wastewater engineering

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Lecture notes on
Prepared By

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Introduction
Definition of Terms
 Wastewater Engineering: Wastewater Engineering is traditionally a branch of
Civil Engineering and subset of Environmental Engineering which deals with
application of engineering methods to improve sanitation of human
communities primarily by providing removal & disposal of human waste in
addition to the supply of safe and potable water.
 Sanitation is the hygienic means of preventing human contact from hazards
of waste to promote health and potable water is that water which is fit for
drinking.
 Wastewater: It may be defined as a combination of liquid and water carried
solid waste from residences, institutions, commercial & industrial
establishment, agricultural field and can encompass a large range of
potentials contaminants and concentrations.

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Constituents of Wastewater
1. pH
2. Total Solids (TSS+TDS)
3. Oil & grease
4. B.O.D. (Biochemical Oxygen Demand)
5. C.O.D.
6. Nitrogen & Phosphorous
7.
Heavy Metals (lead, copper, chromium, cadmium, mercury, bismuth,
nickel, iron)
8. Refractory organic matters (pesticides)
9. Temperature
10. Toxic substances
11. Pathogen & pathogen indicators

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 Sewage: It is also known as domestic wastewater or municipal wastewater.
It is type of wastewater that is produced by a community of people which
is potential source of pollutants as it includes human waste (human
excreta).
 Sullage: Sullage is the term used to indicate the wastewater from household
sinks, showers and bath but not waste liquid or excreta from toilet. In other
words, it is all wastewater generated in household or office building without
fecal contamination.
 Effluent: It is an outflowing of water from a sewage treatment plant or
facility or wastewater discharge from any waste industrial facility. It is
defined by the USEPA (United States Environmental Protection Agency)
as wastewater treated or untreated that flows out from a treatment plant,
sewer or industrial outfall. Generally refers to wastes discharge into surface
water.

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 Sewer: It is an underground conduits or drains through which sewage is
carried to a point of discharge or disposal. It refers to:
 Sanitary sewer: A system of pipes used to transport human waste.
 Storm sewer: carry rainwater from roof & street surfaces.
 Separate sewer: carry households and industrial waste.
 Combined sewer: those which carry both sewage and storm water.
 House sewer: it is a pipe carrying away the sewage from a building to street
sewer.
 Main or trunk sewer: it is a sewer that receives sewage from many tributary
and serving as a outlet for a large territory.
 Branch/ sub main sewer: it is a sewer which receives sewage from a relatively
small area usually a few laterals and discharge into a mains.
 Lateral sewer: it is a sewer which collects sewage directly from the house.
 Outfall sewer: it is a sewer that receives the sewage from the collecting
system to a final discharge or disposal point.

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System of sanitation:
● Conservancy System (Dry System): Different types of wastes (dry or wet,
foul or non-foul) are manually collected separately and transported over
vehicles to the outskirts of the city and disposed off by composting method.
 Disadvantages:
i. Health hazards to the personnel handling the sewage
ii. Nuisance condition prevail while collecting and transporting
iii. It is inhuman practice and social injustice
● Water carriage system: The wastes from water-closets (WC) are collected by
flushing with water and this water is used to transport the wastes to
treatment works by a system of pipes.
 Advantages
i. Hygienic but costly
ii. Waste strength is reduced by dilution and the waste becomes more amenable
for treatment.

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 Advantages of modern water carriage system over old conservancy system:
1. The water carriage system is more hygienic because in this system waste collected
and carried in closed conduit as it is produced, where in old conservancy system
waste is collected and carried by buckets and cart.
2. In old conservancy system, chances of water pollution is more as wastes are buried
underground.
3. In modern water carriage system, problem of creating foul smell and unhygienic
condition does not exists.
4. Modern water carriage system, do not occupy the floor area or don't impair the
beauty of surroundings as sewage is carried by underground pipes (popularly
known as sewers).
5. The water carriage system allows the construction of bathrooms and latrine
together, thus occupying lesser space with their compact design.

