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Design the dimension of a septic tank for a colony of 200 persons provided with an assured water supply from municipal head works at a rate of 100 litres per person per day. Assume any data if require
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Given

Population = 200 persons

Sewage/capita/day = 100 litres

  • Quantity of sewage produced= population X per capita

= 200 X 100

= 20000 lit/day

Assuming 80% of water becomes sewage.

Therefore, Quantity of sewage= 20000 X 80/100

= 16000 lit/day

  • Assuming detention period = 24 hours.

Therefore, quantity of sewage = 16000 X 24/24

During detention period

= 16000 litres

  • Assuming rate of sludge deposited = 30lit/capita/day & sludge periods =1year.

Therefore, volume of sludge

= rate of sludge deposited X population X sludge period

= 30 X 200 X 1

= 6000 litres.

Therefore, required capacity of tank = 16000 + 6000 = 22000 litres.

Or = 22m^3

Assuming depth of tank = 1.5m (Range= 1.5 to 2.5m)

Therefore, surface area available = 22/1.5

= 14.667 m^2

Assuming length & width ratio i.e. (L: B = 4:1)

Therefore, surface area of tank = L X B

14.667 = 4B X B

B = 1.917m

Therefore, L = 4B

L = 4X1.914

L = 7.65m

Area of cross-section = L X B

= 7.65X1.914

= 14.66 m^2 ok

  • Therefore surface area of tank calculated above or equal to the 14.667 m^2 which is satisfy by length width ratio.

  • Assuming free board surface 0.3m.

Therefore, depth (d) = 1.8m

Therefore, a septic tank of size 7.65mX1.917mX1.8m

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