Effect of disinfectant residual, pH, and temperature on microbial abundance in disinfected drinking water distribution systems

Abstract

In piped drinking water distribution systems, microbial water quality depends on the quantities and types of microorganisms present as well as the physicochemical conditions that influence them. Accurately assessing microbial water quality in these systems is important to maintain water quality throughout distribution. Microbial water quality can be assessed directly, using measures of microbial abundance, and indirectly, by measuring pH, temperature, and disinfectant residual. In the United States, total coliform bacteria measurement is the only regulated parameter for microbial abundance, but because levels are required to be maintained below the quantification limit, this parameter provides little insight into the total microbial abundance. In this study, alternate measures of microbial abundance were assessed in six drinking water distribution systems with a wide range of free chlorine (<0.02 to 2.14 mg L⁻¹ as Cl₂) and total chlorine residuals (<0.02 to 2.9 mg L⁻¹ as Cl₂). Five measures of microbial abundance were compared for quantifiability and variability throughout distribution: total and intact cell counts, total and intracellular ATP concentrations, and heterotrophic plate counts. We found that: intracellular ATP and intact cell counts had significant and strong correlations with disinfectant concentration; these correlations were stronger in chlorinated systems compared to chloraminated systems; 97.6% of samples had a quantifiable intact cell counts, with only four samples below the intact cell count quantifiaction limit at the highest residual concentration in chlorinated drinking water distribution systems (1.5–2.0 mg L⁻¹ as Cl₂); and variance between technical replicates was lowest for total ATP followed by total and intact cell counts, as compared to heterotrophic plate counts and intracellular ATP. We also demonstrated that a generalized linear mixed model could be used to estimate the combined effect of common water quality parameters on intact cell counts in a chloraminated distribution system: total chlorine had the greatest inverse effect on intact cells with a greater positive effect of temperature at lower levels of total chlorine. We discuss the purpose and interpretation of typical microbial water quality parameters, such as heterotrophic plate count and disinfectant residual, and consider the future role of ATP and flow cytometry-based methods.