Residual disinfection

  • Consider residual disinfectant in treated water, since it guards against recontamination up to the point of consumption.

Key indicators

  • WHO: residual concentration of free Cl ≥0.5 mg/L after 30 minutes contact time at pH <8.0.
  • Center for Disease Control Safe Water System (SWS): <2.0 mg/L free Cl 30 minutes after addition of NaOCl, >0.2 mg/L free Cl 24 hours after addition of NaOCl.
  • No WHO health-based guidelines for iodine, silver, chlorine dioxide, dichloramine, or trichloramine.
  • Need to define residual concentration levels for other disinfectants (e.g., bromine).

Notes and exceptions

  • Residual disinfectant levels should remain detectable for the anticipated consumption pattern of the users and the lower volume safe storage capacity of the vessel. (For example, the CDC designed a recommendation value [0.2 mg/L < x < 2.0 mg/L] for use of liquid sodium hypochlorite that reflected the field based assessments of decay-rate patterns of liquid hypochlorite against the anticipated consumption of drinking water within 24 hours.)
  • Residual disinfectant levels should not exceed national or international maximum contaminant levels (MCLs) such as those provided by the World Health Organization or the United States Environmental Protection Agency.
  • “Higher levels of silver, up to 0.1 mg/L (this concentration gives a total dose over 70 years of half the human No Observable Adverse Effect Level (NOAEL) of 10 g), could be tolerated in such cases without risk to health” (pg. 434, WHO, 2006, Guidelines for Drinking Water Quality).
  • “Studies in rats indicate that the effects of iodine in drinking water on thyroid hormone concentrations in the blood differ from those of iodide. Available data therefore suggest that derivation of a guideline value for iodine on the basis of information on the effects of iodide is inappropriate, and there are few relevant data on the effects of iodine…iodine is not recommended for long-term disinfection“ (pg. 389, WHO, 2006, Guidelines for Drinking Water Quality).

Supporting evidence