AWWA WQTC60692

Treating drinking water at the treatment plant effluent to meet regulatory requirements does not necessarily ensure high quality water at the consumer's tap. During distribution, water quality degrades, which may lead to increased bacterial counts with possible coliform occurrences and depletion of disinfectant residual. The maintenance of a disinfectant residual in distribution systems has traditionally been used to protect microbial water quality. It is necessary to understand the reasons for the presence and survival of microorganisms in distribution systems despite the continuing presence of disinfectant residuals. The overall aim of this benchscale study was to quantify differences in the inactivation kinetics of E. coli O157:H7 and MS 2 phages in the presence of particulate matter, and compare those kinetics to the kinetics obtained in parallel experiments in pure water. The types of particles that were used in the experiments included iron pipe corrosion debris (at 50 mg/L), soil contamination to simulate a pipe break (at 50 and 200 mg/L), and municipal wastewater solids to simulate cross-contamination (added to reach a water turbidity of 2.0 NTU). A soil sample was collected adjacent to a drinking water pipe in Toronto. Corrosion debris were produced by suspending a metal coupon in a flask filled with tap water, and incubating it for a week. Raw wastewater was collected from a Toronto wastewater plant and sterilized by autoclaving. The inactivation kinetics experiments were conducted in 250-mL flasks at 21°C, with agitation to inactivate planktonic microorganisms. The disinfectants tested were free chlorine and monochloramine. Inactivation was measured as the log₁₀ (N_t / N₀). Ct values were calculated by integration of the disinfectant residual concentration (C) up to the given sampling time (t). Statistical comparisons between the slopes of linear regressions of inactivation data for chlorine and monochloramine inactivation in control reactors or in the presence of corrosion debris, soil, or wastewater were performed to determine if the two data sets were statistically equivalent within a 95 percent confidence level (t-test). The results for E. coli O157:H7 show that the inactivation kinetics with monochloramine in the presence of wastewater debris was significantly different from the inactivation kinetics in control reactors, which suggests that the presence of wastewater debris had a negative (and statistically significant) impact on monochloramine efficacy. In all other cases with E. coli O157:H7 (including with chlorine), there was no significant difference between inactivation in control water versus inactivation in water containing debris, suggesting that corrosion debris and soil particles (at the tested concentrations) did not have an impact on disinfection. Similar results were found with MS 2 phages; in other words, the presence of particulate matter had no effect on phage inactivation, regardless of which disinfectant was tested. In conclusion, it appears that particulate matter does not have a significant impact on the inactivation of freely suspended E. coli O157:H7 cells and MS 2 phages, with the exception of the effect of wastewater debris on the monochloramine inactivation of E. coli. Particulate matter exerted a demand on the disinfectant residuals (especially chlorine); however, when the results were normalized using Ct, the inactivation kinetics of the various treatments were identical. Includes 3 references, tables.