Removal of foreign or harmful substances by means of living organisms.

A method of treating domestic and industrial waste water consisting of the biochemical destruction (mineralisation) by micro-organisms of organic substances (organic pollutants) dissolved and emulsified in the waste water. 

The micro-organisms (bacteria) use these substances as a source of food and energy for their vital functions. During the respiration process, the microorganisms oxidise the organic substances and release the energy needed for their vital functions. Part of the energy is used for cell synthesis processes, i.e. increasing the mass of bacteria, the amount of activated sludge and the biological film in the wastewater treatment plant. 

The mineralisation of organic compounds in waste water is carried out by bacteria, which can be divided into 2 groups according to their relationship with oxygen: Aerobes (which use oxygen dissolved in the water during respiration) and Anaerobes (which develop in the absence of free oxygen). 

In addition to dissolved organic matter, wastewater contains suspended solids, resins and oils that must be removed before the water can be biologically treated. Grids, sand traps and sedimentation tanks are used for this purpose. 

Aerobic water treatment is carried out in near-natural conditions - in irrigation fields, filtration fields, biological ponds and in an artificially created environment where micro-organisms are intensified - in aerotanks, aerofilters, biofilters. Anaerobic treatment uses digesters. The choice of plant type depends on the type and quantity of wastewater, local conditions, quality requirements of the treated water, etc. As a result of complete treatment, biochemically oxidised organic matter in the water is virtually absent. The water loses its ability to decompose, becomes transparent and bacterial contamination is greatly reduced. Wastewater undergoes biological treatment and contains sufficient amounts of biogenic elements (nitrogen, phosphorus and potassium), which are necessary for micro-organisms. In some cases, nutrients in the form of saline solutions are added to the wastewater prior to bioremediation.