Drinking water quality

Drinking water must be safe and acceptable. Safe means free of disease-causing organisms and of chemicals at harmful levels; acceptable means clear, without odour or taste, so people do not turn to unsafe sources. Indian requirements are set in IS 10500, which lists acceptable limits and, for some parameters, permissible limits in the absence of an alternative source.

Microbiological quality is the first priority, because waterborne diseases such as cholera, typhoid and diarrhoeal illnesses kill far more people than chemical contaminants. Since testing for every pathogen is impractical, indicator organisms are used: E. coli and total coliforms should not be detectable in a 100 mL sample of drinking water. Physical parameters include turbidity, colour, taste and odour. Chemical parameters cover pH, total dissolved solids, hardness, alkalinity, chloride, sulphate, nitrate, iron and manganese, plus toxic substances such as fluoride, arsenic, lead, mercury, chromium and pesticides. Fluoride and arsenic are major natural groundwater problems in parts of India, and nitrate from fertilisers and sewage is rising.

Water safety planning manages risk from catchment to tap: protecting sources, providing multiple treatment barriers, maintaining pressure in distribution to stop contamination entering, and monitoring residual chlorine at consumer ends. Sampling frequency increases with population served. Laboratories should be accredited and results acted on.

Treatment is chosen to match the problem. Turbidity and pathogens call for coagulation, filtration and disinfection. Hardness can be reduced by lime-soda softening or ion exchange. Iron and manganese are removed by aeration and filtration. Fluoride needs activated alumina, bone char or the Nalgonda technique at small scale, and arsenic needs adsorption or coagulation-based removal with safe sludge disposal. Reverse osmosis removes most dissolved salts but wastes water and needs remineralisation.