On 12 September 2026, a joint project by Swiss research institutes Eawag, Empa and Agroscope reported the first nationwide estimate of microplastic releases from Swiss wastewater treatment plants and showed that atmospheric deposition to Swiss waters is roughly twice as large as the plant discharge figure. The work, funded by the Federal Office for the Environment (FOEN), places measured plant-level emissions at 4.7 tonnes per year and atmospheric inputs at around 10 tonnes per year AZoCleantech.
Key takeaways
- Swiss WWTPs discharge an estimated 4.7 tonnes of microplastics per year, based on sampling at 51 plants serving about 44% of the population AZoCleantech.
- Empa estimates around 10 tonnes of microplastics per year reach Swiss waters from the atmosphere, roughly double the WWTP figure AZoCleantech.
- Per-capita WWTP release is about 0.5 g of microplastics per person per year AZoCleantech.
- The method measured particles attached to sewage sludge flocs larger than 20 µm rather than free particles in treated effluent, giving more reliable counts AZoCleantech.
- Atmospheric transport of plastic particles, including to remote Alpine sites, is documented separately in nanoplastic snow work across Italy, Switzerland and France Nature.
What happened: a national baseline for WWTP microplastic releases
The project, described in the journal ACS ES&T Water, set out to fix what researchers call a basic data gap: "Analyzing microplastics is a complex process, and many studies use different methods. This makes it difficult to compare results and extrapolate them to larger regions," said PhD student Guillaume Crosset-Perrotin of Eawag's Ralf Kägi group, who led the study AZoCleantech. The team sampled 51 plants covering about 44% of the Swiss population, then scaled up to a national figure of 4.7 tonnes of microplastics per year leaving the plants AZoCleantech. That equals about half a gram per inhabitant per year, "roughly the amount of plastic in a drinking straw" AZoCleantech.
Instead of chasing free-floating particles in treated effluent, the researchers counted microplastics stuck to sewage sludge flocs and used each plant's routine floc discharge data to back-calculate the load leaving the works AZoCleantech. "Microplastics adhere to larger sludge flocs. Although these are also present in treated wastewater, their concentration there is significantly lower," the study notes AZoCleantech. The published estimate excludes tyre-abrasion particles, which are being tackled in separate joint work because of analytical limits AZoCleantech.
Industrial and municipal plants showed no systematic difference, although a few outliers carried unusually high loads of polypropylene particles, with the authors pointing to a PP carrier material used in one treatment stage as a likely in-plant source of particles to the sludge AZoCleantech. Limited comparable international data, for example from Denmark, are described as of a similar order of magnitude AZoCleantech.
What happened: the atmosphere is the bigger pathway
The same programme includes a parallel atmospheric study led by Christoph Hüglin of Empa, which estimated about 10 tonnes of microplastics per year reaching Swiss waters from the air, roughly twice the WWTP figure AZoCleantech. The point, the authors say, is not to rank blame but to size the pathways correctly: "Only with reliable data can we determine where the biggest sources of pollution lie and which measures are appropriate," Crosset-Perrotin said AZoCleantech.
Independent peer-reviewed work published in August 2026 reinforces the atmospheric pathway. A study of high-altitude glacial snow across Italy, Switzerland and France, published in Scientific Reports on 15 August 2026, detected six common plastic polymers in samples from 19 sites between 2,300 m and 3,800 m, with polyethylene and polypropylene contributing more than 60% of the measured mass Nature. The work used thermal desorption proton transfer reaction mass spectrometry (TD-PTR-MS) and citizen-science sampling by trained mountaineers, and flagged local agriculture and plastic factories as possible point sources Nature.
