Design of Septic Tank Along With Soak Pit For 30 Users - Civil Engineering

Tuesday, 14 May 2019

Design of Septic Tank Along With Soak Pit For 30 Users

Design of Septic Tank Along With Soak Pit For 30 Users

Design of septic tank with soak pit for 30 users are as follows:

Design of Septic Tank

Assume, rate of water supply = 135 lit/capita/day. 
Quantity of water supplied = Per capita rate ⨯ Population = 135⨯30 = 4050 lit/day.

Assuming that 80% of water supplied becomes sewage, we have
Quantity of sewage produced = 4050⨯0.8 = 3240 lit/day.

Assuming detention period = 18 hours.
Quantity of sewage produced during the detention period = 3240⨯(18/24) = 2430 litres.
Assuming the rate of deposited sludge = 30 lit/capita/year & cleaning period = 2 years.
The volume of sludge deposited = 30⨯30⨯2 = 1800 litres.
The total required capacity of the tank = Capacity for sewage + Capacity for sludge = 2430 + 1800 = 4230 litres = 4.23 Cu.m

Provide depth of the tank = 1.5m.
The surface area of the tank = 4.23/1.5 = 2.82m.
Provide the ratio of length(L) to width(B) = 3: 1
So, 3B² = 2.82 or, B = 0.97 m.

Provide width of the tank = 1.0m and length of the tank = 3⨯1.0 = 3.0m. 
Provided freeboard = 0.3m, so overall depth of the tank =1.5+0.3 = 1.8m.
Hence, use a tank of size 3.0m⨯1.0m⨯1.8m

Design of Soak Pit

Assume, percolating capacity of the filtering media = 1250 lit/Cu.m/day.
Quantity of sewage produced = 3240 lit/day.

Volume (of filtering media) required for soak pit = 3240/1250 = 2.592 Cm.m.
Provided depth of soak pit 2.0m.

Area of soak pit required = 2.592/2.0 = 1.296m.
Therefore dia of soak pit =√{(1.296⨯4)/π = 1.28m = 1.30m(say).
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