COMBINE SEWER DESIGN

Prepared by : Mujtaba “Ahmadi” 2012-CIV-240

Abstract In this design basic design procedure for selection of suitable sewer diameter , having reasonable

slope , excavation depth for laying of sewer and some other important parameter have been

discussed .

2012-CIV-240

Design Assignment

Introduction:

The land encompasses an area of 59,236 m2 (14.6 acre) and have the following characteristic:

Number of houses = 281

Number of flats = 3

Number of apartments = 3

Number of parks = 2

School = 1

Dispensary = 1

Graveyard = 1

For more detail refer to the attached map.

Objectives:

To find a suitable layout for disposal of waste water.

To find diameter of pipes for each length across sewerage system.

To find required slope for laying of pipes.

To plot longitudinal section of ground level and invert level of sewers from primary

disposal (laterals) to the last disposal station.

To find depth for excavation.

Related theory:

Introduction to sewerage system:

The waste water trip starts from fixture. Fixtures are the devices that draws clean water

from the distribution system and discharge waste water in to the sewage system ,without

allowing any flow in the opposite direction . The most common residential plumbing fixtures

are:

Toilets

Shower

Bath tube

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Sinks

Washing machines

The purpose of house plumbing system is to capture all the waste water from these fixtures and

combine it to a single stream for disposal .

This stream is composed of over 99.9 % water .The number seems quite surprising until we

recognize that combine effluent from sinks , tubes, washing machine far exceeds that of the

toilets .

The remaining 0.1 % is suspended and dissolved solids which include:

Dissolved organic matter :which include both human waste and food.

Suspended inorganic particles

Rags ,papers and plastic and other materials

And high concentration of enteric micro-organisms

These are the materials in which all the sewer system is designed to remove.

For removal of these materials proper pipe diameter and slope maintenance is the basic

objective of sewer system design .

And material is sent for centralized treatment or onsite septic system .

Waste water primarily operates by gravity-driven flow. So the treatment plant are located at

lowest points of the area .

The central spine of residential sewerage system is a large diameter vertical pipe called soil

stack ,extending from the roof to the down base slab .Which is connected to a gentle slope pipe

known as lateral , and lateral moves toward sewage system or septic tanks whatever is designed

.All the pipes which connect the fixture to the stack should have a gentle slope.

The slope of sewer pipes should be designed such that all the waste should be removed

efficiently as in case of more slope solids will remain and water will erode the pipe ,however in

case of lesser slope water won’t move downward so a specified and reasonable slope should be

adopted .

If topography prevents downhill gradient a sewage lift station should be installed to pump the

waste water to a higher elevation at which point it resumes its gravity for downhill flow .

Manholes are provided to give access for repair and maintenance of the pipe system (sewer).

Manhole is a vertically oriented concrete cylinder typically located where under pipes intersects

with another sewer line or changing direction , slope or diameter of pipe.

Drinking water purification rely more on mechanical and chemical process however waste water

treatment rely more on biological process .

2012-CIV-240

Sewerage system:

Sewerage is the system of collection of wastewater and conveying it to the point of disposal with

or without treatment. Essential components of sewerage system is :

1.Collection System Network of Sewer pipes

2.Disposal Sewage Pumping Stations and Outfalls

3.Treatment Works Wastewater treatment Plants

Source of sewage:

1.Domestic: 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

Type of sewer and sewer system:

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

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

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Types of sewer system:

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.

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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

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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)

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Note:

•Sanitary waste water is not allowed to discharge in any stream.

•When there is storm water inside the sewer, some portion of sanitary sewer might go, however

the effects would be less since due to dilution.

Infiltration :

•It is the waste water that enters sewers through joints, cracked pipes, walls

•Infiltration is almost non-existent in dry weather but increases during rainy season

Pipe dia. Up to 600mm 5% avg. Sewage flow

For greater than 600mm 10% avg. Sewage flow

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Procedure:

First of all a surveying should be conducted to find the elevation of points at which sewer

is going to be laid.

