Residual chlorine required for normal supply of water system in order to disinfect water born pathogen is

Ans) a (0.5 mg/L) Ref Park 20th Edn P625The minimum recommended concentration of free chlorine is 0.5 mg/L, for one hour.Tests used for both free and combined chlorine after chlorination isBreak point chlorination- the point at which the residual chlorine appears and when all combined chlorine has been completely destroyed is the break point and the corresponding dosage in break point dosage.Other methods of water disinfectionOzonisationUltra violet irradiationNote- In disinfection of wells by using bleaching powder contact period of one hour is allowed before water is drawn for use.Chlorine demand-the dose of bleaching powder required for disinfec-tion of water. It is estimated by Horrock's apparatus.2.5 gm for 1000Ltrs,0.7 mg for 1 Ltr. Ideally once every week, in epidemic once every day. Con-tact period of 1 hour is essential to kill the bacteria and viruses. But chlo-rine has no effect on spores, protozoal cyst, helminthic ova, needs superchlorination. All viruses except Polio and hepatitis A are killed, these killed, by super chlorination.Indicator used in Horrock's apparatus-Starch -iodide solution. Orthotoludine test-detects both free and combined chlorine simultaneously. OTA test-free and combined chlorine separately