Domestic
Raw water

categories:
and rivers;
water

water

Source:

Treatment

The

various

and Supply
sources

of

water

can

be

classified

into

1. Surface sources,
such as a. Ponds and lakes;
c. Storage reservoirs;
and
d. Oceans, generally
not

supplies,

at

2. Sub-surface
Infiltration

wells

two

b. Streams
used for

present.

sources

or underground

; and

c.

wells

sources,

such as a. Springs;

b.

and Tube-wells.

water Quality:
The raw or treated
water is
chemical and bacteriological
characteristics:
Physical
Characteristics:
Turbidity

analysed

by testing

Colour

their

Taste

physical,

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.
originally
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
found in nature such as Fuller's
earth,
consistency
in standard formulation
was difficult
to achieve.
These days turbidity
is measured by applying
Nephelometry,
a technique
to
measure level
of light
scattered
by the particles
at right
angles to the
incident
light
beam. The scattered
light
level
is proportional
to the particle
concentration
in the sample. The unit of expression
is Nephelometric
Turbidity
Unit (NTU). The IS values for drinking
water is 10 to 25 NTU.
Colour:
dissolved
organic matter from decaying vegetation
or some inorganic
materials
may impart colour
to the water.
It can be measured by comparing the
colour of water sample with other standard
glass tubes containing
solutions
of
different

is

standard

produced

distilled

colour

intensities.

by one milligram

water.

The

IS

The

of platinum

value

for

treated

standard

cobalt
water

unit

of

dissolved
is

5

to

colour

in
25

is

that

one litre

of

cobalt

which

units.

Taste and Odour:
odour depends on the contact
of a stimulating
substance with
the appropriate
human receptor
cell.
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.

A new method to estimate
taste of water sample has been developed based on
flavour
known as Flavour
Profile
Analysis
(FPA). The character
and intensity
of taste and odour discloses
the nature of pollution
or the presence of
microorganisms.

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
Chlorides

Characteristics:
Sulphates

PH
Iron

Acidity
Solids

Alkalinity
Nitrates

Hardness

pH:
pH value denotes the acidic
or alkaline
condition
of water.
It is
expressed on a scale ranging from 0 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,
(b) carbonate
alkalinity,
(c) hydroxide
plus carbonate
alkalinity,
(d) carbonate
plus
bicarbonate
alkalinity,
and (e) bicarbonate
alkalinity,
which is useful mainly
in water softening and boiler feed water processes.
Alkalinity
is an important
parameter in evaluating
the optimum coagulant dosage.
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 300 mg/L as CaC03.
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 50 mg/L, for
some industries.
Desirable
limit
for drinking
water is 150 mg/L. May be extended
upto 400 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 250 to 1000
mg/L.
Iron:
Iron is found on earth mainly as insoluble
ferric
oxide. when it
comes

in

contact

favourable

with

water,

conditions.

This

it

dissolves

ferrous

hydroxide,
which is a precipitate.
reduced to soluble
ferrous
ion.
discolouration

drinking
total

in

water is
Solids:
solids.

clothes

and

Iron

Total

solids

incrustations

is

the

of the sample and its

(103

°c).

105

form

ferrous
is

bicarbonate

oxidised

into

under

ferric

Under anaerobic
conditions,
ferric
ion is
can impart bad taste to the water,
causes
in

0.3 to 1.0 mg/L.
The sum total
of foreign

evaporation
to

to

bicarbonate

matter

water

matter
that

subsequent

mains.

present

remains

drying

as

IS

value

in water
residue

at a defined

for

is

termed

as

after

temperature

Total solids
consist
of volatile
(organic)
and non-volatile
(inorganic
or
fixed)
solids.
Further,
solids
are divided
into suspended and dissolved
solids.
Solids
that can settle
by gravity
are settleable
solids.
The others are nonsettleable
solids.
IS acceptable limit
for total
solids
is 500 mg/L and
tolerable
limit
is 3000 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:
Plate Count Test
Most Probable Number
Membrane Filter

Standard
Technique

Standard Plate Count Test:
In this test,
the 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 per 100 ml of
sample.

Most Probable 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
of E.Coli
in

different

dilution

combination.

MPN value

is

calculated

as

follows:

Five

10 ml (five dilution
combination)
tubes of a sample is tested for E.Coli.
If 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 100 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 0.5 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 100 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 public,
since there are many variable
factors
affecting
water consumption. The various types of 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)
Average
%1Domestic Consumption65-300 160 35 2Industrial
and Commercial Demand 45-450
30 3 Public Uses including
Fire Demand 20-90 45 10 4 Losses and waste 45-150

135
62

25

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 (P in thousand) Q for 1 lakh Population)
1 American Insurance
Association
Q (L/min)=4637
HP (1-0 01 HP) 41760
2 Kuchling's
Formula Q
(L/min)=3182
HP 31800
3 Freeman's
Formula Q (L/min)= 1136.5(P/5+10)
35050
4 Ministry
of Urban Development Manual Formula Q (kilo liters/d)=100
HP for
P>50000

31623

Factors affecting
per capita demand: a. Size of the city:
Per capita
demand for big cities
is generally
large as compared to that for smaller towns
as big cities
have sewered houses.
b. Presence of industries.
c. Climatic
conditions.
d. Habits of people and their economic status.
e. Quality
of water: If water is aesthetically
$ medically safe, the consumption will
increase

f.

as people

Pressure

administration:

will

not

resort

in the distribution
Leaks

in

water

to

mains

can be kept to a minimum by surveys.

private

system.

wells,

etc.

g. Efficiency

and services;

of water works

and unauthorised

h. Cost of water.

i.

use of water

Policy

of

metering
and charging
method: water tax
basis of meter reading and on the basis
Fluctuations

Average
(365

Daily

in

Per Capita

Rate

of

is charged
of certain

in two different
ways:
fixed monthly rate.

on the

Demand

Demand

= Quantity

Required

in

12 Months/

x Population)

If

this

sufficient

average

demand is

to

the

meet

supplied

at all

the

times,

it

will

not be

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
demand)/24
= 2.7 x
average daily
demand/24
= 2.7 x annual average hourly demand
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 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.
Available
resources.
Performance of the
system during initial
period.
Population
Forecasting
Methods: The various
methods adopted for
estimating
future
populations
are given below. The particular
method to be
adopted for a particular
case or for a particular
city
depends largely
on the
factors

discussed

in

the

methods,

and

the

selection

is

left

to

the

discrection

and intelligence
of the designer.
1. Arithmetic
Increase
Method 2. Geometric
Increase
Method
3. Incremental
Increase
Method 4. Decreasing
Rate of Growth
Method
5. Simple Graphical
Method
6. Comparative
Graphical
Method 7. Ratio
Method
8. Logistic
Curve Method
Arithmetic
population
method is

Increase
Method:
This method is based on the assumption
that the
increases
at a constant
rate;
i.e.
dP/dt=constant=k;
Pt= P0+kt. This
most applicable
to large and established
cities.

Geometric Increase
Method: This method is based on the assumption
that
percentage
growth rate is constant
i.e.
dP/dt=kP;
lnP= lnP0+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
increase
is worked out, and is then subtracted
from the latest
percentage
increase
to get the percentage
increase
of next 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
similar
to the city whose future
population
is to
be estimated
are selected.
It is then assumed that the city
under consideration
will
develop,
as the selected
similar
cities
have developed in the past.
Ratio

Method

In this

method,

four

five

to

the local

decades

is

population
obtained

and the country'spopulation

from

the

census

records.

The

for

ratios

the last
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
estimate.
2. Does
not consider
the abnormal or special
conditions
which can lead to population
shifts
from one city
to another.
3. Logistic
Curve Method
4. The three factors
responsible
for changes in
population
are : (i) Births,
(ii)
Deaths and (iii)
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
is to help in
safely
withdrawing
water from the source over predetermined
pool levels
and then
to discharge
this water into the withdrawal
conduit
(normally
called
intake
conduit),
through which it flows up to water treatment
plant.
Factors Governing Location
of Intake
1. As far as possible,
the site
should be near the treatment
plant so that 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
lifts
water from a lower to a higher level
and delivers
Pumps are employed in water supply projects
at various
purposes:
1. To lift

raw

water

from

wells.

