The sight of people launching themselves into an inner-city waterway would be unthinkable in most cities in Europe, the US and many other parts of the world. Three-quarters of Britain’s rivers are in poor ecological health, according to data collected by thousands of citizen scientists in 2024, with experts describing the findings as “truly disturbing”.

Pollution from water companies and agricultural runoff are driving the contamination, which affects all parts of the country, causing toxic algal blooms, mass die-offs of fish, and risks to human health.

Yet across Switzerland, such swimming scenes are normal. This hasn’t always been the case. In the 1960s, Switzerland had among the dirtiest water in Europe, blighted by mats of algae, mountains of foam, scum, and dead fish floating on the surface. For decades, swimming was banned in some rivers such as the Aare and Limmat on health grounds, and people could get ill if they swallowed the water.

Raw sewage and industrial wastewater flowed directly into water bodies – in 1965 only 14% of the population was connected to a wastewater treatment plant. Today, it is 98%, and the country has a reputation for pristine swimming waters, sometimes referred to as its “blue gold” – and it’s all thanks to a complex network of sewage plants.