This lack of nutrient recovery and utilization leads to a linear flow of nutrients from agriculture, through humans, to receiving water bodies. Even when sewage sludge is used in agriculture, only a small part of the nutrients contained in the excrement is reintroduced into the living layer of the soil. Most of it is destroyed in the treatment process (e.g. by removing nitrogen) or enters the water cycle, where it pollutes the environment, causing eutrophication of lakes and waters.