Will the EPA Change Drinking Water Rules for Microbes and Disinfection Byproducts?
The agency must propose changes by July. Here’s what to know.
This is part one of a two-part series on what changes the EPA might propose to the Microbial and Disinfection Byproduct Rules. Part two, coming soon, will cover how to start preparing today.
By July 31, 2025, the Environmental Protection Agency (EPA) is required to propose changes to a set of drinking water regulations sometimes collectively called the Microbial and Disinfection Byproduct Rules, or M/DBP Rules. The rules—which include the Surface Water Treatment Rules and the Stage 1 and Stage 2 Disinfectant and Disinfection Byproduct Rules—were developed over the past few decades to manage two interrelated groups of contaminants: microbes and chemicals called disinfection byproducts (DBPs).
The EPA is considering revising parts of these rules for a routine review. But the July deadline stems from a lawsuit filed by conservation groups claiming the agency moved too slowly with that process.
The National Drinking Water Advisory Council (NDWAC), a panel of 15 water sector experts, recommended roughly a dozen potential changes to the M/DBP Rules. It’s unclear which ones, if any, the EPA might consider. But adopting even one or two could impact utilities across the U.S. Here’s what we think the EPA might propose changing and who could be affected.
The Balancing Act
But first, it’s worth revisiting why these rules exist at all. Hazen’s Meric Selbes, who studied DBPs for his doctorate, sees them as “a balancing act.” On one side are microbes (or pathogens) often present in source waters. They range from microorganisms like Giardia lamblia to the Hepatitis A virus and can cause all sorts of illnesses.
Water towers (like the one above) are often used in water distribution systems to store emergency supplies, control water pressure, and help meet peak demand.
“Disinfection is very important, because you need it to inactivate the pathogens present in water,” Selbes said. “Otherwise, people will get sick immediately—like, starting tomorrow.”
Too much disinfection, however, can generate high levels of DBPs, which form when chlorine and other disinfectants react with natural organic matter. There are hundreds of DBPs. Most aren’t well understood, but research has linked some of them to health risks like bladder cancer.
The EPA’s various regulations around disinfection and DBPs seek to balance those varied risks. It’s not easy, in part because any change to disinfection can impact DBP management, and vice-versa.
The rules are also complex because they apply not just to drinking water treatment plants but to distribution systems: networks of pipes that carry finished (treated) water to customers. Each system is unique, with features like pump stations, water towers, and pipes that can span hundreds, even thousands, of miles.
“All sorts of reactions are happening on the pipe walls,” said Meg Roberts, who leads Hazen’s Distribution System Services group and also did graduate research on DBPs and distribution systems. “There’s organic matter, different microorganisms, corrosion interactions. So, even if your DBPs are pretty low coming out of the plant, they have potential to increase after that point.”
Whatever techniques a utility uses during treatment must protect the water throughout those pipes, all the way to the most distant house. It’s like a balancing act combined with a marathon. And because our industry is always working to improve, the requirements keep evolving. Here’s what we think could change next.
The Bluffs Water Tower just outside Knoxville, Tennessee. Water distribution systems can span vast distances and customer bases in both rural and urban areas. The longer water stays in the system, the more likely it is to pick up microbes and disinfection byproducts.
A Numeric Minimum for Disinfectant Residuals
What’s regulated now: “Disinfectant residual” means the small amount of disinfectant that’s deliberately left in drinking water after it’s treated, to protect it from microbes as it travels through distribution systems to people’s faucets.
The EPA requires most water utilities to maintain a “detectable” amount of disinfectant residual throughout their distribution systems. That means the water can have hardly any residual—so little you can’t reliably quantify it—but as long as testing can detect it, the water meets that standard.
What could change: NDWAC recommended making the minimum as high as 0.5 milligrams per liter (mg/L) of disinfectant residual, but Alan Roberson said 0.2 mg/L is more likely (for systems that use chlorine as their disinfectant residual).
Roberson, the executive director of the Association of State Drinking Water Administrators (ASDWA), thinks this is the change the EPA is most likely to propose.
“The idea is that you have to have higher disinfectant throughout the far reaches of the distribution system,” Roberson said.
Why it might change: Utilities only regularly test the water in select locations in their distribution systems. If the disinfectant residual is barely detectable at a sampling location, especially on a regular basis, it could be completely depleted in other, untested spots.
Free chlorine means active chlorine in the form of bleach (hypochlorite) or hypochlorous acid. Total chlorine includes both free chlorine and chloramine (chlorine combined with ammonia). Both chlorine and chloramine are adequate disinfectants, but chloramine is often used in distribution systems that tend to form high amounts of regulated DBPs, since it produces less of them.
Who could be impacted: A numeric minimum could impact many utilities, said Roberson. Roberts and Selbes agreed. For one thing, about half of all U.S. states don’t have a numeric minimum rule. Even utilities that already follow a numeric minimum could be impacted. For example, a utility in a state with no numeric requirement may still aim to have at least 0.2 mg/L of disinfectant residual in its finished water, just to play it safe, Selbes said.
“If they set the regulation at 0.2 mg/L, [the utility] wouldn’t want to hover around the regulatory limit,” Selbes explained. “So maybe they will aim for 0.5 instead of 0.2, so that they don’t get a hit by the regulatory agency. And because adding more disinfectant increases the risk of DBP formation, they might have to take additional steps to control DBPs.”
Federal Storage Tank Inspection and Cleaning Requirements
What's regulated now: Water towers and other tanks store finished water in distribution systems. They help maintain water pressure, make it easier to distribute water during demand peaks, and provide extra water for emergencies like fires.
