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EPA Microbial & Disinfection Byproducts Rule Update: Key Changes and Timeline
The Microbial and Disinfection Byproducts (MDBP) rules are a critical part of the United States drinking water regulatory framework. These rules aim to balance two key public health priorities:
- Eliminating harmful microbial pathogens
- Minimizing potentially harmful disinfection byproducts (DBPs)
The U.S. Environmental Protection Agency (EPA) is currently evaluating potential revisions to the MDBP rule suite following recommendations from the Microbial Disinfection Byproducts Rule Revisions Working Group (MDBP WG).
This article summarizes the key recommendations, regulatory implications, and expected timeline for potential rule changes. The previous article can be found at the following link. Microbial/Disinfection Byproducts Rules Updates – SAMS.
Key Takeaways
- The MDBP Rule Revisions Working Group met from May 2022 through November 2023.
- A final report was issued on November 13, 2023.
- The report contains 13 major recommendations related to drinking water treatment, monitoring, and system management.
- The EPA is expected to propose MDBP rule revisions by Summer 2027.
Background: MDBP Rule Revision Process
The National Drinking Water Advisory Council (NDWAC) formed the MDBP Rule Revisions Working Group to evaluate potential improvements to the current regulatory framework governing microbial contaminants and disinfection byproducts in drinking water.
The working group included representatives from:
- Public water utilities
- State regulatory agencies
- Environmental organizations
- Public health professionals
- Industry experts
Their goal was to recommend improvements that protect public health while maintaining practical compliance for water utilities. As regulatory expectations evolve, utilities must ensure effective monitoring and well-designed distribution system sampling plans. Digital asset and compliance management platforms such as SAMS can support this effort by centralizing monitoring data, inspection records, and regulatory documentation within a single operational system.
Key Recommendations from the MDBP Working Group
The MDBP Working Group issued 13 major recommendations addressing water treatment, monitoring practices, source protection, and system management.
Recommendation 1: Disinfectant Residual
Current drinking water regulations require that a detectable disinfectant residual be maintained throughout distribution systems.
The working group recommends replacing the vague “detectable” requirement with numerical standards, such as:
- 5 mg/L free chlorine
- 7 mg/L total chlorine for chloraminated systems
The group also recommends that:
- At least 95% of monthly disinfectant residual measurements meet the minimum value.
- Water systems avoid repeated failures to maintain the required residual.
In addition, utilities should develop distribution system disinfectant residual monitoring plans that more accurately represent conditions across the system.
This recommendation received substantial support from working group members.
Recommendation 2: Premise Plumbing
While Public Water Systems do not have control over premise plumbing (e.g. office and commercial buildings, hospitals) nor are they required to do monitoring, these systems have been identified as harboring or allowing the proliferation of opportunistic pathogens, including Legionella bacteria. Therefore, the MDBP WG recommends establishing a partnership between the EPA, State primacy agencies, drinking water professional associations, and building owner/operators and associated organizations to develop and promote programs to address opportunistic pathogens in premise plumbing through regulatory or incentive programs. It would also include, at a minimum, development of water management plans for federal buildings and facilities.
This recommendation received substantial support from WG members.
Recommendation 3: DBPs of Emerging Concern
Current drinking water regulations primarily regulate two groups of DBPs:
- Total Trihalomethanes (TTHM4)
- Five Haloacetic Acids (HAA5)
However, research has identified many additional DBPs that may pose health risks.
Studies indicate that toxicity levels vary among DBPs:
The working group recommends additional data collection to evaluate:
- Occurrence of emerging DBPs
- Health impacts
- Treatment technologies
This information would determine whether future regulations are justified.
Recommendation 4: Multi-Benefit Precursor Control
This recommendation recognizes the role that source water quality and especially DBP precursors play in DBP formation. Source waters may include TOC, bromide, and contributions from wastewater treatment plant discharges. Control of these precursor constituents can reduce DBP formation.
