Design of Sewer and Disposal of Sewage Effluents

By Ashutosh Yadav|Updated : December 2nd, 2021

Through Champion Study Plan for GATE Civil Engineering (CE) 2022, we are providing Design of Sewer and Disposal of Sewage Effluents study notes and other important materials on every topic of each subject.

These topic-wise study notes are useful for the preparation of various upcoming exams like GATE CivilIESBARCISROSSC-JEState Engineering Services examinations and other important upcoming competitive exams.

This article contains fundamental notes on the "Design of Sewer and Disposal of Sewage Effluents" topic of the "Environmental Engineering" subject.

Disposal of The Sewage Effluents

Standards of Dilution for Discharge of Wastewaters into Rivers

  • Standards of Dilution based on Royal Commission Report



  • BIS Standards for Discharge of Sewage and Industrial Effluents in Surface Water Sources and Pub



  • General standards for Discharge of Environment Pollutants from effluents into Surface Water Sources, Public Sewers, and Marine Coasts Under Environment (Protection) Rules, 1986



Dilution and Dispersion



 byjusexamprep The concentration of sewage in mg/lit.

byjusexamprep A flow rate of sewage in m3/sec or lit/sec.

 byjusexamprep The concentration of the river in mg/lit.

byjusexamprep Flow rate (discharge in m3/sec or lit/sec.

 byjusexamprep The concentration of the mixture.


Zone of Pollution in River Stream




 Saturation D.O at 20oC 9.2 mg/lit.

 Saturation D.O at 30oC 7.6 mg/lit.

 Saturation D.O at 0oC 14.6 mg/lit.




·               byjusexamprep 

·               byjusexamprep


 byjusexamprep Theoretical oxygen demand

 byjusexamprep Biological oxygen demand

 byjusexamprep Chemical oxygen demand

 byjusexamprep Ultimate BOD byjusexamprep 

·               byjusexamprep 



·               byjusexamprep 

·               byjusexamprep 

·               byjusexamprep

·               byjusexamprep 

·               byjusexamprep

·               byjusexamprep 


 byjusexamprep D.O deficit in mg/lit after t days.

byjusexamprep Ultimate first stage BOD of the mix at a point of waste discharge in mg/lit.

byjusexamprep Initial oxygen deficit of the mix at the mixing point in mg/lit.

byjusexamprep Reoxygenation constant

byjusexamprep Deoxygenation constant

byjusexamprep Self-purification constant

byjusexamprep Critical time at which minimum dissolved oxygen occurs i.e.

 byjusexamprep byjusexamprep Critical maximum oxygen deficit.









The sewer pipes are laid below the ground level sloping continuously at sufficiently steeper gradient. It is different from water supply conduit as sewage pipes are designed to flow under gravity only. Also, sewage contains lots of suspended particles which may settle down and clog the system. To avoid the clogging, sufficient velocity known as ‘Self cleansing velocity’ is need to be maintained in the system.


2.1.  Important Formulas for Determining Flow Velocity

Following formulas used to determine flow velocities in sewers:

(i) Manning’s formula: The flow velocity is given by


R = Hydraulic radius = A/P

A = Cross sectional area of sewer

P = Wetted Perimeter

S = Ground slope

n = manning’s constant

(ii) Chezy’s formula: The flow velocity is given by


C = Chezy’s constant

2.2.  Design Data

Sewage should be designed for maximum hourly discharge and it should be ensured that velocity of flow will always be greater than self-cleansing velocity.

Maximum hourly discharge = 3 × Average daily discharge

Maximum daily discharge = 2 × Average daily discharge

It is assumed that 80% of water supply goes to sewers.


The self-cleansing velocity can be calculated using the Shield’s formula


G = Specific gravity of particle

dp = Size of particle

K = A constant

R = Hydraulic radius of sewer

n = manning coefficient

2.3.  Circular Sewer running Partially Full



When the sewage is running partially full at depth d such that,



(i) For constant value of manning’s coefficient, the velocity will be maximum when d = 0.81D.

(ii) For constant value of manning’s coefficient, the discharge will be maximum when d = 0.95D.

Equal Degree of Self Cleansing:

For equal degree of self cleansing, the drag force under partial flow should be same as drag force under full flow.



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