TRE Slides Unit V_Wastewater collection and disposal systems.pdf

Module for CEE, CGE, CCE, CSE, CWE students
College of Science and Technology
School of Engineering
Department of Civil, Environmental and Geomatic Engineering
TRE 2161:
Design of Drainage Structures

Unit V. Wastewater collection and
disposal
3

6.0 Basic terms about wastewater
6.1 Sources of Wastewater
6.2 Components of Wastewater Engineering
6.3. Types of Sewers
6.4. Sewage Pumps and their use
6.4. Wastewater collection
Contents of this Unit V

Objectives
1. To Learn Basic Concepts of wastewater
engineering (Origin , Quantities ,
Characteristics and Carriage etc.)
2. To design Wastewater Collection system
3. To Design various Wastewater treatment
processes

• Text Book
Water supply & Sewerage by E.W Steel and Mcghee
• Reference Books
1. Wastewater Engineering, Treatment , Disposal,
Reuse by Metcalf and Eddy , 3rd Edition
2. Water and Wastewater Engineering by Fair and
Geyer
3. Water and Wastewater Technology by Masle
J.Hammer

Basic Terms
• Sewage: It is the Liquid Waste or Wastewater
produced as a result of water use.
• Sewer: It is a pipe or conduit for carrying
sewage. It is generally closed and flow takes
place under gravity .

• Sewerage: Sewerage is the system of collection of
wastewater and conveying it to the point of disposal
with or without treatment.
Sources of Wastewater
1.Dometic: It is wastewater from houses offices,
other buildings, hotels and institutions
2.Industrial: It is the liquid waste from industrial
process
3.Storm-water: It includes surface run-off
generated by rainfall and the street wash

Components of Wastewater
Engineering
1. Collection System  Network of Sewer pipes
2. Disposal  Sewage Pumping Stations and
Outfalls
3.Treatment Works  Wastewater treatment
Plants

Fig: Components of Wastewater Engineering

Types of Sewers
1. Sanitary Sewer- It carries sanitary sewage like waste
from municipalities including domestic and industrial
waste-water
2. Storm Sewer-It carries storm sewage including surface
run-off and street wash
3. Combined Sewer- It carries domestic, industrial and
storm Sewage
4. House Sewer-It is the sewer conveying sewage from
plumbing system of a building to common municipal
system
5. Lateral Sewer- This sewer carries discharge from
houses sewer

Types of Sewers
6.Sub-main-This sewer receives discharge from two or
more laterals
7.Main/Trunk Sewer- Receives discharge from two or
more sub-mains
8.Outfall Sewer- It receives discharge from all
collecting system and conveys it to the point of final
disposal

Lateral
Main/Trunk
Sewer
Sub-main
sewer
Outfall sewer
House
Sewer
Fig: Types of Sewers

Types of Sewer Systems
• 1. Separate System
If storm water is carried separately from domestic
and industrial wastewater the system is called as
separate system.
Separate systems are favored when
(i) There is an immediate need for collection of the
sanitary sewage but not for storm water.
(ii) When sanitary sewage needs treatment but the
storm water does not.

2. Combined System
It is the system in which the sewers carry both
sanitary and storm water, combined system is
favored when;
(i) Combined sewage can be disposed off without
treatment
(ii) Both sanitary and storm water need treatment
(iii) Streets are narrow and two separate sewer
cannot be laid

3. Partially Combined System
If some portion of storm or surface run-off is
allowed to be carried along with sanitary
sewage the system is known as partially
combined system.
(In Urban area of developing countries, mostly
partially combined system is employed as it is
economical)
In Pakistan we use this system

Note
• Sanitary wastewater is not allowed to
discharge in any stream
• When there is storm water inside the sewer
some portion of sanitary sewer might go as its
effects would be less since due to dilution
• Sanitary wastewater remains at the bottom
usually because of high density than storm
water

Infiltration
• It is the wastewater that enters sewers through
joints, cracked pipes, walls and covers of the
wholes
• Infiltration is almost non-existent in dry weather
but increases during rainy season
• Water and Sanitation Agency (WASA) Lahore uses
the following infiltration rates for the design of
sewer system.
Pipe dia. Up to 600mm  5% avg. sewage flow
for greater than 600mm 10% avg. sewage flow

Sewage Generation & Water
Consumption
• Around 70-130% of water consumed gets into
sewers
1. Industries with private point of discharge
2. Poor Sewer joints
General Range 70-90% of water consumption
when infiltration is taken into consideration
then
Avg. sewage flow equals the avg. rate of water
consumption

