Policy and engineering strategies to manage public health risk from disinfection byproduct mixtures in wastewater-impacted drinking waters

The advent of drinking water disinfection to inactivate pathogens was a significant public health achievement. However, disinfectants react with dissolved organics to form disinfection byproducts (DBPs), which have been associated with bladder cancer, colorectal cancer, and adverse reproductive outcomes. After 50 years of research, it remains unclear which DBP classes drive the toxicity of disinfected drinking water. Globally, trihalomethanes (THMs) are the most commonly regulated DBP class. THMs are used as a surrogate for DBP exposure based on the assumption that they are representative of the overall DBP mixture. Two recent trends challenge this assumption: 1) the identification of hundreds of DBP species with precursors and formation pathways distinct from THMs, and 2) an increase in potable reuse of reclaimed municipal wastewater, which contains precursors that promote the formation of nitrogen-containing DBP classes (N-DBPs). Although N-DBPs are unregulated in most countries, in vitro and in vivo toxicity studies indicate they are more toxic than THMs. The four studies comprising this dissertation demonstrate that US and global DBP policy may not lead to treatment choices that effectively minimize health risk, particularly for the large population that relies on wastewater-impacted drinking water without advanced treatment. The first two studies utilized a pilot-scale system to evaluate the potential of several disinfection strategies (one existing, one novel) to minimize DBP-associated toxicity while meeting regulatory limits for pathogens and DBPs in reclaimed wastewater. The first study focused on tradeoffs between regulated and unregulated DBP formation with chlorine-chloramine disinfection. Pre-oxidation with free chlorine is effective for inactivating viruses and reducing the formation of N-Nitrosodimethylamine (NDMA), which is being considered for regulation in the US. Compared to chlorine, chloramines mitigate the formation of THMs and haloacetic acids (HAAs), which are regulated in the US. A minimal pre-chlorine contact time achieved inactivation of the virus indicator MS2 while maintaining THM and HAA concentrations below regulatory limits. A longer pre-chlorination contact time was required to reduce NDMA to target levels; however, this increased the estimated toxicity of the DBP mixture, primarily due to the formation of haloacetonitriles (HANs), an unregulated N-DBP class. The second study introduces a novel disinfection method, distributed chorine injection, to reduce NDMA formation during break-point chlorination. The efficacy of this method was demonstrated at pilot-scale, and the hypothesized mechanism was proven with kinetic modeling and bench experiments. Distributed chlorine injection is a low-cost strategy for utilities to meet low NDMA limits without increasing halogenated DBP formation. The final two studies addressed whether THMs are an effective surrogate for unregulated DBP classes. The first investigated this question in the context of treatment and infrastructure practices in low- and rising-middle income countries through a case study in Rajasthan, India. Most of the water sources were impacted by wastewater due to minimal sanitation infrastructure. The low to moderate levels of THMs measured were not indicative of high concentrations of unregulated DBPs in many drinking waters. The concentrations of toxic, unregulated DBP classes in the largest water system resembled those measured in reclaimed wastewater for nonpotable reuse. HANs were again the dominant contributor to estimated toxicity. Thus, the final study was designed to conclusively determine whether THMs are an effective surrogate for HANs. Multiple statistical models were developed using a large database of DBP concentrations measured in distribution systems of ~250 US public water systems. Multilevel/hierarchical regression models identified substantial systematic variance in the HAN:THM ratio between water systems and within distribution systems. A portion of the variance was attributed to factors such as source water type, disinfectant sequence, distribution system retention time, as well as seasonal effects on surface water. A risk ratio analysis indicated that using THMs as a surrogate for HAN exposure introduces significant classification bias. Overall, these findings underscore the need to identify the toxicity drivers in disinfected waters so that treatment systems can be designed to target those DBP classes. Furthermore, a new global policy paradigm should account for local factors influencing DBP speciation and holistically address the risk posed by complex contaminant mixtures in drinking water

Advisor: Mitch William A. 
Boehm Alexandria
Luthy Richard G.