage:
The quantity
of water required
to be stored in the reservoir
equalising
or balancing
fluctuating
demand against
constant
supply is known as the

balancing

storage

(or equalising

or operating

storage).

The balance storage

for

can be

worked out by mass curve method.
2. Breakdown Storage:
The breakdown storage or often called
emergency storage
is the
storage
preserved
in order to tide over the emergencies
posed by the failure
of pumps,
electricity,
or any othe mechanism driving
the pumps. A value of about 25% of the total
storage
capacity
of reservoirs,
or 1.5 to 2 times of the average hourly
supply,
may be
considered
as enough provision
for accounting
this
storage.
3. Fire Storage:
The third
component of the total
reservoir
storage
is the fire
storage.
This provision
takes care of the requirements
of water for extinguishing
fires.
A provision
of 1 to 4 per person per day is sufficient
to meet the requirement.
The total
reservoir
storage
can finally
be worked out by adding all the three storages.
Pipe Network Analysis
Analysis
of water distribution
system includes
losses in the various
pipe lines,
and resulting
network,
the following
two conditions
must be
1. The algebraic
sum of pressure
drops around
can be no discontinuity
in pressure.
2. The flow entering
a junction
must be equal
the law of continuity
must be satisfied.
Based on these two basic principles,
the pipe
methods of successive
approximation.
The widely
the Hardy-Cross
method.
Hardy-Cross
Method
This method consists
of assuming a distribution
that the principle
of continuity
is satisfied

determining
quantities
of flow and head
residual
pressures.
In any pipe
satisfied:
a closed loop must be zero, i.e.
there
to the

flow

leaving

that

networks are generally
used method of pipe

of flow in the
at each junction.

junction;

solved
network

i.e.

by the
analysis

network in such a way
A correction
to these

is

assumed flows is then computed successively
for each pipe
the correction
is reduced to an acceptable
magnitude.
If Qa is the assumed flow and Q is the actual
flow in the
correction
E, is given by
El=Q-Qa.;

or

loop

in the

pipe,

then

network,

until

the

Q=Qa+El

Now, expressing

the head loss

(HL) as

HL=K.Qx

we have,

the

head loss

in

a pipe

=K.(Qa+E,)x
=K.[Qax
+x.Qax-12,
+.........negligible
terms]
=K.[Qax + x.Qax-1a,]
Now, around a closed

loop,

the

summation

of

head losses

++++++++
=q++++++++++_,K.[Qax + x.Qax-12,]
or ++++++++++_,K.Qax= -++.,Kx Qax-12,
Since,

E, is

the

same for

all

the

pipes

must be zero.

= 6

of the

considered

loop,

it

can be taken

out

of

thesummation.
=,-|&#39;-|&#39;-|&#39;-|&#39;
-|&#39;_.K.Qax
=--"E...
_.Kx
Qax-1
or++++++++ ++a,++=-.1K.Qax/ .1x.KQax-1
Since a, is given the same sign (direction)
of the above equation
summation. Hence,

is

taken

as the

in all

absolute

pipes of the loop,

sum of

the

individual

the denominator
items

in

the

or++++++++ ++a,++=-.,K.Qax/ .1 l x.KQax-1 l
or++++++++ ++E,++=-.1HL / x..,++lHL/Qal
where HL is the head loss for assumed flow Qa.
The numerator
in the above equation
is the algebraic
sum of the head losses in the
various
pipes of the closed loop computed with assumed Flow. Since the direction
and
magnitude of flow in these pipes is already
assumed, their
respective
head losses with
due regard to sign can be easily
calculated
after
assuming their
diameters.
The
absolute
sum of respective
KQax-1 or HL/Qa is then calculated.
Finally
the value

of E,++is found out for
adjustments

each loop,

are made until

the

and the assumed flows

desired

accuracy

is

are corrected.

The value of x in Hardy- Cross method is assumed to be constant
william&#39;s
formula, and 2 for Darcy-weisbach formula)
Domestic

Module
Module

Module

Module

wastewater

Module

and

(i.e.

Treatment

7: Municipal
wastewater
Quantity
and Quality
Lecture 17: wastewater
Quality
and Quantity
Estimation
8: Municipal
wastewater
Collection
and Treatment
Philosophy
Lecture 18: Layout and Design of Municipal
Sewers
Lecture 19: Sewer Appurtenances,
Sump-well and Sewage Pumping
Lecture
26: wastewater
Treatment
Philosophy
9: Preliminary
and Primary wastewater
Treatment
Lecture
21: Bar Rack/Screens
and Equalization
Tank Design
Lecture
22: Grit Chamber and Primary Sedimentation
Tank Design
16: Secondary wastewater
Treatment
Lecture
23: Fundamentals
of Applied
Microbiology
Lecture
24: Activated
Sludge Process Description
Lecture
25: Design of Activated
Sludge Systems
Lecture
26: Design of Activated
Sludge Systems
Lecture
27: Aerator
Design for Activated
Sludge Process
Lecture
28: Trickling
Filter
Fundamentals
and Design
Lecture
29: Other Aerobic
Treatment
Systems
Lecture

Module

Collection

36:

Lecture
31:
Lecture
32:
11: Tertiary
Lecture
33:
Lecture
34:
Lecture
35:
12: Residuals
Lecture
36:

Anaerobic

Treatment

Repeated

obtained.

Fundamentals

Design of Anaerobic
Reactors
Design of UASB Reactors
and Advanced Treatment
Nitrification:
Process Description
and Design
Denitrification:
Process Description
and Design
Phosphorus Removal and Other Advanced Treatment
Management
Fundamentals
of Residual
Management

1.85 for

Hazen-

Module

Lecture
38: Siting
and Hydraulics
of wastewater
Treatment
Plants
14: Treated
Effluent
Disposal
Lecture
39: Treated
Effluent
Discharge,
Reuse and Recycling

Module

15:

Natural

Methods

of

wastewater

Treatment

Lecture 46: Oxidation,
Facultative
and Anaerobic
Ponds
Lecture 41: Phyto-Remediation
and Root-Zone Treatment
Module 16: Hygiene and Sanitation
in Rural/Semi-Rural
Areas
Lecture 42: Septic Tanks, Soak Pits,
Cesspools,
Dry Latrines
wastewater
Quantity
Estimation
The flow of sanitary
sewage alone in the absence of storms in dry season

dry weather flow

is

known as

(DwF).

Quantity:
Per capita
sewage contributed
per day x
Population
Sanitary
sewage is mostly the spent water of the community draining
into the sewer
system. It has been observed that a small portion
of spent water is lost in
evaporation,
seepage in ground,
leakage,
etc. Usually
86% of the water supply may be
expected to reach the sewers.
Fluctuations
in Dry weather Flow
Since dry weather flow depends on the quantity
of water used, and as there are
fluctuations
in rate of water consumption,
there will
be fluctuations
in dry weather

flow also. In general, it can be assumed that (i) Maximum daily flow
daily flow and (ii)
Minimum daily flow = 2/3 x (average daily flow).
Population
Equivalent
Population
equivalent
is a parameter
used in
from industrial
establishments
for accepting
of industrial
sewage is, thus, written
as

Std. BOD5= (Std.

the conversion
into sanitary

= 2 x average

of contribution
sewer systems.

of wastes
The strength

BOD5of domestic sewage per person per day) x
(population
equivalent)

Design Periods & Population
Forecast
This quantity
should be worked out with due provision
for the estimated
requirements
of
the future
. The future
period for which a provision
is made in the water supply scheme
is known as thedesign
period.
It is suggested that the construction
of sewage treatment
plant may be carried
out in phases with an initial
design period
ranging from 5 to 16
years excluding
the construction
period.
Design period is estimated
based on the following:
Useful life
of the component, considering
obsolescence,
wear, tear,
etc.
Expandability
aspect.
Anticipated
rate of growth of population,
including
industrial,
commercial
developments
& migration-immigration.
Available

resources.

Performance
of the system during
initial
period.
Population
forecasting
methods:
The various
methods adopted for estimating
future
particular
method to be adopted for a particular
largely
on the factors
discussed
in the methods,
discrection
and intelligence
of the designer.
. Arithmetic
. Geometric
. Incremental

populations
are given below. The
case or for a particular
city
depends
and the selection
is left
to the

Increase
Method
Increase
Method
Increase
Method

m\lO\U&#39;|. Decreasing
Rate of Growth Method
. Simple Graphical
Method
. Comparative
Graphical
Method
. Ratio

Method

. Logistic

wastewater

Curve Method

Characterization

To design a treatment
process properly,
characterization
of wastewater
is perhaps
most critical
step. wastewater
characteristics
of importance
in the design of the
activated
sludge process can be grouped into the following
categories:
Temperature
pH
Colour and Odour
Carbonaceous substrates

Nitrogen
Toxic

metals

Temperature

Chlorides
and compounds

Total

and volatile

suspended solids

(TSS and VSS)

the

Observation
of temperature
of sewage is useful
in indicating
the solubility
of oxygen
which affects
oxygen transfer
capacity
of aeration
equipments
and rate of biological
activity.
Normally
the temperature
of domestic and municipal
sewage is slightly
higher
than that of the water supply.
pH
The pH of fresh domestic sewage is slightly
more than that of the water supply to the
community.
However, the onset of septic
conditions
may lower the pH while the presence
of industrial
wastes may produce extreme fluctuations.
Colour

and

Odour

Fresh domestic
sewage has slightly
soapy and earthy odour and cloudy appearance
depending upon its concentration.
with the passage of time, the sewage becomes stale,
darkening
in colour with a pronounced smell due to microbial
activity.
Carbonaceous

Constituents:

Carbonaceous constituents
are measured by BOD, COD or TOC analyses.
while the BOD has
been the common parameter to characterize
carbonaceous
material
in wastewater,
COD is
becoming more common in most current
comprehensive
computer simulation
design models.
Biochemical
Oxygen Demand: The BOD test gives a measure of the oxygen utilized
by
bacteria
during the oxidation
of organic
material
contained
in a wastewater
sample. The
test is based on the premise that all the biodegradable
organic
material
contained
in
the wastewater
sample will
be oxidized
to CO2 and H20, using molecular
oxygen as the
electron
acceptor.
Hence, it is a direct
measure of oxygen requirements
and an indirect
measure of biodegradable
organic
matter.
Chemical Oxygen Demand: The COD test is based on the principle
that most organic
compounds are oxidized
to CO2 and H20 by strong oxidizing
agents under acid conditions.
The measurement represents
the oxygen that would be needed for aerobic
microbial
oxidation,
assuming that all organics
are biodegradable.
Total Organic Carbon: The total
carbon analyzer
allows
a total
soluble
carbon analysis
to be made directly
on an aqueous sample. In many cases TOC can be correlated
with COD
and occasionally
with BOD values.
As the time required
for carbon analysis
is generally
short,
such correaltions
are extremely
helpful
when monitoring
treatment
plant flows
for efficiency
control.
Nitrogenous
Constituents:
The principal
nitrogenous
compounds in domestic sewage are proteins,
amines, amino

acids and urea. Nitrogen may be present in different
forms such as (i) organic
nitrogen,
(ii)
albiminoid
nitrogen,
(iii)
ammonia nitrogen,
(iv) nitrite
nitrogen,

(v)

nitrate
nitrogen,
depending on the condition
of sewage. The determination
of various
forms of nitrogen
helps in the selection
of proper biological
treatment
units.
Phosphorus
Municipal
wastewaters
contain
phosphorus in three different
forms:
1. organic
phosphorus,
2. orthophosphorus,
and 3. condensed phosphorus.
when treating
wastewater
biologically,
bacteria
assimilate
orthophosphate
during their
growth process.
However,
condensed phosphates must first
undergo enzymatic
hydrolysis
to the ortho form before
they can be assimilated.
In biological
wastewater
treatment,
assimilation
is the only
means by which phosphorus is removed, except,
possibly,
when the water has an unusual
chemical makeup and precipitation
onto biological
flocs
occurs.
Chlorides

Chloride
ion may be present
in combination
with one or more of the cations
of calcium,
magnesium, iron and sodium. Chlorides
of these minerals
are present
in water because of
their
high solubility
in water.
Each human being consumes about six to eight grams of
sodium chloride
per day, a part of which is discharged
through
urine and night soil.
Thus, excessive
presence of chloride
in water indicates
sewage pollution.
IS value for
drinking
water is 256 to 1666 mg/L.
Solids

Total solids
include
both the suspended solids
and the dissolved
solids
which are
obtained
by separating
the solid
and liquid
phase by evaporation.
Suspended solids
are a combination
of settleable
solids
andnonsettleable
solids,
which
are usually
determined
by filtering
a wastewater
sample through
a glass fiber
filter
contained
in a Gooch crucible
or through
a membrane filter.
Settleable
solids
are those
which usually
settle
in sedimentation
tanks during a normal detention
period.
This
fraction
is determined
by measuring the volume of sludge in the bottom of an Imhoff
cone after
1h of settling.
Solids
remaining
after
evaporation
or filtration
are dried,
weighed,
and then ignited.