The recommendation includes conducting source water screening to identify precursors in the source water. It then recommends developing an enhanced treatment technique requirement to reduce the elevated precursors.
Recommendation 4 received substantial support from the WG with an alternative proposal of source water protection in lieu of achieving the challenging balance between pathogen and DBP control.
Recommendation 5: Finished Water Storage Tanks
Water storage tanks are important components in the distribution system. They store water so that it is available to meet demand at high water use times and provide for fire flow. However, because they are designed to slow water flow and hold it for a period of time, it creates an environment that can degrade disinfectant residuals and allow for DBP formation.
The WG recommends establishing a regular inspection and periodic cleaning requirement for storage tanks. If conducted by adequately trained personnel, inspections can identify openings that can allow debris, birds, and other animals getting into the tank, corrosion of or damage to storage tank surfaces and coatings, and structural problems that can compromise tank integrity. Periodic cleaning can remove biofilm and accumulated sludge that can contribute to disinfectant residual degradation and DBP formation.
Guidance should also be prepared or revised to alter storage tank operations to minimize DBP formation. This may include reducing the volume of water stored in the tank to decrease the water age and subsequent DBP formation potential and structural or hydrologic changes to minimize stratification of the water in the tank.
Recommendation 5 received full support from the WG.
Recommendation 6: Chloramination
Chloramination is a proven disinfection technology to chlorine with the benefit of reducing DBP formation in the distribution system. However, it does require attention to operating parameters including chlorine to ammonia ratios, nitrification control, and formation of unregulated DBPs.
The WG recommendations include developing chloramination operating guidance for water systems. The WG also recommends developing guidance to assist with periodic conversion from chlorine to chloramination to mitigate negative water quality changes. Systems that are converting to chloramination should monitor for regulated DBPs before, during and after conversion. Water systems should prepare Nitrification Control Plans to minimize nitrification in the distribution system.
Recommendation 6 received full support from the WG.
Recommendation 7: Consecutive Systems
Wholesale-consecutive system arrangements are not uncommon in many areas of the country. They may be established to provide the primary source of drinking water or as an emergency supply to the consecutive system. Such arrangements create unique challenges regarding water quality. Existing data shows that consecutive systems show a higher level of non-compliance with disinfectant residuals, microbiological contaminants, and DBPs.
The WG recommends that monitoring of disinfectant residuals and DBPs should be required at the active point of connection from the wholesale system including when violations of MDBP rules occur in the consecutive system. Systems using chloramination should make monitoring part of the Nitrogen Management Plan. The WG also recommended establishing a consultative process between the wholesale system, consecutive system and primacy agency to address problems (e.g. violations) when they occur.
Finally, the WG recommends developing guidance on developing contractual agreements between the wholesale and consecutive systems to document responsibilities of all parties.
Recommendation 7 received full support from the WG.
Recommendation 8: Source Control
Source waters are the primary contributor of contaminants to drinking water. Upstream natural, industrial, and municipal discharges can introduce organics (i.e. TOC), pathogenic organisms, and inorganic constituents (e.g. nitrogen, iodide, bromide), that can challenge drinking water systems. The addition of disinfectants (i.e. chlorine, chloramine, chlorine dioxide, ozone) as part of treatment process trains inactivate pathogens but can generate DBPs.
DBP precursor removal (especially TOC) before the addition of chlorine disinfectants can reduce subsequent DBP formation in the distribution system but adds to the cost of treatment.
Therefore, the WG recommends preventing the addition of drinking water contaminants into the water cycle that can affect water supplies, treatment plants, and public health. It includes a recommendation for EPA to screen source waters for drinking water contaminants. The WG also recommends reducing or eliminating discharges (from municipalities, industries) to surface waters using existing statutory (e.g. Clean Water Act, Clean Air Act, Resource Conservation and Recovery Act) and associated regulations.
Recommendation 8 received full support from the WG.