2.

To

deliver

at desired
pressure.
3. To supply pressured
To boost up pressure in water mains.
5. To fill
6. To back-wash filters.
7. To pump chemical
treatment.

Classification

of Pumps

into hydraulic
energy.
it at high pressure.
stages for following

treated

water for
elevated
solutions,

water

to

the

consumer

fire
hydrants.
4.
overhead water tanks.
needed for water

It

Based on principle
of operation,
pumps may be classified
as follows:
1. Displacement
pumps (reciprocating,
rotary)
2. Velocity
pumps (centrifugal,
turbine
and jet pumps)
3. Buoyancy pumps (air lift
pumps)
4. Impulse pumps
(hydraulic
rams)
Capacity
of Pumps
work done by the pump,
H.P.=E,QH/75
where, EA: specific
weight of water kg/m3, Q: discharge
of pump, m3/s; and H:
total
head against
which pump has to work.
H: Hs + Hd + Hf + (losses
due
to exit,
entrance,
bends, valves,
and so on)
where, Hs=suction
head, Hd = delivery
head, and Hf = friction
loss.
Efficiency
of pump (E) = EAQH/Brake H.P.
Total brake horse power required
2

QH/E

Prgvideevennumber
of motorssay 2,4,... with their total capacitybeingequal
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,
Hazenwilliam'sformula for pressure conduits
and Manning's
formula for freeflow
conduits

are

used.
U=0.85

Hazen-william's
C rH0.63S0.54

formula

Manning's
formula
U=1/n rH2/3S1/2
where, U: velocity,
m/s; rH= hydraulic
S: slope,
C: Hazen-william's coefficient,
and n = Manning's
coefficient.
Darcy-weisbach
formula
hL=(fLU2)/(2gd)

radius,m;

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
modifying
the quality
of raw
Standards for drinking
water
for drinking
water Parameter
The typical
functions
of

the proper sequential
order for the purpose of
water to meet the desired
standards.
Indian
are given in the table below.
Indian Standards
each unit

operations

are given

in

the following

table:

Functions
of water Treatment Units Unit treatment
Function
(removal)
Aeration,
chemicals
use Colour,
Odour, Taste Screening
Floating
matter Chemical
methods Iron,
Manganese, etc. Softening
Hardness Sedimentation
Suspended matter
Coagulation
Suspended matter,
a part of colloidal
matter and bacteria
Filtration
Remaining colloidal
dissolved
matter,
bacteria
Disinfection
Pathogenic
bacteria,
Organic matter and Reducing substances
The types of treatment
required
for
table:
Source Treatment
required
1. Ground water and spring water
or

different
fairly

sources
free

from

are given
contamination

in

the following
No treatment

Chlorination

2. Ground water with chemicals,
minerals
and gases Aeration,
coagulation
(if
necessary),
filtration
and disinfection
3. Lakes, surface water reservoirs
with less amount of pollution
Disinfection
4. Other surface waters such as
rivers,
canals and impounded reservoirs
with a considerable
amount of pollution
Complete

treatment

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 C02 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
(Cascades):
In gravity
aerators,
water is allowed to fall
by gravity
such that a large area of water is exposed to atmosphere,
sometimes
aided 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

0.03

to

0.09

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 10 to 30 min and 0.2 to
0.4 litres
of air is required
for 1 litre
of water.
Mechanical Aerators
: Mixing paddles as in flocculation
are used. Paddles
may be either
submerged or at the surface.
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 treatment.
To
settle
the sludge (biomass)
after
activated
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
each

to

other.

Type IV: Compression
The concentration
of particles
is so high that
sedimentation
can 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:
FQ="PQVP
(2) the buoyant force quantified
by Archimedes as: Fb="gVp
If the density
of the particle
differs
from that of the water,
a net force is
exerted and the particle
is accelaratd
in the direction
of the force:
Fnet=(*pA)QVP
This net force becomes the driving
force.
Once the motion has been initiated,
a third
force is created due to viscous friction.
This force,
called
the drag
force,
is quantified
by:
Fd=CDAp"v2/2
CD: drag coefficient.
Ap =
projected
area of the particle.
Because the drag force acts in the opposite
direction
to the driving
force
and increases
as the square of the velocity,
accelaration
occurs at a decreasing
rate until
a steady velocity
is reached at a point where the drag force equals
the driving
force:
"p-")gVp
= CDAp"v2/2
s number:

Re="vd

Reynolds
10

number less

indicate

Stokes
which

than

turbulent

1.0

flow.

indicate

laminar

Intermediate

values

Flow For laminar
flow,
terminal
v= ("p-')gd2
is known as the stokes equation.

Transition

Flow
3

Need to solve
CD

CD:

24

settling
18

non-linear
+

3

flow,

while

values

indicate

transitional

velocity

equation

equations:

v2=

+0.34

greater

than

flow.

becomes:

4g("p-')d
Re

Re1/2

Re="vd

Calculate
Reynolds

velocity
using Stokes law or turbulent
expression.
Calculate
and check
number. Calculate
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
sludge hopper from where it is pumped out periodically.
A long
zones:

rectangular

settling

tank

can be divided

into

four

different

functional

a

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:
Forcollection
ofsludge
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 shortcircuiting.
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 short-circuiting.
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 300
m3/d/m. It may be necessary to provide special inboard weir designs as shown to
lower

the

Inboard

weir

overflow

rates.

weir Arrangement

to Increase

weir

Length

Circular
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 treatment.
To
settle
the sludge (biomass) after activated
sludge process / tricking
filters.
Doubt

Sludge

removal mechanisms in

circular

tanks are

simpler

and require

less

maintenance.

Settling

operations

Particles
falling
through the settling
basin have two components of velocity:
1) Vertical
component: vt=("p-')gd2
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. 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
v0.
Q/As, which has the units of velocity
and is referred
to as the
overflow rate qo. This overflow rate is the design factor for settling
basins
and corresponds to the terminal setting velocity
of the particle
that is 100%
removed.

Design

Details

1. Detention
sedimentation:

period:
2 to

2.5

for

plain

sedimentation:

3 to 4 h, and for

coagulated

h.

2. Velocity
of flow: Not greater than 30 cm/min (horizontal
flow).
3. Tank dimensions: L:B = 3 to 5:1. Generally L: 30 m (common) maximum 100 m.
Breadth: 6 m to 10 m. Circular:
Diameter not greater than 60 m. generally
20 to
40

m.

4. Depth 2.5

to 5.0

m (3 m).

5. Surface Overflow Rate: For plain sedimentation
12000 to 18000 L/d/m2
area; for thoroughly
flocculated
water 24000 to 30000 L/d/m2 tank area.
Slopes:
Rectangular
1% towards inlet
and circular
8%.

tank
6.

General Properties
of Colloids
1. Colloidal
particles
are so small that
their
surface
area in relation
to mass is very large.
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
of dispersion
medium. This motion is called
Brownian motion (named
after
Robert Brown who first
noticed
it).
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 barrier
must be overcome
before aggregation
occurs.
The magnitude of energy barrier
depends on (1) charge
on the particle,
and (2) ionic
composition
of water.
Destabilization
of Colloidal
Dispersion
Particle
destabilization
can be
achieved by four mechanisms:
Change characteristics
of medium-Compression
double layer.
Change characteristics
of colloid
particles-Adsorption
and
charge neutralization.
Provide bridges1. Enmeshment in a precipitate.
2. Adsorption
and interparticle
bridging.

of

(Double Layer Compression
Colloidal
systems could be destabilized
by the
addition
of ions having a charge opposite
to that of the colloid.
The
coagulating
power of ions increased
in the ratio
of 1:10:1000
as the valence of
the ions increased
in the ratio
from 1 to 2 to 3. 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 a system with an adsorbable
species and cause
restabilization
as a result
of a reversal
of charge on the colloidal
particle.)
(Enmeshment in a Precipitate
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
relationship
for that
coagulant.)

