This lack of nutrient recovery and utilization leads to a linear flow of nutrients from agriculture, via humans to receiving water bodies. Even when sewage sludge is used in agriculture, only a very small fraction of the nutrients contained in the excrement are reintroduced into the living layer of the soil. Most are either destroyed during the treatment process (e.g. by nitrogen removal) or enter the water cycle, where they pollute the environment, causing eutrophication of lakes and waters.