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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