After a study shows San Francisco treatment plants are dealing with microplastic pollution, experts weigh in on possible solutions


So-called “flushable” wipes and other persistent materials are well-known problems for wastewater treatment plants. Now, studies around the country have found other insidious materials — microplastics and microfibers — entering the wastewater stream on a massive scale.

Microplastic particles measuring 5mm or smaller come from beauty products, plastic bags, polystyrene packaging and other disposable plastic items. Microfibers, a more recently identified culprit, are shed in washing machines mainly from polar fleece and other synthetic fabrics.

San Francisco Bay study

Microplastics and microfibers were found at all eight San Francisco treatment plants examined in a recent study conducted by the San Francisco Estuary Institute and the Aquatic Science Center. Collecting particles using a sieve, they found that the plants discharged an average of 33 particles per 100 gallons, and 80 percent of those particles were microfibers.

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The study also skimmed water at nine sites in the Bay Area, finding 14,000 to 2 million microparticles/km2 at each location (see figure at right). California and the U.S. EPA banned microbeads in beauty products starting in 2017, but microbeads accounted for only a small portion of the total microplastic pollution found in the Bay.


This image shows the various types of microplastic pollution researchers found in the Bay Area and other areas in the nation.

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Dr. Rebecca Sutton, the study’s leading researcher spoke to TPO magazine about her findings, saying it’s concerning because wildlife can ingest microplastics and microfibers, leading to digestive blockages or starvation. “It can also expose wildlife to toxic chemicals within the plastic,” she says. “Our understanding of this concern is emerging, so we do not yet have specific toxicity thresholds that we can use to evaluate the risks posed by current levels of microplastic pollution in the Bay.”

Sutton says the dense population of the Bay Area versus its size could explain why they found higher levels of microplastic pollution there than in other studies, including two that examined the Great Lakes and Chesapeake Bay.

While treated wastewater is one gateway for microplastic and microfiber pollution, other pathways like stormwater — which can wash plastic litter into city storm drains — are speculated to be even more consequential. But microbeads and microfibers are particularly relevant to the wastewater industry, Sutton says. “Both these types of particles wash down the drain. While most of the plastic is likely to end up in sludge or biosolids, a small portion can pass through treatment and be discharged to the Bay or other receiving waters.”

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That’s why Sutton is helping lead a two-year investigation into microplastic and microfiber pollution in San Francisco Bay. The effort kicked off earlier this year and aims to provide the nation’s most comprehensive study in the nation on microplastics in water, sediment and fish. The research also will assess stormwater runoff and wastewater as pathways.

A major scientific component of the project is to develop monitoring methods and standards to encourage similar studies across the globe. “The Bay Area is an international hub for innovation and forward-looking science,” says Sutton. “We can lead the search for solutions to this global problem.”

Examining treatment plants

Another leading researcher is Dr. Sherri Mason, chemistry professor and chair of the Department of Geology and Environmental Sciences at the State University of New York-Fredonia. Her study, Microplastic pollution is widely detected in US municipal wastewater treatment plant effluent, found reason for concern about the indirect effects of plastics.

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“Plastics are manufactured using many chemicals — plasticizers, UV stabilizers, colorants and others,” she says. “These chemicals are inherent within the plastics, but are not chemically bound to the plastic polymers, meaning they can migrate out of the plastic. Additionally, as plastics are in water, they can adsorb harmful chemicals from the water onto their surface. The point is that microplastics become little ‘poison pills.’ They carry chemicals from the external environment into the food web. These chemicals are known to have a multitude of effects.”

Mason’s study analyzed 90 effluent samples from 17 treatment plants  and averaged the results. While the number of microparticles per liter were relatively small, the results were significant considering the amount of wastewater being processed daily. The study estimated that treatment plants release more than 4 million microparticles per day in effluent.

The study wasn’t focused on the plants’ efficiency in removing microplastics. “Some other studies have come out that are more focused on that aspect, and those concluded that, yes, the removal is largely happening during skimming and settling,” Mason says. “Of course, this just means it ends up in the biosolids and the question is, how are the biosolids used? If it’s used as fertilizer, then those microplastics have just been re-released into the environment.”

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What’s the solution?

She emphasizes that no one is calling treatment plants a source of microplastic pollution, saying the urban runoff likely accounts for even more, and that the entire issue begins in households.

“People wash their faces and bodies with products that contain microbeads, they wash their synthetic clothes — of which each garment can release a minimum of 1,900 microfibers — they use synthetic sponges to wash their dishes and pieces break off, baggies get flushed,” she says.

Although her group’s study was testing microparticle levels at WWTPs, Mason says this isn’t their problem. “Wastewater treatment plants were designed to remove pee and poop and they are quite good at that. They weren’t designed to remove the multitude of synthetic chemicals now on the marketplace. I think, as a society, we need to be better about thinking of the full life cycle of products from the get-go. Flushable wipes, for example. Did anyone ask you guys what y’all (treatment plant operators) thought of this idea before they were released on the market? By all accounts, they are the bane of operators’ existence because, while they may technically flush, they don’t disintegrate they way toilet paper does. This should have been considered from the outset.”

A multi-faceted solution is best, according to Mason. Some groups are working on filters for washing machines, and other innovative filters like this microfiber catcher being developed by the Rozalia Project could soon be available. Researchers also discovered that top-loading washing machines release five times more microfibers than front-loaders. In any case, it’s likely that consumer education is a key step toward finding a solution.

An operator’s perspective

Chris Lightfoot, assistant utilities director for the city of Panama City, Florida, agrees that public education is necessary, and likely would be more successful than regulation in combating the microplastics problem.

“I personally think that trying to regulate this is virtually impossible,” he says. “There is no feasible way to look at every item a household flushes down the sewer system, and it is impossible to know where something in a lift station or treatment plant came from. The best thing, in my opinion, is education — showing how much damage these products present to our sewer systems.”

If the public is shown how much money is spent on maintenance that could’ve been prevented, it may open their eyes, according to Lightfoot. “When you show someone a $4,500 impeller that has had to be replaced because it was clogged with products that should not have been flushed, it makes them a little more cautious about flushing something — especially when their sewer and water rates are planned to rise each year.”

Lightfoot says that when issues like these present themselves to treatment plant operators, they often come up with their own methods of solving the problem.

“The only process changes I see operators making, is coming up with their own screening process that will better remove the microplastics,” he says. “A lot of times it’s nearly impossible for bar screens — automatic or manual — to catch this type of floatable. Part of an upgrade we’re currently doing includes installing a drum screen in lieu of bar screen, and hopefully this makes removal more efficient.”


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