design of sewer

8/11/2019 Design of Sewer

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Design of Sewer System

DESIGN OF SEWER SYSTEM

INTRODUCTION

Sewer system plays a vital role in the economic development of a country.

Sewers are must for the drainage of waste water. In order to have an effective

sewage system the sewers should be properly designed and more care should be

taken in finding the invert levels otherwise whole design may get wrong.

Sewers are designed for the drainage of waste water coming from houses,

industries, streets, runoff etc to protect the environment and people from

serious diseases, as more than 50 diseases spread from sewage. So for a good

living, the sewers should be properly designed and the sewage should be

treated properly before discharging it into the river.

Some of the important and relevant terms for sewer system are discussed

below.

Sewage

It is Liuid !aste or !aste !ater produce as a result of water use.

Sewer

It is the pipe or conduit for carrying sewage. It is generally closed and flow

takes place undr gravity "#tmospheric $ressure%.

Sewerage

Sewerage is the system of collection of waste water and conveying it to a point

of final disposal with or without treatment.


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Sources of waste water

&ollowing are the principal sources of waste water

'omestic

Industrial

Storm water

Domestic

It is the waste water from houses, offices, other buildings, hotels and

institutions.

Inustria!

It is the liuid waste from the industrial places from their different industrial

processes like dying, paper matting, tanneries, chemical industries, etc.

Storm Water

It includes surface runoff generated from rainfall and the street wash.

Ty"es of Sewer Systems

&ollowing are the types of sewerage
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Se"arate System

It is the system in which storm water is carried separately from domestic and

industrial waste water. (his system is preferred when

(here is an immediate need for collection of sanitary sewage but not forstorm water

!hen sanitary sewage needs treatment but the storm water does not
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Com#ine System

It is the type of system in which sewer carries both the sanitary and storm

water. )ombined system is favored when

)ombined sewage can be disposed off without treatment

*oth sanitary and storm water need treatment

Streets are narrow and two separate sewers can not be laid.

Ty"es of Sewers
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Sanitary Sewers

It carries sanitary sewage i.e. waste water from municipality including

'omestic and Industrial wastewaters.

Storm Sewer

It carries storm sewage including Surface +unoff and Street !ash.
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Com#ine Sewer

It carries domestic, industrial and storm sewage.

$ouse Sewer

It is the sewer conveying sewage from plumbing system of building tocommonmunicipal sewers.

%atera! sewer

(his sewer carries discharge from two or more house sewers.

Su#&Main Sewer

(his sewer carries discharge from two or more laterals.

Main' Trun( Sewer

It receives discharge from two or more sub-mains.

Outfa!! Sewer

It receives discharge from all collecting system and conveys it to the point of

final disposal.
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Sewage f!ow

It is flow derived from the sanitary and industrial sewage that is the raw water

from these industries and houses, so it means it has direct relation with the

amount of water consumed.

enerally /0 to 0 1 of the water consumption is taken as sewage or waste

water flow.

)ariation in sewage f!ow

Like water supply, sewage flow varies from time to time. Since sewers must be

able to accommodate 2a3imum +ate of &low, the variation in the sewage flow

must be studied.


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enerally 4erman &ormula is used to estimate the ratio of 2a3imum to

#verage &low

$ is population in thousands

!#S# Lahore 'esign )onsiders the following relationship for sewer design

#verage Sewage &low "mday% $eak &actor

6 7500 8.0

7500 9 5000 .8

5000 9 :0000 .:

:0000 9 75000 7.;

75000 9 50000 7.5

50000 9 :00000 7.

:00000 9 750000 7.:5
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750000 9 500000 7.0/

< 500000 7.0

Infi!tration

It is amount of water that enters into the sewers through poor =oints, cracked

pipes, walls and covers of manholes.

It is none3istent during dry weather but increases during rainy season. !ater and Sanitation #gency "!#S#% Lahore uses the following

infiltration rates for the design of sewer system.