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Sewerage:
The entire system of collecting, carrying and safe disposal of sewage through sewers is
known as sewerage.
● Types of sewerage systems:
i. Separate system
ii. Combined system
iii. Partially separate system

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● When both sanitary sewage and storm water are carried in a single sewer,
called a combined system.
 Combined system is used in the following circumstances:
1. When both the sanitary sewage and the storm water require pumping
2. When density of population is so high, the space may not be available to
have two separate sewers
3. When the storm sewer exists already, sanitary sewage also can be admitted
in the same sewer provided its quantity is small compared to storm water
4. When the rainfall is evenly distributed during the whole year

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 Combined system:

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● Two separate sets of sewers are installed, one for sanitary sewage and other
for storm water.
 This system is desirable on the following conditions
1. When it is required to treat only sanitary sewage and storm-water is not
treated
2. When the topography is flat which necessitates deep-cutting if combined
sewers are used
3. Frequency and intensity of storms are not of high magnitude, where storm
water can be collected by surface drains
4. Sewer laying requires rock-cutting, where cost of cutting is more for large
combined sewers
5. When the financial position does not permit to have large combined sewers
6. When there is a chance of combined sewers backing-up the flow into the
house sewers

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 Separate system:

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 Partially separate system:

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 Pattern of sewerage layout:
● The network of sewers can be laid in different patterns, depending on the
topography and development pattern of a city or town. The types of
patterns are as follows:
i. Perpendicular pattern
ii. Intercepter pattern
iii. Zone pattern
iv. Fan pattern
v. Radial pattern
 The factors which determine the pattern of sewerage are as follows:
• Type of system (combined or separate)
• Street lay-out
• Topography and hydrological characteristics of the area
• Location of treatment works

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Pattern of sewerage layout:

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Quantity
Estimation
of
Sewage

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 Components of wastewater flow:
Total Wastewater Flow
Dry Weather Flow (DWF) Wet Weather Flow (WWF)
 DWF: Dry weather flow is the flow through the sewer that would normally
be available during non-rainfall period. It consists mainly of Domestic
sewage and Industrial sewage.
 WWF: It is higher than DWF due to the contribution from inflow and
infiltration in the sewer system.

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 Evaluation of sewage discharge:
Apart from the accounted water supply by the water authority that will
converted into wastewater, following quantities are considered while
estimating the sewage quantity:
1. Addition due to unaccounted private water supply.
2. Addition due to infiltration.
3. Subtraction due to water losses
4. Subtraction due to water losses not entering into the sewerage system.
● Net quantity of sewage: [(Accounted quantity of water supply from the
water sources) + (unaccounted private water supplies) + (Infiltration) –
(subtraction due to water loss) – (water not entering into the sewerage
system)]
● 75-80% of water supplied is considered as sewage produced.

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 Variation in sewage flow:
Typical hourly variation in sewage flow

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 Relations:
For estimating design discharge following relations can be considered:
1. Maximum daily flow = 2 x average daily flow
2. Maximum hourly flow = 1.5 x maximum daily flow
= 3 x average daily flow
3. Minimum daily flow = 2/3 x average daily flow
4. Minimum hourly flow = 1/2 x minimum daily flow
= 1/3 average daily flow
1. Subtraction due to water losses
2. Subtraction due to water losses not entering into the sewerage system.

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 Design period:
The future period for which provisions are made in designing capacities of
various components of sewerage scheme.
Design period depends upon the following factors:
1. Ease and difficulty in expansion.
2. Amount and availability of investment.
3. Anticipated rate of population growth including shift of communities,
industries & commercial development.
4. Hydraulic constraints of the system design.
5. Life of the material and equipment.
Components Design Period
Lateral dia.<15 cm Full development
Trunk/main sewer 40 - 50 years
Treatment unit 15 – 20 years
Pumping plant 5 – 10 years

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 Problem-1:
A city has a projected population of 60,000 spread over area of 50 hectare.
Find the design discharge for the separate sewer line by assuming rate of
water supply of 250 litre per capita per day and out of this total supply 75%
reaches into the sewer as wastewater.
● Hints:
(DWF) is defined as = population x per capita rate of sewage contributed per
day (or) DWF = (Density of population x Area served by the sewer) x per
capita rate of flow
Per capita sanitary sewage = 75-80% of per capita water demand
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.

