Importance of residual chlorine

- May 22, 2018-

The SWS Program recommends testing free chlorine in two circumstances:

  • To conduct dosage testing in project areas prior to the start of a program.

  • To monitor and evaluate projects for chlorination compliance by testing stored water in households.

The goal of dosage testing is to determine how much chlorine (sodium hypochlorite solution) to add to water that will be used for drinking to maintain free chlorine in the water for the average time of storage of water in the household (typically 4-24 hours). This goal differs from the goal of infrastructure-based (piped) water treatment systems, whose aim is effective disinfection at the endpoints (i.e., water taps) of the system: defined by the WHO (1993) as: “a residual concentration of free chlorine of greater than or equal to 0.5 mg/L (0.5 ppm or parts per million) after at least 30 minutes contact time at pH less than 8.0.” This definition is only appropriate when users drink water directly from the flowing tap. A free chlorine level of 0.5 mg/L of free chlorine will be enough residual to maintain the quality of water through the distribution network, but is most likely not adequate to maintain the quality of the water when this water is stored in the home in a bucket or jerry can for 24 hours.

Thus, the SWS Program recommends for dosage testing that:

  • At 30 minutes after the addition of sodium hypochlorite there should be no more than 2.0 mg/L of free chlorine present (this ensures the water does not have an unpleasant taste or odor).

  • At 24 hours after the addition of sodium hypochlorite to containers that are used by families to store water there should be a minimum of 0.2 mg/L of free chlorine present (this ensures microbiologically safe water).

The SWS Program methodology leads to free chlorine levels that are significantly lower than the WHO guideline value for free chlorine in drinking water, which is 0.5 mg/L value. The SWS Program recommends testing free chlorine in homes of SWS users for evaluation of whether or not users are using the system and if they are using it correctly. Households can be visited and ‘spot checked’ to determine if, and how much, free chlorine is present in their drinking water. This approach is very useful for program monitoring because the presence of free chlorine in stored water obtained from an unchlorinated source is an objective measure that people are using the hypochlorite solution.