There are no federal requirements for inspecting or maintaining finished water storage tanks. Some states have their own rules; others have none. It’s possible for some tanks to go for years without an inspection or cleaning.
What could change: The NDWAC group recommended creating federal requirements to inspect water storage tanks and clean them as needed, and both Roberts and Roberson think EPA could propose doing so. It’s unclear what the specifics would be. But the group said that if water leaving a tank repeatedly shows no disinfectant residual or too much particulate matter, those issues could be used to trigger a cleaning requirement.
Why it might change: Roberts said the one thing tanks aren’t great for is water quality.
“The lower the velocity in any piece of infrastructure, the higher the potential for microbial growth, disinfectant decay, and DBP formation,” Roberts said. “Tanks have very low velocities. [The water is] just sitting there, stagnant, for a good amount of the day, and they are huge—millions of gallons in one spot.”
Even after treatment, drinking water still contains some particulate matter, Roberts said. It can pick up more inside distribution system pipes. And tanks can act like clarifiers: They can let much of that material sink to the bottom, where it becomes ideal habitat for microbes. The tanks themselves can also corrode, creating the potential for biological and chemical reactions on the tank walls that can trigger disinfectant decay and promote biological activity and DBP formation.
Who could be impacted: Roberts said this could impact many utilities across the country, particularly those that struggle to regularly inspect and clean their tanks.
From Winston-Salem, North Carolina’s otter-themed ground storage water tank (above) to standpipes like this one in Texas (top left) to the Pumpkin Water Tower in Circleville, Ohio (top right), water storage tanks vary greatly in size, material, and environment. All of those factors can affect the chemistry and quality of the water they hold.
Tighter TOC Removal Requirements
What’s regulated now: Before disinfecting water, surface water plants must remove a certain percentage of total organic carbon (TOC) from the water based on the water’s TOC levels and alkalinity. Some utilities are also eligible for alternative compliance criteria that allow them to meet the requirement, even if they are unable to achieve the prescribed TOC removal.
What could change: The EPA may consider requiring utilities to remove more TOC (how much more is unclear) and removing some of the alternative criteria.
Why it might change: TOC removal helps limit DBP formation, since organic matter reacts with disinfectants to form DBPs. Tightening the removal requirements would mean even more control over DBP formation.
Who could be impacted: It depends on how much stricter the requirements could become, said Hazen’s Eric Peterson, who, like Selbes and Roberts, studied DBPs in graduate school. But he said that eliminating any of the alternative compliance criteria would likely have broad impacts for many utilities around the country.
“Many utilities have opted for less costly alternative compliance approaches under the current Stage 2 rules, and there is a real chance this may need to change moving forward,”
Peterson is part of a Hazen-led team that’s helping the American Water Works Association (AWWA) evaluate this potential TOC rule change by developing a decision-making framework and cost estimation guidance for compliance. They’re also creating a nationwide assessment of how the PFAS Rule combined with the potential M/DBP rule changes could impact compliance, particularly for utilities that use granular activated carbon (GAC).
The total organic carbon (TOC) present in rivers, lakes, and other source waters can come from decaying plant matter (like that shown above), human and animal waste, and anthropogenic pollutants such as fertilizers and pesticides.
Regulating More Haloacetic Acids (HAAs)
What’s regulated now: The EPA only regulates a handful of disinfection byproducts, and it regulates them as groups: There’s a limit of 80 parts per billion (ppb) for four trihalomethanes (total THM, or TTHM) and 60 ppb for five haloacetic acids (HAA5) as locational running annual averages.
What could change: The EPA has been considering adding four more HAAs to the regulated group (HAA9). It has already taken some initial steps in that direction—for example, requiring some utilities to collect HAA9 occurrence data for the Fourth Unregulated Contaminant Monitoring Rule (UCMR 4).
Why it might change: More than 600 DBPs have been identified since the 1970s. The TTHM and HAA5 groups were regulated first because besides being linked to health risks like bladder cancer, they were the only DBPs scientists could reliably detect when those laws were established.
“Today, researchers can routinely test water for anywhere between 50 to 100 different DBPs,” Peterson said.
That includes the four additional HAAs under consideration. In 2021, the National Toxicology Program stated that there’s enough evidence to suspect that those four additional HAAs are carcinogenic to humans.
Who could be impacted: The additional HAAs are brominated chemicals, which form when a substance called bromide is present in water, said Hazen Drinking Water Practice Lead Erik Rosenfeldt. Utilities with higher levels of bromide in their raw water will likely have higher amounts of those brominated HAAs. If the EPA expands HAA5 to HAA9 but still requires a collective limit of 60 ppb, Rosenfeldt said, they could struggle to comply.
But Rosenfeldt said it’s relatively straightforward for utilities to understand their potential risk.
“Big utilities that participated in UCMR 4 can just go back to that data to see whether they had a propensity for these brominated HAAs,” he said. “Even if you don’t have that data, lab reports today often include all nine HAAs, because we can now measure all nine routinely.”
Other potential DBP actions: While they wouldn’t be surprised to see HAA9 in the EPA’s proposed M/DBP rule changes, both Peterson and Rosenfeldt said the agency could just as easily take no action on HAAs. If anything, Peterson thinks it’s more likely that the EPA will try to address some of the research gaps around both regulated and unregulated DBPs.
“We still don’t even know exactly how prevalent a lot of the unregulated DBPs are in drinking water, let alone how their potential toxicity to people compares,” Peterson said. “The science around DBPs is so complex. And it’s always evolving.”
