Standard Operating Procedure


Public drinking water supply systems are mandated to provide safe and adequate drinking water to serving communities. Consumption of water contaminated with pathogenic microorganisms is one of the leading causes of child mortality globally. Hence major emphasis is given on removing or killing such microorganisms from water used for public consumption.

One way of ensuring water safety is to disinfect water supplied through distribution lines. Residual protection of treated water supplied to long distances is a major concern in intermittent water supply schemes. All Piped Water Supply Schemes (PWSSs) in the State operationalize in a discontinuous mode, which makes it vulnerable to re-contamination, unless there is sufficient disinfectant residual maintained in treated water till it reaches consumer end.

Free Residual Chlorine concentration in water sample collected from nearest street standpost (SSP) – Samuktala PWSS, Alipurduar
Pump Operator measuring Free Residual Chlorine at pump house of Groundwater Based Water Supply Scheme for Gun Sankrul and Adjoining Mouzas, Malda

The method of choice for disinfecting water for human consumption depends on a variety of factors (Symons et al., 1977). These include:

  • its efficacy against waterborne pathogens (bacteria, viruses, protozoa, and helminths);
  • the accuracy with which the process can be monitored and controlled;
  • its ability to produce a residual that provides an added measure of protection against possible post-treatment contamination resulting from faults in the distribution system;
  • the aesthetic quality of the treated water;
  • the availability of the technology for the adoption of the method on the scale that is required for public water supplies.

Chlorination is the most popular method of disinfection in countries like India which may be attributed to its convenience and satisfactory performance as a disinfectant that also satisfies the above listed conditions. As stated in Drinking Water and Health (National Academy of Sciences, 1977), "chlorination is the standard of disinfection against which others are compared. “Calcium Hypochlorite is widely used to disinfect water supplied through PWSSs in rural areas by West Bengal.

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