ANSWER: (A) 0.5 mg/LREF: Park 20th edition page 625Minimum recommended concentration of free chlorine is 0.5 mg/L for one hour.ALSO KNOW:Most effective and cheapest method of chlorination of wells in rural India is by bleaching powder (chlorination)Bleaching powder contains 33 % of chlorine content, is unstable with rapid and brief action.PERCHLORON (High test hypochloride) is a calcium compound which contains 60-70 % available chlorineChlorine demand of well wTater is estimated by Horrock's Apparatus.Roughly 2.5 gms of bleaching powder is required to disinfect 1000 lit of water.One tablet of chlorine (0.5 gm) can disinfect 20 liters of waterChlorine acts best as a disinfectant wThen the pH of water is around 7 because of the predominance of hypochlorous acid.This wdll give an approximate dose of 0.7 mg of applied chlorine per liter of water.A contact period of 1 hour is allowed before the wTater is drawn for use.At the end of 1 hour the residual chlorine is tested by "ORTHOTOLIDINE ARSENITE TEST" (modification of Orthotolidine test) Orthotolidine test = 0.1 ml of reagent + 1 ml of water (read after 10 sec) Orthotolidine reagent = Orthotolidine + 10% solution of HCLIf free residual chlorine level is less than 0.5mg/litre, the chlorination procedure should be repeated before any water is drawnDose of chlorine = chlorine demand + residual chlorine (0.5mg/lit)WATER QUALITY CRITERIA'S & STANDARDS:Turbidity < 5 Nephelometric turbidity unit (NTU)Color <15 True color unit (TCU)pH 6.5 to 8.5Total dissolved solids (TDS): Good palatability < 600 mg/LAcceptable palatability <1000 mg/LUnpalatable > 1200 mg/LChloride: Standard < 200 mg/LMaximum < 600 mg/L