Recommendation 9: Environmental Justice Improvement Opportunities
The WG calls for EPA to conduct analyses on the gaps that exist in MDBP compliance for environmental justice, disadvantaged and historically underserved communities and develop strategies to fill these gaps. The WG also recommends improvements in public notification to consumers from water systems regarding water quality and compliance status and better access to water system performance data.
Recommendation received full support from the WG.
Recommendation 10: Public Water System (PWS) Technical, Managerial, and Financial (TMF) Capacity
This recommendation recognizes the challenges faced by small, rural, EJ, disadvantaged, and historically underserved communities to comply with drinking water rules, and in this context any revised MDBP rules. In that regard the WG recommends that funding be targeted for additional technical and financial assistance, evaluating operator certification with an emphasis on distribution system management, making permanent the Low-Income Household Water Assistance, or similar, Program (LIHWAP2), and create strong incentives for PWSs to provide training to their council or board members.
Recommendation 10 received full support with one abstention to avoid a potential conflict of interest.
Recommendation 11: Primacy Agency Capacity
The WG recommends that PWSs should be provided with adequate resources to implement new MDBP requirements. Sanitary survey implementation should be adjusted to incorporate MDBP revisions.
Recommendation 11 received full support from the WG.
Recommendation 12: MDBP Overall Data and Analysis Gaps
WG recommends that EPA review data and analysis gaps in the MDBP rules that could benefit from additional research, as follows:
- Source water
- Treatment
- Distribution system
- Premise Plumbing
- Enabling Environment
The report identifies several research topics within these subject areas.
Recommendation 12 received full support from the WG.
Recommendation 13: Ground Water Under the Direct Influence of Surface Water (GWUDI)
It has been many years since the definition of GWUDI was introduced. Due to the advancement of groundwater science and analytical methods EPA should revisit the definition of GWUDI to improve clarity, understandability, and simplicity of implementation.
Recommendation 13 received full support from the WG.
The reader is encouraged to review the final Report of the Microbial Disinfectant Byproducts Rule Revisions Working Group. The full report can be accessed here report-of-the-mdbp-rule-revisions-working-group-to-the-ndwac-november-2023.pdf.’
Timeline for Potential MDBP Rule Revisions
The EPA is currently reviewing the working group’s recommendations.
Based on current information, the agency plans to propose revisions to the MDBP rules by Summer 2027.
Any rulemaking process will likely include:
- Proposed rule publication
- Public comment period
- Final rule issuance
What Water Utilities Should Do Now
While the regulatory process continues, utilities can begin preparing by:
- Evaluating disinfectant residual monitoring programs
- Reviewing storage tank maintenance procedures
- Assessing DBP precursor levels in source waters
- Strengthening source water protection strategies
The evolving regulatory landscape surrounding microbial contaminants and disinfection byproducts highlights the increasing importance of effective data management, infrastructure oversight, and regulatory documentation within drinking water systems. As utilities prepare for MDBP rule revisions, maintaining accurate operational records, monitoring data, and infrastructure maintenance histories will play an important role in demonstrating compliance and supporting sound operational decision-making.
Digital asset and compliance management platforms can assist utilities by integrating inspection records, water quality monitoring data, and infrastructure management activities within a centralized system. The SAMS platform is designed to support water utilities and public infrastructure organizations through tools that facilitate asset management, regulatory documentation, inspection tracking, and operational oversight.
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About The Author
At NJBSoft, we’re proud to collaborate with experts like Robert Hollander, P.E., whose extensive experience in water quality and regulatory compliance allows us to design SAMS in order to meet industry needs. Bob’s leadership and deep industry knowledge supports utilities in staying organized, compliant, and focused on protecting public health every day.
Robert Hollander
- 45 years of experience in water quality, regulatory compliance, and utility management
- President of Hollander Consulting Services
- Held senior roles at the City of Peoria, City of Phoenix, and the Arizona Department of Environmental Quality
- Served as President of the AZ Water Association and the Western Coalition of Arid States
- Licensed Professional Engineer in Arizona with multiple Grade 3 operator certifications
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