Sewer grid should be assigned in such a way so that flow come in a systematic and

regular manner from lateral ,to sub man , main and disposal station.

Disposal stations should be selected at lowest elevation point so that maximum gravity

effect should be utilized and direction of flow is toward disposal station.

After plotting the sewer line in a scaled map , their length should be measure with scale

and correspondingly should be used in calculation.

We allocate the population for each pipe, so that waste water according to defined

population should be discharged in a pipe.

We calculate sewage for each person then multiply with population, we get average

sewage flow.

After multiplying average sewage flow with peak factor we get max or peak sewage

flow.

For combine sewer storm flow is equal to peak flow

Then we calculate design flow from 𝑄𝑑 = 2 ∗ 𝑄𝑝 + 𝑖𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑖𝑜𝑛.

Using Q=V*A we can find diameter of the pipe to be used and we round it to higher

commercially available diameter.

Based on V=0.7 m/s and selected diameter we can calculate slope using Maning’s

formula.

We can calculate imaginary full flow for selected diameter as original flow is less that Q

full.

Actual depth of water and actual velocity of flow can be found from graphs having

relation between full and half full parameters.

As we have ground level corresponding to start and end of each sewer so we can find the

difference in elevation between these two heads of sewer line.so fall = upper ground

level-lower ground level.

At last we find invert level of pipes using following equations:

Upper invert level= Ground level-earth cover-thickness of pipe-dia of pipe.

Lower invert level=upper invert level-(length * slope)

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For more clarity one sewer M1M2 design detail is shown below:

Measured length =72 m

30 plot discharge their waste in M1M2 as each plot has population of 10 persons so total

population is 300 persons.

Per capita water consumption is 350+roll number ( l/c/d ) =350+240 (l/c/d)=0.59

m^3/day

𝑄𝑎𝑣𝑔 = 𝑝𝑜𝑝𝑢𝑙𝑎𝑡𝑖𝑜𝑛 ∗ 𝑝𝑒𝑟 𝑐𝑎𝑝𝑖𝑡𝑎 𝑤𝑎𝑠𝑡𝑒 𝑤𝑎𝑡𝑒𝑟 = (300 ∗ 0.59) ∗ 0.85 =

150.45 𝑚3/𝑠

𝑄𝑝 = 4 ∗ 𝑄𝑎𝑣𝑔 = 4 ∗ 150.45 = 601.8𝑚3

𝑑𝑎𝑦

𝑄𝑠𝑡𝑜𝑟𝑚 = 𝑄𝑝𝑒𝑎𝑘 = 601.8𝑚3

𝑑𝑎𝑦

𝑖𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑖𝑜𝑛 = 0.1 ∗ 𝑄 𝑎𝑣𝑔 = 0.1 ∗ 150.45 = 15.045𝑚3

𝑑𝑎𝑦

𝑄𝑑 = 2 ∗ 𝑄𝑝 + 𝑖𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑖𝑜𝑛 = 2 ∗ 601.8 + 15.045 = 1218.645𝑚3

𝑑𝑎𝑦

𝑉𝑓𝑢𝑙𝑙 = 0.7𝑚

𝑠 𝑓𝑜𝑟 𝑡ℎ𝑖𝑠 𝑐𝑎𝑠𝑒

Using 𝐷 = (𝑄

𝑉) = √(

1218.645

0.7∗24∗60∗60) ∗

4

𝑝𝑖= 0.160𝑚 =

160𝑚𝑚 𝑏𝑢𝑡 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 𝑚𝑖𝑛𝑖𝑚𝑢𝑚 𝑑𝑖𝑎 𝑖𝑠 225 𝑚𝑚 𝑠𝑜 𝑤𝑒 𝑠𝑒𝑙𝑒𝑐𝑡 225𝑚𝑚

Using Maning’s equation 0.7 = (1

0.13) ∗ (

.225

4)

2

3∗ 𝑆 so S =3.842*10^(-3)