The loss of weight
classed as organic

by ignition
material.

at 666°C is
The remaining

a measure of the volatile
solids,
which
solids
are the fixed
solids,
which are

are

considered as inorganic
(mineral) matter. The suspended solids associated with volatile
fraction
are termed volatile
suspended solids (VSS), and the suspended solids
associated with the mineral fraction
are termed fixed suspended solids (FSS).
Toxic Metals and Compounds:
Some heavy metals and compounds such as chromium, copper,
cyanide,
which are toxic
may
find their
way into municipal
sewage through
industrial
discharges.
Determination
of
these assume importance
if such waste is to be treated
by biological
process or
disposed of in stream or on land.
Design of Sewers
The hydraulic
design of sewers and drains,
which means finding
out their
sections
and
gradients,
is generally
carried
out on the same lines
as that of the water supply
pipes.
However, there are two major differences
between characteristics
of flows in
sewers and water supply pipes.
They are:
The sewage contain
particles
in suspension,
the heavier
of which may settle
down at the
bottom of the sewers, as and when the flow velocity
reduces,
resulting
in the clogging
of sewers. To avoid silting
of sewers, it is necessary that the sewer pipes be laid at
such a gradient,
as to generate
self cleansing
velocities
at different
possible
discharges.
The sewer pipes carry sewage as gravity
conduits,
and are therefore
laid at a
continuous
gradient
in the downward direction
upto the outfall
point,
from where it
will
be lifted
up, treated
and disposed of.
Hazen-william&#39;s
formula
U=6.85
C rH6.63S6.54

Manning&#39;s
formula
U=1/n
rH2/3S1/2
where, U= velocity,
m/s; rH= hydraulic
radius,m;
coefficient,
and n = Manning&#39;s
coefficient.
Darcy-weisbach
formula

S= slope,

C= Hazen-william&#39;s

hL=(fLU2)/(2gd)
Minimum Velocity
The flow velocity
in the sewers should be such that the suspended materials
in sewage
do not get silted
up; i.e.
the velocity
should be such as to cause automatic
selfcleansing
effect.
The generation
of such a minimum self
cleansing
velocity
in the
sewer, atleast
once a day, is important,
because if certain
deposition
takes place and
is not removed, it will
obstruct
free flow,
causing further
deposition
and finally
leading
to the complete blocking
of the sewer.
Maximum Velocity
The smooth interior
surface
of a sewer pipe gets scoured due to continuous
abrasion
caused by the suspended solids
present
in sewage. It is, therefore,
necessary to limit
the maximum velocity
in the sewer pipe. This limiting
or non-scouring
velocity
will
mainly depend upon the material
of the sewer.
Effects
of Flow Variation
on Velocity
in a Sewer
Due to variation
in discharge,
the depth of flow varies,
and hence the hydraulic
mean

depth (r) varies. Due to the change in the hydraulic mean depth, the flow velocity
(which depends directly
on r2/3) gets affected from time to time. It is necessary to
check the

sewer for

minimum flow

maintaining

a minimum velocity

(assumed to be 1/3rd

of average flow).

of

about

6.45

The designer

m/s at the

time

of

should also ensure

that a velocity
of 6.9 m/s is developed atleast
at the time of maximum flow and
preferably
during the average flow periods
also.
Moreover,
care should be taken to see
that at the time of maximum flow,
the velocity
generated
does not exceed the scouring
value.

Sewer Appurtenances
Sewer appurtenances
are the various
accessories
on the sewerage system and are
necessary for the efficient
operation
of the system. They include
man holes,
lamp
holes,
street
inlets,
catch basins,
inverted
siphons,
and so on.
Man-holes:
Man holes are the openings of either
circular
or rectangular
in shape
constructed
on the alignment
of a sewer line to enable a person to enter the sewer for
inspection,
cleaning
and flushing.
They serve as ventilators
for sewers, by the
provisions
of perforated
man-hole covers.
Also they facilitate
the laying
of sewer
lines
in convenient
length.

Man-holes are provided
at all junctions
of two or more sewers, whenever
sewer changes, whenever direction
of sewer line changes and when sewers
elevations
join together.
Special
Man-holes:
Junction
chambers: Man-hole constructed
at the intersection
of two large
Drop man-hole:
when the difference
in elevation
of the invert
levels
of
and outgoing
sewers of the man-hole is more than 66 cm, the interception
dropping
the incoming sewer vertically
outside
and then it is jointed
to

diameter
of
of different

sewers.
the incoming
is made by
the man-hole

chamber.

Flushing
in

the

man-holes:
sewer

with

They are

located

at the

head of

a sewer to

flush

out

the

deposits

water.

Lamp-holes:
Lamp holes are the openings constructed
on the straight
sewer lines
between
two man-holes which are far apart and permit the insertion
of a lamp into the sewer to
find out obstructions
if any inside
the sewers from the next man-hole.
Street
inlets:
Street
inlets
are the openings through which storm water is admitted
and
conveyed to the storm sewer or combined sewer. The inlets
are located
by the sides of
pavement with maximum spacing of 36 m.
Catch Basins:
Catch basins are small settling
chambers of diameter
66 - 96 cm and 66 75 cm deep, which are constructed
below the street
inlets.
They interrupt
the velocity
of storm water entering
through the inlets
and allow grit,
sand, debris
and so on to
settle
in the basin,
instead
of allowing
them to enter into the sewers.
Inverted
siphons:
These are depressed portions
of sewers, which flow full
under
pressure
more than the atmospheric
pressure
due to flow line
being below the hydraulic
grade line.
They are constructed
when a sewer crosses a stream or deep cut or road or
railway
line.
To clean the siphon pipe sluice
valve is opened, thus increasing
the head
causing flow.
Due to increased
velocity
deposits
of siphon pipe are washed into the
sump, from where they are removed.
Pumping of Sewage
Pumping of sewage is required
when it is not possible
to have a gravitational
flow for
the entire
sewerage project.
Sufficient
pumping capacity
has to be provided
to meet the peak flow,
atleast
56% as
stand by.
Types of pumps :
1. Centrifugal
pumps either
axial,
mixed and radial
flow.
2. Pneumatic ejector
pumps.
water

Treatment

The raw sewage must be treated
before it is discharged
into the river
stream. The
extent
of treatment
required
to be given depends not only upon the characteristics
and
quality
of the sewage but also upon the source of disposal,
its quality
and capacity
to
tolerate
the impurities
present
in the sewage effluents
without
itself
getting
potentially
polluted.

The layout

Indian

of

conventional

Standards

wastewater

treatment

plant

is

for

discharge

of

sewage in

surface

waters

for

Discharge

of

Sewage in

Surface

waters

as follows:

are given

in the

table

below.

Indian

Standards

Characteristic

of

the

Tolerance
limit
for
BOD5 20 mg/L TSS 39
The unit operations
their
functions
and

Effluent

Discharge
of Sewage in Suface water Sources
mg/L
and processes
commonly employed in domestic wastewater
units
used to achieve these functions
are given in the

treatment,
following

table:

Unit

Operations/Processes,

Treatment

Their

Functions

and Units

Used for

Domestic

wastewater

Screening
A screen is a device with openings for removing bigger
suspended or floating
sewage which would otherwise
damage equipment or interfere
with satisfactory
of

treatment

matter in
operation

units.

Types of Screens
Coarse Screens:
Coarse screens also called
racks,
are usually
bar screens,
composed of
vertical
or inclined
bars spaced at equal intervals
across a channel through which
sewage flows.
Bar screens with relatively
large openings of 75 to 156 mm are provided
ahead of pumps, while those ahead of sedimentation
tanks have smaller
openings of 56
mm.

Bar screens are usually
hand cleaned and sometimes provided
with mechanical
devices.
These cleaning
devices are rakes which periodically
sweep the entire
screen removing
the solids
for further
processing
or disposal.
Hand cleaned racks are set usually
at an
angle of 45° to the horizontal
to increase
the effective
cleaning
surface
and also
facilitate
the raking operations.
Mechanical
cleaned racks are generally
erected
almost
vertically.
Such bar screens have openings 25% in excess of the cross section
of the
sewage channel.
Medium Screens:
Medium screens have clear openings of 26 to 56 mm.Bar are usually
16 mm
thick
on the upstream side and taper slightly
to the downstream side.
The bars used for
screens are rectangular
in cross section
usually
about 16 x 56 mm, placed with larger
dimension
parallel
to the flow.
Fine Screens:
Fine screens are mechanically
cleaned devices
using perforated
plates,
woven wire cloth or very closely
spaced bars with clear openings of less than 26 mm.
Fine screens are not normally
suitable
for sewage because of clogging
possibilities.
The most commonly used bar type screen is shown in figure:
Velocity
The velocity
of flow ahead of and through the screen varies
and affects
its operation.
The lower the velocity
through the screen,
the greater
is the amount of screenings
that
would be removed from sewage. However, the lower the velocity,
the greater
would be the
amount of solids
deposited
in the channel.
Hence, the design velocity
should be such as
to permit 166% removal of material
of certain
size without
undue depositions.
Velocities
of 6.6 to 1.2 mps through the open area for the peak flows
have been used
satisfactorily.
Further,
the velocity
at low flows in the approach channel should not
be less than 6.3 mps to avoid deposition
of solids.
Head

loss

Head loss varies
with the quantity
and nature of screenings
allowed to accumulate
between cleanings.
The head loss created
by a clean screen may be calculated
by
considering
the flow and the effective
areas of screen openings,
the latter
being
sum of the vertical
projections
of the openings.
The head loss through
clean flat
screens is calculated
from the following
formula:

the
bar

h = 9.9729 (v2 - v2)
where,

h = head loss in m
V = velocity
through the screen in mps
v = velocity
before the screen in mps
Another formula
often used to determine
the head loss
equation:

h = L; (W/b)4/3

where

hv sin

a bar

rack

is

Kirschmer&#39;s

H

h = head loss,m

,; = bar shape factor
with

through

semicircle

(2.42 for

upstream,

1.79

sharp edge rectangular
for

circular

u/s and d/s face as semicircular).
w = maximum width of bar u/s of flow,
m
b = minimum clear spacing between bars, m
hv = velocity
head of flow approaching
rack, m = v2/2g
H = angle of inclination
of rack with horizontal
The head loss through fine screen is given by

h = (1/2g)

(Q/CA)

where, h = head loss,
Q = discharge,
m3/s

C = coefficient

m

of discharge

(typical

bar, 1.83 for

bar and 1.67

value 6.6)

for

rectangular

rectangular

bar with

bar
both

A = effective
The quantity
openings.
Equalization

submerged open area,
of screenings
depends

m2
on the

nature

of

the

wastewater

(i) to balance fluctuating
flows or concentrations,
or (iii)
to even out the effect of a periodic

"slug"
discharge
from a batch process.
Types of Equalization
Tanks
Equalization
tanks are generally
of three types:
1. Flow through type
2. Intermittent
flow type
3. Variable
inflow/constant
discharge
type
The simple flow through type equalization
tank is mainly
neutralization
or evening out of fluctuating
concentrations,
since a flow through type tank once filled,
gives output
Flow balancing
and self-neutralization
are both achieved
intermittently
one after
another.
One tank is allowed to

checked for

pH (or any other

goes through
from

screen

Tanks

The equalization
tanks are provided
(ii)
to assist self neutralization,

flows

and the

a similar

routine.

parameter)

useful
in assisting
self
not for balancing
of flows
equal to input.
by using two tanks,
fill
up after
which it is

and then allowed to empty out.

