Wastewater from industry, households and runoff from agriculture will severe the water conditions of the recipient. In Norway the governmental authorities have imposed strong recommendations on the municipalities to construct wastewater purification plants. Purification can be achieved by using one of three main methods: a mechanical process, a chemical process or a biological one. Very often a combination of two or all three methods will be used in the same purification plant. Removal of high levels of nitrogen compounds from the wastewater requires special procedures.

Mechanical processes.
The wastewater will first be sent through a course filtering device to remove larger parts of the sewerage. Thereafter, the water goes to one or more large basins, where sedimentable material can be removed from the bottom and fatty materials be skimmed from the surface. Finally the water is pumped through large filters.

The method removes mostly particles from the wastewater, but to some extent dissolved nutrition salts also can be captured in the materials taken out.

Chemical processes.
Addition of chemicals containing iron- or aluminum-ions to the wastewater binds the phosphate ions as insoluble salts which mix with organic sedimentable materials (flocculation). Combined with procedures from the mechanical method, this can give a high degree of removal of phosphates from the wastewater.

The nitrogen compounds, however, are chemically highly soluble in water. Therefore, addition of chemicals will not lead to the formation of sedimentable salts of nitrogen.

Biological processes.
The wastewater enters a system of large basins where different kinds of microbiological organisms thrives. The organic materials of the sewerage are then broken down, and the nutrition salts are taken up by the microorganisms and used for their growth. At the end of the process the contents of the water are allowed to sediment and can be removed.

The microorganisms of the biological process have a high requirement of oxygen, and air is bubbled through the water. The pH of the water must not be allowed to fluctuate too much, and too low temperatures must be avoided. The biological processes can be halted if the wastewater contains toxic waste.

Removal of nitrogen compounds.
Ordinary chemical, biological and mechanical processes does not reduce the amount of nitrogen compounds in the wastewater by more than 25 %. Therefore specially designed biological processes are developed to remove up to 70 % of the nitrogen compounds. By adding selected types of bacteria and carefully regulate the oxygen conditions throughout the process, the nitrogen compounds can be transformed via nitrates to nitrogen gas. This gas is not harmful and can be released to the air.