Using 𝑄𝑓𝑢𝑙𝑙 = 0.7 ∗𝑝𝑖

4∗ 02252 = 2401.92

𝑚3

𝑑𝑎𝑦

Upper invert level 100 − 1 − 0.05 − 0.225 = 99.422 𝑚

Lower invert level𝑢𝑝𝑝𝑒𝑟 𝑖𝑛𝑣𝑒𝑟𝑡 𝑙𝑒𝑣𝑒𝑙 − 𝑝𝑖𝑝𝑒 𝑙𝑒𝑛𝑔𝑡ℎ ∗ 𝑠𝑙𝑜𝑝𝑒 = 99.422 − 72 ∗ 3.842 ∗

10−3 = 99.145𝑚

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Design criteria

Design period: 75 years

Per capita water consumption =350+Roll number L/c/d which is equal to 350+240=0.59

m^3/day.

And sewage flow is taken as 85% of water consumption.

Peak factor is denoted by M and is the ratio of maximum flow to average flow. In this

design M=4

Roughness coefficient is n=0.013

Thickness of the pipes assumed as 50 mm

Earth cover assumed as one meter.

Population for each plot, flat and apartment assumed as 10,500,800 persons respectively.

Design equations that are used in this job are:

o 𝑄𝑎𝑣𝑔 = 0.85 ∗ 𝑤𝑎𝑡𝑒𝑟 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛

o 𝑄 𝑝𝑒𝑎𝑘 = 4 ∗ 𝑄 𝑎𝑣𝑔

o 𝐼𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑖𝑜𝑛 = 0.1 ∗ 𝑄𝑎𝑣𝑔

o 𝑄 𝑠𝑡𝑟𝑜𝑚 = 𝑄 𝑝𝑒𝑎𝑘

o 𝑄 𝑑𝑒𝑠𝑖𝑔𝑛 = 2 ∗ 𝑄𝑝 + 0.1 ∗ 𝑄 𝑎𝑣𝑔

o V full is 0.7 m/s for combine sewer system.

o 𝐴 = 𝑉/𝑄𝑑

o Diameter should be rounded to commercial available diameter.

o 𝑉 = (1

𝑛∗ (

𝐷

4)

2

3∗ 𝑆

1

2 ) from this formula slope is obtained. (where D is rounded

diameter in this formula.)

o After rounding pipe diameter to commercial available dia the level of water in

pipe decreases .we can find full discharge corresponding to full flow in

commercial dia 𝑄𝑓𝑢𝑙𝑙 = 𝑉𝑓𝑢𝑙𝑙 ∗ 𝐴𝑟𝑒𝑎

o Now find ratio of 𝑄𝑑/𝑄𝑓𝑢𝑙𝑙

o From graphs which is between Qd/Qf ,y/D, V/V full ,having the value of

Qd/Qfull we can find V/Vf and corresponding y/D values.

o Having full velocity =0.7 m/s we can find actual velocity from velocity ratio.

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o And corresponding actual depth of water can be found from graph.

Conclusions and comments:

o All our objectives regarding design which was mentioned like :

o To find a suitable layout for disposal of waste water.

o To find diameter of pipes for each length across sewerage system.

o To find required slope for laying of pipes.

o To plot longitudinal section of ground level and invert level of sewers from

primary disposal (laterals) to the last disposal station.

o To find depth for excavation.

Was achieved.

The difference between M1M2 upper invert level and M19M25 lower invert

level is 4.837 meters which shows that slope is very gentle in this long length

indicating that solid waste will always float in water .

Small variation while calculating length may occur.

Manholes should be provided in case of change of direction and slope.

As area is small activated sludge process is suggested for its treatment.

As we observe location of treatment plant is at lower elevation indicating that this

is the best location of treatment plant, as it should be in lowest part of area.

Ground level between two ends of sewer is assumed as straight.

Reference:

Terence J, M., & E.W. Steel. (1990). Water Supply and Sewerage (1 ed.). McGraw-Hill Companies.

Department, T. R. (2012, February 22). How the sewer system works. Retrieved from Massachusets

Water Resources Authority: www.mwra.com