Biological purification plants, or activated sludge plants, are among the most common and traditional systems of wastewater treatment.
The activated sludge process is an aerobic treatment conducted by more or less prolonged aeration of the wastewater in a biological reactor in which, through the work of the active biomass (consisting of numerous micro-organisms such as bacteria, protozoa, metazoa, rotifers, etc.), there is the transformation of organic matter into simple substances such as CO2, H2O and energy.
The different plant solutions that can be adopted in the construction of a biological plant also allow the degradation of other nutrients, such as nitrogen N and phosphorus P.
From the biological oxidation process resulting the formation of the so-called biological sludge that is highly putrescible and must undergo a series of treatments (thickening, digestion and dewatering that make up the line sludge of wastewater treatment plants) before it can be disposed of.
Biological wastewater treatment plants have the advantage of consuming fewer chemicals and produce few sewage sludge, compared to other types of systems.
These systems are, therefore, mainly used for the treatment of wastewaters of housing, food industry (dairies, farms, slaughterhouses and meat processing, processing of vegetables, wine cellars etc.), textile industry and dyeworks.