Disinfection serves as the stalwart final barrier in many water-treatment processes around the world, and it is credited with saving more lives than any other health development in human history.1 Of course, too much of a good thing always has downsides.

Drinking-water disinfectants must be carefully applied to balance the human health risks posed by microbial pathogens with those posed by the generation of harmful disinfection by-products (DBPs) during reactions between oxidizing disinfectants and chemical species in the water (natural organic matter, anthropogenic chemicals, and halides). Since their initial discovery in the 1970s,2,3 DBPs have been shown to have adverse human health impacts4 and are thus widely regulated in public water systems. The DBPs regulated in the U.S., Canada, and European Union are shown in the Examples of DBP Regulations table.

Examples of DBP Regulations

DBPRegulatory Concentrations of DBPs (micrograms per liter)
U.S. Environmental Protection Agency 5Health Canada 6European Union 7
Total trihalomethanesa80100100
Haloacetic acidsb608060
a Total trihalomethanes = sum of four trihalomethanes: chloroform, bromodichloromethane, dibromochloromethane, bromoform

b Haloacetic acids = sum of five haloacetic acids: monochloroacetic acid, dichloroacetic acid, trichloroacetic acid, monobromoacetic acid, dibromoacetic acid

In response to the Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules in the U.S., alternatives to chlorine disinfection (chloramines, ozone, chlorine dioxide, and UV) increased in use (see Figure 1). However, even for those water-treatment systems using alternative disinfectants, DBPs can remain a problem. For example, ozone increases bromate production, and although a switch from free chlorine to chloramines may reduce regulated DBPs by as much as 90 percent,4 nitrosamines, which are not currently regulated, are produced during chloramination.

Figure 1. Temporal trends in disinfectant use in municipal drinking-water treatment in the U.S. Data are from the American Water Works Association’s Water Utility Disinfection surveys. The surveys reported the use of multiple types of disinfectants for some systems (yielding totals in excess of 100 percent of the number of systems surveyed); here the relative distribution of type of disinfectant used is shown, resulting in a maximum of 100 percent. Updated and modified from Mayer and Ryan (2019).8

The handful of commonly regulated DBPs is only the tip of the proverbial iceberg, as more than 700 DBPs have been identified, and more than half of the total organic halogen (TOX) formed during chlorination have yet to be identified (Figure 2).4,9,10 Moreover, many of the unregulated DBPs are much more toxic, with certain emerging DBPs, such as some nitrogenous DBPs (N-DBPs), being orders of magnitude more cyto- and genotoxic than the regulated trihalomethanes (THMs) and haloacetic acids (HAAs).4,11 Drinking-water sources with higher wastewater-derived precursors tend to form more N-DBPs, which are significant drivers of toxicity in drinking water.4,10 Further, compared to their chlorine analogues, brominated DBPs are more toxic and carcinogenic, and iodinated DBPs pose an even greater risk.12

While early DBP regulations likely greatly improved the safety of drinking water, considerations of if the existing regulatory approach can be improved—and if so, then how—are ongoing.4 For example, every version of the Unregulated Contaminant Monitoring Rule (UCMR)14 set forth by the U.S. Environmental Protection Agency (EPA), except for the most recent, included at least one DBP on the monitoring or screening list. Likewise, the EPA’s Contaminant Candidate Lists (CCLs)15 have included multiple DBPs identified for priority research (see Figure 3). Informed by this research and monitoring, the EPA is expected to propose revisions to existing microbial and DBP rules by 2025, with both total trihalomethanes (TTHMs) and HAAs identified as candidates for revision.16

Figure 2. Relative distribution of characterized TOX in chlorinated drinking water, including THMs, HAAs, unknown TOX, and several classes of emerging halogenated DBPs that together account for less than 5 percent of the total DBPs. Data from Mitch et al. (2009).13

No revisions to the U.S. DBP regulations have been implemented since the 2006 Stage 2 Disinfectants and Disinfection Byproducts Rule was published. Forthcoming regulatory revisions may or may not add DBPs, given the historic belief that control of the currently regulated DBPs can also provide adequate control of emerging and unidentified DBPs.10 Unfortunately, many unregulated DBPs have been shown to track poorly against those that are currently regulated.17,18 Accordingly, Richardson and Plewa (2020)4 pose the question of whether or not we are regulating the right DBPs to protect public health, and if not, what should be done about it.

In 2011, the EPA announced that it would regulate perchlorate, but the regulation has yet to be implemented (the proposed regulation is expected by the end of 2025). Additionally, in the last decade, the prospect of nitrosamine regulations seemed likely, but the EPA appears to have backed away for now.4 If the U.S. EPA were to adopt an N-nitrosodimethylamine regulatory level equivalent to the California notification level of 10 nanograms per liter, more than 10 percent of chloraminated water systems would be at risk of noncompliance.19,20 If additional DBPs were introduced into the regulatory framework, regulators and water-treatment systems would need to consider simultaneous compliance, risk balancing, and the operational and cost impact of responsive DBP control strategies.19

Figure 3. Potential DBPs on versions 1-5 of the EPA CCL15 and UCMR.14

One frequently debated possible regulation is that of HAA9 rather than the current HAA5 standard. HAA9 includes HAA5 plus bromochloroacetic acid, dibromochloroacetic acid, dibromoacetic acid, and tribromoacetic acid. The brominated species tend to be more toxic but occur at lower concentrations, leading to multiple pathways to high HAA exposure: high concentrations of low-toxicity HAAs, low concentrations of high-toxicity HAAs, or a combination thereof.21

Peterson et al. (2023)21 argued that regulating HAA9 would not be an effective approach for effectively mitigating health impacts, since it would be more specific to the HAA species that occur at high concentrations rather than the most toxic species. Samson and Seidel (2022)22 estimated that if HAA9 were to be regulated at the existing HAA5 maximum contaminant level of 60 micrograms per liter, approximately 2 percent of public water systems that are currently in compliance with TTHM and HAA5 regulations would likely have compliance challenges. The impact would be most substantial for small systems serving populations between 500 and 3,300 and very large systems serving populations in excess of 100,000.22

Beyond the traditional pathway of establishing DBP maximum contaminant levels based on animal toxicity studies and human epidemiological studies, potential new approaches for DBP management outlined by Peterson et al. (2023)21 and Richardson and Plewa (2020)4 include the following:

Improve source water control in systems with impaired source waters, e.g., better control of point sources of DBP precursors, such as municipal wastewater and/or improved precursor removal.

Use in vitro data (tests outside of animals or people, e.g., cell culture) coupled with the precautionary principle.

Use surrogate metrics of finished water quality, e.g., total organic bromine or iodine, total nitrosamines, or total organic nitrogen.

Apply toxicity assays to whole drinking-water extracts to identify potential problems, and use subsequent chemical analyses to identify toxic agents.

Leverage different treatment strategies to reduce toxicity, e.g., use of granular activated carbon or membranes to remove DBP precursors, thereby reducing the necessary chlorine dose, or use UV followed by a lower dose of chlorine or chlorine dioxide.

Given that DBP exposure is constant today,4 considering these or other strategies for improved DBP management is critical.

About the author

Dr. Brooke K. Mayer is a professor in the Department of Civil, Construction and Environmental Engineering as part of the Opus College of Engineering at Marquette University. She holds master of science and doctorate degrees in civil engineering with an emphasis in environmental engineering from Arizona State University.


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