Test: Quality & Characteristics of Sewage - 1 - Civil Engineering (CE) MCQ

The quantity of settleable solids (S_s) can be determined easily with the help of a specially designed conical glass vessel called Imhoff cone. The capacity of the cone is 1 litre, and it is graduated up to about 50 ml.

Fresh sewage is generally alkaline in nature (with pH more than 7); but as time passes, its pH tends to fall due to production of acids by bacterial action in anaerobic or nitrification processes. The pH however, rises upon treatment of sewage.

pH = -log H⁺
⇒ pH = 3
⇒ H⁺ = 10^-3
⇒ pH = 3
⇒ H⁺ = 10^-5

= 10² times more acidic

COD is the oxygen required to oxidise the total organic matter present in waste water. COD is determined by oxidising a known quantity of waste water with known quantity of strong oxidising agent like K₂Cr₂O₇ or KMnO₄. BOD gives only the total biologically active organic matter present in the sewage.

To ensure 4 ppm of D.O. in sewage to be disposed in river D.O. content is determined by Winkler’s method

The BOD of water during 5 days at 20° generally taken as the standard demand, and is about 68% of the total demand. A 10 day BOD is about 90% of the total.