Humans produce large amounts of waste water. For example, Finns produce over 200 liters worth of waste water per capita each year. Waste water is full of nutrients and is one of the most important causes of eutrophication. This is why sewage treatment is important.

Waste water is treated in sewage treatment facilities. These facilities receive water from houses and buildings via a sewer system and filter the water through various mechanical, chemical and biological processes. After being processed, the water can be safely let into the lake and river system.