Proposed Basin Plan Amendment – Chlorine Water Quality Objectives and Total Residual Chlorine Effluent Limitations for Wastewater Discharges

Wastewater treatment plants that use chlorine for disinfection or have the potential to discharge chlorine must comply with the technology-based effluent limit of 0.0 milligrams per liter (mg/L) for total residual chlorine, as specified in Table 4-2 of the Water Quality Control Plan for the San Francisco Bay Basin (the Basin Plan).

To meet this strict effluent limit, wastewater treatment plant operators in the region have routinely added excess dechlorination chemicals—typically sodium bisulfite—to neutralize any remaining chlorine. However, this practice increases operational costs and raises potential water quality concerns without delivering additional environmental benefits.

To address this issue, San Francisco Bay Regional Water Board staff proposed a Basin Plan amendment that would incorporate chlorine water quality objectives consistent with the U.S. EPA’s national ambient water quality criteria for chlorine. This amendment aimed to replace the technology-based effluent limit with water quality-based effluent limits based on the new chlorine water quality objectives.

The Regional Water Board, the State Water Resources Control Board, and the Office of Administrative Law (OAL) adopted and approved the proposed Basin Plan amendment, as detailed in the sections below. However, during its review of the amendment, the U.S. EPA expressed concerns that the proposed chlorine water quality objectives might not be sufficiently protective of all beneficial uses, including those intended to protect threatened and endangered species in the San Francisco Bay region. Therefore, in a letter dated June 5, 2023, the Regional Water Board staff formally requested to withdraw its request for U.S. EPA approval of the chlorine water quality objectives and related implementation provisions, including the establishment of water quality-based effluent limits for total residual chlorine.

Despite this withdrawal, other approved changes to the Basin Plan remain in effect. These include:

• Removing the total residual chlorine technology-based effluent limit of 0.0 mg/L from Table 4-2.
• Clarifying that the oil and grease effluent limits in Table 4-2 apply only to secondary-level wastewater treatment facilities.
• Updating footnotes in Tables 3-3 and 3-4 to align the Basin Plan’s mercury objectives with the Water Quality Control Plan for Inland Surface Waters, Enclosed Bays, and Estuaries of California—Tribal and Subsistence Fishing Beneficial Uses and Mercury Provisions (Statewide Mercury Provisions).

Regional Water Board staff is currently working on a new Basin Plan amendment to reinstate the total residual chlorine technology-based effluent limit for wastewater discharges.