Microplastics in Wastewater – Poison Accumulating in Sewage Sludge
In the first two parts of our series on micro- and nanoplastics, we wrote about their physiological effects and their presence in our environment.
To close the series, we now look at how large a share of micro- and nanoplastic particles wastewater treatment can remove, and what it would take to stop burdening our natural environment with microplastics through the rivers that receive treated effluent.
Looking for solutions
The most common biological wastewater treatment plants are optimised to remove organic matter and inorganic nutrients, so they can remove only part – albeit a significant part – of microplastics, and they cannot filter out nanoplastic particles, even though plastic particles of 0.001–5 millimetres enter the sewer network from almost every household and industrial process. Currently only membrane technology can filter out plastics this small, but above a certain plant size it places an unbearable financial burden on the operator.
That is why more and more research focuses on new biological methods that could offer a cheaper alternative to expensive membrane technologies. In this article we follow the path of microplastics through wastewater, look at current treatment results, environmental risks, EU requirements and future solutions.
How do microplastics get into wastewater?
Wastewater is one of the most important collection points for microplastics. Households are the largest source. Most people have no idea that a single cycle of an average washing machine can release up to 700,000 tiny synthetic fibres from polyester, polyamide or acrylic clothing. These fibres go straight into the sewer with the wash water. Similarly large amounts come from cleaning products and cosmetics (microbeads in scrubs, toothpastes and shower gels).
On the industrial side, the main sources are waste from plastics manufacturing, textiles and packaging discharged to sewers, while in combined sewer networks dust from traffic and particles from tyre wear are also washed in through the urban stormwater system. Atmospheric deposition and urban dust continuously feed the system.
The most common polymers are polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS) and polyamide (PA). They appear in wastewater as fibres, fragments, films or spheres, on average a few hundred particles per litre.
How much does wastewater treatment remove, and where does the rest go?
Conventional mechanical-biological-chemical plants were not specifically designed to remove microplastics, yet they are surprisingly effective. According to international meta-analyses, removal ranges between 70% and 99% (Iyare et al., 2020).
According to a comprehensive study by Hu et al. (2024), the primary treatment stage (screening, grit and grease removal, primary settling) already achieves 57–88% efficiency. The secondary, biological stage (activated sludge) can add a further 42–99%. The tertiary stage (biological nutrient removal) improves the result even further.
The vast majority (90–99%) of the microplastics removed end up in sewage sludge. The part not removed reaches rivers and lakes with the treated effluent.
Even so, the environmental impact is worrying: microplastics reaching rivers are taken up by plankton, fish and shellfish, bioaccumulate in the food chain, reduce organisms' reproductive capacity and carry toxic additives (phthalates, heavy metals).
Tightening requirements
The recast EU Urban Wastewater Treatment Directive (EU 2024/3019) entered into force on 1 January 2025 and makes microplastic monitoring mandatory at plants above 10,000 population equivalent – for influent, effluent and sludge alike. Delegated Decision (EU) 2024/1441 lays down a harmonised measurement methodology. These rules are part of the EU's Zero Pollution Action Plan. Compared with conventional treatment systems, membrane technologies offer a more reliable solution:
- Membrane bioreactor (MBR): combining UF/MF membranes with biological treatment achieves 95–99.9% removal.
- Ultrafiltration (UF) and nanofiltration (NF): 95–99.9% efficiency.
- Reverse osmosis (RO): >99.9%, especially for nanoplastics.
With these methods, the microplastic concentration in treated water can be reduced to practically zero, but their capital and operating costs are very high, which puts them out of reach for most plant operators.
Promising biological alternatives
More and more research is looking at microbiological methods that promise technical solutions which can be built into existing activated sludge plants, while offering a chance to replace expensive membrane technologies.
Researchers at the University of Waterloo introduced PET-degrading genes into ordinary wastewater bacteria by conjugation. The modified strains degraded 40% of a PET film in just four days at 50 °C (Yip et al., 2024).
A team at Northwestern University achieved similar results with Comamonas testosteroni, a naturally occurring wastewater bacterium that breaks PET down into nanoplastics, then with an enzyme into monomers, and finally uses it as a carbon source (Wilkes et al., 2024).
