California State Board proposes new pathogen log reduction targets for wastewater regulations

The State Water Resources Control Board held a public hearing on June 6, 2025, to discuss proposed regulations for the onsite treatment and reuse of nonpotable water, a critical step in addressing California's water scarcity issues. The proposed regulations aim to establish pathogen log reduction targets to ensure public health safety when using treated nonpotable water for various applications.

Key among the discussions was the introduction of pathogen log reduction targets derived from a quantitative microbial risk assessment. These targets specify the necessary reductions for different pathogens based on the intended use of the treated water. For instance, water treated for indoor uses, such as toilet flushing, must achieve an 8-log reduction of viruses, a 6-log reduction of giardia, and a 5.5-log reduction of cryptosporidium. These standards are informed by a 2017 framework from the Water Environment and Reuse Foundation and have been updated based on recent scientific data.

The proposed regulations outline two compliance options for treatment systems. The first involves using predefined pathogen control treatment trains, which include a combination of advanced technologies like membrane bioreactors and UV disinfection. The second option allows for alternative treatment trains, provided they can demonstrate equivalent public health protection. Both systems must include continuous monitoring to ensure compliance with the pathogen reduction targets.

Another significant aspect of the regulations is the requirement to control opportunistic pathogens in storage and distribution systems. This includes maintaining temperature control and disinfectant levels to prevent pathogen regrowth, with ongoing monitoring mandated.

The public comment period for these proposed regulations is open until noon on June 7, 2025, with the State Board aiming for adoption by December 31, 2025. Following this, local jurisdictions will have the opportunity to adopt corresponding ordinances. The Department of Housing and Community Development will also work on building standards to align with these regulations, targeting readiness for the 2027 triennial code adoption cycle.

As California continues to grapple with water scarcity, these proposed regulations represent a proactive approach to enhancing water reuse practices while safeguarding public health. The outcomes of this hearing and subsequent regulations will play a crucial role in shaping the state's water management strategies in the years to come.