# ENVIRONMENTAL ENGINEERING :Disposal of sewage effluents and design of sewer system Notes

By Deepanshu Rastogi|Updated : April 19th, 2021

## Disposing 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,

CS = The concentration of sewage in mg/lit.

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

CR= The concentration of the river in mg/lit.

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

Cmix = 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,

TOD= Theoretical oxygen demand

BOD= Biological oxygen demand

COD= Chemical oxygen demand

(BOD)U = Ultimate BOD (Yu

•

Stretcher-PHELPS EQUATION

•
•

Where,

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

L= 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

f=Self-purification constant

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

Critical maximum oxygen deficit.

HYDRAULIC DESIGN OF SEWERS

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

V =C (RS)0.5

Where,

C = Chezy’s constant

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

Circular Sewer running Partially Full

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

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.

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