Sewer 'iameter Infiltration

775 mm to >00 mm 5 1 of #vg. Sewage &low

< >00 mm :0 1 of #vg. Sewage &low

Design *erio

Sewer System

$eriod of design is indefinite. (he system is designed to take care for the

ma3imum development of the area. *ut we take design period of 70 years for

our sewer system.

Sewer *um"ing Station


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'esign period is :0-years.

+ate of &low are average daily, peak and minimum flow including

Infiltration.

*RO+ECT DESCRI*TION(he pro=ect is based on the design of $artially combined sewerage system of a

community which is having appro3imately a flat terrain because the difference

in the reduce levels is very small. I have designed the sewage pumping station

along with the hydraulic statement and the reuired drawings plus some e3tra

drawings. I am confident that my design will work successfully and there will

be not any problems. (he Layout of the community is given as under.
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DESIGN CRITERI,

Design F!ow

&irst of all calculate the average sewage flow on the basis of water

consumption and the population at the end of the design period. i.e at the full

development of the area. (hen the design flow for sanitary sewer and partially

combined sewers can by calculated by using the following formulae.

&or Sanitary Sewer

?design@ $eak sewage flow A infiltration

&or partially combined sewer "!#S# )riteria%

?design@ 73$eak sewage flow A infiltration

Design E-uation

2anningBs Cuation is used for sewers flowing under gravity

!here

D @ Delocity of flow in msec

+ @ 4ydraulic mean depth "#$% @ '8 when pipe is flowing full or half full

S @ Slope of the sewer

n @ )oefficient of roughness for pipes

Minimum .Se!f C!eansing/ )e!ocity
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Sewage should flow at all times with sufficient velocity to prevent the

settlement of solid matter in the sewer. Self )leansing Delocity is the minimum

velocity that ensures non settlement of suspended matter in the sewer.

(he following minimum velocities are generally employed

Sanitary sewer @ 0.> msec

Storm sewer @ :.0 msec

$artially combined sewer @ 0.; msec

Ma0imum 1e!ocity

(he ma3imum velocities in the sewer pipes should not e3ceed more than 7.8msec. (his ma3 velocity in the sewer should not e3ceed this limit of 7.8 msec.

It is to avoid the e3cessive sewer abrasion and also to avoid steep slopes.

Minimum Sewer Si2e

775mm is taken as the minimum sewer siEe. (he reason being that, the choking

does not take place even with the bigger siEe particles, which are usually

thrown into the sewer through manholes.

Minimum Co1er of Sewer

:m is taken as the minimum cover over the sewers to avoid damage from live

loads coming on the sewer.

S"acing of Man3o!e .W,S,4 Criteria/

&or "Sewer SiEe% 775mm to /0mm spacing not more than :00m

&or "Sewer SiEe% 8>0mm to ;>0mm spacing not more than :70m

&or "Sewer SiEe% greater than ;>0mm spacing not more than :50m


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Direction of Sewer %ine

Sewer should flow, as for as possible the Fatural Slope.

Design of Sewer

SiEe of Sewer

Gse the following relation to find the diameter of sewer

?f@ # 3 D

Slope of Sewer

Select the minimum velocity value and use the 2anningBs formula

In1ert %e1e!

(he lowest inside level at any cross-section of a sewer pipe is known as InvertLevel at that )ross-section.

Invert Level @ FSL+oad Level 9 'epth of Sewer 9 (hickness of Sewer 9

'ia. of Sewer

+oints in Sewers
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*ell H Spigot oint

(ongue Hroove oint

Man3o!es

(hese are provided for

)leaning

inspection and

house connection

#t

)hange in Sewer direction

)hange in sewer diameter

)hange in slope

Jne man hole to be provided for 7-8 plots

SEWER,GE DESIGN D,T,

Fo of $lots @ 7/:

Fo of #partments @
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Fo of &lats @

'esign period @ 70 years

$J$GL#(IJF &J+C)#S(

Present (2009) Design (202

Persons/plot ; :0

Persons/apartment 800 >00

Persons/flat 700 800

POPULATION FOR!A"TIN#

$resent $opulation $p@ :% 7/:K;A800KA700K @ ;>;