Variation in Sewage Flow
• Like Water Supply the sewage flow varies from
time to time since the sewers must be able to
accommodate the max. rate of flow the
variation in sewage flow need to studied
(1). HERMAN FORMULA: is used to estimate the
ratio of max. to avg. flow
M=
𝑄𝑚𝑎𝑥
.𝑄𝑎𝑣𝑔
.
= 1 +
14
4+ 𝑃
P = Pop. In 1000
M = Peak Factor

Water Supply & Sanitation Agency(WASA), Lahore consider the
following relationship for sewer design
Average Flow (m3/day) Peak Factor
< 2500 4
2500-5000 3.4
5000-10000 3.1
10000-25000 2.7
25000-50000 2.5
50000-100000 2.3
100000-250000 2.15
>500000 2

• Minimum rate of Sewage Flow
Generally taken as 50% of avg. sewage
- It is used in the design of sewage pumping
station
- To investigate the velocities in sewer during
Low Flow periods

Numerical
• The residential area of a city has a population
density of 15000 per/ Km2 and an area of
120,000 m2. If the average water consumption
in 400 lpcd. Find the average sewage flow and
the maximum sewage flow that can be
excepted in m3/day.

Design Period and Use of Sewage Flow
Data
1. Design of Sewer System.
Period of design is indefinite as the system is
designed to care for the maximum development
of area
-Use of Qmax (maximum flow) for sewer design
-Use of Qmin (minimum flow) to check velocities
during low flow

Design Period and Use of Sewage Flow
Data
2. Design of sewage pumping station
-Design period is usually 10 years
-We consider average daily flow , peak and
minimum flow including infiltration
3. Design of sewage treatment Plants
-Design period is usually 15-20years,
-Require data of average flow , infiltration , peak
flow

Sewage Pumps
&
Their Uses
Submitted by www.pumpkart.com

What Is a Sewage Pump?
A sewage pump is
located in the bottom
of basin. It is a
device used to move
liquids with solids all
over a sewer system.

Sewage Pumps designed to drain out wasted
liquids and semi-solids that contaminated in the
home basements and other secluded areas

They Work Automatically
The pump switch is activated automatically as the liquid reachesa specific
point
The pump switchesoff when the liquid goes down to a certain point
Users Safefor functioning

These pumps
can be
submersed into
the liquid

They can
work
outside a
basin

Most of them are efficient
to handle up to 2” solids

More powerful
pumps need to be
used for a higher
and longer lifting of
the waste

When thinking of
buying a pump,
don’t forget to
take advice from a
plumber to select
the right
horsepower
sewage pump!

Horsepower can
be calculated by
the number of
drain outlets
connected to the
sewage basin

Must Visit at
www.pumpkart.com to Buy
a Long Lasting Sewage
Pump

L-9
Sewage Pumping
Environmental Engineering-
II

The need for sewer pumping
stations
arises when
• The existing topography and required
minimum sewer grades create deep
sewers that have high construction
costs. The sewage is raised and then
conveyed by gravity.
• Basements are too low to discharge
sewage to the main sewer.
• Sewage must be conveyed over a ridge.

• The sewage must be raised to get
head for gravity flow through a
treatment plant.
• Discharge outlets are below the
level of the receiving body of
water.

Problems is sewage
pumping
1. Sewage has foul characteristics
2. Sewage has lot of suspended solids and
floating matter. Which may make running
of pumps difficult and may cause frequent
clogging of pumps.
3. Sewage may contain organic and inorganic
waste, which may cause corrosion and
errosion of parts of the pumps and reduce
life of pumps.

4.Sewage contains pathogens which may
cause health problems to working persons
at sewage pumping station.
5.The rate sewage flow varies hourly hence
rate of pumping has to be adjusted
accordingly
6.Size of sump is limited since large size
of sump results in settlement of solids
and organic matter at its bottom.
7.Pumps should be of high order reliability
since failure of pumps will lead to flooding
of adjoining areas.

