TRE Slides Unit V_Wastewater collection and disposal systems.pdf

drainage design

1. 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

2. Unit V. Wastewater collection and
disposal
3

3. 6.0Basic terms about wastewater
6.1 Sources of Wastewater
6.2Components of Wastewater Engineering
6.3. Types of Sewers
6.4.Sewage Pumps and their use
6.4.Wastewater collection
Contents of this Unit V

4. Basic terms about
wastewater

5. Objectives
1.ToLearn Basic Conceptsofwastewater
engineering(Origin, Quantities,
CharacteristicsandCarriageetc.)
2.Todesign WastewaterCollectionsystem
3.ToDesignvariousWastewater treatment
processes

6. •TextBook
Watersupply&SeweragebyE.WSteel and Mcghee
•ReferenceBooks
1.Wastewater Engineering, Treatment , Disposal,
ReusebyMetcalfandEddy ,3
rd
Edition
2.WaterandWastewaterEngineeringbyFairand
Geyer
3.WaterandWastewaterTechnologybyMasle
J.Hammer

7. BasicTerms
•Sewage:Itisthe LiquidWasteorWastewater
produced as a resultofwateruse.
•Sewer:Itisa pipe or conduitforcarrying
sewage.Itis generally closed and flowtakes
placeundergravity.

8. •Sewerage:Sewerageisthesystemofcollectionof
wastewaterandconveyingittothepointofdisposal
withorwithouttreatment.
SourcesofWastewater
1.Dometic:It iswastewaterfromhouses offices,
otherbuildings,hotelsandinstitutions
2.Industrial:Itistheliquidwastefromindustrial
process
3.Storm-water:Itincludessurfacerun-off
generatedbyrainfallandthe streetwash

9. ComponentsofWastewater
Engineering
1.CollectionSystemNetworkofSewerpipes
2.DisposalSewagePumpingStations and
Outfalls
3.Treatment Works Wastewater treatment
Plants

10. Fig:ComponentsofWastewaterEngineering

11. TypesofSewers
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-offandstreetwash
3.CombinedSewer-Itcarriesdomestic,industrialand
stormSewage
4.HouseSewer-Itisthesewerconveyingsewagefrom
plumbingsystemofabuildingtocommonmunicipal
system
5.LateralSewer-Thissewercarriesdischargefrom
housessewer

12. TypesofSewers
6.Sub-main-Thissewerreceivesdischargefromtwoor
morelaterals
7.Main/TrunkSewer-Receivesdischargefromtwoor
moresub-mains
8.OutfallSewer-Itreceivesdischargefromall
collectingsystemandconveysittothepointoffinal
disposal

13. Lateral
Main/Trunk
Sewer
Sub-main
sewer
Outfallsewer
House
Sewer
Fig:TypesofSewers

14. TypesofSewerSystems
•1.SeparateSystem
Ifstormwateriscarriedseparatelyfromdomestic
andindustrialwastewaterthesystemiscalledas
separatesystem.
Separatesystemsare favoredwhen
(i)There is an immediate need for collection of the
sanitarysewagebutnotforstormwater.
(ii)When sanitary sewage needs treatment but the
stormwaterdoesnot.

17. TypesofSewerSystems
2.CombinedSystem
Itisthesysteminwhichthesewerscarryboth
sanitaryandstormwater,combinedsystemis
favoredwhen;
(i)Combined sewage can be disposed off without
treatment
(ii)Bothsanitaryandstormwaterneedtreatment
(iii)Streetsare narrowand twoseparate sewer
cannotbelaid

19. TypesofSewerSystems
3.PartiallyCombinedSystem
If someportionof stormor surface run-offis
allowed tobe carriedalongwithsanitary
sewagethe systemis knownaspartially
combinedsystem.
(InUrbanareaofdevelopingcountries,mostly
partiallycombinedsystemis employedas itis
economical)
InPakistanweusethissystem

21. Note
•Sanitary wastewater is not allowed to
dischargeinanystream
•Whenthereisstormwaterinsidethe sewer
someportionof sanitarysewermightgoas its
effectswouldbe less sincedue todilution
•Sanitarywastewaterremainsatthebottom
usuallybecauseofhighdensitythanstorm
water

22. Infiltration
•It is the wastewater that enters sewers through
joints,crackedpipes,wallsandcoversofthe
wholes
•Infiltration isalmost non-existentindry weather
butincreasesduringrainyseason
•WaterandSanitationAgency(WASA)Lahoreuses
thefollowinginfiltrationratesforthe designof
sewersystem.
Pipe dia. Up to 600mm 5% avg. sewage flow
forgreater than600mm10% avg.sewageflow

25. Sewage Generation & Water
Consumption
•Around 70-130% of water consumed gets into
sewers
1.Industrieswithprivatepointofdischarge
2.PoorSewerjoints
GeneralRange70-90%of water consumption
wheninfiltrationistakenintoconsideration
then
Avg.sewageflow equalsthe avg.rateof water
consumption

