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Gases, which are generally evolved during aerobic decomposition of sewage, are:
Imhoff cone is used to measure, in sewage:
The quantity of settleable solids (Ss) 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.
The pH of fresh sewage is usually
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 = 3, when compared to pH = 5, will be more acidic by
pH = -log H+
⇒ pH = 3
⇒ H+ = 10-3
⇒ pH = 3
⇒ H+ = 10-5
= 102 times more acidic
Chemical oxygen Demand (COD) of sewage is the
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.
Minimum D.O. prescribed for a river stream, to avoid fish kills, is
To ensure 4 ppm of D.O. in sewage to be disposed in river D.O. content is determined by Winkler’s method
BOD5 represents 5 days-biochemical oxygen demand at a temperature of
Standard 5 day BOD at 20°C, when compared to ultimate BOD, is about
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.