Pumping
station
• Dry well :- For housing the pumps
• Wet well: - For incoming sewage.
• Rising main:- To led the pumped
sewageto high leveled gravity sewer.
• Pumps used:- Centrifugal, Reciprocating,
Propeller of axial flow, Air pressure
pumps or ejectors

Design
criteria
1. Hydraulic retention time HRT:- retention
time of wet well usually does not exceed
20 min.
2. High and low water levels:- These are
fixed in the wet well in order to
determine position of suction pipes for
the pumps.
3. Screens :- these are required to remove
floating matter that may clog and
damage the pump

4.Standby pumps :- At least one extra pump
more than the pumps required per design,
should always be provided to act as standby
so that it can be used when one of the
working pimps under maintainance or
repair
5.Additional Space :- Provision for extra space
for dy wells in the design of the pump house
to install additional pumps in future should
be kept at initial planning stage itself

Formulae
used
1. For finding frictional losses
in pipe
hf= (fLv2/2gd)
v= velocity of flow in rising
main d= dia of
rising main
L= length of pipe

2. Power of pump
P=(w.Qp.H)/(75ηpηm
)
w= density of water in
kg/m3 Qp=Flow to be lifted
by pump H= total head
ηp ηm= Efficiency of pump and
driving motor resp.

Objective
Questions
1. matter may
clog the pump.
2. and wells
are provided in pumping stations.
3.Dry well is required for
.
4.Wet well is required for
.

Theory
questions
Q1. Write short note on
1. Sewage pumping
2. Difficulties in sewage pumping
3. Design of sewage pumping
station

 Combined
• In wet weather, both wastewater and stormwater
flow in same system
• If flow is too high, untreated water is discharged
into river (or channels in tehran)
 Separate
• Wastewater and stormwater flow in separate
sewer systems

Sewer Basics
 Collection and transport of wastewater from each
home/building to the point where treatment occurs.
 Wastewater Characterization
 Solids
 Liquids
 Pipe System
 Plastic
 Ductile iron
 Concrete/lined
 Bricks

Satellite Wastewater Management
 Also called “decentralized” or “distributed”
 Undertaken by utilities for a variety of reasons:
 Economics
 To reuse water locally
 To avoid expanding “centralized facilities
 As communities expand, the distances from the new
developments to existing wastewater treatment facilities
becomes so great as to not be economically feasible
 Sustainable
 Cost Effective, good for the public and the environment

Collection System Alternatives
1. Conventional Gravity sewers
2. Septic Tank Effluent Gravity (STEG)
3. Septic Tank Effluent Pump (STEP)
4. Pressure Sewers with Grinder Pumps
5. Vacuum Sewers

Gravity Flow
 Least expensive
method of operation
 Gravity sewers are
never full
 More vulnerable to
H2S corrosion

Conventional Gravity Sewer
 Large pipe (200 minimum), manholes spaced 90-150 m
 Designed to transport solids
 Minimum velocity > 0.6 m/s (to avoid the deposition of solids)
 Max velocity = 4.5 m/s
 Infiltration and Inflow (I & I)
 Uniform Slope between manholes
 Sewer size Minimum slope (m/100 m)
 8-inch (200) 0.40
 10-inch (250) 0.28
 12-inch 0.22
 14-inch 0.17
 16-inch 0.14
 18-inch 0.12
 24-inch 0.08

Alignment: 24-inch sewers -600 (or smaller) should be laid
with straight alignment between manholes.
Changes in pipe size: When a smaller sewer joins a larger
one (at a manhole), the invert (bottom) of the larger sewer
should be lowered sufficiently to overcome head losses. An
approximate method is to place the 0.8 depth point of both
sewers at the same elevation.
Sewer Materials: many types, materials and bedding shall
prevent damage from external loads, and joints shall prevent
leakage
0.8 depths are aligned

Manholes
* Located at changes in sewer size, direction, or slope
* Or every 90 – 150 m
* Provides access for maintenance (cleanout, etc.)
* Problematic because of Infiltration and Inflow (I&I)

Small Diameter Gravity Sewer Systems
 Wastewater flows from home to
interceptor (septic) tank where settleable
solids and grease are removed
 Wastewater flows by gravity to central
collector pipe
 Central collector pipe can flow full in
certain areas
 Pipe sizes are typically 4 and 6-inch
diameter (100 – 150 mm)

Small Diameter Sewer Layout
Pipe will flow full, under pressure in these areas

Interceptor Tank (same as Septic Tank)

Most Sewers Rely on Gravity Flow
 Most sewers are designed to flow by gravity
(water flows down-hill)
 Includes sewer pipe from home to septic tank or
to a municipal collector pipe
 Gravity sewers must follow the topography of
the land
 Where gravity flow is not possible, pumps are
used
• Individual unit pump
• Large municipal lift (pump) station