26. VariationinSewage Flow
•LikeWaterSupplythesewageflowvariesfrom
timetotimesincethesewersmustbeableto
accommodatethemax.rateofflowthe
variationinsewageflowneedtostudied
(1).HERMANFORMULA:isusedtoestimatethe
ratioof max.toavg.flow
M=
�??????????????????
.�??????????????????
.
=1+
14
4+�
P=Pop.In1000
M=PeakFactor

27. VariationinSewage Flow
Water Supply & Sanitation Agency(WASA), Lahore consider the
followingrelationshipforsewerdesign
AverageFlow(m3/day) PeakFactor
<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

28. VariationinSewage Flow
•MinimumrateofSewageFlow
Generallytakenas50%ofavg.sewage
-Itis used inthe designof sewagepumping
station
-Toinvestigatethevelocitiesinsewerduring
LowFlowperiods

29. Numerical
•The residential area of a city has a population
density of 15000 per/ Km
2
and an area of
120,000m
2
.If theaveragewaterconsumption
in400lpcd.Findthe averagesewageflowand
themaximumsewageflowthatcanbe
exceptedinm
3
/day.

30. DesignPeriodandUseofSewageFlow
Data
1.Design ofSewerSystem.
Periodofdesign isindefiniteasthe systemis
designedtocareforthemaximumdevelopment
ofarea
-UseofQmax (maximumflow) forsewerdesign
-UseofQmin (minimumflow)tocheckvelocities
duringlowflow

31. DesignPeriodandUseofSewageFlow
Data
2.Designofsewagepumpingstation
-Designperiodisusually10 years
-Weconsideraveragedailyflow, peakand
minimumflowincludinginfiltration
3.Design ofsewagetreatmentPlants
-Design periodisusually15-20years,
-Requiredataofaverage flow,infiltration, peak
flow

32. SewagePumps
&
TheirUses
Submitted bywww.pumpkart.com

33. WhatIsaSewagePump?
A sewage pump is
locatedin thebottom
ofbasin.Itisa
device used to move
liquids with solids all
overasewer system.
Submitted bywww.pumpkart.com

34. Sewage Pumps designed to drain out wasted
liquids and semi-solids that contaminated in the
homebasementsandothersecludedareas
Submitted bywww.pumpkart.com

35. TheyWorkAutomatically
Thepumpswitchisactivatedautomaticallyasthe liquidreachesaspecific
point
Thepumpswitches offwhentheliquidgoes downtoacertainpoint
UsersSafeforfunctioning
Submitted bywww.pumpkart.com

36. These pumps
can be
submersedinto
the liquid
Submitted bywww.pumpkart.com

37. Theycan
work
outside a
basin
Submitted bywww.pumpkart.com

38. Mostof themare efficient
tohandleupto 2”solids
Submitted bywww.pumpkart.com

39. Morepowerful
pumps need to be
used for a higher
andlongerliftingof
thewaste
Submitted bywww.pumpkart.com

40. When thinking of
buying a pump,
don’tforgetto
takeadvicefroma
plumber to select
the right
horsepower
sewagepump!
Submitted bywww.pumpkart.com

41. Horsepower can
be calculated by
the number of
drain outlets
connectedtothe
sewagebasin
Submitted bywww.pumpkart.com

42. MustVisitat
www.pumpkart.comtoBuy
aLongLastingSewage
Pump
Submitted bywww.pumpkart.com

43. L-9
SewagePumping
EnvironmentalEngineering-
II

44. WET
WEL
L

45. Modula
r pump
well

46. RISING
MAIN

47. The need for sewer pumping
stations
ariseswhen
•The existing topography and required
minimum sewer grades create deep
sewers that have high construction
costs. The sewage is raised and then
conveyed bygravity.
•Basements are too low todischarge
sewage to the mainsewer.
•Sewage must be conveyed over aridge.

48. •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.

49. Problems is sewage
pumping
1.Sewage has foulcharacteristics
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 ofpumps.

50. 4.Sewagecontainspathogenswhichmay
causehealthproblemstoworkingpersons
atsewagepumpingstation.
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 itsbottom.
7.Pumps should be of high order reliability
since failure of pumps will lead to flooding
of adjoining areas.

52. Pumping
station
•Dry well:-For housing thepumps
•Wet well: -For incomingsewage.
•Rising main:-To led the pumped
sewageto high leveled gravitysewer.
•Pumps used:-Centrifugal, Reciprocating,
Propeller of axial flow, Air pressure
pumps orejectors

53. 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
thepumps.
3.Screens:-these are required to remove
floating matter thatmayclogand
damage thepump

54. 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 stageitself

55. Formulae
used
1.For finding frictional losses
in pipe
h
f=(fLv
2
/2gd)
v=velocityofflow inrising
main d= diaof
risingmain
L=lengthofpipe

56. 2. Power ofpump
P=(w.Q
p.H)/(75η
pη
m
)w=densityofwaterin
kg/m3Q
p=Flowtobelifted
bypumpH=totalhead
η
p η
m= Efficiency of pump and
driving motorresp.