(Adsorption
and Interparticle
Bridging
Many different
natural
compounds such
as starch,
cellulose,
polysaccharide
gums, and proteineous
materials,
as well as
a wide variety
of synthetic
polymeric
compounds are known to be effective
coagulating
agents.
Research has revealed
that both positive
and negative
polymers are capable of destabilizing
negatively
charged colloidal
particles.)
Flocculation

Flocculation
is stimulation
by mechanical
means to agglomerate
destabilised
particles
into compact, fast settleable
particles
(or flocs).
Flocculation
or
gentle agitation
results
from velocity
differences
or gradients
in the
coagulated
water, which causes the fine moving, destabilized
particles
to come
into contact
and become large,
readily
settleable
flocs.
It is a common practice
to provide an initial
rapid (or) 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 30 to 60 seconds and the head loss is 20 to
60 cms
formed.

of

water.

Here

colloids

are

Slow mixing brings
the contacts
formed during rapid mixing.
Perikinetic

and

The flocculation
and

Orthokinetic

process

destabilised

between

and

the

nucleus

the finely

divided

classified

into

for

the

destabilised

floc

is

matter

Flocculation

can be broadly

two types,

perikinetic

orthokinetic.

Perikinetic
flocculation
refers
to flocculation
(contact
or collisions
of
colloidal
particles)
due to Brownian motion of colloidal
particles.
The random
motion of colloidal
particles
results
from their
rapid and random bombardment by
the

molecules
Orthokinetic

of

the
fluid.
flocculation

refers

to

contacts

or

collisions

of

colloidal

particles
resulting
from bulk fluid
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/EEV)1/2,
where
P=Power, V=channel volume, and EE= Absolute
viscosity.
Mechanism

of

Flocculation

Gravitational
flocculation:
Baffle
type mixing basins are examples of
gravitational
flocculation.
water flows by gravity
and baffles
are provided
the basins which induce the required
velocity
gradients
for achieving
floc
formation.

in

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(0H)2+ and Al(0H)2+.
These products
are
quite effective
as coagulants
as they adsorb very strongly
onto the surface
of
most negative
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 iso-electric
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 amorphous metal hydroxide
is necessary for sweep-floc
coagulation.

The solubility
of Al(0H)3(s)
and Fe(0H)3(s)
is minimal at a particular
pH and
increases
as the pH increases
or decreases from that value.
Thus, pH must be
controlled
to establish
optimum conditions
for coagulation.
Alum and Ferric
Chloride
reacts with natural
alkalinity
in water as follows:
Al2(S04)3.14H20
FeCl3 + 3 Hco3

+ 6 HC03Fe(0H)3(S)

Jar Test
The jar
optimum operating
allows adjustments
mixing speeds, or
scale in order to
Jar

Testing

2 Al(0H)3(s)
+3 C02 + 3 c1-

+ 6C02 +14 H20 + 3 S042-

test is a common laboratory
procedure used to determine
the
conditions
for water or wastewater
treatment.
This method
in pH, variations
in coagulant
or polymer dose, alternating
testing
of different
coagulant
or polymer types,
on a small
predict
the functioning
of a large scale treatment
operation.

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

Jar

Test

the

containers.
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
100 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

30

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 20 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

How Filters
There
are

work:
Filtration
four
basic
filtration

materials

is

known as Filtration.

Mechanisms
mechanisms:

SEDIMENTATION : The mechanism of sedimentation
is due to force of gravity
and
the associate settling
velocity
of the particle,
which causes it to cross the
streamlines

and

reach

the

collector.

INTERCEPTION : Interception
of particles
is commonfor 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
to travel
off their streamlines
Filter

occurs when larger particles
and bump into media grains.

move fast

enough

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.
D10 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,
i.e.
(D60/D10) may be defined as the ratio of the sieve size in mm through which
60 percent of the sample of sand will pass, to the effective
size of the 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 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 physicalchemical 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. 0.30.4 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 30 to 75 cm in depth
while in RSF it varies from 3 to 40 mm in size and its depth is slightly
more,
i.e.

about

60

to

90

cm.

Filter
sand: In SSF the effective
size ranges between 0.2 to 0.4 mm and
uniformity
coefficient
between 1.8 to 2.5 or 3.0. In RSF the effective
size
ranges between 0.35 to 0.55 and uniformity
coefficient
between 1.2 to 1.8.
Rate of filtration:
In SSF it is small, such as 100 to 200 L/h/sq.m.
of filter
area while in RSF it is large,
such as 3000 to 6000 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. 10 cm is the initial
loss, and 0.8 to 1.2m
is the final
limit when cleaning is required.
For RSF 0.3m is the initial
loss,
and 2.5 to 3.5m is the final
limit when cleaning is required.
Typical Rapid Gravity Filter
Flow Operation
Isometric view of Rapid Sand
Filter
Clean

water

Headloss

Several equations have been developed to describe the flow of clean water
through a porous medium. Carman-Kozeny equation used to calculate
head loss
as

is

follows:

where,

h = headloss,

m

f = friction
factor
d = grain size diameter,
m
vs = superficial
(approach) filtration
velocity,
m/s
g
accelaration
due to gravity,
9.81 m/s2
p = fraction
of particles
( based on mass) within adjacent
sieve sizes
dg = geometric mean diameter between sieve
sizes d1 and
d2
Ng = Reynolds number
= viscosity,
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

when is
The
The
desired
in the

of

the

filter.

Backwashing Needed
filter
should be backwashed when the following
conditions
have been met:
head loss is so high that the filter
no longer produces water at the
rate; and/or
Floc starts
to break through the filter
and the turbidity
filter
effluent
increases;
and/or
A filter
run reaches a given hour of

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 10 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

the optimum allowable
Formation

of

Mud

filters

are

cleaned

as soon

as the

head

loss

exceeds

value.

Balls

.

The mud from the atmosphere usually
accumulates
on the sand surface
to form a
dense mat. During inadequate
washing this 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:
problem
harmful disease producing
bacteria
to make the water safe for drinking.
known
as
Disinfection

Disinfection
Kinetics

when a single
disinfectant,

or

order
is

Sterilization.

of
reaction.

N=N0e-kt

This equation
microorganism

is known as Chick&#39;s
Law:(N0 is initial
number)
t

of

= contact

survival

N = number of
k = disinfection

time

Disinfection

1. Boiling:
The bacteria
long time. However it is
Moreover it cannot take
2. Treatment with Excess
softening.
But if excess
the bacteria
also.
Lime
extremely
alkaline.
This
water

it.
These bacteria
must be killed
in
The process of killing
these bacteria

unit of microorganisms
is exposed to a single
unit
the reduction
in microorganisms
follows
a first-order

dN/dt=-kN

constant
Methods

in

of

bacteria.

before

recarbonation

it

present in water can be destroyed
by boiling
it
not practically
possible
to boil huge amounts of
care of future
possible
contaminations.
Lime: Lime is used in water treatment
plant for
lime is added to the water,
it can in addition,
when added raises
the pH value o water making it
extreme alkalinity
has been found detrimental
to

This

method

can be supplied
for

lime

removal

needs

the

removal

to the general
should

be

used

of

public.
after

excess

lime

Treatment

from

for a
water.
kill
the

the

like

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 also can be added as a salt,
such as sodium
hypochlorite
(Na0Cl) or bleach.
Chlorine
gas dissolves
in water following
Henry&#39;sLaw.

Once dissolved,

Cl2(g)

Cl2(aq)

the following

reaction

Cl2(aq)+H20

Hypochlorous

acid

is

KH =6.2

occurs

x 10-2

forming

hypochlorous

acid

(Hocl):

HOCl + H+ + Cl-

a weak acid

that

dissociates

to form

hypochlorite

ion

(0Cl-).
HOCl
0Cl-+ H+ Ka= 3.2X10-8
All forms of chlorine
are measured as mg/L of C12 (Mw = 2 x 35.45 = 70.9 g/mol)
Hypochlorous
acid and hypochlorite
ion compose what is called
the free chlorine

residual.
inorganic
reactions
residual.
to

form

These free chlorine
compounds can react with many organic
and
compounds to form chlorinated
compounds. If the products
of these
posses oxidizing
potential,
they are considered
the combined chlorine
A common compound in drinking
water systems that reacts with chlorine
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
utilities

use

monochloramine

as

a disinfectant.