Hydraulics of Sewers
 Most sewers designed to flow at a velocity
of 0.6 m/sec when flowing half-full
 Do not want pipe to flow full at peak flows
 Do not want pipe flow velocity too low—can’t
transport solids
 Most designs are complex, but use basic
hydraulic equations for computing
necessary size and slope

Manning Equation for Pipe Flow
 Manning Equation
V = velocity (m/sec)
n = coefficient of roughness (dependent upon pipe
material/condition)
R = hydraulic radius = area/wetted perimeter (m)
S = hydraulic slope (assumed to be slope of pipe)
(m/m)
2
/
1
3
/
2
1
S
R
n
V 

Basic Sewer Design
 Collector pipes (pipe in street) is minimum 8
inches (200) diameter (to allow cleaning)
 Service pipes (home or building to collector
is 4 to 6 inches (100-150 mm)diameter
 Gravity sewer pipes have no bends,
manholes used to make transitions in
direction and pipe size
 Pipe sections between manholes are at a
constant grade or slope (S)

Proportional velocity and discharge

Proportional geometry elements
2
8
sin
D
Ad 




 



2
D
Pd 

0.015 0.4911 28 5.48648E-05 0.0368 0.1164 0.00036
0.016 0.5073 29 6.04235E-05 0.0380 0.1215 0.00042
0.017 0.5230 30 6.61557E-05 0.0392 0.1265 0.00047
d/D  radian  Ad Pd v/V q/Q

Bedding: specified by an engineer for pipe type and anticipated loads

Backfill: suitable material, free of debris, stones, etc.
DO NOT DISTURB SEWER ALIGNMENT
Separation from Water Lines:
 3 m horizontal
 Water line 0.5 m above sewer

Crown Corrosion
H2S + H2O    H2SO4
H2S
H2SO4

Pressure Flow
 Wastewater is actively
pumped
 Used when gravity flow will
not work, due to terrain or
other factors
 Construction and operation
usually more expensive than
gravity flow

Pressure Sewers
 Each
home/building has
individual pump
 Wastewater
pumped to a
central treatment
location
 Pumps “grind”
sewage solids

Pressure Systems Can Pump Wastewater Treated
by Septic Tank – Used for Homes Previously on
Septic Systems

Conventional Gravity vs. Pressure Sewers

Sewer Alternatives and Characteristics

Septic Tank Effluent Gravity
STEG
 Small diameter plastic pipe
 Conveys effluent from a septic tank
 with an effluent filter
 No solids to transport (no minimum velocity required)
 Can be installed at variable (flat) grades
 No manholes
 Air-release valves needed at high points

Septic Tank Effluent Gravity
STEG

Septic Tank Effluent Pump
STEP
 High-head turbine pump used to pump screened septic tank
effluent into a pressurized collection system.
 Small diameter, plastic pipe (50 mm)
 No Solids transport (no minimum velocities required)
 Installed shallow
 Can follow terrain
 Air-release valves incorporated

STEP Components
 Building sewer (from house to septic tank)
 Septic Tank (or interceptor tank)
 Vaults/pump basins (effluent filter and pump)
 Pumps (submersible, high-head, turbine)
 Service lateral (1.25-inch typical)
 Check Valves (at pump outlet and at edge of property)

Septic Tank Effluent Pump
STEP

Pressure Sewer with Grinder Pumps
 Discharge pump with chopper blades in a small pump basin
 Small diameter, pressure line, installed shallow
 Solids and greases are transported
 Relatively simple installation
 Somewhat higher O&M
 No I&I

Pressure Sewer
Grinder Pump basin

Vacuum Flow
 Not very common, but use is gradually
increasing
 Vacuum pumps used to move waste
through sewer
 Same advantages and disadvantages as
pressure sewers
 Less water needed to transport waste

Vacuum Sewers
 Wastewater flows by gravity to a central
collector well (up to four homes per well)
 When well fills a vacuum lines pulls
wastewater to a central vacuum tank
 Wastewater pumped from central vacuum
tank to treatment or a gravity sewer

Vacuum System Components
Could also come
from existing
septic tank

Vacuum Sewer
 Central vacuum source maintains a
vacuum on a small diameter sewer
 Pulls wastewater to a central location
 Ideal application:
 Flat terrain
 High water table
 70-100 connections to be economical

