Waste water engineering

♦️ Septic tank

👉 Septic tanks are used in unsewered areas.

👉 Septic tanks are best suitable if the number of users are less than 300.

👉 Septic tank undertakes process anaerobic digestion of solids

👉 The BOD5 of the effluent obtained from septic tank is of order of 100 - 200 mg/l

👉 Detention time :- 24 to 48 hrs.

👉 Rate of sludge accumulation :- 30 to 40 l/person/year

👉 Length to width ratio :- 2 to 3

👉 Depth :- 1.2 to 1.8 m

👉 Free board :- 0.3 to 0.5 m

👉 Cleaning period :- 6 month to 1 year

♦️ Imhoff tank (Two storey digestion tank)

👉 It is improvement over septic tank.

👉 If Number of users are more than 300, Imhoff tank is preferred.

👉 Imhoff tank is a two compartment

~ upper compartment :- Settling of solids

~ lower compartment :- anaerobic digestion of settled solid.

👉 Detention time :- 2 to 4 hrs (usually 2hr)

👉 Depth :- 9 to 11 m

👉 L/B ratio :- 3 to 5 (L
 ≯ 30 m)

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