If

once all ammonia nitrogen
has been converted
to
species are oxidized
through what is termed the
overall
reactions
of free chlorine
and nitrogen
simplified
reactions
as follows:
Monochloramine
Formation Reaction.
This reaction
nitrogen
is combined with free chlorine
up to a
HOCl

Breakpoint
molar

Reaction:

ratio,

+NH3

when excess

free

NH2Cl

The formation
relationship

of chloramines
between chlorine

illustrated

free

chlorine

water
exits

monochloramine,
chloramine
breakpoint
reactions.
The
can be represented
by two
occurs rapidly
when ammonia
molar ratio
of 1:1.

+ HOCl

chlorine

monochloramine
is removed
2NH2Cl + HOCl N2(g)+

excess

is

as follows:
3H++ 3c1+

added beyond the 1:1

initial

H20

and the breakpoint
reaction
create a unique
dose and the amount and form of chlorine
as

below.

Free Chlorine,
Chlorine

Chloramine,

and Ammonia Nitrogen

Reactions

Demand

Free chlorine
and chloramines
readily
react with a variety
compounds, including
organic
substances,
and inorganic
substances like
iron and manganese. The
stoichiometry
of chlorine
reactions
with organics
can be represented
as shown
below:
HOCl:
1/10C5H702N
+ HOCl4/10C02
+ 1/10HC03+ 1/10NH4++
H+ + Cl0Cl-:
1/10C5H702N
+ 0Cl4/10C02
+ 1/10HC03+ 1/10NH4++
Cl+ 1/10H20
NH2Cl:
1/10C5H702N
+ NH2Cl
+ 9/10H204/10C02
+ 1/10HC03+ 11/10NH4++
Cl-

Chlorine
reduced
forms

of

demand can be increased
iron at corrosion
sites
chlorine

and

iron

are

+ 1/10H20

by oxidation
reactions
with inorganics,
such as
at the pipe wall.
Possible
reactions
with all
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
requirements
and locate

can facilitate
normal sequence

gravity
flow that will
reduce pumping
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
and that

in
in

water
surface
the
effluent
of

less than the
In such cases

elevations
in
last
treatment

the
influent
unit.
If

the

of first
total

head loss through the plant,
flow by gravity
pumping is needed to raise the head so that

treatment
available

unit
head is

cannot be achieved.
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
profiles
are prepared at peak and average design flows and at
minimum

initial

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
treatment
units and the connecting
piping
and appurtenances.
4. The head losses through the treatment
unit include
the following:
a.

Head

losses

structure.

at

c.

surface

the

influent

Head losses

b.

the

Head

unit.

losses

at

the

effluent

d. Miscellaneous

and free

fall

allowance.

5. The total
loss through
the sum of following:
a.

structure.

through

the

Head

loss

due

to

the connecting

entrance.

b.

pipings,

Head

loss

due

channels
to

and appurtenances

exit.

c.

Head

contraction
and enlargement.
d. Head loss due to friction.
e. Head loss due to bends, fittings,
gates, valves,
and meters.
required
over weir and other hydraulic
controls.
g. Free-fall

loss

due

is
to

f. Head
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 the repair
of any section
of the
system.

5. All
the

6. It

the distribution

sewer

pipes

should

be preferably

laid

one metre

away or above

lines.

should

be fairly

water-tight

as to keep losses

due to leakage

to the

minimum.
Layouts
ofDistribution
Network
The
distribution
pipes
aregenerally
laid
below the road pavements, and as such their
layouts
generally
follow
the layouts
of roads. There are, in general,
four different
types of pipe networks;
any one
of which either
singly
or in combinations,
can be used 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
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 absence of dead
ends.
some

2.
other

In

the

cases

of

a breakdown

in

some section,

water

is

available

from

direction.

Disadvantages
1. Exact calculation
provision
of valves on all branches.

of sizes

of pipes

is

not

possible

due to

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 be supplied
to any point from at least
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 demand.
Functions
of 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 permit gravity
flow at an adequate pressure.
Types of Reservoirs
1. Underground reservoirs.
2. Small ground level
reservoirs.
3. Large ground
level

reservoirs.

Storage
distribution

4.

Capacity

Overhead

of Distribution

reservoir

is

the

tanks.

Reservoirs
summation

The total

storage

capacity

of a

of:

1. Balancing
Storage:
The quantity
of water required
to be stored in the
reservoir
for equalising
or balancing
fluctuating
demand against
constant
supply
is known as the balancing
storage
(or equalising
or operating
storage).
The
balance storage 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
determining
quantities
of flow
and head losses in the various
pipe lines,
and resulting
residual
pressures.
In
any pipe network,
the following
two conditions
must be satisfied:
1. The algebraic
sum of pressure drops around a closed loop must be zero, i.e.
there can be no discontinuity
in pressure.
2. The flow entering
a junction
must be equal to the flow leaving
that
junction;
i.e.
the law of continuity
must be satisfied.
Based on these two basic principles,
the pipe networks are generally
solved by
the methods of successive
approximation.
The widely
used method of pipe network
analysis
is the Hardy-Cross
method.
Hardy-Cross
Method
This method consists
of assuming a distribution
of flow in the network in
such a way that the principle
of continuity
is satisfied
at each junction.
A
correction
to these assumed flows is then computed successively
for each pipe
loop in the network,
until
the correction
is reduced to an acceptable
magnitude.

Domestic

wastewater

Collection

wastewater
Quantity
absence of storms in
Quantity:
Per capita

and

Treatment

Estimation
The flow of sanitary
sewage alone in
dry season is known as dry weather flow (DwF).
sewage contributed
per day X

the

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
80% 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 = 2 x average daily
flow and (ii)
Minimum daily
flow = 2/3 x (average daily
flow).
Population

Equivalent

Population
equivalent
is a parameter used in the conversion
of contribution
of wastes from industrial
establishments
for accepting
into sanitary
sewer
systems. The strength
of industrial
sewage is, thus, written
as
Std. BOD5 =
(Std. BOD5 of domestic sewage per person per day) x
(population

Design Periods
due provision
for

& Population
the estimated

equivalent)

Forecast
This quantity
should be worked
requirements
of the future
. The future

out with
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 10 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
populations
are given
below. The particular
method to be adopted for a particular
case or for a
particular
city
depends largely
on the factors
discussed
in the methods, and the
selection
is left
to the discrection
and intelligence
of the designer.
1.
Arithmetic

Increase

Increase
Method

Method

2.

Method
4. Decreasing
6. comparative
Graphical

Method
wastewater

Geometric

Increase

Method

3.

Incremental

Rate of Growth Method
5. Simple Graphical
Method
7. Ratio Method
8. Logistic
curve

characterization

To design a treatment
perhaps the most critical
design of the activated

process properly,
characterization
step. wastewater
characteristics
sludge process can be grouped into

of wastewater
of importance
the following

is
in

the

categories:

Temperature
Nitrogen

pH
colour
chlorides

and Odour
carbonaceous
substrates
Total and volatile
suspended solids
(TSS and

vss)

Toxic

metals

and compounds

Temperature

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
testis based
ontheprinciple
thatmost
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 250 to 1000
mg/L.
Solids

solids

Total solids
include
which are obtained

both the suspended solids
and the dissolved
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 by ignition
at 600°C is a measure of the volatile
solids,
which are classed as organic material.
The remaining
solids
are
the fixed
solids,
which 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=0.85

C rH0.63S0.54

Manning&#39;s
formula
U=1/n rH2/3S1/2
where, U: velocity,
m/s; rH= hydraulic
radius,m;
S: slope,
C: Hazen-william&#39;s coefficient,
and n = Manning&#39;s
coefficient.
Darcy-weisbach
formula
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
self-cleansing
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 nonscouring
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 maintaining
a minimum velocity
of about 0.45
m/s at the time of minimum flow (assumed to be 1/3rd of average flow).
The
designer
should also ensure that a velocity
of 0.9 m/s is developed atleast
at
the time of maximum flow and preferably
during the average flow periods also.
Moreover,

velocity
Sewer

care

should

generated

be taken

to

see

that

at

does not exceed the scouring

the

time

of

maximum

flow,

the

value.

