On 31 August 2026, a report from bluewin.ch detailed how prolonged dry weather in Switzerland has pushed the proportion of treated wastewater inside some small streams to as much as 80 percent, with water protection experts already flagging exceedances of pollutant thresholds in random samples. The picture is one of low dilution, residual micropollutants and heat stress for aquatic life, and it is being treated by cantonal authorities as a live operational issue for treatment plant outfalls as well as for swimmers.
Key takeaways
- Up to 80 percent of the water in some small Swiss streams currently consists of treated wastewater, according to Aargau water protection expert Martin Märki, as reported by bluewin.ch.
- Over the past 90 days, only 50 to 70 percent of the usual precipitation has fallen across large parts of the northern side of the Alps, and around 30 percent in the Basel region, per federal data cited by bluewin.ch.
- Märki told SRF, via bluewin.ch, that certain pollutant thresholds had been exceeded in random samples in Aargau.
- The Federal Office for the Environment (FOEN) describes the input of micropollutants as the greatest current challenge for Swiss water quality, according to bluewin.ch.
What happened: drought, dilution and rising effluent share
Households continue to discharge the same volume of wastewater during dry spells, but the receiving streams carry far less water, so the share of treated effluent in the channel rises. "As SRF reports, the water in some small streams may currently consist of up to 80 percent treated wastewater," wrote bluewin.ch. The figure comes from Martin Märki of the Aargau Department of the Environment and primarily applies to small water bodies receiving one or more wastewater treatment plant discharges, the report said.
Federal precipitation data underline how unusual the hydrology is. Over the past 90 days, only 50 to 70 percent of the usual rainfall has fallen across large parts of the northern side of the Alps, and in the Basel region the figure was as low as about 30 percent, as cited by bluewin.ch. The Federal Drought Bulletin expects low water levels in the north to worsen again after a brief recovery, bluewin.ch added.
What happened: micropollutants, ecotoxicology and what comes next
Conventional treatment removes most contaminants but cannot fully strip substances such as medication residues, pesticides and other everyday chemicals, classed as micropollutants. FOEN describes their introduction as the greatest challenge currently facing the quality of Switzerland's waterways, bluewin.ch reported. Diclofenac, a common anti-inflammatory active ingredient, is poorly broken down in conventional plants and can damage the liver, kidneys and gills of fish, with one metabolite shown by Eawag to be even more toxic to aquatic organisms than the parent compound, the same report said.
"Water protection expert Martin Märki told SRF that certain pollutant thresholds had been exceeded in random samples," bluewin.ch wrote. A 2022 sampling round found 18 of 22 tested pesticides and pharmaceuticals exceeding their ecotoxicological threshold in at least one of 38 water bodies, with only six of those 38 meeting all limits, according to bluewin.ch. Exceeding an ecotoxicological threshold is not the same as a swimming ban; more than 97 percent of assessable Swiss swimming areas still meet at least adequate bathing water quality under FOEN's E. coli and intestinal enterococci checks, the report noted.
The Canton of Aargau currently recommends avoiding entering small and medium-sized water bodies where possible to protect heat-stressed fish, and the Canton of Zurich advises waiting two to three days after heavy rainfall before swimming in rivers, because stormwater flushes accumulated pollutants into combined sewers and can trigger overflow discharges, as bluewin.ch detailed. Long-term, Switzerland's broader effort to control combined sewer overflows and aging infrastructure has played out for years elsewhere as well; for example, a separate April 2026 Contrarian Boston report described the Massachusetts Water Resources Authority preparing plans to plug remaining combined sewer overflows into the Charles and Mystic rivers.
Specification read: what 80 percent effluent share means at the outfall
The reported figure is a volumetric share, not a load: a small receiving stream at, for example, 500 m³/day of baseflow (general industry range, not from the sources) would see roughly 400 m³/day of treated effluent and only about 100 m³/day of natural flow during this drought pattern. In population-equivalent terms, a single mid-sized municipal plant discharging around 20,000 m³/day to a small receiving water could effectively define the river's chemistry at low flow, which is why consent limits on BOD, COD, ammonia and total phosphorus become acutely binding in dry summers. The event bears directly on the biological and tertiary stages of a conventional municipal treatment train: activated sludge or MBBR for organics and nitrification, a tertiary polishing step (general industry range, not from the sources) for residual BOD/COD and phosphorus, and advanced treatment for the micropollutants that conventional plants only partially remove, with diclofenac as the worked example flagged by the Swiss report.
If your plant is a small-to-medium municipal works on a low-flow receiving water, discharging on the order of 5,000 to 50,000 m³/day with conventional BOD/COD loads, and you are facing tightening micropollutant expectations or summer low-flow consent limits, this is the case for reviewing tertiary polishing capacity and planning an advanced treatment upgrade rather than relying on dilution. The integrated treatment train for that profile is laid out on our Municipal hub and in the dedicated Integrated Sewage Treatment range.
FAQ
What does "80 percent treated wastewater" actually mean for effluent quality?
It is a volumetric share of the receiving stream, not a pollutant concentration, according to bluewin.ch. A stream that is 80 percent treated effluent is not automatically 80 percent "dirty"; what matters is the measured pollutant load at that specific location.
Is it still safe to swim in Swiss rivers and lakes during this drought?
More than 97 percent of assessable Swiss swimming areas still have at least adequate bathing water quality under FOEN's bacterial checks, bluewin.ch reported, but Aargau currently recommends avoiding small and medium-sized water bodies, and Zurich advises waiting two to three days after heavy rain before swimming in rivers.
How long does an advanced treatment upgrade for micropollutants typically take to procure and install?
Lead times vary widely with capacity, technology choice and site conditions (general industry range, not from the sources), and a buyer should confirm current timelines with shortlisted suppliers when scoping.
How do I shortlist suppliers for a tertiary or advanced treatment stage?
Match the proposed train to the reported effluent profile, check reference plants on similar receiving waters, and compare whole-life cost against consent risk. For a buyer-focused walkthrough of selection criteria, see the Effluent Treatment Plant in London: 2026 Buyer's Engineering Guide and the Residential Wastewater Treatment in the UK: 2026 Buyer's Guide.