| Pathway | Reported load to Swiss waters | Source |
|---|---|---|
| Wastewater treatment plants (51 sampled, 44% population coverage, scaled nationally) | 4.7 t/yr (~0.5 g per inhabitant per year) | AZoCleantech |
| Atmospheric deposition (Empa estimate) | ~10 t/yr | AZoCleantech |
| Atmospheric nanoplastic signal in remote Alpine snow (Italy, Switzerland, France, 2,300–3,800 m) | Mean 85 ng mL⁻¹ (range 3.6–470 ng mL⁻¹) | Nature |
Specification read: what this means for plant operators and buyers
Converting the reported number to procurement units: 4.7 t/yr from plants serving roughly 44% of Switzerland's population, which is about 3.9 million people on an ~8.8 million national base, implies an order-of-magnitude load of about 1.2 tonnes of microplastics per million inhabitants per year exiting secondary treatment, or roughly 0.5 g per person per year AZoCleantech. For a mid-sized municipal works in the 50,000 m³/day to 200,000 m³/day band (general industry range, not from the sources), that scales to an effluent-side microplastic loss on the order of a few kilograms per day across the catchment, which is well below the visible solids load that screens, primary clarifiers and tertiary filters are normally sized on, but it sits in the same effluent envelope that disinfection and reuse polishing steps must demonstrate control of.
The reported event bears on the secondary biological stage and on sludge handling, because the Swiss method deliberately tracked particles bound to sludge flocs rather than free particles in the clarified stream AZoCleantech. In a conventional train (screening, grit removal, primary clarification, biological treatment such as A2O, MBR, MBBR, SBR or UASB, followed by tertiary clarification, filtration and disinfection, with DAF or ZLD where reuse or brine limits apply) the lever is the sludge side: WAS thickening, digestion and dewatering determine how much of the plastic load leaves in biosolids versus in final effluent, and the Swiss data suggest that for some polymers, the carrier materials used inside the process itself can be a measurable in-plant source. Buyers looking at Municipal trains should expect to see microplastic mass-balance questions asked in tender documents alongside BOD, COD and TSS envelopes, and operators of Food and Oil sites with high polymer use in process aids should expect the same scrutiny on the sludge line.
Recognition sentence: if your plant runs between 10,000 and 200,000 m³/day, treats mixed municipal sewage at typical BOD of 150–350 mg/L and COD of 300–700 mg/L (general industry range, not from the sources), and you are chasing a reuse permit, a tighter effluent consent or a ZLD envelope, this study is the kind of dataset that lets a regulator argue for a microplastic mass balance across the sludge line as well as the effluent, so any procurement step that touches floc handling, polymer dosing or tertiary filtration should now be specified with that audit in mind. For deeper dives on high-recovery RO and ZLD trains that sit downstream of this kind of secondary-plus-tertiary envelope, see the comparisons on ZLD vs High-Recovery RO for High-BOD FOG Wastewater: 2026 Brine Mandate Guide and ZLD vs High-Recovery RO for Paint & Battery Wastewater at EV Plants: 2026 Brine Mandate Comparison.
FAQ
How much microplastic is a typical Swiss WWTP releasing per year?
The national average implied by the 51-plant sample is about 0.5 g per inhabitant per year, or 4.7 tonnes per year across the country AZoCleantech. A plant in the 50,000 m³/day class (general industry range, not from the sources) would therefore be expected to release on the order of a kilogram or two of microplastics per day, with significant plant-to-plant scatter.
What does this change in a tender specification?
Expect requests for a microplastic mass balance across the sludge line as well as the effluent, sampling tied to sludge flocs rather than grab samples of treated water, and explicit exclusion or inclusion of tyre-abrasion particles, which the Swiss study did not cover AZoCleantech. Lead time impact is modest for monitoring (general industry range, not from the sources): a floc-based sampling protocol can be added within a few weeks of a routine sludge survey.
Is the atmosphere a bigger problem than the plant?
Which suppliers or technologies does this favour?
The reported evidence points at sludge-side handling and tertiary filtration rather than at any single vendor. WWAS thickening, anaerobic digestion, dewatering, and fine-pore membrane or disc-filter stages are the parts of the train where a floc-borne microplastic load is most visible, so procurement evaluations should weight those stages' microplastic removal data alongside BOD, COD and TSS performance.