These methods need little energy, do not require high pressure and could be cheaper than reverse osmosis in the long run. Although still largely experimental, they look promising for retrofitting conventional plants.
What happens to the microplastics that end up in sewage sludge?
So only a small fraction of microplastics reach nature with treated wastewater, since well-functioning plants already remove most of them. But the microplastic particles that end up in sewage sludge remain a problem to be solved.
Producing biogas from sewage sludge, which can be used as an energy carrier, is an advanced solution in energy terms. The question is whether anaerobic digestion also solves the microplastic problem. Unfortunately, the answer is no!
Microplastics (MPs) and nanoplastics (NPs) do not break down significantly during anaerobic digestion.
The process is optimised mainly for breaking down organic matter (to produce biogas), but plastic polymers are extremely stable, and digester conditions (35–55 °C, an oxygen-free environment, a retention time of 15–30 days) are not enough to break them down.
Since organic matter breaks down during digestion, the microplastic content becomes relatively concentrated in the remaining dry matter. So what happens if digested sewage sludge is used as fill or soil improver? It pollutes nature directly – and the microplastics accumulate in the soil for the long term. They persist for decades, even 20–100 years, and with repeated sludge application the microplastic content of the soil rises sharply (by as much as 700–1,400% within a few years).
The effects are far from reassuring: microplastics alter soil structure, reduce water-holding capacity, harm the microbial community, can inhibit plant root growth and can enter the food chain.
Thermal processes (incineration, pyrolysis, hydrothermal treatment), which effectively break down plastics, offer a possible way to remove plastics from contaminated sewage sludge. Advanced composting and enzymatic/biotechnological post-treatment are still at an experimental stage.
So if sludge is to be used in agriculture, its microplastic content poses a significant environmental risk, which is why alternative uses (such as land reclamation or fuel for cement kilns) are coming to the fore in many places.
A partial result is just an expensive self-deception
EU requirements, membrane technologies and new biological methods (modified bacteria and enzymes) together make it possible to tackle this modern challenge ever more effectively and economically. Wastewater treatment plants are thus increasingly dealing not only with conventional pollutants but also with hidden plastic pollution – protecting our rivers and drinking water. Yet the spotlight still does not fall on sludge treatment and the final elimination of microplastics from sewage sludge – and without that we have achieved only partial success: we are freeing our rivers of plastics while continuing to pollute our land.
Conventional wastewater treatment plants remove 70–99% of microplastics, but the vast majority of the extracted particles end up in sewage sludge. Anaerobic digestion – although beneficial for biogas production – cannot break down plastic polymers. If the sludge is spread on farmland, microplastics go straight into the soil, where they accumulate for decades. Membrane technologies offer near-complete removal, but their high cost means few plants can use them. PET-degrading bacteria and enzymes are a promising alternative. The EU's Directive 2024/3019 requires mandatory monitoring, but a full solution also requires sludge treatment to be sorted out.
Tackling micro- and nanoplastic pollution is one of the most complex challenges facing modern wastewater treatment. Plastic particles – fibres, fragments, microbeads – from households, industry and urban stormwater continuously reach treatment plants through the sewer network.
The good news is that conventional mechanical-biological treatment plants – although not designed for this – remove 70–99% of incoming microplastics. According to a comprehensive study by Hu et al. (2024), the primary treatment stage alone achieves 57–88% efficiency, and the biological stage can add a further 42–99%. The vast majority of the removed particles, 90–99%, end up in sewage sludge, while the rest reaches receiving surface waters, where they enter the food chain and bioaccumulate.
The bad news is that the fate of microplastics concentrated in sludge remains unresolved. Anaerobic digestion – an advanced and beneficial process for biogas production – cannot break down plastic polymers, because digester conditions are not sufficient. If the sludge is used on farmland, microplastics go straight into the soil, where they persist for decades, alter soil structure and enter the food chain.
The most reliable technical solutions are membrane technologies, which remove microplastics with nearly 100% efficiency, but their high cost puts them out of reach for most plants. That is why research into plastic-degrading bacteria and enzymes, such as the work of the University of Waterloo (Yip et al.) and Northwestern University (Wilkes et al.), is so important. The EU's Directive 2024/3019 requires mandatory monitoring, but a complete solution also requires the question of sludge treatment to be resolved.
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