TYPES OF FLOW
Dry Weather Flow
1. Domestic waste water
2. Industrial waste water
3. Waste water from public facilities
4. Ground water infiltration
Storm Water Flow
Factors Affecting Dry Weather Flow
Rate of water supply Population growth
(Design period) Type of area served Infiltration of ground water.
Factors affecting infiltration of subsoil water into sewer lines
Depth of sewer below ground water levelSize of sewerLength of sewer through infiltration prone zoneNature and type of soil through which the sewer is
laidType of joints, workmanship etc.Sewer material
Storm water flow
The storm water flow depends upon the following factors
Catchment areaGround slopePermeability of groundExtent of impervious areaExtent of vegetation growthRainfall intensity
Method for estimating storm water flow
Rational method
Q=K*A*I*R Here Q= runoff or storm water flow K= constant A= catchment area in hectares R= intensity of rainfall in mm per hour I= impermeability factor.
Hydraulic Design Of Sewers
1) Chezy’s Formula:
V=C(RS)^0.5
where,
C = Chezy’s coefficient
(depends upon shape, size, roughness of
sewer, etc)
2. Manning’s formula:
V=1/n*R^(2/3)*S^(1/2).
where,
n = manning’s coefficient
(the Value of manning’s coefficient n varies
from 0.01 to 0.05 depending upon material of
sewer line)
By comparing Chezy’s & Manning formula
C=1/n*(R^1/6)
3) 3) Hazen–William’s formulaHazen–William’s formula : :
V=0.85*H*(R^0.63)*(S^0.50).V=0.85*H*(R^0.63)*(S^0.50).
where,where, H is Hazen- William’s coefficientH is Hazen- William’s coefficient (the value of H varies from 100 to 150(the value of H varies from 100 to 150 depending upon sewer line’s material)depending upon sewer line’s material)
Minimum Velocity of flow
Minimum or self cleansing velocity is the minimum
velocity required to prevent deposition & clogging of
sediments.
It depends upon the shape, size & specific gravity of
particles.
Required at least once in a day.
Maximum velocity of flow
The maximum velocity
at which no scouring
action or abrasion takes
place is known as non
scouring velocity
Such a velocity depends
upon the material used
for the construction of
sewers
Material of sewerMaterial of sewer Non scouring Non scouring velocity(cm/sec)velocity(cm/sec)
Earth channelsEarth channels 60-12060-120
Ordinary brick-lined Ordinary brick-lined sewerssewers
150-250150-250
Cement concrete Cement concrete sewerssewers
250-300250-300
Cast iron sewer pipesCast iron sewer pipes 350-450350-450
Vitrified tile and Vitrified tile and glazed bricksglazed bricks
450-500450-500
Rectangular
Horse-shoe
U-shape
Parabolic
1. Circular Sewers
2. Non Circular Sewers :
Forms of sewer
Significance of Circular Sewers
I. Circular section has least perimeter & max R for given area so it is most efficient.
II. It requires minimum quantity of materials, making it economical too.
III. Having uniform curvature, it prevents the possibility of deposition.
IV. They are easily manufactured
Crown Corrosion
H2SO4 DropsAir
H2S
Wastewater
Cement concrete pipes
Measures to overcome Crown Corrosion
1.Ventilating the sewers
2.Operating sewers to full run
3.Pretreatment to reduce sulphate
4.Prohibit entry of sulphides
5.Lining the concrete pipes by vitrified clay blocks.
6.Neutralization of sulphides by chemicals.