Residual chlorine required for normal supply water system:

Ans: a (0.5 mg/1) Ref: Park, 19 ed, p. 575The minimum recommended concentration of free chlorine is 0.5 mg/L at the end of one hour.Tests used for both free and combined chlorine after chlorination are:Orthotoludine test - free and combined chlorine simultaneouslyOrthotoludine arsenite test - tests both separatelyBreak point chlorinationThe point at which the residual chlorine appears and when all combined chlorine have been completely destroyed is the breakpoint and corresponding dosage is breakpoint dosage.Other methods for water disinfectionOzonizationUltraviolet irradiationNote:In disinfection of wells using bleaching powder, contact period of one hour is allowed before the water is drawn for use.Chlorine demand: It is the dose of bleaching powder required for disinfection of water. It is estimated by Horrocks apparatus; The test uses 2.5 grains bleaching powder for 1000 litres.Ideally once every weekIn epidemics it is done once every dayContact period: bacteria/ virus- 1 hourNo effect on spores/ protozoal cyst / ova- needs superchlorinationVirus (exception- polio/ hepatitis A)BRIDGE:1 mg/L of free residual chlorine destroys Hep- A virus in 30 mins.Indicator used in Horrocks apparatus is starch - iodide solution.