Intermittent

flow

type

tanks

are

The second tank

economic

for

small

industries.

when flows are large an equalization
tank of such a size may have to be provided
that inflow
can be variable
while outflow
is at a constant
rate,
generally
by a
pump.The capacity
required
is determined
from a plot of the cumulative
inflow
and a
plot of the constant
rate outflow
and measuring the gaps between the two plots.
A
factor
of safety
may be applied
if desired.
Generally,
detention
time vary from 2 to 8 hours but may be even 12 hours or more in
some cases. when larger
detention
times are required,
the equalization
unit is
sometimes provided
in the form of facultative
aerated
lagoon.
Grit

Chambers

Grit chambers are basin to remove the inorganic
particles
to prevent
damage to the
pumps, and to prevent their
accumulation
in sludge digestors.
Types of Grit Chambers
Grit chambers are of two types:
mechanically
cleaned and manually
cleaned.
In mechanically
cleaned grit
chamber, scraper
blades collect
the grit
settled
on the
floor
of the grit
chamber. The grit
so collected
is elevated
to the ground level
by
several
mechanisms such as bucket elevators,
jet pump and air lift.
The grit
washing
mechanisms are also of several
designs most of which are agitation
devices
using either
water or air to produce washing action.Manually
cleaned grit
chambers should be cleaned
atleast
once a week. The simplest
method of cleaning
is by means of shovel.
Aerated

Grit

Chamber

An aerated grit
chamber consists
of a standard
spiral
flow aeration
tank provided
with
air diffusion
tubes placed on one side of the tank. The grit
particles
tend to settle
down to the bottom of the tank at rates dependant upon the particle
size and the bottom
velocity
of roll
of the spiral
flow,
which in turn depends on the rate of air diffusion
through
diffuser
tubes and shape of aeration
tank. The heavier
particles
settle
down
whereas the lighter
organic
particles
are carried
with roll
of the spiral
motion.
Principle
of working of Grit Chamber
Grit chambers are nothing
but like
sedimentation
tanks,
designed to separate
the

intended

heavier

inorganic

materials

(specific

gravity

about 2.65) and to pass forward

the lighter
organic
materials.
Hence, the flow velocity
should neither
be too low as to
cause the settling
of lighter
organic
matter,
nor should it be too high as not to cause
the settlement
of the silt
and grit
present
in the sewage. This velocity
is called
"differential
sedimentation
and differential
scouring
velocity".
The scouring
velocity
determines
the optimum flow through velocity.
This may be explained
by the fact that
the critical
velocity
of flow
&#39;vc&#39;
beyond which particles
of a certain
size and density
once settled,
may be again introduced
into the stream of flow.
It should always be less
than the scouring
velocity
of grit
particles.
The critical
velocity
of scour is given
by Schield&#39;sformula:

V = 3 to 4.5 (g(Ss - 1)d)1/2
A horizontal
velocity
of flow of 15 to 36 cm/sec is used at peak flows.
velocity
is to be maintained
at all fluctuation
of flow to ensure that
solids
and not the grit
is scoured from the bottom.

This same
only organic

Types of Velocity
Control
Devices
1. A sutro weir in a channel of rectangular
the

cross

section,

with

free

fall

downstream

of

channel.

2. A parabolic
shaped channel with a rectangular
weir.
3. A rectangular
shaped channel with a parshall
flume at the
easy flow measurement.
Design of Grit Chambers
Settling
Velocity
The settling
velocity
of discrete
particles
can be determined
equation
depending upon Reynolds number.

end which

using

would

also

help

appropriate

Stoke&#39;s
law: v= g(Ss-1)d2
18*

Stoke&#39;s
law holds good for

Reynolds number,Re-fbelow

1.

Re=vd
5:

For grit

particles

of specific

gravity

2.65 and liquid

temperature

at 16°C, *=1.61

16-6m2/s.
This corresponds
to particles
of size less than 6.1 mm.
Transition
law: The design of grit
chamber is based on removal of grit
minimum size of 6.15 mm and therefore
Stoke&#39;s
law is not applicable
to
settling
velocity
of grit
particles
for design purposes.

particles
determine

x

with
the

V2= 4g("p-")d
3 CD
where, CD= drag coefficient
Transition
number,Re between 1 and 1666. In this
CD= 18.5

=

conditions
hold good for Reynolds
CD can be approximated
by

18.5

Re6.6
Substituting

v =

flow
range

(vd/_ )6.6
the

value

[6.767(Ss-1)dE..AE

of CD in

settling

velocity

equation

and simplifying,

we get

"-6.6]6.714

Primary Sedimentation
Primary sedimentation
in a municipal
wastewater
treatment
plant is generally
plain
sedimentation
without
the use of chemicals.
In treating
certain
industrial
wastes
chemically
aided sedimentation
may be involved.
In either
case, it
constitutes
flocculent
settling,
and the particles
do not remain discrete
as in the
case of grit,
but tend to agglomerate
or coagulate
during
settling.
Thus, their
diameter
keeps increasing
and settlement
proceeds at an over increasing
velocity.
Consequently,
they trace a curved profile.
The settling
tank design in such cases depends on both surface
loading
and detention
time.

Long tube settling
tests
can be performed
in order to estimate
specific
value of
surface
loading
and detention
time for desired
efficiency
of clarification
for a given
industrial
wastewater
using recommended methods of testing.
Scale-up
factors
used in
this
case range from 1.25 to 1.75 for the overflow
rate,
and from 1.5 to 2.6 for
detention
time when converting
laboratory
results
to the prototype
design.
For primary
settling
tanks treating
municipal
or domestic
sewage, laboratory
tests
are
generally
not necessary,
and recommended design values given in table
may be used.
Using an appropriate
value of surface
loading
from table,
the required
tank area is
computed. Knowing the average depth, the detention
time is then computed. Excessively

high detention

time (longer

than 2.5 h) must be avoided especially

in warm climates

where anaerobicity
can be quickly
induced.
Design parameters
for settling
tank
Types of settling
Overflow
rate m3m2/day
Solids
loading
kg/m2/day
Depth
Detention

time

Average Peak Average Peak Primary settling
only 25-36
settling
followed
by secondary treatment
35-56 66-126 - 2.5-3.5
Primary settling
with activated
56-66

56-66
sludge

--

2.5-3.5

return

2.6-2.5

25-35

Primary

Classification
of Micro organisms
1. Nutritional
Requirements:
On the basis of chemical form of carbon required,
microorganisms
are classified
as
1. Autotrophic:
organisms that use CO2 or HCO3- as their
sole source of carbon.
2. Heterotrophic:
organisms that use carbon from organic
compounds.
b. Energy Requirements:
On the basis of energy source required,
microorganisms
are
classified

as

1. Phototrophs:
2. Chemotrophs:

organisms
organisms

They are further
donor)

that
that

classified

1. Chemoorganotrophs:

use light
as their
energy
employ oxidation-reduction

source.
reactions

to

provide

on the basis of chemical compounds oxidized

Organisms

that

use complex

organic

molecules

(i.e.,

as their

energy.

electron
electron

donor.

2. Chemoautotrophs:
sulfide

or

ammonia

Organisms
as their

that
electron

use simple

inorganic

molecules

such as hydrogen

donor.

b. Temperature
Range: On the basis of temperature
range within
which they
proliferate,
microorganisms
are classified
as
i. Psychrophilic:
organisms whose growth is optimum within
15 to 36°C.
ii.
Mesophilic:
organisms whose growth is optimum within
36 to 45°C.
iii.
Thermophilic:
organisms whose growth is optimum within
45 to 76°C.
b. Oxygen Requirements:
On the basis of oxygen requirement
microorganisms
classified

can

are

as

i. Aerobes:
organisms that use molecular
oxygen as electron
acceptor.
ii.
Anaerobes:
organisms that use some molecule other than molecular
oxygen as electron
acceptor.
iii.
Facultative
organisms
: organisms that can use either
molecular
oxygen or some
other chemical
compound as electron
acceptor.
Growth Pattern
of Micro organisms
when a small number of viable
bacterial
cells
are placed in a close vessel containing
excessive
food supply in a suitable
environment,
conditions
are established
in which
unrestricted
growth takes place.
However, growth of an organism do not go on
indefinitely,
and after
a characteristic
size is reached,
the cell
divides
due to
hereditary
and internal
limitations.
The growth rate may follow
a pattern
similar
to as
shown in figure:
The curve shown may be divided
into six well defined
phases:
1. Lag Phase:adaptation
to new environment,
long generation
time and null growth rate.
2. Accelaration
phase: decreasing
generation
time and increasing
growth rate.
3. Exponential
phase: minimal and constant
generation
time, maximal and constant
specific
growth rate and maximum rate of substrate
conversion.
4. Declining
growth phase: increasing
generation
time and decreasing
specific
growth
rate due to gradual
decrease in substrate
concentration
and increased
accumulation
of
toxic

metabolites.

5. Stationary
phase:
and cells
in a state
6. Endogenous phase:
Biomass

Growth

exaustion
of nutrients,
of suspended animation.
endogenous metabolism,

high
high

concentration
death

rate

of
and cell

used expression

for

the

growth

rate

of

micro

organisms

Monod:

rate of microbial

growth,dx

= L&#39;mXS
dt

Ks+ S

where,

L&#39;m=++maximum
specific
X = micro

organism

S = substrate

growth rate

concentration

concentration

Ks= substrate
concentration
Similarly,
rate of substrate
dS = k X S
dt

where,

metabolites,

lysis.

Rate

The most widely

Total

toxic

Ks+ S

at one half
utilization,

the

maximum growth

rate

is

given

by

k = maximum specific
substrate
utilization
rate
Maintenance
as Endogenous Respiration
Net growth rate of micro organisms
is computed by subtracting
from the total
growth
rate,
the rate of micro organisms endogenously
decayed to satisfy
maintenance
energy
requirement.
Therefore,

Net rate of microbial

growth =a*L&#39;mX
S+*,A++kdX
Ks+ S

where,
kd = endogenous
Growth

decay

coefficient

Yield

Growth yield
is defined
as the
utilization
of the incremental

incremental
increase
amount of substrate.

in biomass which results
from the
The maximum specific
growth rate is

given by:++L&#39;m
=Y.k
where, Y
of cells
designated
nature of
reactor.
or without
Activated
The most
activated

is the maximum yield
coefficient
and is defined
as the ratio
of maximum mass
formed to the mass of substrate
utilized.
The coefficients
Y, kd, k and Ks are
as kinetic
coefficients.
The values of kinetic
coefficients
depend upon the
wastewater
and operational
and environmental
conditions
in biological
The biological
reactors
can be completely
mixed flow or plug flow reactor
with
recycle.
Sludge Process
common suspended growth process used for municipal
wastewater
treatment
is the
sludge process as shown in figure:

Activated
sludge plant
involves:
1. wastewater
aeration
in the presence of a microbial
suspension,
2. solid-liquid
separation
following
aeration,
3. discharge
of clarified
effluent,
4. wasting
of excess biomass, and
5. return
of remaining
biomass to the aeration
tank.
In activated
sludge process wastewater
containing
organic
matter is aerated
in an
aeration
basin in which micro-organisms
metabolize
the suspended and soluble
organic
matter.
Part of organic
matter is synthesized
into new cells
and part is oxidized
to
CO2 and water to derive
energy.
In activated
sludge systems the new cells
formed in the
reaction
are removed from the liquid
stream in the form of a flocculent
sludge in
settling
tanks.
A part of this
settled
biomass, described
as activated
sludge is
returned
to the aeration
tank and the remaining
forms waste or excess sludge.
Activated
Sludge Process Variables
The main variables
of activated
sludge process are the mixing regime,
loading
rate,
and
the

flow

scheme.

