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SIG wastewater treatment

To enable wastewater to enjoy a second life, almost 75 million m3 of water is treated each year in the canton of Geneva. This a drop in the ocean compared to the 89 km3 in the lake and local water table, but is an essential process for the environment and population health.

Treating wastewater for a population of nearly half a million people and local economic activity is crucial for preserving this precious resource, the environment and public health. Each year, SIG makes significant investments in wastewater facilities in order to keep up with and integrate technical developments in this field and provide local citizens the most advanced sanitary conditions possible.


Greater awareness around treatment needs for over 50 years

The first treatment facilities in the greater Geneva area date back to the 1960s, designed to meet the waste needs of the time. In the 1990s, the Swiss Confederation and local cantons set specific new criteria for wastewater discharge for each water treatment station. These criteria vary in line with the quality levels required for the various waterways which receive water once it has been processed by the water treatment plants, or STations d’EPuration (“STEP”, in French).

Wastewater makes up over 50% of cantonal drinking water

Around 2,500 litres of wastewater are collected every second in Geneva drainage systems. Rainwater and wastewater from daily activity are each collected differently. The first are carried to the nearest waterway, the second are taken to a ‘STEP’ (water treatment plant) to undergo treatment before being discharged back into a river: Solid waste extracted from water is treated as landfill at the waste treatment and recycling factory at Cheneviers. Some 12,000 tonnes of sludge and waste are transformed into energy (biogas) and used to produce cement.

Organised recycling of wastewater: vital to the water cycle

More than ever, treating wastewater is part of the virtuous circle of water recycling, which is so essential to life. The first way to limit costs and taxes on water treatment is to limit pollution at source. This is why SIG encourages everyone to adopt ‘eco-actions’ in each community, as well as investing in new STEPs (treatment plants) that are even more high performance. Geneva is one of the cities in Switzerland with the lowest costs of water treatment.



Overview of the wastewater treatment process in the canton of Geneva, with four main stages: collection, purification, release back into the waterways and optimisation of waste remaining after the purification process.

Bois-de-Bay treatment plant: a new generation of biological purification plants

SIG has harnessed new cutting edge technologies to help manage the wide range of wastewater types that require treating and to handle the complexity of the purification process required nowadays, due to the level of micro-pollutants present in wastewater. The new treatment plant in Bois-de-Bay has replaced that of Nant d’Avril, which closed after 47 years of service. The new plant, which was up and running in 2009, has a treatment capacity of 130,000 person-equivalents and treats water in the areas of Meyrin, Satigny, Aire-la-Ville and the whole of the Pays de Gex. Its new system of biological waste treatment is extremely effective and enables highly efficient elimination of pollutants contained in wastewater. Purified water released back into the Rhone once treated has produced dramatic results for the Allondon river, a tributary considered a nationally important floodplain biotope.