In less polluted environments, a similar task is carried out by freshwater mussels.
Carbon is essential for the removal of nitrogen or for its conversion by bacteria, and it is generally added as carbohydrate, which can be liquid such as methanol, ethanol, or acetic acids, many or all of them derived from plant residues. This is to encourage bacteria to work. Surplus nitrogen is released by bacteria to air. Unless bacteria are encouraged and allowed to work, nitrates move easily through sub-soils, in which no plants or bacteria can live, and can emerge in wells and streams.
Filtration can be combined also with pH adjustment. This is as simple as placing crushed marble or limestone as a layer in a tank, or casting burnt lime over a pond before filling and, if necessary, after filling. Crushed shells or even whole shells in water tanks and ponds have the same effect. Lime flocculates particles, causing them to settle out of the water.
There are several techniques for filtration, some or all of which can be used in series. First, trickle filters of loose pebbles (2,5-10cm) can be used to form an active bacterial surface layer to absorb nutrients, then a sand filter can be used to absorb bacterial pollution. Water rising through a sand column is fairly clear.
Water, now fairly clean, can be passed through a bed of watercress to remove dyes and nitrates, and the cress cut and fed to animals or dried and burnt to ash. As a final process, the water can be trickled through a column (a concrete pipe on end) of active carbon (10%) and silicon dioxide (90%), otherwise known as burnt rice, oat, or wheat husks.
The results should be clear, sparkling, safe water to drink. No machinery is involved if the system is laid out downslope to permit gravity flow.
Lime (freshly burnt) is often used to remove phosphorus and sludge in a primary settling lagoon, and then water is passed to a trickle tower for ammonia removal by bacteria. In small towns, the water can be passed from filter towers to sewage lagoons, which in fact may become rich waterfowl and forest sanctuaries. It can then be routed to field crop such as forest, pasture, and to crops to be distilled or burnt, which does not directly re-enter the food chain.

Sewage treatment using natural processes

Raw sewage is a mixture of nutrients, elements, heavy metals, and carbon compounds; it also contains quite dangerous levels of bacteria, viruses, and intestinal worm eggs. Raw sewage runs into the pond, where it settles out. This settling pond is anaerobic, and gives off biogas, a mixture of methane, carbon dioxide and ammonia gas, with traces of nitrous sulphide or nitrogen dioxide. Biogas is, of course, a useful fuel gas for engines, or a cooking gas for homes.