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California currently recycles treated wastewater at a volume of approximately 8.0 × 10 8 m 3 of water per year, with a potential to recycle an additional 1.9 × 10 9 m 3 per year. A key challenge in promoting the expansion of water recycling for agricultural purposes was addressing the perceived concern about whether recycled water produced in conformance with California law is protective of public health. The California Department of Public Health (CDPH) established an expert panel to consider the concern. The panel found, based on quantitative microbial risk assessment (QMRA), that the annualized median risks of infection for full tertiary treatment ranges from 10 −8 to 10 −4 (for human enteric viruses Cryptosporidium parvum and Giardia lamblia, and Escherichia coli O157:H7) based on the assumption of daily exposure. The panel found that risk estimates are consistent with previous CDPH estimates and concluded that current agricultural water recycling regulations do not measurably increase public health risk.

A case study suggests that the use of site‐specific reactivity coefficients and chlorine consumption results in more accurate models of trihalomethane formation. Because of increasing concern about balancing health risks for microbiological control and disinfection by‐product formation, utilities are closely examining and optimizing disinfection practices. The authors present a methodology for developing site‐specific, inplant (finished water) chlorine (Cl2) residual and trihalomethane (THM) formation models. In a case study, the methodology was applied at three operating water treatment plants in the Paris suburbs. A key obstacle was the limited historical record of bromide (Br–) occurrence. However, lab chlorination experiments indicated that approximately 10 percent of Br– was typically incorporated into THMs. In‐plant Cl2 residuals were accurately simulated with a simple first‐order Cl2 consumption model. The most accurate THM simulations were obtained using a recently developed US Environmental Protection Agency model that incorporates species‐specific reactivity parameters.

Droughts, which are difficult to predict, are a natural feature of the hydrology in most regions.Climate change, however, has the potential to increase the frequency and magnitude of future droughts. While the lack of water availability during droughts is widely publicized, there are equally severe water quality impacts that occur during and after droughts as well. Recent droughts have led to water quality implications for drinking water supplies including turbidity, taste and odor, pathogen concerns, and challenges in managing disinfection byproducts (DBPs). This paper presents the results from a series of case studies prepared for a Water Research Foundation study on the effects of extreme weather on drinking water quality in order to help utilities prepare for vulnerabilities under future climate change. A key finding from the case studies is that droughts can fundamentally alter nutrient cycling and biota within both watersheds and reservoirs that influence water quality for months or years after the event. A few of the critical management actions for responding to degraded water quality related to droughts include awareness of potential impacts, increased monitoring during and after the event, and capacity to quickly adjust treatment processes.

A better understanding of relationships among weather patterns and water quality will allow utilities to more effectively plan for the effects of climate change. Best practice requires understanding as much as possible about changes that can occur and their implications for utility operations and management. This article provides information compiled from published literature on weather and water quality to help utilities develop a series of event‐outcome relationships to describe specific changes to surface water quality relative to climate change.