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 diameter
of sewer changes, whenever direction
of sewer line changes and when sewers of
different
elevations
join together.
Special
Man-holes:
Junction
chambers: Man-hole constructed
at the intersection
of two large sewers.
Drop man-hole:
when the difference
in elevation
of the invert
levels
of the
incoming and outgoing
sewers of the man-hole is more than 60 cm, the
interception
is made by dropping
the incoming sewer vertically
outside
and then
it is jointed
to the man-hole chamber.
Flushing
man-holes:
They are located
at the head of a sewer to flush
out the
deposits
in the sewer with 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 30 m.
Catch Basins: Catch basins are small settling
chambers of diameter
60 - 90 cm
and 60 - 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
50% as stand by.
Types of pumps : 1. Centrifugal
pumps either
axial,
mixed and radial
flow.
2.
Pneumatic

water
river

ejector

pumps.

Treatment
The raw sewage must be treated
stream. The extent of treatment
required

before it is discharged
into the
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
table

Indian

of conventional
wastewater treatment
plant is as follows:
Standards for discharge of sewage in surface waters are given in the

below.

Standards

Characteristic

for

of

the

Discharge

of Sewage in Surface

waters

Effluent

Tolerance limit
for Discharge of Sewage in Suface water Sources
BOD5 20 mg/L
TSS
30 mg/L
The unit operations and processes commonly employed in domestic wastewater
treatment,
their functions
and units used to achieve these functions
are given
in the following
table:
Unit Operations/Processes,
Their Functions and Units Used for Domestic
wastewater

Treatment

Unit Operations/Processes
Functions Treatment Devices
Screening
Removal of large floating,
suspended and settleable
solids
Bar racks and screens of various description
Grit Removal Removal of inorganic suspended solids Grit chamber Primary
Sedimentation
Removal of organic/inorganic
settleable
solids
Primary sedimentation
tank Aerobic Biological
Suspended Growth Process
Conversion of colloidal,
dissolved and residual suspended organic matter into
settleable
biofloc
and stable inorganics
Activated
sludge process units and its modifications,
waste stabilisation
ponds, Aerated lagoons, Aerobic Biological
Attached Growth Process same as above
Trickling
filter,
Rotating biological
contactor
Anaerobic biological
growth processes
Conversion of organic matter into CH4 & C02 and relatively
stable organic
residue

Anaerobic filter,
Fluid bed submerged media anaerobic reactor,
anaerobic sludge blanket reactor,
Anaerobic rotating
biological
Anaerobic Stabilization
of Organic Sludges same as above
Anaerobic digestor

Upflow
contactor

Screening

A screen is a device with openings for removing bigger suspended or floating
matter in sewage which would otherwise damage equipment or interfere
with
satisfactory
operation of treatment 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
150 mm are provided ahead of pumps, while those ahead of sedimentation
tanks
have smaller openings of 50 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 20 to 50 mm.Bar are
usually 10 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
10 X 50 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 20 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
offlow
ahead
ofand
through
thescreen
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 100% removal of material
of certain
size without
undue depositions.
Velocities
of 0.6 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 0.3 mps to
avoid deposition
of solids.
Head

loss

Head loss varies
with the quantity
and
accumulate between cleanings.
The head
calculated
by considering
the flow and
the latter
being the sum of the vertical
loss through clean flat
bar screens is
h = 0.0729 (v2
v2)
where,

h = head

loss

in

nature of screenings
allowed to
loss created by a clean screen may be
the effective
areas of screen openings,
projections
of the openings.
The head
calculated
from the following
formula:

m

V = velocity
through the screen in mps
v = velocity
before the screen in mps
Another formula often used to determine
the head loss through a bar rack is
Kirschmer&#39;sequation:
h = M (w/b)4/3
hv sin H
The quantity
of screenings
depends on the nature of the wastewater
and the
screen

openings.

Equalization
Tanks
The equalization
tanks are provided
(i) to balance fluctuating
flows or
concentrations,
(ii)
to assist
self neutralization,
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 useful
in assisting
self neutralization
or evening out of fluctuating
concentrations,
not for
balancing
of flows since a flow through type tank once filled,
gives output
equal

to

input.

Flow balancing
and self-neutralization
are both achieved by using two tanks,
intermittently
one after
another.
One tank is allowed to fill
up after
which it
is checked for pH (or any other parameter)
and then allowed to empty out. The
second tank goes through a similar
routine.
Intermittent
flow type tanks are
economic

for

small

flows

from

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;s formula:

v = 3 to 4.5 (g(Ss - 1)d)1/2
A horizontal
velocity
of flow of 15 to 30 cm/sec is used at peak flows. This
same velocity
is to be maintained at all fluctuation
of flow to ensure that only
organic solids and not the grit is scoured from the bottom.
Types of Velocity
Control Devices
1. A sutro weir in a channel of rectangular
cross section,
with free fall
downstream

of

the

channel.

2. A parabolic
shaped channel with a rectangular
3. A rectangular
shaped channel with a parshall
also help easy flow measurement.
Design of Grit Chambers
Settling

weir.
flume at the end which would

Velocity

The settling
velocity
of discrete
particles
equation depending upon Reynolds number.
Stoke&#39;s
law: v= g(Ss-1)d2

can be determined

using appropriate

18*

Stoke&#39;s
law holds good for
Re=vd

Reynolds number,Re++below 1.

=<

For grit particles
of specific
gravity
2.65 and liquid
10°C, *=1.01
x 10-6m2/s. This corresponds to particles

temperature at
of size less than 0.1

mm.

Transition
law: The design of grit chamber is based on removal of grit particles
with minimum size of 0.15 mm and therefore
Stoke&#39;s
law is not applicable
to
determine the settling
velocity
of grit particles
for design purposes.
V2 =

4g("p-&#39;)d
3

CD

where, CD: drag coefficient
Transition
number,Re between 1 and 1000. In this
CD:

18.5

=

flow conditions
hold good for Reynolds
range CD can be approximated by

18.5

Re0.6
(vd/*)0.6
Substituting
the value of CD in settling

velocity

equation

and simplifying,

we

get

v =
[0.707(Ss-1)dErME
*0.6]0.714
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 2.0 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
25-30
50-60-

-

Primary
35-50

2.5-3.5

2.0-2.5

settling
60-120

-

followed
-

by secondary

Primary settling
with activated
Secondary settling
for trickling
15-25

40-50

70-120

Secondary

settling

50 70-140
3.5-4.5

210

Secondary

settling

8-15

25-35

25-120

treatment

2.5-3.5

190

2.5-3.5

for

activated

for
170

sludge
filters

25-35

50-60

- - 3.5-4.5

-

1.5-2.0

sludge

extended
3.5-4.5

return

(excluding

extended

aeration)

15-35

40-

aeration
-

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 C02 or HC03- 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:
organisms that use light
as their
energy source.
2.
Chemotrophs:
organisms that employ oxidation-reduction
reactions
to provide
energy. They are further
classified
on the basis of chemical compounds oxidized
(i.e.,
electron
donor)
1. Chemoorganotrophs:
Organisms that use complex organic molecules
as their
electron

donor.

2. Chemoautotrophs:
Organisms that use simple inorganic
molecules
such as
hydrogen sulfide
or ammonia as their
electron
donor.
b. Temperature Range: On the basis of temperature
range within
which they can
proliferate,
microorganisms
are classified
as i. Psychrophilic:
organisms whose
growth is optimum within
15 to 30°C.
ii.
Mesophilic:
organisms whose growth is
optimum within
30 to 45°C.
iii.
Thermophilic:
organisms whose growth is
optimum within
45 to 70°C.
b. Oxygen Requirements:
On the basis of oxygen requirement
microorganisms
are
classified

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
andconstant
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:
metabolites,
and cells
6. Endogenous phase:
Biomass

Growth

exaustion
of nutrients,
high concentration
of toxic
in a state of suspended animation.
endogenous metabolism,
high death rate and cell lysis.