Mixing Regime
Generally
two types of mixing regimes are of major interest
in activated
sludge
process:
plug flow and complete mixing.
In the first
one, the regime is characterized
by orderly
flow of mixed liquor
through the aeration
tank with no element of mixed
liquor
overtaking
or mixing with any other element.
There may be lateral
mixing of
mixed liquor
but there must be no mixing along the path of flow.
In complete mixing,
the contents
of aeration
tank are well stirred
and uniform
throughout.
Thus, at steady state,
the effluent
from the aeration
tank has the same
composition
as the aeration
tank contents.

The type of mixing regime is very important as it affects
(1) oxygen transfer
requirements in the aeration tank, (2) susceptibility
of biomass to shock loads, (3)
local environmental conditions
in the aeration tank, and (4) the kinetics
governing the
treatment
process.
Loading Rate
A loading
parameter

that

has been developed

over

the

years

is

thehydraulic

retention

time (HRT), H, d
H

= V
Q

V= volume of aeration
tank, m3, and Q= sewage inflow,
m3/d
Another empirical
loading
parameter
is volumetric
organic
loadingwhich
is defined
the BOD applied
per unit volume of aeration
tank,
per day.
A rational
loading
parameter which has found wider acceptance
and is preferred
is specific
substrate
utilization
rate,
q, per day.

as

q= Q (SO - Se)
VX

A similar

loading

parameter

is

mean cell

residence

time

organic

matter

or

sludge

retention

time (SRT), H c, d
H c =

V X

QwXr + (Q-QwXe)
where SO and Se are

influent

measured as BOD5 (g/m3),

and effluent

concentration

X, Xe and Xrare MLSS concentration

and return
sludge respectively,
Under steady state operation

and Qw= waste activated
the mass of waste activated

respectively,

in aeration

tank,

sludge rate.
sludge is given

QwXr = YQ (SO - Se) - kd XV
where Y= maximum yield coefficient
(microbial
mass synthesized
utilized)
and kd = endogenous decay rate (d-1) .

effluent

by

/ mass of substrate

From the above equation
it is seen that 1/Hc = Yq - kd
If the value of Se is small as compared SO, q may also be expressed
Microorganism
ratio,
F/M

as Food to

F/M = Q(SO- Se) / XV = QSO/ XV
The H c value adopted for design controls
the effluent
quality,
and settleability
and
drainability
of biomass, oxygen requirement
and quantity
of waste activated
sludge.
Flow Scheme
The flow
scheme

involves:

the pattern
of sewage addition
the pattern
of sludge return
to the aeration
tank and
the pattern
of aeration.
Sewage addition
may be at a single
point at the inlet
end or it may be at several
points
along the aeration
tank.
The sludge return
may be directly
from the settling
tank to the aeration
tank or through
a sludge reaeration
tank. Aeration
may be at a
uniform
rate or it may be varied
from the head of the aeration
tank to its end.
Conventional
System and its Modifications
The conventional
system maintains
a plug flow hydraulic
regime. Over the years,
several
modifications
to the conventional
system have been developed to meet specific
treatment
objectives.
Instep
aeration
settled
sewage is introduced
at several
points
along the
tank length which produces more uniform oxygen demand throughout.
Tapered
aeration
attempts
to supply air to match oxygen demand along the length of the
tank. Contact stabilizationprovides
for reaeration
of return
activated
sludge from from
the final
clarifier,
which allows a smaller
aeration
or contact
tank.Completely
mixed process aims at instantaneous
mixing of the influent
waste and return
sludge with
the entire
contents
of the aeration
tank.
Extended aeration
process operates
at a low
organic
load producing
lesser
quantity
of well stabilized
sludge.
Design Consideration
The items for consideration
in the design
capacity
and dimensions,
aeration
facilities,
excess sludge wasting.
Aeration

of

activated
sludge plant are aeration
tank
secondary sludge settling
and recycle
and

Tank

The volume of aeration
tank is calculated
suitable
value of MLSS concentration,
X.

for

the

selected

value

ofHc

by assuming

a

vx = YQHc(SO - s)
1+++kdH

c

Alternately,
the
F/M = QSO / xv

tank

capacity

may be designed

Hence, the first

step in designing

from

is to choose a suitable

value ofHc

(or F/M) which

depends on the expected winter
temperature
of mixed liquor,
the type of reactor,
expected settling
characteristics
of the sludge and the nitrification
required.
The
choice generally
lies
between 5 days in warmer climates
to 16 days in temperate
ones
where nitrification
is desired
alongwith
good BOD removal,
and complete mixing systems
are employed.
The second step is to select
two interrelated
parameters
HRT, t and MLSS concentration.
It is seen that economy in reactor
volume can be achieved
by assuming a large value of
X. However, it is seldom taken to be more than 5666 g/m3. For typical
domestic sewage,
the MLSS value of 2666-3666 mg/l if conventional
plug flow type aeration
system is
provided,
or 3666-5666 mg/l for completely
mixed types.
Considerations
which govern the

upper limit
are: initial
and running
cost of sludge recirculation
system to maintain
a
high value of MLSS, limitations
of oxygen transfer
equipment to supply oxygen at
required
rate in small reactor
volume, increased
solids
loading
on secondary clarifier
which may necessitate
a larger
surface
area, design criteria
for the tank and minimum
HRT for

the

aeration

tank.

The length
of the tank depends upon the type of activated
sludge plant.
Except in the
case of extended aeration
plants
and completely
mixed plants,
the aeration
tanks are
designed as long narrow channels.
The width and depth of the aeration
tank depends on
the type of aeration
equipment employed. The depth control
the aeration
efficiency
and
usually
ranges from 3 to 4.5 m. The width controls
the mixing and is usually
kept
between 5 to 16 m. width-depth
ratio
should be adjusted
to be between 1.2 to 2.2. The
length
should not be less than 36 or not ordinarily
longer than 166 m.
Oxygen Requirements
Oxygen is reqiured
in the activated
sludge process for the oxidation
of a part of the
influent
organic
matter and also for the endogenous respiration
of the micro-organisms
in the system. The total
oxygen requirement
of the process may be formulated
as
follows:

02 required

(g/d)

= Q(SO - s)

1.42 QwXr
f

where, f = ratio
The formula
removal.
Aeration

of BOD5to ultimate

does not

allow

for

BODand 1.42 = oxygen demand of biomass (g/g)

nitrification

but

allows

only

for

carbonaceous

BOD

Facilities

The aeration
facilities
of the activated
sludge plant are designed to provide
the
calculated
oxygen demand of the wastewater
against
a specific
level
of dissolved
oxygen
in

the

wastewater.

Secondary Settling
Secondary settling
tanks,
which receive
the biologically
treated
flow undergo zone or
compression
settling.
Zone settling
occurs beyond a certain
concentration
when the
particles
are close enough together
that interparticulate
forces
may hold the particles
fixed
relative
to one another
so that the whole mass tends to settle
as a single
layer
or "blanket"
of sludge.
The rate at which a sludge blanket
settles
can be determined
by
timing
its position
in a settling
column test whose results
can be plotted
as shown in
figure.
Compression settling
may occur at the bottom of a tank if particles
are in such a
concentration
as to be in physical
contact
with one another.
The weight of particles
is
partly
supported
by the lower layers
of particles,
leading
to progressively
greater
compression
with depth and thickening
of sludge.
From the settling
column test,
the
limiting
solids
flux
required
to reach any desired
underflow
concentration
can be
estimated,
from which the rquired
tank area can be computed.

The solids load on the clarifier
is estimated in terms of (Q+R)X, while the overflow
rate or surface loading is estimated in terms of flow Q only (not Q+R) since the
quantity
R is withdrawn
from the bottom and does not contribute
to the overflow
from
the tank. The secondary settling
tank is particularly
sensitive
to fluctuations
in flow
rate and on this
account it is recommended that the units
be designed not only for
average overflow
rate but also for peak overflow
rates.
Beyond an MLSS concentration
of
2666 mg/l the clarifier
design is often
controlled
by the solids
loading
rate rather
than the overflow
rate.
Recommended design values for treating
domestic sewage in final

clarifiers
settling)

and mechanical thickeners
are given in lecture 22.

Sludge Recycle
The MLSS concentration
in the
rate and the sludge settleability
Qr =
Q

(which also fall

aeration
tank is
and thickening

in this

controlled
in the

category

of compression

by the sludge recirculation
secondary sedimentation
tank.

X
Xr-X

where Qr = Sludge

recirculation

The sludge settleability

rate,

is determined

m3/d

by sludge volume index (SVI) defined

as volume

occupied
in mL by one gram of solids
in the mixed liquor
after
settling
for 36 min. If
it is assumed that sedimentation
of suspended solids
in the laboratory
is similar
to
that in sedimentation
tank, then Xr = 166/SVI.
Values of SVI between 166 and 156 ml/g

indicate
good settling
of suspended solids.
The Xr value may not be taken more than
16,666 g/m3unless
separate
thickeners
are provided
to concentrate
the settled
solids
or
secondary sedimentation
tank is designed to yield
a higher value.
Excess Sludge wasting
The sludge in the aeration
tank has to be wasted to maintain
a steady level
of MLSS in
the system. The excess sludge quantity
will
increase
with increasing
F/M and decrease
with increasing
temperature.
Excess sludge may be wasted either
from the sludge return
line or directly
from the aeration
tank as mixed liquor.
The latter
is preferred
as the
sludge concentration
is fairly
steady in that case. The excess sludge generated
under
steady state operation
may be estimated
by
H c =

VX
QwXr

or QwXr = YQ (SO - S) - kd XV
Design
Design
a final
following
Sewage
BOD5 =

Total

of Completely
Mixed Activated
Sludge System
a completely
mixed activated
sludge system to serve 66666 people that will
effluent
that is nitrified
and has 5-day BOD not exceeding
25 mg/l. The
design data is available.
flow = 156 l/person-day
= 9666 m3/day
54 g/person-day
= 366 mg/l 3 BODu = 1.47 BOD5

kjeldahl

nitrogen

(TKN) = 8 g/person-day

= 53 mg/l

Phosphorus = 2 g/person-day
= 13.3 mg/l
winter
temperature
in aeration
tank = 18°C
Yield
coefficient
Y = 6.6 3 Decay constant
Kd = 6.67

utilization

rate = (6.638 mg/l)-1

Assume 36% raw BOD5 is

therefore,

removed in

(h)-1

give

per day 3 Specific

substrate

at 18°C

primary

sedimentation,

and BOD5 going

to

aeration

fear

and good
of toxic

is,

252 mg/l (6.7 x 366 mg/l).

Design:

(a) Selection

of H c, t and MLSS concentration:

Considering
the operating
temperature
and the desire
to have nitrification
sludge settling
characteristics,
adopt H c = 5d. As there is no special
inflows,
the HRT, t may be kept between 3-4 h, and MLSS = 4666 mg/l.

(b) Effluent
BOD5:
Substrate concentration,

S =

1 (1/H c + kd)=
qY

1

(1/aE+++ 6.67)
(6.638)(6.6)

S = 12 mg/l.

Assume suspended solids (SS) in effluent
= 26 mg/l and VSS/SS =6.8.
If degradable fraction
of volatile
suspended solids (VSS) =6.7 (check later),
VSS in effluent
= 6.7(6.8x26)
= 11mg/l.
Thus, total effluent
BOD5 = 12 + 11 = 23 mg/l (acceptable).
(c) Aeration Tank:
VX = YQHc(SO S) where X = 6.8(4666) = 3266 mg/l

BOD5 of

1+ kdH c

or3299
v =(6.6)(5)(9666)(252-12)
[1+(6.67)(5)]
V = 1566

Detention

time,

m3

t = 1566 x 24 = 4h
9666

F/M = (252-12)(9666) = 6.45 kg BOD5 per kg MLSS per day
(3266) (1566)
Let the
parallel

aeration
tank be in the form of four
rows, each with two cells
measuring

square shaped compartments
11m x 11m x 3.1m

operated

(d) Return Sludge Pumping:
If

suspended

R =

solids

MLSS

concentration

of

return

flow

is

1% = 16,666

mg/l

= 6.67

(16666)-MLSS
Qr = 6.67

X 9666 = 6666 m3/d

(e) Surplus Sludge Production:
Net vss produced QwXr = vx =

(3266)(1566)(163/166)
Hc

or SS produced =966/6.8
= 1266 kg/d
If SS are removed as underflow
with
gravity
of sludge as 1.6,

solids

= 966 kg/d
(5)

concentration

1% and assuming

specific

in

two

Liquid
sludge
toberemoved
=1266
x 166/1
=126,666
kg/d
=126
m3/d
(f)
1.