Estimating Flow rate
 Equivalent Dwelling Unit (EDU)
• A residence with a given number of people (say 4)
• Represents the average household flowrate
 Design Peak Flowrate (DPF)
• Flowrate expected in the collection system, assuming a given number of
EDUs are discharging at the same time
• Typical values for systems with >50 EDUs is 1.3 to 1.9 L/min-EDU
• Total DPF QDP = 0.5 N

How to Obtain Quantity (lpd) numbers
 Estimated design numbers in codes
 These are generally higher than actual use
 They provide a factor of safety
 Metered flow
 Add a factor of safety
 Average flows from similar facilities
 Add a factor of safety
 Peak flows
 Design

How much wastewater do we
produce each day?
Wastewater Characteristics
Source Average Daily Flow
Domestic sewage 60-120 gal/capita
Shopping centers 60-120 gal/1000 ft2
total floor
area
Hospitals 240-480 gal/bed
Schools 18-36 gal/student
Travel trailer parks
Without individual
hookups
90 gal/site
With individual
hookups
210 gal/site
Campgrounds 60-150 gal/campsite
Mobile home parks 265 gal/unit
Motels 40-53 gal/bed
Hotels 60 gal/bed
Industrial areas
Light industrial area 3750 gal/acre
Heavy industrial 5350 gal/acre
Source: Droste, R.L., 1997. Theory and Practice of
Water and Wastewater Treatment
These values are
rough estimates only
and vary greatly by
locale.

Typical data on unit loading factor and expected
wastewater contaminant loads for individual
residences. (Taken from Wisc. Comm. 83 code)

Time of day
MN Noon MN
Flow
–
gallons/hr/person
6 AM 6 pm
Daily flow pattern for a home
Average flow
Peak flow

What are the wastewater flows?
 Domestic (sanitary)
 Industrial
 Infiltration/inflow
 Stormwater

Wastewater flow rates
 Rule of thumb:
 Iran domestic is 250 lpd/capita (ref. Tbl. 3-1 M&E)
 Developing countries: 20 to 200 lpd/capita ( Tbl.
3-9 M&E)
 Other: depends on facility (industry,
commercial, etc.)
 I/I can be significant (Fig. 3.2 M&E)

Factors affecting flow rates
 Geographical location & socioeconomic
conditions
 Type of development
 Season
 Time of Day
 Climate (rain or dry)

“Diurnal variations” in domestic
wastewater flows

Wastewater flows
 Flows are either normally distributed or
log-normal (log of flows are normally
distributed)
 Statistical procedures based on flow
history used to determine average (dry
weather, wet weather, annual daily), peak
(instantaneous, hour), maximum (day,
month), minimum (hour, day, month)
(Table 3-11 M&E)

Wastewater loads
 Treatment is to decrease chemical
constituents, but quantity to be treated
depends on:
 Concentration (typ. mg/L)
 Flow rate (typ. lpd or MLD)
 Mass loading:
Concentration x Flow = mass /time

Day
S S
Flow
–
gallons/day
W
M T T F
Weekly flow
Average flow
Peak flow

Commercial Sources
 Many sources
 Variable from site to site
 High organic matter
 High fat, oil and grease
 Nitrogen
 Phosphorus
 Cleaning agents
 Heat (high water temperature)

Influences on Wastewater Characteristics
 Organic matter
 BOD and TSS
 Nitrogen
 Organic nitrogen, ammonia, nitrate
 Pathogens
 Bacterial –use fecal coliform as indicator organism
 Viral – coliphage as indicator organisms
 Fats, oils and greases
 Phosphorus
 Cleaning agents/antibacterial/VOC
 Excessive laundry wastes
 Medications
 What else????

Many styles of low
Flow toilets today.
All toilets sold today
has 1.6 gal./flush.

Manholes
 Needed for:
• Change in direction
• Change in pipe size
• To route sewer under roads, rivers, etc.
• Clean-out and inspection
 Spaced every 100 to 120 m along the sewer

Manhole Construction
 Brick was once common
 Most manholes are now
concrete
 Smaller sizes are pre-cast
concrete
 Large manholes are
constructed in-place

Typical Manhole
Fig 5.1, p 134

Problems in Sewers
 Blockage
 Corrosion in sewers made of
• Concrete
• Iron
 Inflow
 Infiltration
 Leakage

Blockage in Sewers
 Caused by sand, rocks, tree roots, various
wastes
 Can cause flooding
• In streets
• In basements

