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
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.
Theoretical oxygen demand
Biological oxygen demand
Chemical oxygen demand
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.
Critical time at which minimum dissolved oxygen occurs i.e.
Critical maximum oxygen deficit.
DESIGN OF SEWERAGE SYSTEM AND APPURTENANCES
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.
- 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
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,
Area of flow
Proportional wetted perimeter
Proportional hydraulic radius
Proportional velocity of flow
(Using manning’s formula)
(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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