Is Your Water System Ready for the Potential MDBP Rule Revisions?

Understanding the MDBP rules and the six-year review requirement

The Microbial Disinfection Byproducts rules are a series of EPA regulations focused on protecting drinking water from microbial (M) pathogens and addressing risks associated with disinfectants and disinfection byproducts (DBPs). The rules focus on surface water sources and include:

• Stage 1 and Stage 2 Disinfectants and Disinfection Byproduct Rules (DBPR)
• Surface Water Treatment Rule (SWTR)
• Interim Enhanced Surface Water Treatment Rule (IESWTR)
• Long Term 1 Enhanced Surface Water Treatment Rule (LT1)

The 1996 Amendments to the Safe Drinking Water Act (SDWA) require the EPA to review primary drinking water regulations every six years and determine whether there are opportunities to improve public health and/or reduce implementation costs for water systems.

Lawsuit sparks rule revision recommendations and new schedule

Several environmental organizations filed a lawsuit in 2019 against the EPA regarding the third six-year review, claiming that the EPA unreasonably delayed, neglected duties, and failed deadlines outlined in the SDWA. A 2020 settlement agreement mandated the EPA to propose revisions to the MDBP rule by July 2025.

Based on this mandate, EPA tasked the National Drinking Water Advisory Council (NDWAC) and its Microbial and Disinfection Byproducts Rule Revisions Working Group with reviewing the MDBP rules. In late 2023, the NDWAC made 13 recommendations and presented them to the EPA. The recommendations touched upon many topics, including minimum disinfectant residuals, storage tank inspection, chloramination, and consecutive systems, with numerous implications for water systems.

Since the recommendations were presented, the EPA has been reviewing and working on how to implement the rules. However, as stated above, the EPA has been granted an extension for the proposed rule until July 2027, with the final version published in late 2028.

Potential Revisions and Implications

We won’t know until we know, but here are four potential MDBP rule revisions and implications for your water system as you begin to consider planning efforts :

1. Minimum disinfection residual in distribution systems
   In many states, an MDBP rule revision may set a federal minimum numeric value for surface water systems instead of the current “detectable” disinfectant level. Values as high as 0.5 mg/L for free chlorine (chlorinated systems) and 0.7 mg/L for total chlorine (chloraminated systems) have been proposed.
   What this could mean for you: This would provide additional protection from microbial pathogens such as Legionella. Lower values may be acceptable under a rule revision but would likely require increased monitoring to ensure water quality within the distribution system. Currently approved methods for free chlorine and total chlorine monitoring allow this level to be accurately measured. However, higher residual values may require increased disinfectant levels dosed at the treatment plant and/or additional operation and maintenance (O&M) (e.g., flushing) to limit water age in the system and keep disinfectant levels up. Systems with potentially higher required values will likely have additional dosing and/or monitoring requirements, increasing O&M costs.
2. Storage tank inspection requirements
   Requirements for tank cleaning and inspection are inconsistent across states, and no federal standard exists. MDPB rule revisions may establish inspection requirements for storage tanks with cleaning as needed and updated O&M guidance to control water age, improve water quality, and limit some types of DBP formation.
   What this could mean for you: Additional O&M may be necessary for water systems to ensure excellent water quality in their storage tanks. Many systems routinely have their tanks inspected every two years and, therefore, would not experience additional O&M.
3. Improve chloramine practices
   Chloraminated systems (chlorine combined with ammonia) have become more prevalent in the US to minimize regulated DBPs associated with chlorine alone. Although some emerging contaminants are associated with chloramination, the primary public health concern is the potential for nitrification, as chloramines degrade in distribution systems with water age. A rule revision will likely require all chloraminated systems to have a formal Nitrification Action Plan if they don’t already.
   What this could mean for you: Implications for chloraminated systems would be developing or modifying action plans to meet new guidelines for nationwide consistency. These plans are critical to maintaining consistent, safe water delivered to customers. Primacy agencies would likely require review and updates to the plan at various frequencies (e.g., every five years).
4. Improve water quality in consecutive systems
   With more regional water systems in both rural and metropolitan areas, the interface between the wholesale water provider and the receiving distribution system is critical. Rule revisions may involve federal guidelines for additional monitoring at the entry point, requirements for a consultative process between stakeholders, and clarifications on water service contracts. Regulatory compliance within the consecutive system highly depends on the water quality received from the wholesaler.
   What this could mean for you: Implications for wholesalers and consecutive systems may enhance partnerships, overall compliance, and protection of public health. Additional monitoring for compliance and consultation with primacy agencies concerning compliance will improve water quality for consecutive systems.

Take Aways

Although the EPA’s potential MDBP rule revisions appear to have been delayed until 2027, this extension offers a valuable opportunity for water systems to proactively assess their operations, plan for possible regulatory changes, and invest in long-term improvements to protect and improve water quality. By understanding the likely direction of the revisions and beginning preparations, communities can better manage costs, ensure compliance, and continue delivering safe, high-quality drinking water to the public.

In Part 2 of this article, we will explore one of the more interesting NDWAC recommendations to the EPA, which includes investigating increases to DBP precursor control. DBP precursors are naturally occurring organic matter measured as total organic carbon (TOC ), and some inorganic compounds such as bromide and nitrogen.

In the meantime, please contact me if you have questions about anything in this article. I’m always happy to discuss this topic and others related to community water systems.

Stay tuned.