Corrosion in Sewers
 Concrete is made of cement, which is basic
 H2S gas formed by anaerobic bacteria
 H2S converted to sulfuric acid (H2SO4),
chemically or by bacteria
 Sulfuric acid reacts with cement
(neutralization)
 Result: cement is dissolved by sulfuric acid

Effects of long detention times

Sources of Inflow
 Water from roof and cellar drains
 Water leaking around manhole covers
 Cause excess hydraulic load on treatment
plant

Sources of Infiltration
 Seepage of groundwater into pipe joints
and manholes
 Controlled by
• Good construction materials and practices
• Avoiding construction in saturated soil, if possible

Leakage from Sewers
 Caused by
• Cracked, broken, or corroded pipe
• Bad connections at joints
 Can cause groundwater contamination,
and damage street and building
foundations

Pump Stations
 Pump (lift) sewage from low to higher
elevation, generally from end of one
gravity sewer section to another, higher
section
 Consist of a wet well and pumps
 Wet well forms a place for wastewater to
collect and be pumped from

Source: Metcalf & Eddy, Inc. Wastewater Engineering: Collection, Treatment and Disposal. McGraw-Hill:New
York, 1972.
Large Pump Station

Design of W.W. Collection System
Design criteria:
Waste water flow: Flow varies according to:
The season (monthly variations)
Weather conditions
Week of the month , day of the week, time of the day.
Estimation of the design flow Qdes: Data needed:
Average daily water consumption per capita for domestic areas (L/c/d), (Qavg).
Average daily water consumption per capita for institution ( school, offices, ….etc. ), (Qavg).
Average daily water consumption for commercial and industrial areas.
Infiltration, inflow:
Qinfil is taken as [24-95 m3/day/km] or [0.5 m3/day/diamwter (cm)], take the bigger value of
the two.
Qinflo is taken as 0.2-30 [m3/ha/day]. ( hectare = 10,000 m2 )
Qdes = Qmax + QI/I ( if found)
QI/I = Qinfil + Qinflo
Qmax = [0.80* Qavg] * Pƒ ( 0.8 > 80% return from water supply).
This equation is for domestic users only. Qmax for institutions, commercial activities, and
industries are calculated according to the type of industry, and cannot be calculated from
this equation. Each industry has its specific average wastewater production and peaking
factor that can be taken from published references or from the records of these industries
or institutions.

149
86
43
0 2 4 6 8 10 12 14 16 18 20 22 24
hour
1.8
1.5
1.0
0.5
0.0
Flow
coefficient
Flow
(L/s)
Peak coefficient
Average day flow
Average 24 hr flow
Average night flow
Sewage flow diagram for a small town

- Pƒ : peak factor for domestic wastewater can be calculated
from one of the following formulas :
P
f
P



4
14
1 , ( P: population in thousands)
Or 167
.
0
5
P
f
P 
 The minimum domestic wastewater flow (Qmin) is necessary to check
for the minimum velocity in the sanitary sewers, it is estimated from
the following formula:
W
avg
Q
P
Q 





 *
6
1
2
.
0
min
A typical value of
W
avg
Q
Q 







3
1
min
Note: [Qavg]w = 0.8 Qavg , which is
the average domestic wastewater
production , while Qavg is the
average water consumption.

Example
A gravity pipe serving a community of 50,000 inh. The length of the pipe
is 200 m, and the average water consumption is 120 L/c/d. Use an
infiltration rate of 30 m3
/day.km, and a wastewater production rate of
80% of the water supply. Neglect the inflow for this example. Calculate
Qdes and Qmin.
a. Calculate the average domestic WW flow:
[Qavg]w = 0.8 Qavg = 0.80 * 120 L/c/d * 50,000 capita* 10-3
= 4800 m3
/d
b. Calculate the peak factor:
P
f
P



4
14
1 = 26
.
2
50
4
14
1 


Solution

a. Calculate the maximum wastewater flow:
Qmax = [Qavg]w * Pƒ = 2.26 * 4800 = 10848 m3
/d
b. Calculate the minimum wastewater flow:
W
avg
Q
P
Q 





 *
6
1
2
.
0
min 1845
4800
*
6
1
)
50
(
2
.
0 

m3
/d
c. Calculate the infiltration flow:
Qinfil = 30 *0.20 = 6 m3
/d
d. Calculate the design flow:
Qdes = Qmax + QI/I = 10848 + 6 = 10854 m3
/d

