A wastewater treatment plant is generally installed at the end of an effluent collection network (domestic and urban wastewater and, by extension, industrial or agricultural wastewater) and just upstream of the outlet of the water to be treated.
The first treatments are present in all the stations, they consist of:
To remove the remaining materials, different systems are used involving:
Elimination of organic compounds
They use aerobic bacteria that oxidatively degrade organic compounds that contaminate water. These microorganisms have virtually unlimited activity. They are able to transform many organic or mineral molecules thanks to their extreme richness in enzymes which catalyze the necessary reactions on the one hand, to their breathing, on the other hand to the synthesis of living matter by biodegradation of the medium.
The elimination of nitrogen:
Organic nitrogen is converted in the wastewater into ammonia nitrogen (NH4 +). The removal of ammoniacal nitrogen is most often obtained through biological treatments, "nitrification-denitrification". Nitrification consists of a transformation, by bacterial cultures, of ammoniacal nitrogen into nitrates (NO3), an oxidized form of nitrogen. A second phase, denitrification, completes the process. Nitrates, under the action of "denitrifying" bacteria, are transformed into nitrogen gas. This gas escapes into the atmosphere.
The elimination of phosphorus:
They allow to agglomerate the particles by:
which will then be removed by settling or flotation.
Some treatment plants only use physicochemical treatments (about a hundred units in France). They are adapted to seasonal tourist contexts where load variations can be very abrupt over a short period and can remove up to 90% of suspended solids: Dissolved pollution is only partially treated.
Note: The removal of phosphorus is by the addition of reagents, such as iron or aluminum salts, and allows a precipitation of insoluble phosphates and their removal by decantation.
These techniques, the most currently used, eliminate between 80 and 90% of phosphorus, but generate a large production of sludge.
It is used for small-scale collective sanitation, used water undergoes pre-treatments (degreasers, de-oilers, grit traps ...) then they circulate slowly by gravity in a succession of basins (called lagoons) shallow. During this journey, the degradation of the organic matter is ensured mainly by aerobic microorganisms (which need oxygen to live). The first basins are basins with micro-organisms, where the organic matter (microphytes) contained in the wastewater is degraded.
The last basin contains irises, reeds, rushes. (macrophytes) which absorb (for their growth) the mineral elements resulting from the degradation of the organic matter, the solar radiation can also eliminate biological contaminants