Examination of discrete and counfounding effects of water quality parameters during the inactivation of MS2 phages and Bacillus subtilis pores with free chlorine

Abstract

The role of water quality (pH, temperature, turbidity, and natural organic matter (NOM)) on the efficacy of chlorine to inactivate Bacillus subtilis spores and MS2 phages was investigated in synthetic waters. A half-factorial statistical experimental design was employed to reduce the number of assays. Temperature and pH were found to be by far the most important variables in explaining chlorine efficacy, that is 78% of the variability in the experimental domain studied (pH, 6.5–8.5; temperature, 7–22 °C; dissolved organic carbon (DOC), 0–5 mg/L, 0–5 nephelometric turbidity units (NTUs)). The interaction between pH and temperature was also significant, being the third most important factor, explaining 11% of the variability. For both MS2 phages and B. subtilis spores, hypochlorous acid efficacy was less sensitive than hypochlorite ions to a modification of water temperature. Such effect is not taken into account in the current United States Environmental Protection Agency (USEPA) concentration–time (Ct) tables. Once the consumption of disinfectant by natural organic matter (NOM) was taken into account, the presence of NOM had either no impact on MS2 phage inactivation or significantly improved B. subtilis spore inactivation. The role of inorganic turbidity was either not statistically significant or negligible. Little or no protection was therefore provided by kaolin particles, up to a concentration of 5 mg/L ([Formula: see text]5 NTU).Key words: drinking water, disinfection, spores, MS2 coliphages, chlorine, water quality, turbidity, natural organic matter.