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 Quantity estimation of storm water:
A portion of rainfall that falls on the ground is lost as evaporation and
percolation. The remaining flows over the ground surface as storm water.
The quantity of storm water that goes to the sewers depends on the following
factors:
i. Intensity and duration of rainfall
ii. Time of concentration
iii. Run off coefficient
iv. Catchment area.
a. Area of the catchment
b. Slope & shape of the catchment
c. Obstruction in the flow of water
d. Initial state of catchment area with respect to wetness
e. Number & size of the ditches present in catchment area
f. Porosity of soil
g. Atmospheric temperature & humidity

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● Time of concentration:
The period after which the entire catchment area will start contributing to the
runoff is called “Time of concentration” and it is the summation of inlet
time and travel time.
A
B
Tt
Ti
= Inlet time; A = inlet point of sewer
= Travel time; B = point of concentration
Tc = Ti + Tt
The runoff will be maximum when the duration
of rainfall is equal to the time of concentration,
this is called critical rainfall duration.
Ti
Tt
● Inlet time (Ti) ● Travel time (Tt)
= (Length of drain / velocity)

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 methods for quantity estimation of storm water:
b
T
a
I


4
.
25 Duration of storm a b
5 – 20 minutes 30 10
20 – 100 minutes 40 20
*The value is prescribed by US
Ministry of Health

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● Empirical Formula:
1. Burkli-Zeigler Formula
2. Dicken’s Formula
Where,
Qp = Peak runoff in cumecs
C = co-efficient of runoff having average value as 0.7
I = maximum rainfall intensity, usually adopted as 2.5 -7.5 cm/hr
A = area of the basin (drainage area) in hectares
S = slope of the ground surface of the basin in m per thousand metres.
Where,
C = co-efficient of runoff having average value as 0.7
M = catchment area in sq.km
C = constant depends upon nature of catchment area and intensity of rainfall, the
value usually taken as 11.5.

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3. Ryve’s Formula
Value of C1
Location Value
Area within 24 km from the coast 6.8
Areas within 24 – 16 km from the coast 8.8
Limited areas near hill 10.1
4. Inglis Formula 5. Nawab Jung Bahadur Formula
The value of constant C2 varies between
48 – 60 and M’ is the catchment area in
acres.

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 Worked out example:
The catchment area is 300 hectares. The surface cover in the catchment can
be classified as given below:
Types of cover Runoff co-efficient percentage
Roofs 0.9 15
Pavements & yards 0.8 15
Lawn & gardens 0.15 25
Roads 0.40 20
Open ground 0.10 15
Single family dwelling 0.50 10
Calculate the runoff co-efficient and quantity of storm water runoff, if
intensity of rainfall is 30 mm/h for rain with duration equal to time of
concentration. If population density in the area is 350 persons per hectare and
rate of water supply is 200 LPCD, calculate design discharge for separate
system, partially separate system and combined system.

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Hydraulic
Design of
Sewer

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 Difference between water supply pipe and sewer:
1. In water supply pipe, velocity greater than self cleansing velocity is not
essential because of solids is not present in the suspension. But in sewer, to
avoid deposition of solids self cleansing velocity is necessary at all possible
discharge.
2. Water supply pipes carries water under pressure. Hence, the pipe can be
laid up & down the hills & valley within certain limit. Sewer pipes carries
sewage under gravity. Therefore it is required to be laid at a continuous
falling gradient in the downward direction towards the outfall.
3. Water supply pipes flowing full under pressure. Sewers are design to run
partial full at maximum discharge. The extra space ensure non pressure
gravity flow. This will minimize the leakage from sewer from faulty joints or
cracks if any.

