Sewer Agency Faces Costly Bay Pollution Mandate In Marin County

MILL VALLEY, CA — The Sewer Agency of Southern Marin is preparing for major new regulatory requirements that could drive significant long‑term costs for Mill Valley and neighboring districts, Mill Valley city council members discussed at their Tuesday meeting.

SASM General Manager Mark Newman described a forthcoming San Francisco Bay Regional Water Quality Control Board order that will require wastewater plants around the Bay to reduce nitrogen discharges by 50 percent by 2035. The order follows a 2022 algal bloom that killed tens of thousands of fish in the Bay, officials said.

“In 2022, an $11 billion projected total cost region‑wide was estimated for this nutrient reduction requirement,” Newman said. SASM has retained original plant designer Black & Veatch to analyze options and expects preliminary cost estimates later in 2026.

Newman outlined three main compliance paths: in‑plant upgrades, multi‑benefit projects and nitrogen credit trading. In‑plant solutions could include adding a moving bed biofilm reactor to convert nitrates into atmospheric nitrogen. Multi‑benefit options include a proposed horizontal levee near Tamalpais High School and Bothin Marsh that could combine nutrient treatment, flood protection and sea‑level rise adaptation.

“The feasibility is uncertain,” Newman said, noting SASM has twice applied for grants to fund a detailed feasibility study of the horizontal levee concept.

Councilmembers pressed for clarity on financial impacts. Councilmember Urban Carmel focused on SASM’s long‑range 20‑year rate plan, asking whether residents should expect steady increases. Newman said that under current projections, they can expect a 4 percent annual increase in charges to the City of Mill Valley through 2031.

Mayor Max Perrey highlighted the link to regional water reuse discussions, asking how SASM’s work relates to emerging direct potable reuse concepts in the Bay Area. Newman said SASM is willing to be a partner and suggested that multi‑benefit solutions might better position the agency for future reverse osmosis and DPR options.

Construction related to nutrient reduction would likely not begin before 2030, with most rate impacts expected in the 2030–2035 period, Newman said.