Rate

The most widely
used expression
for the growth rate of micro organisms is given
by Monod:
Total rate of microbial
growth,dx
= L&#39;mXS
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 =+mX S++MgJkdX
Ks+ S where, kd = endogenous decay coefficient
Growth

Yield

Growth yield
from

the

specific

is

utilization

growth

coefficient

and

rate
is

defined

as the incremental

of

the

is

given

defined

incremental
as

by:+m
the

ratio

increase

amount

=Y.k
of

of

where,
maximum

in

substrate.

Y is
mass

biomass
The

which

results

maximum

the maximum yield
of

cells

formed

to

the

mass of substrate
utilized.
The coefficients
Y, kd, k and Ks are designated
as
kinetic
coefficients.
The values of kinetic
coefficients
depend upon the nature
of wastewater
and operational
and environmental
conditions
in biological
reactor.
The biological
reactors
can be completely
mixed flow or plug flow
reactor
with or without
recycle.
Activated
Sludge Process
The most common suspended growth process used for municipal
wastewater
treatment
is the activated
sludge process as shown in figure:
Activated
sludge plant involves:
1. wastewater
aeration
in the presence of a microbial
suspension,
2. solidliquid
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 C02 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,
Mixing

and the
Regime

flow

scheme.

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* = V
Q
V= volume of aeration
tank, m3, and Q: sewage inflow,
m3/d
Another empirical
loading
parameter is volumetric
organic loadingwhich
is
defined
as 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.

q= Q (so

Se)

v

x

_

A similar
loading
parameter is mean cell
residence
time or sludge retention
time (SRT), c,
d
c
=
V X
QwXr + (Q-QwXe)
where S0 and Se are influent
and effluent
organic matter concentration
respectively,
measured as BOD5 (g/m3), X, Xe and Xrare MLSS concentration
in
aeration
tank, effluent
and return
sludge respectively,
and Qw= waste activated
sludge rate.
Under steady state operation
the mass of waste activated
sludge is given by
QwXr = YQ (S0 - Se) - kd XV where Y: maximum yield
coefficient
(microbial
mass
synthesized
/ mass of substrate
utilized)
and kd = endogenous decay rate (d-1)
From the above equation
it is seen that 1/Hc = Yq - kd
If the value of Se is small as compared S0, q may also be expressed as Food to
Microorganism
ratio,
F/M
F/M = Q(S0- Se) / XV = QSO / XV
The 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
and return
sludge with the entire
contents
of the aeration
tank. Extended
aeration
process operates
at a low organic load producing
lesser
quantity
well stabilized
sludge.
Design Consideration
The items for consideration
in the design
sludge plant are aeration
tank capacity
and dimensions,
aeration
secondary sludge settling
and recycle
and excess sludge wasting.
Aeration

waste
of

of activated
facilities,

Tank

The volume of aeration
tank is calculated
for the selected
value ofc
by
assuming a suitable
value of MLSS concentration,
X.
VX = YQc(S0
- S)
1+++kdc
Alternately,
the tank capacity
may be designed from F/M =
QSO /

xv

Hence, the first
step in designing
is to choose a suitable
value ofc
(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 10 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 5000
g/m3. For typical
domestic sewage, the MLSS value of 2000-3000 mg/l if
conventional
plug flow type aeration
system is provided,
or 3000-5000 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 10 m. width-depth
ratio should be adjusted to be between 1.2 to 2.2. The length should not be less
than 30 or not ordinarily
longer than 100 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:
2 required (g/d) = Q(S0 - S) 1.42

QwXr
f

where, f = ratio of BOD5 to ultimate
BOD and 1.42 = oxygen demand of biomass
(9/9)
The formula does not allow for nitrification
but allows only for carbonaceous
BOD removal.
Aeration
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 2000 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
and mechanical
thickeners
(which also fall
in this category of compression settling)
are given
in

lecture

22.

Sludge Recycle
The MLSS concentration
in the aeration
sludge recirculation
rate and the sludge settleability
secondary sedimentation
tank.
Qr =

x

Q

tank is controlled
by the
and thickening
in the

Xr-X

where Qr = Sludge recirculation
rate, m3/d
The sludge settleability
is determined by sludge volume index (SVI) defined as
volume occupied in mL by one gram of solids in the mixed liquor after settling
for 30 min. If it is assumed that sedimentation
of suspended solids in the
laboratory
is similar
to that in sedimentation
tank, then Xr = 106/SVI. Values

of SVI between 100 and 150 ml/g indicate
good settling
of suspended solids.
The
Xr value may not be taken more than 10,000 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
c = VX
Design

QwXr
orQwXr
=YQ
(S0- S)- kdXV

of Completely

Mixed Activated

Sludge

System

Theory of Aeration
Aeration
is a gas-liquid
mass transfer
process in which the
driving
force in the liquid
phase is the concentration
gradient
(Cs - C) for
slightly
soluble
gases.
Mass transfer
per unit time =KL.a (Cs - C) where, KL = Liquid film coefficient
=Diffusion
coefficient
of liquid
(D)
Thickness
of film
(Y)
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 time, is determined
by
experimentation.

Adjustment
for Field Conditions
The oxygen transfer
capacity
under field
conditions
can be calculated
from
the standard
oxygen transfer
capacity
by the formula:
N = [Ns(Cs- CL)x 1.o24T2o,,.~+]/9.2
where, N = oxygen transferred
under field
conditions,
kg 02/h.
Ns= oxygen
transfer
capacity
under standard
conditions,
kg 02/h.
Cs: D0 saturation
value
for sewage at operating
temperature.
CL= operating
D0 level
in aeration
tank
usually
1 to 2 mg/L.
T = Temperature,
degree C.
1, = Correction
factor
for
oxygen transfer
for sewage, usually
0.8 to 0.85.
Aeration

Facilities

oxygen may be supplied
fine

or

coarse

either

by surface

aerators

or diffused

aerators

employing

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 20°C, 760 mm Hg barometric
pressure and zero D0.
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 Filter
High Rate Filter
1. Hydraulic
loading,
m3/m2 d 1 - 4 10 - 40
2. Organic loading,kg
BOD / m3.d
0.08 - 0.32 0.32 - 1.0
3. Depth, m. 1.8 - 3.0 0.9 - 2.5
4. Recirculation
ratio
0 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
notadopted
in lowratefilters.
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
design is based on empirical
relationships
to
find the required
filter
volume for a designed degree of wastewater
treatment.
Types of equations:
1. NRC equations
(National
Research Council of USA) 2. Rankins equation
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 recirculation,
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:
These
equations
areapplicable
toboth
lowrateand
highrate
filters.
The efficiency
E2 is given by
E2:

of single

stage

or first

stage

of two stage

filters,

100

1+0.44(F1.BOD/V1.Rf1)1/2
For the second stage filter,
the efficiency
E3 is given by
E3:
100
[(1+o.44)/(1E2)]
(F2.BOD/V2.Rf2)1/2
where E2: % efficiency
in BOD removal of single
stage or first
stage of twostage filter,
E3=% efficiency
of second stage filter,F1.BOD=
BOD loading
of
settled
raw sewage in single
stage of the two-stage
filter
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.

filter
(including
recirculation)
is not more than three times the BOD expected
in effluent.
This implies
that as long as the above conditions
are satisfied
efficiency
is only a function
of recirculation
and is given by:
=
(R/Q) + 1
(R/Q) + 1.5
other

Aerobic

Treatment

E

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:

The

oxidation

ditch

is

a modified

form

of

"extended

aeration"
of activated
sludge process.
The ditch consists
of a long continuous
channel oval in shape with two surface
rotors
placed across the channel.
(Aerated
Lagoon
Aerated lagoons fall
in between the algal
ponds and activated
sludge systems.
Oxygen is supplied
through aeration.
The unit is deeper (3 to 5 m) and hence
require
much less land than algal
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
process is an effective
method for the
treatment
of many organic wastes. The treatment
has a number of advantages over
aerobic
treatment
process,
namely,
the energy input of the system is low as
no energy is requred for oxygenation,
lower production
of excess
sludge(
biological
synthesis)
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.
The first
stage is
commonly referred
to as "acid fermentation".
However in the second stage the end
products
of the first
stage are converted
to gases (mainly methane and carbon
dioxide)
by several
different
species of strictly
anaerobic
bacteria.
This stage
is generally
referred
to 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
Upflow anaerobic
media to support
reported
to work
interstices.