Oxygen Requirement:
For carbonaceous

oxygen required

demand,

= (BODu removed) - (BODu of solids leaving)
= 1.47 (2166 kg/d) - 1.42 (966 kg/d)
= 72.5

2.

kg/h

For nitrification,

oxygen required

= 4.33 (TKN oxidized,

kg/d)

Incoming TKN at 8.6 g/ person-day
= 486 kg/day.
Assume 36% is removed in primary
sedimentation
and the balance 336 kg/day is oxidized
to nitrates.
Thus, oxygen required
= 4.33 x 336 = 1455 kg/day = 66.6 kg/h
3. Total oxygen required
= 72.5 + 66.6 = 133 kg/h = 1.6 kg/kg of BODu removed.
Oxygen uptake rate per unit tank volume = 133/1566
= 96.6 mg/h/l
tank volume

(g) Power Requirement:
Assume oxygenation
capacity
of aerators
at field
conditions
at standard
conditions
and mechanical
aerators
are capable
kwh at

standard

is only
of giving

76% of the capacity
2 kg oyxgen per

conditions.

Power required

=

136

= 97 kw (136 hp)
6.7

x 2

= (97 x 24 x 365) / 66,666 = 14.2 kwh/year/person
Theory
Aeration

of Aeration
is a gas-liquid

mass transfer

process

in which

phase is the concentration
gradient (Cs - C) for
Mass transfer
per unit time =KL.a (Cs - C)
where,

KL = Liquid

film

the

slightly

driving

soluble

force

in

the

liquid

gases.

coefficient

=Diffusion
coefficient
Thickness of film

of liquid
(Y)

(D)

a = Interficial
area per unit volume
Cs =saturation
concentration
at the gas-liquid
interface
and
C =
some lower value in the body of the liquid.
The value of a increases
as finer
and finer
droplets
are formed, thus increasing
the
gas transfer.
However, in practice,
it is not possible
to measure this
area and hence

the overall

coefficient

(KL.a)

per unit

Adjustment
for Field Conditions
The oxygen transfer
capacity
under field
oxygen transfer
capacity
by the formula:

time,

is determined

conditions

by experimentation.

can be calculated

from

the

standard

N = [Ns(Cs- CL)x 1.e24T-2o,,++]/9.2
where,
N = oxygen transferred
under field
conditions,
kg O2/h.
Ns= oxygen transfer
capacity
under standard
conditions,
kg O2/h.
Cs= DO saturation
value for sewage at operating
temperature.
CL= operating
DO level
in aeration
tank usually
1 to 2 mg/L.
T = Temperature,
degree C.
1, = Correction
factor
for oxygen transfer
for sewage, usually
6.8
Aeration

Oxygen may be supplied
or

coarse

to

6.85.

Facilities

either

by surface

aerators

or diffused

aerators

employing

fine

diffusers.

The aeration
devices apart from supplying
the required
oxygen shall
also provide
adequate mixing in order that the entire
MLSS present
in the aeration
tank will
be
available
for biological
activity.
Aerators
are rated based on the amount of oxygen they can transfer
to tap water under
standard
conditions
of 26°C, 766 mm Hg barometric
pressure
and zero DO.
Trickling
Filters
Trickling
filter
is an attached
growth process i.e.
process in which microorganisms
responsible
for treatment
are attached
to an inert
packing material.
Packing material
used in attached
growth processes
include
rock, gravel,
slag,
sand, redwood, and a wide
range of plastic
and other synthetic
materials.
Process

Description

The wastewater
in trickling
filter
is distributed
over the top area of a vessel
containing
non-submerged
packing material.
Air circulation
in the void space, by either
natural
draft
or blowers,
provides
oxygen
for the microorganisms
growing as an attached
biofilm.
During operation,
the organic
material
present
in the wastewater
is metabolised
by the
biomass attached
to the medium. The biological
slime grows in thickness
as the organic
matter abstracted
from the flowing
wastewater
is synthesized
into new cellular
material.

The thickness
of the aerobic
layer is limited
by the depth of penetration
of oxygen
into the microbial
layer.
The micro-organisms
near the medium face enter the endogenous phase as the substrate
is
metabolised
before it can reach the micro-organisms
near the medium face as a result
of
increased
thickness
of the slime layer and loose their
ability
to cling
to the media
surface.
The liquid
then washes the slime off the medium and a new slime layer starts
to grow. This phenomenon of losing
the slime layer is calledsloughing.
The sloughed off film
and treated
wastewater
are collected
by an underdrainage
which
also allows
circulation
of air through filter.
The collected
liquid
is passed to a
settling
tank used for solidliquid
separation.
Types of Filters
Trickling
filters
are classified
as high rate or low rate,
based on the organic
and
hydraulic
loading
applied
to the unit.
S.No.

Design

Feature

Low Rate

High

Filter

Rate Filter

1.
Hydraulic
loading,
m3/m2.d
1-4
10

40

2.

Organic
0.08
0.32

loading,kg

BOD / m3.d

0.32
1.0

Recirculation
0

ratio

0.5 - 3.0 (domestic

wastewater)

upto 8 for

strong

industrial

wastewater.

The hydraulic
loading
rate is the total
flow including
recirculation
appied on unit
area of the filter
in a day, while the organic
loading
rate is the 5 day 20°C BOD,
excluding
the BOD of the recirculant,
applied
per unit volume in a day.
Recirculation
is generally
not adopted in low rate filters.
A well operated
low rate trickling
filter
in combination
with secondary settling
tank
may remove 75 to 90% BOD and produce highly
nitrified
effluent.
It is suitable
for
treatment
of low to medium strength
domestic wastewaters.
The high rate trickling
filter,
single
stage or two stage are recommended for medium to
relatively
high strength
domestic and industrial
wastewater.
The BOD removal efficiency
is around 75 to 90% but the effluent
is only partially
nitrified.
Single
stage unit consists
of a primary
settling
tank,
filter,
secondary settling
tank
and facilities
for recirculation
of the effluent.
Two stage filters
consist
of two
filters
in series
with a primary
settling
tank,
an intermediate
settling
tank which may
be omitted
in certain
cases and a final
settling
tank.
Process Design
Generally
trickling
filter
required
filter
volume for
equations:

design is based on empirical
relationships
a designed degree of wastewater
treatment.

1. NRC equations

Research Council

2. Rankins

(National

equation

of USA)

to find the
Types of

3. Eckenfilder
equation
4. Galler
and Gotaas equation
NRC and Rankin&#39;s
equations
are commonly used. NRC equations
give satisfactory
values
when there is no re-circulation,
the seasonal variations
in temperature
are not large
and fluctuations
with high organic
loading.
Rankin&#39;s
equation
is used for high rate
filters.

NRC equations:
The efficiency

These equations
of single
stage

E2:

are applicable
or first
stage

to both low rate and high rate filters.
of two stage filters,
E2 is given by

100

1+0.44(F1.BOD/V1.Rf1)1/2
For the

second

stage

filter,

E3=

the

efficiency

E3 is

given

by

100

[(1+0.44)/(1where
filter,

E2)](F2.BOD/V2.Rf2)1/2

E2= % efficiency
E3=% efficiency

sewage in single

in BOD removal of single
stage
of second stage filter,F1.BOD=

stage of the two-stage

filter

or first
stage of two-stage
BOD loading
of settled
raw

in kg/d,

F2.BOD= F1.BOD(1- E2)= BOD

loading
on second-stage
filter
in kg/d, V1= volume of first
stage filter,
m3; V2=
volume of second stage filter,
m3; Rf1= Recirculation
factor
for first
stage,
R1=
Recirculation
ratio
for first
stage filter,
Rf2= Recirculation
factor
for second stage,
R2= Recirculation
ratio
for second stage filter.
Rankins equation:
This equation
also known as Tentative
Method of Ten States USA has
been successfully
used over wide range of temperature.
It requires
following
conditions
to be observed for single
stage filters:
1. Raw settled
domestic
sewage BOD applied
to filters
should not exceed 1.2 kg
BOD5/day/ m3 filter
volume.

2. Hydraulic

load (including

recirculation)

should not exceed 36 m3/m2 filter

surface-

day.

3. Recirculation
ratio (R/Q) should be such that BODentering filter
(including
recirculation)
is not more than three times the BODexpected in effluent.
This implies
that as long
recirculation

as the above conditions
and is given by:

E=
Other

Aerobic

are

satisfied

efficiency

is

only

a function

of

(R/Q) + 1
(R/Q) + 1.5

Treatment

Units

1. Stabilization
ponds: The stabilization
ponds are open flow through
basins
specifically
designed and constructed
to treat
sewage and biodegradable
industrial
wastes.
They provide
long detention
periods
extending
from a few to several
days.
2. Aerated lagoons:
Pond systems,
in which oxygen is provided
through mechanical
aeration
rather
than algal
photosynthesis
are called
aerated
lagoons.
3.

Oxidation

ditch:

activated
sludge
with two surface

(Aerated
Aerated

oxidation

ditch

is

a modified

form

of

"extended

continuous

aeration"

channel

oval

of

in

shape

Lagoon
lagoons

supplied
land

The

process.
The ditch
consists
of a long
rotors
placed across the channel.

than

fall

through
algal

in

between

aeration.

the

The unit

algal

ponds and activated

sludge

systems.

is deeper (3 to 5 m) and hence require

Oxygen is

much less

ponds.

)
(Extended Aeration

Process

In extended aeration
process the raw sewage goes straight
to the aeration
tank for
traetment.
The whole process is aerobic.
This simplification
implies
longer aeration
time which has earned for the process the name "extended
aeration".
The BOD removal
efficiency
of the extended aeration
process is higher than activated
sludge process
which makes it especially
desirable
to use where it is to be followed
by tertiary
treatment

for

reuse.

)
Anaerobic

Treatment

The anaerobic
waste treatment
organic
wastes.
The treatment
process,
namely,

process is an effective
method for the treatment
has a number of advantages over aerobic
treatment

of

many

the

energy

input

of

lower production

the

system

is

low as no energy

of excess sludge(

biological

is

requred

synthesis)

for

oxygenation,

per unit

mass of substrate

utilized,
lower nutrient
requirement
due to lower biological
synthesis,
and
degradation
leads to production
of biogas which is a valuable
source of energy.
Fundamental Microbiology
The anaerobic
treatment
of organic
wastes resulting
in the production
of carbon dioxide
and methane, involves
two distinct
stages.
In the first
stage,
complex waste
components,
including
fats,
proteins,
and polysaccharides
are first
hydrolyzed
by a
heterogeneous
group of facultative
and anaerobic
bacteria.
These bacteria
then subject
the products
of hydrolysis
to fermentations,L;-oxidations,
and other metabolic
processes
leading
to the formation
of simple organic
compounds, mainly short-chain

(volatile)

acids and alcohols.

fermentation.

converted
strictly

However in the

to gases (mainly
anaerobic

The first
second

stage is commonly referred

stage

the

end products

methane and carbon dioxide)

bacteria.

This

stage

is

generally

of

by several
referred

to as acid

the
to

first

stage

different

are

species of

as "methane

fermentation.

