What happens to wastewater treated by purifiers during drought?

During the summer of 2026, Swiss rivers reached drastically reduced levels, but wastewater from sewage treatment plants remained constant. In Aargau, up to 80% of the flow of some rivers consisted of purified water. Despite the treatment, these waters remain contaminated by pollutants and nutrients, which previously underwent strong dilution in normal rivers. With reduced flow, the concentration of contaminants increases, favoring the abnormal proliferation of algae.

Why are many Swiss purifiers unable to filter water completely?

Most Swiss sewage treatment plants still lack the 'fourth stage' of cleaning, a technology that eliminates the smallest micropollutants. These purifiers are designed to handle certain normal water volumes, not to intervene quickly during extreme conditions such as summer drought. As a result, many micropollutants still end up in rivers.

How many Swiss sewage treatment plants will need to be upgraded by 2040?

According to federal data, at the end of 2024 there were almost 40 plants upgraded to the fourth stage of cleaning. The goal by 2040 is to bring the total to 140 facilities in Switzerland capable of removing critical micropollutants. This means that around 100 plants are still awaiting significant technical updates over the next 16 years.

What is the plan of the Canton of Aargau to solve the problem of purification?

The Argovian Government has decided to radically consolidate the purification network, going from the current 41 small and scattered plants to only 20 large regional plants. These new facilities will be located near major rivers where dilution is guaranteed even during periods of drought. This approach reduces operating costs and significantly increases treatment efficiency and environmental protection.

What are the visible damages of residual contamination in Swiss rivers?

Chemical elements not completely filtered have favored an abnormal proliferation of algae, especially during drought when temperatures rise and the current slows down. The environmental conditions become similar to those of a pond rather than a river: the water is watertight, the chemical elements are concentrated, the fish fauna suffers from the lack of oxygen, and the aquatic ecosystems undergo radical changes in biological composition.

Droughts and purifiers: rivers in crisis (cross-border guide)

Di Redazione Frontaliere Ticino · Published on 31 August 2026

Aerial view of a Swiss Alpine lake during summer drought with reduced water levels and diminished waterstreams

Swiss rivers drained by drought: wastewater remains constant. In Aargau up to 80% of the flow is contaminated purified water. Federal program 2024-2040 to upgrade 140 plants.

Context

In a nutshell

Swiss rivers have drastically reduced levels due to summer drought 2026
Wastewater from purifiers remains constant, creating imbalance
In Aargau up to 80% of the river flow comes from sewage treatment plants
Purified water still contaminated with pollutants and nutrients

Key facts

What: Environmental imbalance caused by droughts and sewage treatment plants
When: Summer 2026
Where: Canton Aargau, Swiss rivers (Limmat and others)
Who: Cantonal Office for the Environment Aargau, Martin Märki (expert), Felix Kreidler (president of the Aargau Sewage Treatment Association)
Critical percentage: 80% of the river flow from sewage treatment plants (in some Argoviesi rivers)
Federal timeline: 40 plants updated by the end of 2024; target 140 by 2040
Cantonal investments: Aargau will reduce from 41 to 20 regional plants

The extreme drought of recent months has led to a drastic drop in Swiss river levels. However, the amount of water consumed by the population — and therefore of wastewater that ends up in sewage networks — has remained almost constant. This imbalance generates a concrete problem that the cantonal environmental office of Aargau cannot ignore: concentrated pollution.

Martin Märki, water protection expert at the Argovian body, explains the mechanism with mathematical clarity: "If the natural flow of a watercourse falls while the volume discharged by the purifiers remains unchanged, the share of the latter,

Operational details

Obsolete technologies: the 4th stage is still absent

Most Swiss sewage treatment plants are not yet equipped with the so-called "fourth stage" of cleaning — a technology that eliminates the smallest micropollutants, the very ones that today still end up in rivers. Felix Kreidler, president of the Association of Water Treatment Plants of the Canton of Aargau, frankly acknowledges the limit: "These facilities are designed to handle certain volumes of water, therefore it is not possible to intervene in a timely manner. In addition, many purifiers are not yet equipped with the fourth stage of cleaning, which allows the elimination of micropollutants that still end up in rivers today. "

The problem is twofold. On the one hand, plants are “calculated” for normal volumes — when the natural water cycle is in equilibrium. During the summer drought, this balance collapses, and purifiers find themselves dealing with disproportionate percentages of effluent relative to the total volume of the rivers. On the other hand, the technology available to most plants is simply not advanced enough to filter out the smallest molecules.

Visible effects on river ecosystems

The consequences are already tangible. Abnormal algal blooms are not an isolated phenomenon — they are a symptom of a deeper imbalance in aquatic ecosystems. When temperatures rise, the current slows, and the waters do not

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Key points

Consolidation of the network: fewer plants, larger, more efficient

The proposed solution is not only to upgrade the plants one by one, but to rethink the entire Swiss purification network. The strategy of Kreidler and the cantons is to consolidate: to decommission small and old local plants in favour of new state-of-the-art and centralised facilities, located near large rivers where effective dilution is guaranteed even during periods of drought.

Canton Aargau is a pioneer of this radical approach. It currently manages 41 small and medium-sized plants scattered throughout the territory. The goal of the Argovian Government is drastic: to halve the number of facilities, passing to only 20 large regional plants. This transition is not a simple administrative optimisation — it is a massive investment in public infrastructure that will require years of planning, cantonal authorisations and significant funding. However, the return — in terms of water quality, ecosystem protection, and operational efficiency — justifies the expense.

Federal 2024-2040 Timeline: Counting Remaining Jobs

The federal modernization program will proceed for the next 16 years on a precise trajectory. The current situation is clear:

End of 2024: 40 plants upgraded with micropollutant removal technology Target 2040: 140 plants with fully operational fourth stage Plants still in

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Source: rsi.ch

Frequently Asked Questions

What happens to wastewater treated by purifiers during drought?: During the summer of 2026, Swiss rivers reached drastically reduced levels, but wastewater from sewage treatment plants remained constant. In Aargau, up to 80% of the flow of some rivers consisted of purified water. Despite the treatment, these waters remain contaminated by pollutants and nutrients, which previously underwent strong dilution in normal rivers. With reduced flow, the concentration of contaminants increases, favoring the abnormal proliferation of algae.
Why are many Swiss purifiers unable to filter water completely?: Most Swiss sewage treatment plants still lack the 'fourth stage' of cleaning, a technology that eliminates the smallest micropollutants. These purifiers are designed to handle certain normal water volumes, not to intervene quickly during extreme conditions such as summer drought. As a result, many micropollutants still end up in rivers.
How many Swiss sewage treatment plants will need to be upgraded by 2040?: According to federal data, at the end of 2024 there were almost 40 plants upgraded to the fourth stage of cleaning. The goal by 2040 is to bring the total to 140 facilities in Switzerland capable of removing critical micropollutants. This means that around 100 plants are still awaiting significant technical updates over the next 16 years.
What is the plan of the Canton of Aargau to solve the problem of purification?: The Argovian Government has decided to radically consolidate the purification network, going from the current 41 small and scattered plants to only 20 large regional plants. These new facilities will be located near major rivers where dilution is guaranteed even during periods of drought. This approach reduces operating costs and significantly increases treatment efficiency and environmental protection.
What are the visible damages of residual contamination in Swiss rivers?: Chemical elements not completely filtered have favored an abnormal proliferation of algae, especially during drought when temperatures rise and the current slows down. The environmental conditions become similar to those of a pond rather than a river: the water is watertight, the chemical elements are concentrated, the fish fauna suffers from the lack of oxygen, and the aquatic ecosystems undergo radical changes in biological composition.