Maximum and minimum velocities:
Minimum velocity of 0.6 m/s should be maintained to prevent solids settling, it
is called self cleansing velocity.
Maximum velocity should not be lighter than 3 m/s to prevent erosion of pipes
and manholes.
Minimum size of pipes:
Minimum diameter is 8 inches (20 cm).
Minimum Slope and maximum slope of sanitary sewers:
Minimum slope is a function of the minimum velocity of 0.60 m/s (See the
table). The maximum slope is related to the maximum velocity (3 m/s or any
other velocity selected by the designer) according to the pipe material and the
expected amount of sand carried with the wastewater.
Depth of excavation:
Minimum cover on the top of sewers
Depth of excavation depends on:
water table
topography
lowest point to be served
other factors
Depth below
design level (m)
D (inch)
0.7
4
0.8
6
1.0
8
1.0
10
1.0
12
1.2
14
1.3
16

Manholes: Manholes are constructed in the following cases:
• when pipes change in diameter
• change of direction
• change of slope
• intersection of pipes
• at interval, ( 20-100 m)
Lay-out plan
1. Find the intermediate points of collection from topographic maps.
2. Assign the final points of collection.
3. Draw lines indicating the flow of sewage by gravity (different alternatives).
4. The pipes can be considered as :
• Building sewers.
• Lateral sewers.
• Sub main sewers.
• Main sewers.
• Trunk sewers.
• Intercepting sewers.
5. The contributory area for each pipe is defined by drawing lines depending
on topography and points of connection to manholes.
6. Manholes and pipes are given numbers to facilitate the design of each pipe.

Building sewer
Main sewer
Sub main sewer
Lateral sewer
Trunk sewer
Intercepting sewer
Outlet sewer
Manholes

Alternative Lay-outs for main sewer

Sanitary Sewers Appurtenances
Manholes
The following table gives the allowable intervals of manholes relative to the
diameter:
Pipe diameter ( inch ) Max. distance between manholes
(m)
8 30
8-10 40
10-12 50
12-16 60
16-36 100
≥ 36 150
Note: The distance depends on the maintenance equipments available.

Manhole dimensions
The diameter of the manhole or its side's dimensions depends on the depth of excavation.
The following table gives their relation.
Depth of manhole Manhole dimensions
0.60 x 0.60 m [Square]
Depth ≤ 0.90 m
Φ 0.60 m [ Circular]
1 . 0 0 x 1 . 0 0 [ S q u a r e ]
Φ 1 . 0 0 m [ C i r c u l a r ]
1 . 5 0 - 2 . 0 0 m
0 . 8 0 x 1 . 2 0 m [ R e c t a n g u l a r ]
≥ 2 . 0 0 m Φ 1 . 5 0 m [ C i r c u l a r ]
• The cover of the manhole should be strong enough to withstand the loads of traffic.
• It is usually made of cast iron to carry a minimum concentrated load of 25 ton.
• The manhole should be supplied with steps to allow for maintenance access.
• The floor of the manhole should be lined with cement mortar which is called
benching.

Drop manhole: are used when the difference of elevation between the inlet pipe and
the outlet pipe is ≥ 60cm. The drop manhole has a vertical pipe to prevent
turbulence in the manhole and to allow the maintenance works to enter the
manholes safely.
Normal manhole
Drop
manhole
Grease and oil traps
Grease and oil traps: For the institutions, commercial
units, restaurants and other places which produce oil
and grease in there effluent, a grease and oil trap
should be used to remove oil and grease before they
enter the sewage pipes. Grease and oil affect the
sewers and the treatment plant equipments that is why
they should be removed. In case of the pipes, grease
sticks to the walls and collects sand and other solids
leading eventually to the decrease in the pipe diameter
and some times to complete clogging.

Inverted siphon: When an obstacle such as railway or a river obstructs the sewer line the
sewers can be lowered below these obstacles.
• For the inverted siphon: Minimum velocity = 0.9 m/s
• Pipes flow full (under pressure).
More than one pipe is used to overcome these variations. Usually Three pipes are used.
• The first pipe is used to carry the minimum wastewater flow (Qmin),
• The second carry the difference between the average flow and the minimum flow (Qavg-Qmin)
• The third carries the difference between the average and the maximum flow (Qmax – Qavg) .

TRE Slides Unit V_Wastewater collection and disposal systems.pdf

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TRE Slides Unit V_Wastewater collection and disposal systems.pdf