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 provisions of free boards in sewer:
● Free board is provided to counteract the following factors:
1. Safeguard against lower estimation of the quantity of wastewater to be
collected at the end of design period due to private water supplies by
industries and public. Thus to ensure that sewers will never flow full,
eliminating pressure flow inside the sewers.
2. Large scale of infiltration of storm water through wrong or illegal
connection, cracks or open joints in the sewers.
3. Unforeseen increase in population or water consumption & consequent
increase in sewage production.
 Sewers with dia <400 mm are design to run half full at maximum discharge
 Dia. >400 mm are designed to flow 2/3rd to 3/4th full at maximum discharge

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Values of freeboard to be adopted for the design of S.W. Drains
Peak Discharge (cumecs) Freeboard (m)
<0.3 0.3
0.3 – 1.0 0.4
1.0 – 5 0.5
5 – 10 0.6
10 – 30 0.75
30 – 150 0.90
>150 1.0
 Minimum dia. of sewer will be adopted 200 mm for cities having present or
base year population over 1,00,000.
 In hilly location, minimum dia. of 150 mm shall be adopted
 For house sewers connection pipe and public sewers, minimum dia. of 100
mm will be adopted based on the number of house.

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 Hydraulic formula for determining flow velocity :
Q = A x V
1. Manning’s formula
Where,
V = velocity of flow in sewer in m/s
r = hydraulic mean depth of flow in m
s = hydraulic gradient or slope
n = rugosity coefficient (0.011 – 0.015)
Q = discharge in cumecs
A = area of flow in sq.m

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2. Chezy’s formula
3. Crimp & Burge’s formula
4. William - Hazens’s formula

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 Minimum velocity/ self-cleansing velocity :
*Maximum velocity should
be limited to 3 m/s

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● Design of circular sewers (Running Full)
 Hydraulic elements of sewer are
 Sectional area (A) or diameter (D)
 Hydraulic mean radius (R)
 Velocity (U)
 Slope (S)
 Out of the four quantities Q, D, U and S, if any two quantities are known or
assumed then the remaining two quantities can be found by sewer
formulae.
 These quantities have been solved for wide range of values and readymade
charts are available by which the design of sewers flowing just full can be
done very easily.
 Such readymade chart is known as Nomogram.

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Nomogram based on Manning’s formula for n= 0.013 for sewers running full

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● Flow through partially full circular sewer
 Most of the time, the sewers do not run full, i.e. run partially full.
 Such conditions exits in the following cases
 Upper reaches of laterals and branch sewers
 Initial stages after commissioning
 During the lean hours of the day
 Readymade charts are available for the analysis of such problems.
 The different parameters are d/D, u/U, a/A, q/Q.
 If any parameter is known then other parameters could be found out from
the charts.

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Hydraulic elements of circular sewer for all values of
roughness and slope

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d/D a/A p/P r/R v/V q/Q
1.0 1 1 1 1 1
0.9 0.949 0.857 1.192 1.124 1.066
0.8 0.858 0.703 1.217 1.140 0.988
0.7 0.748 0.631 1.185 1.120 0.838
0.6 0.626 0.564 1.110 1.072 0.671
0.5 0.5 0.5 1 1 0.5
0.4 0.373 0.444 0.857 0.902 0.337
0.3 0.252 0.369 0.684 0.776 0.196
0.2 0.143 0.296 0.482 0.615 0.088
0.1 0.052 0.205 0.254 0.401 0.021
0.0 0 0 0 0 0
Proportionate values of hydraulic elements for circular sewer when
flowing partially full (for constant ‘n’)

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PROBLEM-1:
A combined sewer was designed to serve an area of 60 sq.km. with an average
population of 185 persons per hectare. Average rate of sewer flow is 350 LPCD,
maximum flow is 50% in excess of average sewage flow. Rainfall equivalent of
12 mm in 24 hours can be considered for design, all of which is contributing in
surface runoff. What will be the discharge in the sewer. Find the diameter of
sewer if running full at maximum discharge.
PROBLEM-2:
Design a sewer running 0.7 times full for a maximum discharge of a town
provided with the separate system serving a population of 80,000 persons. The
water supplied from water works to the town at a rate of 190 LPCD. Given
n=0.013, S= 1 in 600, neglect variation of n. Check for minimum & maximum
velocity, assuming min. flow 1/3rd of avg. flow & max. flow is 3 times of avg.
flow.

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