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Separate Sanitary and stormwater sewersystems


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Sanitary sewer system


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)ypica!manho!es


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Design of Sewer Network

A sewer system is a network of pipes usedto convey storm runo and/or wastewaterin an area.

The design of sewer system involves thedetermination ofdiameters,

slopes, and

crown or invert elevations for each pipe in thesystem


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Constraints and assumptions

Free surface ow eits for the designdischarges! that is, the sewer system is designedfor "gravity ow#! pumping stations andpressuri$ed sewers should %e avoided as much aspossi%le

The sewers are of commercially availa%lecircular si$es

The design diameter is the smallestcommercially availa%le pipe having owcapacity e&ual to or greater than the designdischarge and satisfying all the appropriate

constraints


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'ewers must %e placed at a depth such thatthey will not %e suscepti%le to frost, will %e a%le to drain %asements, and will have su(cient cushioning to prevent

%reakage due to ground surface loading.

To these ends, minimum cover depths must%e speci)ed.The sewers are *oined at *unctions such thatthe crown elevation of the upstream sewer isnot lower than that of the downstream sewer


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To prevent or reduce ecessive deposition ofsolid material in the sewers, a minimumpermissi%le ow velocity at design dischargeor at %arely full+pipe gravity ow is speci)edTo prevent scour and other undesira%leeects of high+velocity ow, a maimumpermissi%le ow velocity is also speci)edAt any *unction or manhole, the downstreamsewer cannot %e smaller than any of theupstream sewers at that *unctionThe sewer system is a dendritic, or %ranching,network converging in the downstreamdirection without closed loops


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Sewer hydra"!ics

=2

cos12

Dh

=8

sin2 DA

2

DP=

2/1

0

3/2

S

P

A

n

AQ

=

( )0

16.20sin

2/1

0

3/83/2

3/5

=

SD

nQ


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*ain Speci+cations

inimum velocity -self+cleansing velocity 0.1 m/s

aimum velocity 2.3 m/s

inimum pipe diameter 430 mm.

'anitary sewers up to 253 mm diameter should %edesigned to run half full.

6arger pipes may run three+fourths full.


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S"!+de generation

B

P

QS

EBOD

Z = 33.05.00

308.0

2007.1 = TBODEBOD

BODday-5=BOD sewertheofSloe0 =

SDischar!e=Q eri"eter#etted=P

flowofwidth$o=B

, a!"es S"!+de Condition

7 8 3,000 'ul)de rarely generated

3,0008 7 840,000

arginal condition forsul)de generation

7 9 40,000 'ul)de generation common


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Design Steps

Step - . )opographica! map:%tain or develop a map of the contri%uting areaAdd location and level of eisting or proposed detailssuch as;

Contours

uildings 'ewers and other services :utfall point -e.g. near lowest point, net to

receiving water %ody


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Design Steps

Step / . Pre!iminary hori0onta!!ayo"t'ketch preliminary system layout -hori$ontalalignment;

locate pipes so that all potential users canreadily connect into the system

try to locate pipes perpendicular to contours try to follow natural drainage patterns

locate manholes in readily+accessi%le positions

Step 1. Pre!iminary sewer si0ing?sta%lish preliminary pipe si$es and gradients


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Design Steps

Step 2 . Pre!iminary ertica! !ayo"t@raw preliminary longitudinal pro)les -verticalalignment;

?nsure pipes are deep enough so all users can

connect into the system try to locate pipes parallel to the ground surface ensure pipes arrive a%ove outfall level avoid pumping if possi%le

Step 5 . 3eise !ayo"t=evise the hori$ontal and/or vertical alignment tominimi$e system cost %y reducing pipe;

6engths 'i$es depths

design sewer network

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