Simplified sewerage is an important sanitation option in peri-urban areas of developing countries, especially as it is often the only technically feasible solution in these high-density areas.
Domestic waste water Industrial waste water Waste water from public facilities Ground water infiltration
Storm Water Flow
2. 3.
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 level Size of sewer Length of sewer through infiltration prone zone Nature and type of soil through which the sewer is
laid Type of joints, workmanship etc. Sewer material
Storm water flow The storm water flow depends upon the following factors Catchment area Ground slope Permeability of ground Extent of impervious area Extent of vegetation growth Rainfall 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) Hazen–William’s formula : V=0.85*H*(R^0.63)*(S^0.50). where, H is Hazen- William’s coefficient (the value of H varies from 100 to 150 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
Material of sewer
Non scouring velocity(cm/sec)
action or abrasion takes
Earth channels
60-120
place is known as non
Ordinary brick-lined sewers
150-250
at which no scouring
scouring velocity Such a velocity depends
upon the material used for the construction of sewers
Cement concrete sewers 250-300 Cast iron sewer pipes
350-450
Vitrified tile and glazed 450-500 bricks
Forms of sewer 1. Circular Sewers 2. Non Circular Sewers :
Horse-shoe
Rectangular
U-shape
Parabolic
Significance of Circular Sewers I. II. III. IV.
Circular section has least perimeter & max R for given area so it is most efficient. It requires minimum quantity of materials, making it economical too. Having uniform curvature, it prevents the possibility of deposition. They are easily manufactured
Crown Corrosion Air
H2SO4 Drops
H2S Cement concrete pipes
Wastewater
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.
