Wastewater Plants Catch Most Microplastics — Then Pass Them to the Sludge

Wastewater treatment is good at getting microplastics out of the water. The catch, according to a new article in WaterWorld, is where those particles end up: mostly in biosolids, the treated sewage sludge that is often spread on land.

What the article reports

Dr. Charlotte Haberstroh is Black & Veatch's Microplastics Research & Strategy Leader. She writes that conventional treatment plants often remove more than 90% of microplastics from incoming wastewater, mainly through settling, flotation and filtration. Citing earlier studies, she notes that advanced filter-based processes such as biofilters, ultrafiltration and rapid sand filters achieve high removal efficiencies.

That removal, though, is largely a transfer. Research cited in the article puts 69–80% of the microplastics in raw wastewater in the sludge, where concentrations are "several orders of magnitude higher" than in treated effluent. Microplastics persist through stabilization steps like digestion, thickening and dewatering because they resist biological degradation, and in some cases they may fragment into smaller pieces.

The scale is large. According to the article, the U.S. produces 6 to 7 million dry metric tons of wastewater solids each year from more than 14,000 publicly owned treatment works serving over 230 million people. Citing EPA data, it says land application makes up about 60% of biosolids management, landfilling about 25% and incineration 14%.

Once biosolids reach the land, the article says, microplastics can be carried off by surface runoff, erosion, infiltration and wind. That can take them back into stormwater systems, streams and rivers, or down through soils, with "potential implications for groundwater quality." Haberstroh calls this "a feedback loop": plastic captured at the plant can make its way back into the aquatic environment.

The article also flags nanoplastics, particles smaller than one micrometer, as the "smaller (and likely bigger) concern." They are expected wherever microplastics are found, but they are far harder to measure in sludge and much less studied.

On regulation, the article notes that microplastics are now on the EPA's Contaminant Candidate List. EPA's April 2026 draft of its sixth list (CCL 6) includes microplastics as a contaminant group; the list flags unregulated drinking-water contaminants that may be considered for future regulation, and it is not yet final.

Why it matters

Biosolids sit where wastewater treatment, agriculture and water quality meet. The article argues that the high concentration of microplastics in biosolids is also an opportunity for streamlined management, rather than letting the particles re-enter the water cycle through diffuse sources. It frames choices about treatment, stabilization and end use (land application, landfill or thermal treatment) as decisions that shape where the plastic ultimately goes.

What the author recommends

• Source control first. Strengthen industrial pretreatment programs and work with commercial and industrial dischargers to cut what enters the sewer.
• Better solids capture. Enhanced primary and secondary treatment and advanced filtration, while minimizing fragmentation into smaller particles.
• Measure more. Targeted sampling and monitoring across treatment stages, plus pilot testing of emerging removal or destruction technologies.
• Share the load. In the article's words, "the burden of managing microplastics cannot rest with utilities alone." Regulators, manufacturers and industry all have a role.

Source: Dr. Charlotte Haberstroh (Black & Veatch), "Microplastics in Biosolids," WaterWorld magazine, Sept/Oct 2026 (Wastewater Digest section, pp. 32–34); originally published online by Wastewater Digest, May 28, 2026, as Microplastics in biosolids: An emerging challenge for the urban water cycle. Figures and quotes are from that article; this is our summary.

Video B-roll via Wikimedia Commons: Microplastics in sediments (Martin Wagner et al., CC BY 4.0); Overflowing water drain Werrington March 2022 (Xyxyzyz, CC BY 3.0); Rio Grande River at Pilar, New Mexico August 2022 (New Mexico Outdoor Sports Guide - NMOSG, CC BY 3.0); Rioolwaterzuivering Dokhaven (waterschap Hollandse Delta, CC BY 3.0); Rioolwaterzuivering Goedereede (waterschap Hollandse Delta, CC BY 3.0); Rioolwaterzuivering Piershil (waterschap Hollandse Delta, CC BY 3.0); Rioolwaterzuivering Spijkenisse (waterschap Hollandse Delta, CC BY 3.0); Secondary settlement tank Viikinmäki Wastewater Treatment Plant 1 (Antti Leppänen, CC BY 4.0); Sewage sludge applied (6305085533) (SuSanA Secretariat, CC BY 2.0); Street drain in downtown LA during atmospheric river - February 2024 - Sarah Stierch (Missvain, CC BY 4.0). Public-domain and CC0 footage and photos: USDA, U.S. Fish and Wildlife Service and Wikimedia Commons contributors. Featured image: our composite on a public-domain USDA photo.