An excess of nutrients (phosphorus, nitrogen) in water bodies (lakes, rivers, seas) leads to an increased and rapid growth of algae. Since algae also need sunlight for growth (photosynthesis), algae mainly stay in the exposed part of the water. The high density of algae then leads to less light penetrating to the plants growing further down. As a result, the entire biodiversity of the water body is disturbed or negatively affected. Some plants will gradually die off and be replaced by less sensitive species. When the excessively proliferating algae die off in large quantities and sink to the bottom of the water body, this biomass is broken down by bacteria using up oxygen. The strong oxygen consumption is then another negative effect, so that living organisms such as mussels and fish may die.
This whole process is called eutrophication and eutrophication is a worldwide problem. The cause of eutrophication is the excessive input of nutrients into water bodies. Input via untreated or insufficiently treated wastewater is one of several input pathways. Other input pathways are agriculture and industry.
In order to minimise nutrient inputs through wastewater, the concentration of phosphorus and nitrogen in treated wastewater must be as low as possible.