The primary
acids produced during acid fermentation
are propionic
and acetic
acid.
It
is reported
that only one group of methane bacteria
is necessary for methane
fermentation
of acetic
acid, whereas propionic
acid, which is fermented
through
acetic
acid requires
two different
groups of methane bacteria.
The methane fermentation
reactions

for

these

two

acids

are:

The bacteria
responsible
for acid fermentation
are relatively
tolerant
to changes in pH
and temperature
and have a much higher
rate of growth than the bacteria
responsible
for
methane fermentation.
As a result,
methane fermentation
is generally
assumed to be the
rate limiting
step in anaerobic
wastewater
treatment.
Anaerobic

Reactor

Various
types of anaerobic
units that have been developed are as follows:
Upflow anaerobic
filters
packed with either
pebbles,
stones,
PVC sheets,

to support

submerged biological

growths

(fixed

film).

The units

etc.

are reported

as media

to work

well but a likely
problem is accumulation
of solids
in the interstices.
Downflow anaerobic
filters
packed with similar
media as above but not to be confused
with usual trickling
filters
which are aerobic.
In the anaerobic
units,
the inlet
and
outlet
are so placed that the media and fixed
film
stay submerged.
UASB type units
in which no special
media have to be used since the sludge granules
themselves
act as the &#39;media&#39;
and stay in suspension.
These are commonly preffered.
Fluidized
bed units
filled
with sand or plastic
granules
are used with recirculation
under required
pressure
to keep the entire
mass fluidized
and the sludge distributed
over the entire
reactor
volume. Their power consumption
is higher.
UASB Units

UASB type units
are one in which no special
media have to be used since the sludge
granules
themselves
act as the &#39;media&#39;
and stay in suspension.
UASB system is not
patented.
A typical
arrangement
of a UASB type treatment
plant for municipal
sewage
would

be as follows:

. Initial
pumping
. Screening
and degritting

OIU1-kl)-JIUI. Main

.
.
.
In

UASB reactor

Gas collection
and conversion
or conveyance
Sludge drying
bed
Post treatment
facility
the UASB process,
the whole waste is passed through

upflow mode, with

a hydraulic

retention

the

anaerobic

reactor

in

an

time (HRT) of only about 8-16 hours at average

flow.
No prior
sedimentation
is required.
The anaerobic
unit does not need to be filled
with stones or any other media; the upflowing
sewage itself
forms millions
of small
"granules"
or particles
of sludge which are held in suspension
and provide
a large
surface
area on which organic
matter can attach
and undergo biodegradation.
A high

solid

retention

time (SRT) of 36-56 or more days occurs within

the unit.

No mixers or

aerators
are required.
The gas produced can be collected
and used if desired.
Anaerobic
systems function
satisfactorily
when temperatures
inside
the reactor
are above 18-26°C.
Excess sludge is removed from time to time through
a separate
pipe and sent to a simple

sand bed for drying.
Design Approach
Size of Reactor:
Generally,
UASBs are considered
where temperature
in the reactors
will
be above 26°C. At equilibrium
condition,
sludge withdrawn
has to be equal to sludge
produced daily.
The sludge produced daily
depends on the characteristics
of the raw

wastewater since it is the sum total of (i) the new VSS produced as a result of BOD
removal, the yield coefficient
being assumed as 6.1 g VSS/ g BOD removed, (ii)
the nondegradable

residue

and residue

of

the

VSS coming

is 66%, and (iii)

in

the

Ash received

inflow

assuming

in the inflow,

46% of

the

VSS are degraded

namely TSS-VSS mg/l.

Thus, at

steady state conditions,
SRT= Total sludge present
in reactor,
kg
Sludge withdrawn
per day, kg/d
= 36 to 56 days.
Another parameter
is HRT which is given by:
HRT= Reactor volume, m3
Flow rate,
m3/h
= 8 to 16 h or more at average flow.
The

reactor

volume

has

to

be so

chosen

that

the

desired

SRT value

is

achieved.

This

is

done by solving for HRT from SRT equation assuming (i) depth of reactor (ii)
the
effective
depth of the sludge blanket, and (iii)
the average concentration
of sludge in
the blanket (76 kg/m3). The full depth of the reactor for treating
low BODmunicipal
sewage is often 4.5 to 5.6 m of which the sludge blanket
itself
may be 2.6 to 2.5 m
depth.
For high BOD wastes,
the depth of both the sludge blanket
and the reactor
may
have to be increased
so that the organic
loading
on solids
may be kept within
the
prescribed
range.
Once the size of the reactor
is fixed,
the upflow velocity
can be determined
from
Upflow velocity
m/h = Reactor height
HRT, h
Using average flow rate one gets the average HRT while the peak flow rate gives the
minimum HRT at which minimum exposure to treatment
occurs.
In order to retain
any
flocculent
sludge in reactor
at all times,
experience
has shown that the upflow
velocity
should not be more than 6.5 m/h at average flow and not more than 1.2 m/h at
peak flow.
At higher velocities,
carry over of solids
might occur and effluent
quality
may be deteriorated.
The feed inlet
system is next designed so that the required
length
and

width

of

the

UASB reactor

are

determined.

The settling
compartment is formed by the sloping
hoods for
of the compartment is 2.6 to 2.5 m and the surface
overflow

m3/m2-day (1 to 1.2 m/h) at peak flow.
connecting
the reaction
zone with
5 m/h at peak flow.
Due attention

hydraulics

The flow velocity

gas collection.
rate kept at

through

The depth
26 to 28

the aperture

the settling
compartment is limited
to not
has to be paid to the geometry of the unit

to ensure proper working of the Gas-Liquid-Solid-Separator

more than
and to its

(GLSS) the gas

collection
hood, the incoming flow distribution
to get spatial
uniformity
and the
outflowing
effluent.
Physical
Parameters
A single
module can handle 16 to 15 MLD of sewage. For large flows a number of modules
could be provided.
Some physical
details
of a typical
UASB reactor
module are given
below:

Reactor configuration
Rectangular
or circular.
Depth
4.5 to 5.6 m for sewage.
width

or

Rectangular

preferred

diameter

To limit
lengths
of inlet
laterals
distribution
and sludge withdrawal.
Length
As necessary.
Inlet

shape is

to

around

16-12

m for

facilitating

uniform

flow

feed

gravity
through

feed from top (preferred
for municipal sewage) or pumped feed from bottom
manifold and laterals
(preferred
in case of soluble industrial
wastewaters).

Sludge blanket
2 to 2.5 m for
Deflector/GLSS

depth
sewage.

More depth

is

needed for

stronger

wastes.

This is a deflector
liquid-solid-separator"

beam which together with the gas hood (slope 66) forms a gas(GLSS) letting
the gas go to the gas collection
channel at top,

while the liquid
rises
into the settler
compartment and the sludge solids
fall
back
into the sludge compartment.
The flow velocity
through the aperture
connecting
the
reaction
zone with the settling
compartmentt
is generally
limited
to about 5m/h at peak
flow.

Settler
compartment
2.6-2.5
m in depth. Surface overflow
rate equals 26-28 m3/m2/d at peak flow.
Process Design Parameters
A few process design parameters
for UASBs are listed
below for municipal
sewages with
BOD about 266-366 mg/l and temperatures
above 26°C.
HRT

8-16 hours at average flow

(minimum 4 hours at peak flow)

SRT

36-56

days or more

Sludge blanket

concentration

(average)

15-36 kg VSS per m3. About 76 kg TSS per m3.
Organic loading
on sludge blanket

6.3-1.6

kg COD/kg VSS day (even upto 16 kg COD/ kg VSS day for

Volumetric

organic

agro-industrial

wastes).

agro-industrial

wastes)

loading

1-3 kg COD/m3day for

domestic sewage (16-15 kg COD/m3day for

BOD/COD removal efficiency
Sewage 75-85% for BOD. 74-78% for COD.
Inlet
points
Minimum 1 point
per 3.7-4.6
m2 floor
area.
Flow regime
Either
constant
rate for pumped inflows
or typically
fluctuating
systems.
Upflow velocity
About 6.5 m/h at average flow,
or 1.2 m/h at peak flow,
whichever
Sludge production
6.15-6.25
kg TS per m3 sewage treated.
Sludge drying
time

Seven days (in

flows

is

for

gravity

low.

India)

Gas production
Theoretical
6.38

m3/kg COD removed.

Actual

6.1-6.3

m3per kg COD removed.

Gas utilization

Method of use is optional.
1 m3 biogas with
kwh electricity.
Nutrients
nitrogen
and phosphorus removal
5 to 16% only.
Nitrification-Denitrification

A certain

75% methane

content

is

equivalent

to

1.4

Systems

amount of nitrogen

removal (26-36%) occurs in conventional

activated

sludge

systems.
Nitrogen
removal ranging from 76 to 96 % can be obtained
by use of
nitrification-denitrification
method in plants
based on activated
sludge and other
suspended growth systems.
Biological
denitrification
requires
prior
nitrification
of
all ammonia and organic
nitrogen
in the incoming waste.
Nitrification

There

are two groups

of

chemoautotrophic

bacteria

that

can be associated

with

the

process of nitrification.
One group (Nitrosomonas) derives its energy through the
oxidation of ammoniumto nitrite,
whereas the other group (Nitrobacter)
obtains energy
through the oxidation
of nitrite
to nitrate.
Both the groups,
collectively
called
Nitrifiers,
obtain
carbon required,
from inorganic
carbon forms.
of ammonia to nitrate
is a two step process:
Nitrosomonas
NH3

NH4 N02

Nitrification

Nitrobacter

N03

Stoichiometrically,
4.6 kg of oxygen is required
for nitrifying
1 kg of nitrogen.
Under
steady state conditions,
experimental
evidence
has shown nitrite
accumulation
to be
insignificant.
This suggests that the rate-limiting
step for the conversion
of ammonium
to nitrate
is the oxidation
of ammonium to nitrite
by the genus Nitrosomonas.
c =

1

where

|_
|_

is

the

growth

age (or mean cell
if

nitrification

rate

residence
is

of

nitrosomonas

time),

at the

worst

H c in a treatment

operating

plant

temperature.

and

high

desired.

Combined and Separate Systems of Biological
Oxidation
& Nitrification
Following
figure
shows flow sheets for combined and separate
systems
oxidation

Sludge

must be sufficiently

for

biological

nitrification.

Combined system is favoured
method of operation
as it is less sensitive
to load
variations
- owing to larger
sized aeration
tank - generally
produces a smaller
volume
of surplus
sludge owing to higher values of H c adopted,
and better
sludge
settleability.
Care should be taken to ensure that the oxygenation
capacity
of aeration
tank is
sufficient
to meet oxygen uptake due to carbonaceous
demand and nitrification.

Recycling

of sludge must be rapid

enough to prevent

denitrification

(and rising

sludge)

owing to anoxic conditions
in the settling
tank.
In separate
system, the first
tank can be smaller
in size since a higher
F/M ratio
can
be used, but this makes the system somewhat more sensitive
to load variations
and also
tends to produce more sludge for disposal.
An additional
settling
tank is also
necessary
between the two aeration
tanks to keep the two sludges separate.
A principal
advantage of this
system is its higher efficiency
of nitrification
and its better
performance
when toxic
substances
are feared to be in the inflow.
Biological
Denitrification
when a treatment
plant discharges
into receivingstream
with lowavailable
nitrogen
concentration
and with a flow much larger
than the effluent,
the presence of nitrate
in
the effluent
generally
does not adversely
affect
stream quality.
However,if
the nitrate
concentration
in the stream is significant,
it may be desirable
to control
the nitrogen
contentof
the effluent,
as highly
nitrified
effluents
can stillaccelarate
algal
blooms.
Even more critical
is the case where treatment
plant effluent
is dischargeddirectly
into relatively
still
bodies of water such as lakes or reservoirs.
Another argument for
the controlof
nitrogen
in the aquatic
environment
is theoccurence
of infantile
methemoglobinemia,which
results
from high concentration
of nitrates
indrinking
water.