anaerobic
units that have been developed are as follows:
filters
packed with either
pebbles,
stones,
PVC sheets,
etc.
submerged biological
growths (fixed
film).
The units are
well but a likely
problem is accumulation
of solids
in the

as

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:
1. Initial
pumping
2. Screening
and degritting
3. Main UASB reactor
4. Gas collection
and conversion
or conveyance
5. Sludge drying bed
6. Post
treatment
facility
In the UASB process,
the whole waste is passed through the anaerobic
reactor
in
an upflow mode, with a hydraulic
retention
time (HRT) of only about 8-10 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 30-50 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 18-20°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 20°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
0.1 g VSS/ g BOD removed, (ii)
the non-degradable
residue of the VSS coming in
the inflow assuming 40% of the VSS are degraded and residue is 60%, and (iii)
Ash received
in the inflow,
namely TSS-VSS mg/l. Thus, at steady state
conditions,
SRT= Total sludge present in reactor,
kg
Sludge withdrawn
per day, kg/d
= 30 to 50 days.
Another parameter is HRT which is given by:
HRT:

Reactor

volume,

Flow rate,
The

reactor

volume

m3

m3/h
has

to

= 8 to 10 h or more at average
be

so

chosen

that

the

desired

flow.

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
(70 kg/m3). The full
depth of the reactor
for treating
low BOD municipal
sewage is often 4.5 to 5.0 m of which the sludge
blanket
itself
may be 2.0 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 0.5 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 gas collection.
The
depth of the compartment is 2.0 to 2.5 m and the surface
overflow
rate kept at
20 to 28 m3/m2-day (1 to 1.2 m/h) at peak flow.
The flow velocity
through the
aperture
connecting the reaction
zone with the settling
compartment is limited
to not more than 5 m/h at peak flow.
Due attention
has to be paid to the
geometry of the unit and to its hydraulics
to ensure proper working of the "GasLiquid-Solid-Separator
(GLSS)" the gas collection
hood, the incoming flow
distribution
to get spatial
uniformity
and the outflowing
effluent.
Physical
Parameters
A single
module can handle 10 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.
Rectangular
shape is preferred
Depth
4.5 to 5.0 m for sewage.
width or diameter
To limit
lengths
of inlet laterals
to around 10-12 m for
facilitating
uniform flow distribution
and sludge withdrawal.
Length As necessary.
Inlet feed
gravity
feed from top (preferred
for municipal
sewage) or pumped
feed from bottom through manifold
and laterals
(preferred
in case of soluble
industrial
wastewaters).
Sludge blanket
depth
2 to 2.5 m for sewage. More depth is needed for stronger
wastes.
Deflector/GLSS

This is a deflector
beam which together
with the gas hood (slope 60) forms a
"gas-liquid-solid-separator"
(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

2.0-2.5

m in depth.

Process

Design

Surface

overflow

rate

equals

20-28

m3/m2/d at peak flow.

Parameters

A few process design parameters for UASBs are listed
below for municipal
sewages
with BOD about 200-300 mg/l and temperatures
above 20°C. HRT 8-10 hours at
average flow (minimum 4 hours at peak flow)
SRT 30-50 days or more
Sludge
blanket
concentration
(average)
15-30 kg VSS per m3. About 70 kg TSS per m3. Organic loading on sludge blanket
0.3-1.0
kg COD/kg VSS day (even upto 10 kg COD/ kg VSS day for agro-industrial
wastes).

Volumetric
organic loading
1-3 kg COD/m3 day for domestic sewage (10-15 kg
COD/m3 day for agro-industrial
wastes)
BOD/COD removal efficiency
Sewage 75-85% for BOD. 74-78% for COD.
Inlet
points Minimum 1 point per 3.7-4.0
m2 floor
area.
Flow regime
Either
constant rate for pumped inflows
or typically
fluctuating
flows for gravity
systems.
Upflow velocity
About 0.5 m/h at average flow,
or 1.2 m/h at peak flow,
whichever

is

low.

Sludge production
0.15-0.25
kg TS per m3 sewage treated.
Sludge drying
time
Seven days (in India)
Gas production
Theoretical
0.38 m3/kg COD removed. Actual

0.1-0.3

m3per kg COD

removed.

Gas utilization
is equivalent

Method of use is optional.
1 m3 biogas with 75% methane content
to 1.4 kwh electricity.
Nutrients
nitrogen
and phosphorus

removal

5 to

10% only.

Nitrification-Denitrification
Systems
A certain
amount of nitrogen
removal (20-30%) occurs in conventional
activated
sludge systems. Nitrogen
removal ranging from 70 to 90 % 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 ammonium to 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. Nitrification
of ammonia to nitrate
is a two step

process:
Nitrosomonas
NH3

NH4

N02

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
I_

where
is the growth rate of nitrosomonas
at the worst operating
temperature.
Sludge age (or mean cell
residence
time),
c in a treatment
must be sufficiently
high if nitrification
is desired.
Combined and Separate Systems of Biological
Oxidation
& Nitrification
Following
figure
shows flow sheets for combined and separate
systems for
biological
oxidation
and nitrification.

plant

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 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: (1) ammonia stripping,
(2) selective
ion exchange, (3) break point chlorination,
and
(4) biological
nitrification/denitrification.
Biological
nitrification/denitrification
is a two step process.
The first
step
is nitrification,
which is conversion
of ammonia to nitrate
through the action
of nitrifying
bacteria.
The second step is nitrate
conversion
(denitrification),
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

place. This
or artificially
higher ratio
economical

is

also

essential

as

electron

donor

for

denitrification

to

take

source may be in the form of carbon internally
available
in sewage
added (eg. as methanol).
Since most community wastewaters
have a
of BOD N, the internally
available
carbon becomes attractive
and
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
waste

itself.

arrangement

However,

the

would

anoxic

tank

be to use the carbon
has

to

be of

for denitrification
to occur which, has a slower rate;
oxygen uptake rate of the mixed liquor
is mainly due
and is thus low. The denitrification
rate,
therefore,
the F/M ratio
in the prior
aeration
tank.
Consequently,
if desired,
a portion
of the raw waste
directly
into the anoxic tank and thus contribute
to
rate.

This

reduces

denitrification
not

be

the

sizes

efficiency

of

is

both

reduced

the

anoxic

contained

sufficient

the
time

since the corresponding
to endogenous respiration
in a way also depends on
may be bypassed to enter
an increased
respiration

and aeration

as the bypassed

in

detention

tanks,

unnitrified

but

the

ammonia can

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
arrangement.
The first
anoxic tank has the advantage of higher
rate while the nitrates
remaining
in the liquor
passing out of
denitrified
further
in a second anoxic tank through endogenous
The

flow

off

nitrogen

from

anoxic

tank

gas bubbles

is

desirable

to

reaerate

and add oxygen prior

more hydroxide,

that

lime

is

difficult

to

dewater.

Use

of

10-15

minutes

to

drive

to sedimentation.

Phosphorus Removal
Phosphorus precipitation
is ususally
calcium hydroxide,
ferrous
or ferric
the final
settling
tank.
Alum is more expensive and generates

achieved
chloride,

for

Bardenpho
denitrification
the tank can be
respiration.

by addition
of chemicals
like
or alum, either
in the primary

results

which
in

an

creates

increase

extra

or

sludge,

of

approximately
50% in surplus
sludge,
but the sludge is reported
to have good
dewatering
properties.
when using iron salts,
a molar ratio
of 1.0:1.4
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.

Residual Management
In all biological
waste treatment
processes
The objective
of residual
management is:
Reduction

of

water

some surplus

sludge

is

produced.

content.

Stabilization
of sludge solids.
Reduction in sludge solids
volume.
In facultative
type aerated lagoons and algal waste stabilization
ponds, the
surplus
sludge settles
out in the unit itself
and is removed only once in a few
years after
emptying the unit,
exposing the wet sludge to natural
drying,
and
carting
away the dried sludge for agricultural
use or land filling.
In extended aeration
process where aerobic
digestion
of surplus
sludge is done,
the sludge can be taken directly
for dewatering
and disposal.
In case of activated
sludge and trickling
filter
plants,
the sludge is taken
(along with the primary sludge)
to a sludge digester
for further
demineralization

and

thereafter

it

is

dewatered.

