The excretions of animals rich in ammonia add some nitrogen to the water. There are some bacteria that can transform nitrates into free molecular nitrogen. It also causes eutrophication which increases plant growth and decay which will increase the demand for biochemical oxygen. Nitrates mainly come from sewage that is from sewage plants, sewer connections, and bad septic systems. The household water goes through a main pipe into a box that is called a septic tank.
Water is washed through while materials are collected.
In systems that are working properly, the solid particles remove nutrients before they are able to reach the groundwater. These systems must be properly placed away from the water resources and must be emptied ever once and a while so it doesn't clog and some materials won't enter the waterways. The problem with most filters is that they can't stop all nitrates. Also, when wells build up with nitrates, it is best to just build a new well. A problem called methemoglobinemia is caused when water with high levels of nitrates is put into infant milk and causes the blood not to carry oxygen.