57. Objective
Questions
1. matter may
clog thepump.
2. and wells
are provided in pumpingstations.
3.Dry well is requiredfor
.
4.Wetwellis requiredfor
.

58. Theory
questions
Q1. Write short noteon
1.Sewagepumping
2.Difficulties in sewagepumping
3.Design of sewage pumping
station

59. Wastewater Collection

60. Types of Sewer Systems
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

61. Sewer systems

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

63. 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

64. 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

65. Gravity Flow
Least expensive
method of operation
Gravity sewers are
never full
More vulnerable to
H
2S corrosion

66. 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

67. Conventional Gravity Sewer

68. Conventional Gravity Sewer
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

69. 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)

70. 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)

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

72. Interceptor Tank (same as Septic Tank)

73. 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

74. 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

75. 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)2/13/21
SR
n
V

76. 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)

77. Proportional velocity and discharge

78. Proportional geometry elements2
8
sin
DA
d 






 2
DP
d
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  A
d
P
d
v/V q/Q

79. Conventional Gravity Sewer
Bedding:specified by an engineer for pipe type and anticipated loads

80. Conventional Gravity Sewer
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

81. Crown Corrosion
H
2S + H
2O H
2SO
4
H
2S
H
2SO
4

82. 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

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

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

85. Wet Well Design

86. Conventional Gravity vs. Pressure Sewers

87. Sewer Alternatives and Characteristics

88. 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

89. Septic Tank Effluent Gravity
STEG

90. STEG Sewer Components

91. Pipe Size and Velocity

93. 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

94. 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)

95. Septic Tank Effluent Pump
STEP

96. STEP System Design Data

97. STEP System Interceptor Tank

98. 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

99. Pressure Sewer
Grinder Pump basin

100. Grinder
Pump
Basin

101. 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

102. 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

103. Vacuum System Components
Could also come
from existing
septic tank

104. 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

105. Vacuum Sewer

106. Vacuum Sewer Components

107. Vacuum Station

108. 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 Q
DP= 0.5 N

109. 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

110. Estimated
Wastewater
Flows

111. How much wastewater do we
produce each day?
Wastewater CharacteristicsSource Average Daily Flow
Domestic sewage 60-120 gal/capita
Shopping centers 60-120 gal/1000 ft
2
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 area3750 gal/acre
Heavy industrial5350 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.

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

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

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

115. 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)

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

117. “Diurnal variations” in domestic
wastewater flows

118. 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)

119. 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

120. Day
S
S
Flow
–
gallons/day
WM T T F
Weekly flow
Average flow
Peak flow

121. 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)

122. 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????

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

125. 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

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

127. Typical Manhole
Fig 5.1, p 134

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

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

130. Corrosion in Sewers
Concrete is made of cement, which is basic
H
2S gas formed by anaerobic bacteria
H
2S converted to sulfuric acid (H2SO4),
chemically or by bacteria
Sulfuric acid reacts with cement
(neutralization)
Result: cement is dissolved by sulfuric acid

131. Effects of long detention times

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

133. 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

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

135. 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

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

137. Small Pump Station

138. Pump Station

139. Pump Station
Details

140. Main Pump Station

142. 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:
Qinfilis taken as [24-95 m3/day/km] or [0.5 m3/day/diamwter (cm)], take the bigger value of
the two.
Qinflois 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. Qmaxfor 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.

143. 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

144. - 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
QPQ






 *
6
1
2.0
min

A typical value of
W
avg
QQ







3
1
min
Note:[Qavg]w= 0.8 Qavg, which is
the average domestic wastewater
production , while Qavgis the
average water consumption.

145. ExampleA 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 m
3
/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 m
3
/d
b. Calculate the peak factor:
P
f
P


4
14
1 = 26.2
504
14
1 


Solution

146. a. Calculate the maximum wastewater flow:
Qmax = [Qavg]w * Pƒ = 2.26 * 4800 = 10848 m
3
/d
b. Calculate the minimum wastewater flow: W
avg
QPQ






 *
6
1
2.0
min 18454800*
6
1
)50(2.0 
m
3
/d
c. Calculate the infiltration flow:
Qinfil = 30 *0.20 = 6 m
3
/d
d. Calculate the design flow:
Qdes = Qmax + QI/I = 10848 + 6 = 10854 m
3
/d

147. Maximum and minimum velocities:
Minimum velocity of 0.6 m/sshould be maintained to prevent solids settling, it
is called self cleansing velocity.
Maximum velocity should not be lighter than 3 m/sto 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)
0.74
0.86
1.08
1.010
1.012
1.214
1.316

148. 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.

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

150. Alternative Lay-outs for main sewer

151. Lay-out of sewer Scheme

152. Solving the Network by Table

153. 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.

154. 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.

155. 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.

156. 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 pipeis used to carry the minimum wastewater flow (Qmin),
•The secondcarry the difference between the average flow and the minimum flow (Qavg-Qmin)
•The thirdcarries the difference between the average and the maximum flow (Qmax–Qavg) .

157. END OF Unit V