Sludge Dewatering
Methods
Natural:
sludge drying beds, sludge lagoons
Mechanical:
sludge thickeners,
centrifuges,
vacuum filters,
filter
press
Physical:
heat drying,
incineration
Disposal
of Sludge Final disposal
of sludge is to land and sometimes to the sea,
in one of the following
ways: Agricultural
use of dried or wet sludge.
Use of
dried sludge as landfill
in absence of agricultural
demand. Spreading wet
sludge on eroded or waste land, contouring
the field,
so as to gradually
build
up a top soil
of agricultural
value.
Disposing
off wet sludge along with solid wastes for (i) composting,
or (ii)
sanitary
landfill.
Transporting
and dumping into the sea.
Sludge Characteristics
For the rational
design of sludge drying systems, it is esstential
to know a few
characteristics
of sludges,
such as moisture
content
as affected
by the nature
and extent of organic
and other matter contained
in them, their
specific
gravity,
weight and volume relationships,
their
dewatering
characteristics,
etc.
The specific
gravity
of sludge is very close to that of water itself,
1.01 for
biological
sludge and 1.02 from alum sludge.
Stepwise reduction
in moisture
content
in dewatering
extended aeration
sludge
It

is

evident

that

the

bulk

of

the

water

is

removed

Thereafter,
the bulk of the remaining
moisture
is
Evaporation
removes the least
but, in fact,
takes
"dried"
sludge still
has considerable
moisture
in

in

the

thickener.

removed in free drainage.
the longest
time. The final
it,
but the sludge is now

"handleable".

Sand Beds for Sludge Drying
Sand beds are generally
constructed

as shown in

the

typical

cross-sectional

view.

Sludge is generally
through which is laid
3 m apart and sloping
drying beds are often
50 m long. The drying
or

even

Sludge
Solids

more

source
water

in

spread over the sand which is supported
on a gravel bed,
an open-joint
earthen pipe 15 cm in diameter
spaced about
at a gradient
of 1 in 150 towards the filtrate
sump. The
subdivided
into smaller
units,
each bed 5-8 m wide and 15time averages about 1-2 weeks in warmer climates,
and 3-6

unfavourable

Moisture
Total

ones.

content

Initial

weight,

moisture

g/person-day

% by weight

content

99 30 2970 3000 After
thickening
96 30 720 750 After
other mechanical
process 90
30 270 300 After
natural
or physical
drying 60 30 45 75
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,
-

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 6.0,
methane formation essentially
ceases, and more acid accumulates, thus bringing
the digestion
process to a standstill.
As a result,
methane fermentation
is
generally
assumed to be the rate limiting
step in anaerobic wastewater
treatment.
The methane bacteria
are highly active in mesophilic (27-43°C) with
digestion
period of four weeks and thermophilic
range (35-40°C) with digestion
period of 15-18 days. But thermophilic
range is not practised
because of odour
and operational
difficulties.
Digestion Tanks or Digesters
A sludge digestion
tank is a RCC or steel tank of cylindrical
shape with hopper
bottom and is covered with fixed or floating
type of roofs.
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 30 to 60 d. In a high rate digestion
process, the digester
contents are heated and completely mixed. The required
detention
period is 10 to 20 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
1.

Tank

digesters
sizes

are

are designed
not

less

than

to treat
6 m diameter

for

a capacity
and

not

more

upto 4 MLD.
than

55

m diameter.

2. Liquid depth may be 4.5 to 6 m and not greater than 9 m.
3. The digester
capacity may be determined from the relationship
V = [Vf-2/3
(Vf - Vd)]t1 + Vdt2
where V = capacity of digester
in m3, Vf = volume of fresh sludge m3/d, Vd =
volume of daily digested sludge accumulation in tank m3/d, t1= digestion
time in
days required for digestion,
d, and t2 = period of digested sludge storage.
Gas

Collection

The amount of sludge gas produced varies from 0.014 to 0.028 m3per capita.
The
sludge gas is normally composed of 65% methane and 30% carbondioxide
and
remaining 5% of nitrogen and other inert gases, with a calorific
value of 5400
to

5850

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
requirements and locate

can facilitate
gravity flow that will
reduce pumping
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
profiles
are prepared at peak and average design flows and at
minimum initial
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:
a. Head
structure.

c.

losses

Head losses

at

the

influent

through

the

structure.

unit.

b.

Head

d. Miscellaneous

losses

at

and free

the

fall

effluent

surface

allowance.

5. The total
loss through
the sum of following:
a.

Head

loss

due

to

entrance.

the connecting
b.

Head

pipings,
loss

due

channels
to

exit.

and appurtenances
c.

Head

loss

contraction
and enlargement.
d. Head loss due to friction.
e. Head loss
bends, fittings,
gates, valves,
and meters.
f. Head required
over weir
other hydraulic
controls.
g. Free-fall
surface allowance.
Typical
Hydraulic
Profile
Through Treatment Facility

due

is
to

due to
and

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
climatic
conditions
large impoundments
evaporation.
Therefore,
considerations
requirements,
and possible
percolation
particularly
beneficial
where recovery
disposal
of brines.
Groundwater Recharge
Methods for groundwater
recharge include
application
or impoundment, intermittent
all cases risks for groundwater
pollution
implies
high costs of treating
effluent

with no discharge.
Depending on the
may be necessary if precipitation
exceeds
must be given to net evaporation,
storage
and groundwater
pollution.
This method is
of residues
is desirable
such as for

rapid infiltration
by effluent
percolation,
and direct
injection.
In
exists.
Furthermore,
direct
injection
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.
However, field
crops such as cotton,
sugar beets, and crops for seed production
are grown with wastewater
effluent.
wastewater
effluent
has been used for watering
parks, golf courses and highway
medians.
Recreational

Lakes

The effluent
from the secondary treatment
facility
is stored in a lagoon for
approximately
30 days. The effluent
from the lagoon is chlorinated
and then
percolated
through an area of sand and gravel,
through which it travels
for
approximately
0.5 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
been practiced
for centuries
primarily
for production
of food,
fertilizer.
Lagoons are used for aquaculture,
although
artificial

and fauna),
has
fiber
and
and natural

wetlands
hyacinths
ruin

are also being considered.
is itself
a great concern

water

Municipal

However, the uncontrolled
spread of water
because the flora
can clog waterways and

bodies.

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 into Natural
waters
Discharge into natural
waters is the most common disposal
practice.
The selfpurification
or assimilative
capacity
of natural
waters is thus utilized
to
provide
the remaining
treatment.
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.
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 0.5 m and BOD loading
of
40-120 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 400-3000 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 C02 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
wastewater

Affecting

Pond Reactions

characteristics

and

Various

factors

affect

pond design:

fluctuations.

environmental
factors
(solar
radiation,
light,
temperature)
algal growth
patterns
and their
diurnal
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 6000 calories
per gram
of algae.
Depending on the sky clearance
factor
for an area, the average visible
radiation
received

Avg.

can

radiation:

be

estimated

Oxygen production
following
equation:
106C02

+ 16N03

as

follows:

Min.

radiation
+ [(Max. radiation
sky clearance
factor]
occurs concurrently
with algal

+ HP04

+ 122H20

+ 18H+

- Min.

radiation)x

production

in

C106H2630110N16P1

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
(1 + 0.003 EL)
where EL is the elevation
of the pond site above MSL in hundred meters.
Detention

Time

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

either
plug flow or complete mixing or
described
by first
order reaction,
the pond

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 1.0 - 1.5 m.

Aquatic

Plant

Systems

Aquatic
systems in waste treatment
are either
free floating
growths harnessed
the form of 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

in

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.
Longitudinal
Section

They have rooted
Through a Typical

Soil

Flow

with

Horizontal

of

vegetation.
Reed Bed with

Gravel,

Sand or Selected

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

side

walls.

worked-out
Examples:
Population
Forecast by Different
Problem: Predict
the population

Methods
for the years

19