The four basic processes that are used are:
(2) selective
ion exchange, (3) break point
(4) biological
nitrification/denitrification.
Biological
nitrification,

nitrification/denitrification
which is conversion

nitrifying

bacteria.

of

(1) ammonia stripping,
chlorination,
and

is a two step process.
The first
step
ammonia to nitrate
through the action
of

The second step is nitrate

conversion

(denitrification),

is

which is

carried
out by facultative
heterotrophic
bacteria
under anoxic conditions.
Microbiological
Aspects of Denitrification
Nitrate
conversion
takes place through
both assimilatory
and dissimilatory
cellular
functions.
In assimilatory
denitrification,
nitrate
is reduced to ammonia, which then
serves as a nitrogen
source for cell
synthesis.
Thus, nitrogen
is removed from the
liquid
stream by incorporating
it into cytoplasmic
material.
In dissimilatory
denitrification,
nitrate
serves as the electron
acceptor
in energy
metabolism
and is converted
to various
gaseous end products
but principally
molecular
nitrogen,
N2, which is then stripped
from the liquid
stream.
Because the microbial
yield
under anoxic conditions
is considerably
lower than under
aerobic
conditions,
a relatively
small fraction
of the nitrogen
is removed through
assimilation.
Dissimilatory
denitrification
is, therfore,
the primary means by which
nitrogen
removal is achieved.
A carbon source is also essential
as electron
donor for denitrification
to take place.
This source may be in the form of carbon internally
available
in sewage or artificially

added (eg. as methanol).

Since most community wastewaters

have a higher

ratio

of BOD:N,

the internally
available
carbon becomes attractive
and economical
for denitrification.
Denitrification
releases
nitrogen
which escapes as an inert
gas to the atmosphere while
oxygen released
stays dissolved
in the liquid
and thus reduces the oxygen input needed
into the system. Each molecule of nitrogen
needs 4 molecules
of oxygen during
nitrification
but releases
back 2.5 molecules
in denitrification.
Thus, theoretically,
62.5% of the oxygen used is released
back in denitrification.
Typical
Flowsheets
for Denitrification

Denitrification
in suspended growth systems can be achieved
using anyone of the typical
flowsheets
shown in the figure.
The use of methanol or any other artificial
carbon source should be avoided as far as
possible
since it adds to the cost of treatment
and also some operating
difficulties
may arise fro dosing rate of methanol.
Too much would introduce
an unnecessary
BOD in
the

effluent

while

too

little

would

leave

some

nitrates

undernitrified.

A more satisfactory
arrangement
would be to use the carbon contained
in the waste
itself.
However, the anoxic tank has to be of sufficient
detention
time for
denitrification
to occur which,
has a slower rate;
since the corresponding
oxygen
uptake rate of the mixed liquor
is mainly due to endogenous respiration
and is thus
low. The denitrification
rate,
therefore,
in a way also depends on the F/M ratio
in the
prior
aeration
tank.
Consequently,
if desired,
a portion
of the raw waste may be bypassed to enter directly
into the anoxic tank and thus contribute
to an increased
respiration
rate.
This reduces
the sizes of both the anoxic and aeration
tanks,
but the denitrification
efficiency
is
reduced as the bypassed unnitrified
ammonia can not be denitrified.
By reversing
the relative
positions
of anoxic and aerobic
tanks,
the oxygen requirement
of the waste in its anoxic state is met by the release
of oxygen from nitrates
in the
recycled
flow taken from the end of nitrification
tank.
Primary settling
of the raw
waste may be omitted
so as to bring more carbon into the anoxic tank.
More complete nitrification-denitrification
can be achieved
by Bardenpho arrangement.
The first
anoxic tank has the advantage of higher denitrification
rate while the
nitrates
remaining
in the liquor
passing out of the tank can be denitrified
further
in
a second anoxic tank through
endogenous respiration.
The

flow

nitrogen

from

anoxic

gas bubbles

tank

is

desirable

and add oxygen

to

prior

reaerate

to

for

16-15

minutes

to

drive

off

sedimentation.

Phosphorus Removal
Phosphorus precipitation
is ususally
achieved
by addition
of chemicals
like
calcium
hydroxide,
ferrous
or ferric
chloride,
or alum, either
in the primary or the final
settling
tank.
Alum is more expensive
and generates
more hydroxide,
which creates
extra sludge,
that
is difficult
to dewater.
Use of lime results
in an increase
of approximately
56% in
surplus
sludge,
but the sludge is reported
to have good dewatering
properties.
when
using iron salts,
a molar ratio
of 1.6:1.4
of iron to phosphorus is reported
to give
91-96% removal of total
phosphorus using ferrous
chloride
dosed directly
beneath the
aerator.

Chemical addition
prior
to biological
treatment
is feasible
if a primary
settling
tank
exists
as in the case of the conventional
activated
sludge process.
The dose
requirement
then increases,
but chemical
precipitation
also improves organic
removal,
thus reducing
BOD load on the biological
treatment.
For extended aeration
plants
there
is no primary
settling;
chemical addition
has to be done in the final
settling
tank.
Sludge Digestion
Sludge digestion
involves
the treatment
of highly
concentrated
organic
wastes in the
absence of oxygen by anaerobic
bacteria.The
anaerobic
treatment
of organic
wastes
resulting
in the production
of carbon dioxide
and methane, involves
two distinct
stages.
In the first
stage,
referred
to as acid fermentation,
complex waste
components,
including
fats,
proteins,
and polysaccharides
are first
hydrolyzed
by a
heterogeneous
group of facultative
and anaerobic
bacteria.
These bacteria
then subject
the products
of hydrolysis
to fermentations,
L;-oxidations,
and other metabolic
processes
leading
to the formation
of simple organic
compounds, mainly short-chain

(volatile)
acids and alcohols.
However in the second stage, referred to as "methane
fermentation,
the end products of the first
stage are converted to gases (mainly
methane and carbon dioxide) by several different
species of strictly
anaerobic
bacteria.

The bacteria
responsible
for acid fermentation
are relatively
tolerant
to changes in pH
and temperature
and have a much higher
rate of growth than the bacteria
responsible
for
methane fermentation.
If the pH drops below 6.6, methane formation
essentially
ceases,
and more acid accumulates,
thus bringing
the digestion
process to a standstill.
As a

result,
methane fermentation
is
anaerobic
wastewater
treatment.

generally
assumed to
The methane bacteria

(27-43°C) with

of four

digestion

period

be the rate limiting
are highly
active
in

weeks and thermophilic

digestion
period of 15-18 days. But thermophilic
range is not
odour and operational
difficulties.
Digestion
Tanks or Digesters
A sludge digestion
tank is a RCC or steel tank of cylindrical
and is covered with fixed
or floating
type of roofs.

step in
mesophilic

range (35-46°C) with
practised

shape with

because

hopper

of

bottom

Types of Anaerobic
Digesters
The anaerobic
digesters
are of two types:
standard
rate and high rate.
In the standard
rate digestion
process,
the digester
contents
are usually
unheated and unmixed. The
digestion
period may vary from 36 to 66 d. In a high rate digestion
process,
the
digester
contents
are heated and completely
mixed. The required
detention
period is 16
to

26 d.

Often a combination
of standard
and high rate digestion
is achieved
in two-stage
digestion.
The second stage digester
mainly separates
the digested
solids
from the
supernatant
liquor:
although
additional
digestion
and gas recovery
may also be
achieved.

Design Details
Generally
digesters
1.

Tank

sizes

are

2. Liquid
depth
3. The digester

are
not

designed

less

than

to

treat

for

6 m diameter

a capacity
and

not

more

upto
than

4 MLD.
55 m diameter.

may be 4.5 to 6 m and not greater
than 9 m.
capacity
may be determined
from the relationship

v = [v+=2/3 (vf

- Vd)]t1

+ Vdt2

where V = capacity
of digester
in m3, Vf = volume of fresh sludge m3/d, Vd = volume
daily
digested
sludge accumulation
in tank m3/d, t1= digestion
time in days required
for digestion,
d, and t2 = period of digested
sludge storage.

of

Gas Collection

The amount of sludge gas produced varies
from 6.614 to 6.628 m3per capita.
The sludge
gas is normally
composed of 65% methane and 36% carbondioxide
and remaining
5% of
nitrogen
and other inert
gases, with a calorific
value of 5466 to 5856 kcal/m3.
Treatment
Plant Layout and Siting
Plant layout
is the arrangement
of designed treatment
units
on the selected
site.
The
components that need to be included
in a treatment
plant,
should be so laid out as to
optimize
land requirement,
minimize
lengths
of interconnecting
pipes and pumping heads.
Access for sludge and chemicals
transporting,
and for possible
repairs,
should be
provided
in the layout.
Siting
is the selection
of site for treatment
plant based on features
as character,
topography,
and shoreline.
Site development
should take the advantage of the existing
site topography.
The following
principles
are important
to consider:
1. A site on a side-hill
can facilitate
gravity
flow that will
reduce pumping
requirements
and locate
normal sequence of units without
excessive
excavation
or fill.
2. when landscaping
is utilized
it should reflect
the character
of the surrounding
area. Site development
should alter
existing
naturally
stabilized
site
contours
and
drainage
as little
as possible.
3. The developed site
should be compatible
with the existing
land uses and the
comprehensive
development
plan.
Treatment
Plant Hydraulics
Hydraulic
profile
is the graphical
representation
of the hydraulic
grade line through
the treatment
plant.
If the high water level
in the receiving
water is known, this
level
is used as a control
point,
and the head loss computations
are started
backward
through the plant.
The total
available
head at the treatment
plant is the difference
in
water surface
elevations
in the interceptor
and the water surface
elevation
in the
receiving
water at high flood
level.
If the total
available
head is less than the head
loss through the plant,
flow by gravity
cannot be achieved.
In such cases pumping is
needed to raise the head so that flow by gravity
can occur.
There are many basic principles
that must be considered
when preparing
the hydraulic
profile
through the plant.
Some are listed
below:

1. The hydraulic
initial

profiles

are

prepared

at

peak and average

design

flows

and at minimum

flow.

2. The hydraulic
profile
is generally
prepared for all main paths of flow through the
plant.
3. The head loss through the treatment
plant is the sum of head losses in the treatment
units
and the connecting
piping
and appurtenances.
4. The head losses through the treatment
unit include
the following:
. Head
. Head

losses
losses

at
at

the
the

influent
effluent

structure.
structure.

OU&#39;|Q.O&#39
. Head losses

through

. Miscellaneous

. The total
f following:

a.
b.

Head
Head

loss
loss

and

loss

due
due

the

unit.

free

fall

through

the

to
to

c.

Head loss

due to

contraction

Head

due

to

friction.

due to

bends,

e. Head loss

allowance.

pipings,

channels

and appurtenances

is

the

sum

entrance.
exit.

d.

loss

surface

connecting

and enlargement.

fittings,

gates,

valves,

and meters.

f . Head required
over weir and other hydraulic
controls.
surface
allowance.
S . Free-fall
T ypical
Hydraulic
Profile
Through Treatment
Facility

Treated
Effluent
Disposal
The proper disposal
of treatment
plant effluent
or reuse requirements
is an essential
part of planning
and designing
wastewater
treatment
facilities.
Different
methods of
ultimate
disposal
of secondary effluents
are discussed
as follows.
Natural
Evaporation
The process involves
large impoundments with no discharge.
Depending on the climatic
conditions
large impoundments may be necessary
if precipitation
exceeds evaporation.
Therefore,
considerations
must be given to net evaporation,
storage
requirements,
and
possible
percolation
and groundwater
pollution.
This method is particularly
beneficial
where recovery
of residues
is desirable
such as for disposal
of brines.
Groundwater
Recharge
Methods for groundwater
recharge include
rapid infiltration
by effluent
application
or
impoundment,
intermittent
percolation,
and direct
injection.
In all cases risks
for
groundwater
pollution
exists.
Furthermore,
direct
injection
implies
high costs of
treating
effluent
and injection
facilities.
Irrigation
Irrigation
has been practiced
primarily
as a substitute
for scarce natural
waters or

sparse rainfall
in arid areas. In most cases food chain crops (i.e.
crops consumed by
humans and those animals whose products are consumed by humans) may not be irrigated
by
effluent.
production
wastewater

However, field
crops such as cotton,
are grown with wastewater
effluent.
effluent
has been used for watering

Recreational

sugar

beets,

parks,

golf

and crops
courses

for

seed

and highway

medians.

