Environmental Engineering : disposal of sewage effluents and design of sewer system and appurtances

By Aditya Kumar|Updated : September 13th, 2021

                                                                                         

 

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

 

 Where,

  The concentration of sewage in mg/lit.

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

  The concentration of the river in mg/lit.

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

  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.

 

  

·                

·               

 Where,

  Theoretical oxygen demand

  Biological oxygen demand

  Chemical oxygen demand

  Ultimate BOD  

·                

 

Stretcher-PHELPS EQUATION

·                

·                

·               

·                

·               

·                

 Where,

  D.O deficit in mg/lit after t days.

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

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

 Reoxygenation constant

 Deoxygenation constant

 Self-purification constant

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

   Critical maximum oxygen deficit.

 

 

 

 

                       DESIGN OF SEWERAGE SYSTEM AND APPURTENANCES

        1.INTRODUCTION

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.

  1. HYDRAULIC DESIGN OF SEWERS

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

Where,

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

Where,

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

Where,

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,

Proportional depth

Area of flow

Proportional area

Wetted Perimeter

Proportional wetted perimeter

Hydraulic radius

Proportional hydraulic radius

Proportional velocity of flow

                                      (Using manning’s formula)

Proportional discharge

Note:

(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.

The proportional velocity in the above case will be equal to

If, the slope of both the sewer is same,

r = R

This is possible only when the sewer is running either half full or completely full.

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Aditya Kumar
Aditya KumarMember since Feb 2021
B-Tech(Civil Engg) NIT Allahabad
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Dr Lbn Bhat

Dr Lbn BhatMay 30, 2021

At last missing content

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