Question

Design a rectangular sedimentation tank to treat 2.4 million litres of raw water per day. The detention period may be assumed to be 3 hours. Find overflow rate?

(a) 28,000 l/d/m^2

(b) 12,000 l/d/m^2

(c) 24,000 l/d/m^2

(d) 4,670 l/d/m^2

I had been asked this question during an interview for a job.

Question is taken from Principle for the Preparation of Water Supply topic in section Valuation, Reports Technical and Design Data of Civil Engineering Drawing

Answer 1

Correct answer is (c) 24,000 l/d/m^2

Easy explanation: Raw water flow per day is 2.4 x 106 l. Detention period is 3h.

Volume of tank = Flow x Detention period = 2.4 x 103 x 3/24 = 300 m^3

Assume depth of tank = 3.0 m.

Surface area = 300/3 = 100 m^2

L/B = 3 (assumed). L = 3B.

      3B2 = 100 m^2 i.e. B = 5.8 m

       L = 3B = 5.8 X 3 = 17.4 m

Hence surface loading (Overflow rate) = 2.4 x 106 = 24,000 l/d/m^2 < 40,000 l/d/m^2.