Lakes

The effluent
from the secondary treatment
facility
is stored in a lagoon for
approximately
36 days. The effluent
from the lagoon is chlorinated
and then percolated
through
an area of sand and gravel,
through which it travels
for approximately
6.5 km
and is collected
in an interceptor
trench.
It is discharged
into a series
of lakes used
for swimming, boating
and fishing.
Aquaculture

Aquaculture,

or the production

of aquatic

organisms

(both flora

and fauna),

has been

practiced
for centuries
primarily
for production
of food, fiber
and fertilizer.
Lagoons
are used for aquaculture,
although
artificial
and natural
wetlands
are also being
considered.
However, the uncontrolled
spread of water hyacinths
is itself
a great
concern because the flora
can clog waterways and ruin water bodies.
Municipal
Uses
Technology
is now available
to treat
wastewater
to the extent
that it will
meet
drinking
water quality
standards.
However, direct
reuse of treated
wastewater
is
practicable
only on an emergency basis.
Many natural
bodies of water that are used for
municipal
water supply are also used for effluent
disposal
which is done to supplement
the natural
water resources
by reusing
the effluent
many times before it finally
flows

to the sea.
Industrial
Uses

Effluent

has been successfully

used as a cooling

water

or boiler

feed

water.

Deciding

factors for effluent
reuse by the industry include (1) availability
of natural water,
(2) quality
and quantity of effluent,
and cost of processing,
(3) pumping and transport
cost of effluent,
and (4) industrial
process water that does not involve public health
considerations.

Discharge
Discharge
purification
remaining

into
into

Natural
waters
natural
waters is the
or assimilative
capacity
treatment.

Stabilization

most common disposal
practice.
The selfof natural
waters is thus utilized
to provide

the

Ponds

The stabilization
ponds are open flow through
basins specifically
designed and
constructed
to treat
sewage and biodegradable
industrial
wastes.
They provide
long
detention
periods
extending
from a few to several
days.
Pond systems,
in which oxygen is provided
through mechanical
aeration
rather
than algal
photosynthesis
are called
aerated lagoons.
Lightly
loaded ponds used as tertiary
step in waste treatment
for polishing
of
secondary effluents
and removal of bacteria
are called
maturation
ponds.
Classification

of

Stabilization

Ponds

Stabilization
ponds may be aerobic,
anaerobic
or facultative.
Aerobic
ponds are shallow
ponds with depth less than 6.5 m and BOD loading
of 46-126
kg/ha.d
so as to maximize penetration
of light
throughout
the liquid
depth. Such ponds
develop intense
algal
growth.
Anaerobic
ponds are used as pretreatment
of high strength
wastes with BOD load of 4663666 kg/ha.d
Such ponds are constructed
with a depth of 2.5-5m as light
penetration
is
unimportant.
Facultative
pond functions
aerobically
at the surface while anaerobic
conditions
prevail
at the bottom.They
are often about 1 to 2 m in depth.
The aerobic
layer acts as
a good check against
odour evolution
from the pond.
Mechanism

of

Purification

The functioning
of a facultative
stabilization
pond and symbiotic
relationship
in the
pond are shown below. Sewage organics
are stabilized
by both aerobic
and anaerobic
reactions.
In the top aerobic
layer,
where oxygen is supplied
through
algal
photosynthesis,
the non-settleable
and dissolved
organic
matter is oxidized
to CO2 and
water.
In addition,
some of the end products
of partial
anaerobic
decomposition
such as
volatile
acids and alcohols,
which may permeate to upper layers
are also oxidized
periodically.
The settled
sludge mass originating
from raw waste and microbial
synthesis
in the aerobic
layer and dissolved
and suspended organics
in the lower layers
undergo stabilization
through
conversion
to methane which escapes the pond in form of
bubbles.

Factors
Various
wastewater

Affecting
factors

Pond Reactions
affect
pond design:

characteristics

environmental factors
algal growth patterns

and

fluctuations.

(solar radiation,
and their diurnal

light,
temperature)
and seasonal variation)

bacterial
growth patterns
and decay rates.
solids
settlement,
gasification,
upward diffusion,
sludge accumulation.
The depth of aerobic
layer in a facultative
pond is a function
of solar
radiation,
waste characteristics,
loading
and temperature.
As the organic
loading
is increased,
oxygen production
by algae falls
short of the oxygen requirement
and the depth of
aerobic
layer decreases.
Further,
there is a decrease in the photosynthetic
activity
of
algae because of greater
turbidity
and inhibitory
effect
of higher concentration
of
organic
matter.
Gasification
of organic
matter to methane is carried
out in distinct
steps of acid
production
by acid forming
bacteria
and acid utilization
by methane bacteria.
If the
second step does not proceed satisfactorily,
there is an accumulation
of organic
acids
resulting
in decrease of pH which would result
in complete inhibition
of methane
bacteria.
Two possible
reasons for imbalance
between activities
of methane bacteria

are:

(1) the waste may contain

inhibitory

substances which would retard

the activity

of

methane bacteria

and not affect

the activity

of acid producers

to the same extent.

(2)

The activity
of methane bacteria
decreases much more rapidly
with fall
in temperature
as compared to the acid formers.
Thus, year round warm temperature
and sunshine provide
an ideal
environment
for
operation
of facultative
ponds.
Algal Growth and Oxygen Production
Algal growth converts
solar energy to chemical
energy in the organic
form. Empirical
studies
have shown that generally
about 6% of visible
light
energy can be converted
to
algal
energy.
The chemical
energy contained
in an algal
cell
averages 6666 calories
per gram of
algae.
Depending on the sky clearance
factor
for an area, the average visible
radiation
received

can

be estimated

Avg. radiation=

Oxygen production
following
equation:
106C02

as follows:

Min. radiation

+ [(Max.

sky clearance
occurs concurrently

+ 16NO3 + HPO4 + 122H2O

radiation

factor]
with

+ 18H+

algal

- Min. radiation)x
production

C106H2630116N16P1

in

accordance

with

+ 13802

On weight basis,
the oxygen production
is 1.3 times the algal
production.
Areal Organic Loading
The permissible
areal organic
loading
for the pond expressed as kg BOD/ha.d will
depend
on the minimum incidence
of sunlight
that can be expected at a location
and also on the
percentage
of influent
BOD that would have to be satisfied
aerobically.
The Bureau of
Indian
Standards
has related
the permissible
loading
to the latitude
of the pond
location
to aerobically
stabilize
the organic
matter and keep the pond odour free.
The
values are applicable
to towns at sea levels
and where sky is clear for nearly
75% of
the days in a year. The values may be modified
for elevations
above sea level
by

dividing

by a factor

MSL in hundred
Detention
Time

(1 + 6.663 EL) where EL is the elevation

of the pond site

above

meters.

The flow of sewage can approximate
flow.
If BOD exertion
is described
given by:
for plug flow:
Le/Li = e-k1t
for complete mixing:
Le/Li =
1

either
plug flow or complete
by first
order reaction,
the

mixing or dispersed
pond efficiency
is

1+k1t

For dispersed
flow the efficiency
of treatment
for different
dgrees of intermixing
is
characterized
by dispersion
numbers.Choice
of a larger
value for dispersion
number or
assumption
of complete mixing would give a conservative
design and is recommended.
Depth
Having determined
the surface
area and detention
capacity,
it becomes necessary to
consider
the depth of the pond only in regard to its limiting
value.
The optimum range
of depth for facultative
ponds is 1.6 - 1.5 m.

Aquatic
Aquatic
form of

Plant Systems
systems in waste treatment
are either
free floating
growths harnessed in the
built-up
ponds for waste treatment
such as duckweed and hyacinth
ponds or

rooted vegetations

(reeds)

which emerge out of shallow waters cultivated

in constructed

wetlands.

Natural
wetlands
exists
all over the world.
They generally
have saturated
soil
conditions
and abound in rooted vegetation
which emerges out of shallow waters in the
euphotic
zone. They may also have phytoplankton.
Natural
wetlands
can be integrated
with wastewater
treatment
systems.
Constructed
wetlands
are man-made for treatment
of wastewater,
mine drainage,
storm
drainage,
etc. They have rooted vegetation.
Longitudinal
Section
Through a Typical
Reed Bed with Gravel,
Sand or Selected
Soil with
Horizontal

Flow

of

wastewater

Aquatic
plant ponds consisting
of free floating
macrophytes,
such as water hyacinths,
duckweeds, etc. have been cultured
in ponds either
for their
ability
to remove heavy
metals,
phenols,
nutrients,
etc. from wastewaters
or to assist
in giving
further

treatment
same time
Conceptual

to pretreated
wastewaters
to meet stringent
discharge
standards
while
producing
new plant growths for their
gas production
or food value.
flowsheet
showing waste treatment
using an aquatic
plant pond

at the

Septic Tank
Septic tanks are horizontal
continuous
flow,
small sedimentation
tanks through which
sewage is allowed to flow slowly to enable the sewage solids
to settle
to the bottom of
the tank, where they are digested
anaerobically.
The tank is de-sludged
at regular
intervals
usually
once every 1-5 years.
Cesspool
It is a pit excavated
in soil with water tight
lining
and loose lining
by stone or
brick to provide
for leaching
of wastewater
by sides and the pit is covered.
The
leaching
type is suitable
for porous soils.
The capacity
should not be less than one
day&#39;s
flow into the pit.
If all the water in a test pit of one meter diameter
and 2 m
deep, disappears
in 24 hours, such soil
is best suitable
for cesspools.
The bottom of
the cesspool must be well above the ground water level.
After
sometime the sides of pit
get clogged by the sewage solids,
reducing
the leaching
capacity.
At overflow
level,
an
outlet
is provided
to take-off
unleached liquid
into a seepage pit.
The settled
matter
is removed at intervals.
water tight
cesspools
are cleaned every 6 months and their
capacity
must not be less than 70 l/person/month.
Seepage Pit
The seepage pit is needed to discharge
the effluent
of cesspool,
aquaprivy,
septic
tank
or sullage
from bathrooms and kitchens.
The difference
between seepage pit and cesspool
is that the seepage pit is completely
filled
up with stones.
The fine
suspended solids
adhere to the surface
of stones and get decomposed by the zoogleal
film,
which are on
the

stones

and

the

effluent

is

leached

into

the

worked-out
Examples:
Population
Forecast
by Different
Methods
Problem: Predict
the population
for the years
following
census figures
of a town by different

side

walls.

1981, 1991,
methods.

1994,

and 2001 from

the

Year
1901
10.66,
n = 3
= 120 X 1.355
= 162.60

Population

for

1994 = 146.95 + (15.84 x 3/10) = 151.70

Sedimentation
Tank Design
Problem: Design a rectangular
sedimentation
tank to treat
2.4 million
water per day. The detention
period may be assumed to be 3 hours.
Solution:
Raw water flow per
Dia of inlet
pipe coming from two filter
= 50 cm.
Velocity
<0.6 m/s. Diameter of washwater pipe to overhead tank = 67.5
Air compressor unit = 1000 l of air/
min/ m2 bed area.
For 5 min, air required
= 1000 x 5 x 7.5 x 5.77 x 2 = 4.32 m3 of air.

litres

of

raw

cm.

Flow in Pipes of a Distribution
Network by Hardy Cross Method
Problem: Calculate
the head losses and the corrected
flows in the various
pipes of a
distribution
network as shown in figure.
The diameters
and the lengths
of the pipes
used are given against
each pipe. Compute corrected
flows after
one corrections.
Solution:
First
of all,
the magnitudes
as well as the directions
of the possible
flows
in each pipe are assumed keeping in consideration
the law of continuity
at each
junction.
The two closed loops,
ABCD and CDEF are then analyzed by Hardy Cross method
as per tables
1 & 2 respectively,
and the corrected
flows are computed.
- 15.2
- 12.2

Trickling
Problem:

Filter
Design

Design
a low rate

filter

to

treat

6.0

Mld of

sewage of

BOD of

210 mg/l.

The

final
Solution:

effluent

should

Assume

be 36 mg/l

36% of

BOD l

and organic

loading

rate

is

326 g/m3/d.