RESUMO
As global temperatures rise with climate change, the negative effects of heat on drinking water distribution systems (DWDS) are of increasing concern. High DWDS temperatures are associated with degradation of water quality through physical, chemical and microbial mechanisms. Perhaps the most pressing concern is proliferation of thermotolerant opportunistic pathogens (OPs) like Legionella pneumophila and Naegleria Fowleri. Many OPs can be controlled in DWDS by residual disinfectants such as chlorine or chloramine, but maintaining protective residuals can be challenging at high temperatures. This critical review evaluates the literature on DWDS temperature, residual disinfectant decay, and OP survival and growth with respect to high temperatures. The findings are synthesized to determine the state of knowledge and future research priorities regarding OP proliferation and control at high DWDS temperatures. Temperatures above 40 °C were reported from multiple DWDS, with a maximum of 52 °C. Substantial diurnal temperature swings from â¼30-50 °C occurred in one DWDS. Many OPs can survive or even replicate at these temperatures. However, most studies focused on just a few OP species, and substantial knowledge gaps remain regarding persistence, infectivity, and shifts in microbial community structure at high temperatures relative to lower water temperatures. Chlorine decay rates substantially increase with temperature in some waters but not in others, for reasons that are not well understood. Decay rates within real DWDS are difficult to accurately characterize, presenting practical limitations for application of temperature-dependent decay models at full scale. Chloramine decay is slower than chlorine except in the presence of nitrifiers, which are especially known to grow in DWDS in warmer seasons and climates, though the high temperature range for nitrification is unknown. Lack of knowledge about DWDS nitrifier communities may hinder development of solutions. Fundamental knowledge gaps remain which prevent understanding even the occurrence of high temperatures in DWDS, much less the overall effect on exposure risk. Potential solutions to minimize DWDS temperatures or mitigate the impacts of heat were identified, many which could be aided by proven models for predicting DWDS temperature. Industry leadership and collaboration is needed to generate practical knowledge for protecting DWDS water quality as temperatures rise.
RESUMO
BACKGROUND: Disinfection byproducts (DBPs) in public water systems (PWS) are an unintended consequence resulting from reactions between mostly chlorine-based disinfectants and organic and inorganic compounds in source waters. Epidemiology studies have shown that exposure to DBP (specifically trihalomethanes) was associated with an increased risk of bladder cancer. OBJECTIVE: Our goal was to characterize the relative differences in exposures and estimated potential bladder cancer risks for people served by different strata of PWS in the United States and to evaluate uncertainties associated with these estimates. METHODS: We stratified PWS by source water type (surface vs. groundwater) and population served (large, medium, and small) and calculated population-weighted mean trihalomethane-4 (THM4) concentrations for each stratum. For each stratum, we calculated a population attributable risk (PAR) for bladder cancer using odds ratios derived from published pooled epidemiology estimates as a function of the mean THM4 concentration and the fraction of the total U.S. population served by each stratum of systems. We then applied the stratum-specific PARs to the total annual number of new bladder cancer cases in the U.S. population to estimate bladder cancer incidence in each stratum. RESULTS: Our results show that approximately 8,000 of the 79,000 annual bladder cancer cases in the United States were potentially attributable to DBPs in drinking water systems. The estimated attributable cases vary based on source water type and system size. Approximately 74% of the estimated attributable cases were from surface water systems serving populations of >10,000 people. We also identified several uncertainties that may affect the results from this study, primarily related to the use of THM4 as a surrogate measure for DBPs relevant to bladder cancer. DISCUSSION: Despite significant reductions in exposure over the past several decades, our study suggests that â¼10% of the bladder cancer cases in the United States may still be attributed to exposure to DBPs found in drinking water systems. https://doi.org/10.1289/EHP9985.