'esign $opulation $d@ 7% 7/:K:0A>00KA800K @ 5/:0

#nnual rowth +ate @ 7.:1 "&or $akistan, 700/ report%

Design Pop$lation P%

:% $d @ $p 3 ":A7.::00%70

$d @ ;>;3":A7.::00%70 @ 5;0

$d @ 5/:0 " &rom (able%$er capita water consumption @ 50 A 88@ 8 lpcd "liters per capita per day%

#verage 'esign flow @ $d3 water consumption 3 0./ :000

"/01 goes to sewers as waste water%

@ "8 3 5/:0 3 0./ % :000


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&a'g *+,+2 m+/%a-

$eak factor @ 8 "from !#S# table%

To !.e Infiltration rates
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DESIGN OF WET WE%%

?ma3 @ :8;87.:mday @ :0.7; mmin

$umping capacity

$ @ ?ma3 @ :0.7; mmin

1inim$m -le time

2inimum )ycle time must not be less than 5-minutes

&or smaller pumps t min @ :5 min

Dolume @ D @ $ 3 t"min%M8

Cffective Dolume @ " :0.7; 3 :5 % 8 @ /. m

DIMENSIONS OF WET WE%%

Length @ .> m

!idth @ .>m

4eight @ m

Dolume @ .>N.>N @ /.//m
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P$mp m$st r$n for at least 2 min$tes

)heck the cycle time , should be greater than 7 minutes

t @ D"$-?min% @ /. ":0.7; 9 0.>5/% @ .O@8

So 8 minutes is greater than 7 minutes .." JP%

Cyc!e Time for Minimum an ,1erage F!ow

)Q)LC (I2C @ t @ "D"$-?%%A"D?%

&or ?min @ /. ":0.7; 9 0.>5/% A /./0.>5/ @ >8.: min < :5min

"JP%

&or ?avg @ /. ":0.7; -:.7;:% A /.:.7;: @ 8.8/ min < :5min "JP%


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http://civilengineerspk.com/design/design-of-sewer-system/pumping-statiion/


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M,N$O%E
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http://civilengineerspk.com/design/design-of-sewer-system/manhole-2/


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DRO* M,N$O%E

SEWER +OINTS
http://civilengineerspk.com/design/design-of-sewer-system/drop-manhole/


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http://civilengineerspk.com/design/design-of-sewer-system/sewer-joints/


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SEWER 5EDDINGS


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http://civilengineerspk.com/design/design-of-sewer-system/sewer-beddings/


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COMMENTS

(his design is based on partially combined sewerage system thus is

economic.

#ll the necessary things are taken from the !#S# tables and 2inimum

velocity is taken as 0.> msec which is the self cleansing velocity and velocity

must not be more than 7.8 msec.

2inimum diameter of sewer is taken as 775 mm and other diameters are

rounded to the locally available in the market according to !#S# standards.

2inimum rate of sewage flow is taken as 501 of average sewage flow.

2inimum clear cover of :-m is provided above the sewer in order to

avoid from impact of live loading.

&lush tanks are provided where velocity is less than 0.> mSec.

RESU%TS

'iameters are less then >00mm so Infiltration rate used is 51 of average

sewage flow.

*ell H Spigot =oints have been used as the diameters are less then

>00mm.

Jne 'rop 2anhole is coming at 2:5 as the vertical drop is more than

0.>m.

In Sewers "2-2/, 2/-2::, 2:0-2::, 2:7-2:, 25-2:/, 270-

27:, 278-27:, 27:-277, 275-27>%, Delocity is less than the self cleansing

velocity So, &lush tanks will be provided here.

2ost of the diameters are of 775mm ensuring the economic side of the

pro=ect.

!et well dimensions are 7.5K.5K8.8.

)ycle time of :5 minutes is satisfied ensuring the adaptability of small

pumps so more economic.

RECOMMEND,TIONS

&lush tanks should be flushed once in 78 hours to avoid sediment

deposition.


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If the sewer is to be laid under the water table then crushed stone

bedding should be used.

Sewers should be =oined in a manhole keeping the crowns at the same

level.

design of sewer

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