The same disinfectant chemicals that flooded households during the pandemic may now be creeping into San Francisco Bay, according to a new analysis from the San Francisco Estuary Institute. The study zeroes in on quaternary ammonium compounds, or QACs, which show up in a long list of wipes, sprays and sanitizers. Researchers say wastewater plants pull most QACs out of the liquid stream, but in the process, shift large amounts into biosolids and sediments. Even the relatively small share that escapes into open Bay waters may already be high enough to harm fish, shellfish and the microbes that keep treatment systems working.
What the study shows
According to the San Francisco Estuary Institute, new measurements indicate that regional wastewater treatment plants remove about 98% of QACs from the water column on average. That success comes with a catch. Much of the chemical load sticks to solids and becomes concentrated in biosolids that are later land‑applied or otherwise managed. The report documents QACs in Bay water, sediment and urban stormwater samples, and it recommends that the Regional Monitoring Program expand screening in the environment to see how often levels approach thresholds of concern. The authors emphasize that current treatment tends to relocate QACs into solid waste streams rather than fully breaking them down.
Why wastewater plants are vulnerable
“It's an antimicrobial agent, and a lot of what cleans wastewater are bacteria themselves,” Miguel Méndez, an author of the study and an SFEI scientist, told Maven's Notebook. That basic conflict matters for plant operators. At high concentrations or under long‑term exposure, QACs can stress or kill the nitrifying and denitrifying microbes that remove nutrients, which can erode treatment performance and drive up operating costs. The SFEI write‑up also notes that the one facility in the study that broke down nearly all measured QACs relied on a more expensive treatment train, underscoring trade‑offs between better removal and higher expense.
Risks for Bay wildlife
The SFEI analysis classifies QACs as a "Possible Concern" within the Regional Monitoring Program's tiered screening system for the Bay. It also pinpoints individual compounds whose measured 90th‑percentile concentrations exceed marine predicted no‑effect concentrations, according to the San Francisco Estuary Institute. In particular, the report flags some BACs and ATMACs at levels that could impair growth or survival in sensitive invertebrates and fish. The authors stress that the overall sampling record is still limited, yet they say the findings are strong enough to warrant more monitoring and efforts to cut QACs off at the source.
QACs and public health
Use of QAC‑based products surged during the COVID‑19 pandemic, a trend documented in scientific reviews and used to help explain higher loads in wastewater, according to Environmental Science & Technology Letters. Laboratory and field studies have found that sub‑lethal exposure to QACs can spur bacterial adaptations and mechanisms linked to antibiotic resistance, as reported in Science of the Total Environment. More recent work summarized in Water Research reinforces those concerns, which is one reason public‑health and environmental scientists continue to press for source control alongside monitoring.
What agencies and residents can do
Local guidance already urges residents to pick less hazardous cleaning products and save true disinfectants for situations that clearly call for them. The "Screen for a Safer Clean" materials outline safer options and label‑reading tips from the San Francisco Environment Department. Rebecca Sutton, SFEI's lead scientist on emerging contaminants, told Maven's Notebook that "more [wastewater] treatment is not the answer" and that it is typically more effective and less expensive to keep QACs out of the sewer system in the first place. In practice, that looks like using soap and water for day‑to‑day cleaning, steering clear of QAC‑heavy wipes and sprays when they are not necessary and checking ingredient lists before buying.
SFEI is urging Bay managers to expand ambient monitoring, reconsider how biosolids are handled and evaluate whether some plants should test alternative technologies, while signaling that source reduction, not endless treatment upgrades, is likely to be the most durable way to protect the Bay. For Bay Area residents, the message is straightforward: cut back on unnecessary disinfectant use and choose safer products when actual disinfection is needed.
