SBR plants constitute a special type of the activated sludge plants. The biological wastewater purification is based on the activated sludge process. The biological wastewater purification is here accomplished by microorganisms which float in the activation tank and form activated sludge flocs. The bacteria need oxygen for the degradation of the organic contaminants in the wastewater. The oxygen is added through technical aeration systems, which furthermore provide the physical mechanism for mixing the sewage and sludge.

Fixed bed reactors are based on the biofilm technology. In the biofilm process, the biological wastewater treatment is accomplished by microorganisms that are attached to a carrier material and form a biofilm. In fixed bed reactors the carrier material usually consists of plastic particles. These are firmly installed in the reactor and permanently covered with the water. The bacteria need oxygen for the degradation of the organic contaminants in the wastewater. The oxygen is provided through high pressure aeration systems which are installed under the fixed bed.

Trickling filters are based on the biofilm technology. In the biofilm process, the biological wastewater treatment is accomplished by microorganisms that are attached to a carrier material and form a biofilm. The carrier material in trickling filters consists of lava rock, formed plastic pieces or fixed bed particles. Pretreated wastewater is applied evenly to the trickling filter surface and percolates vertically through the trickling filter. The bacteria need oxygen for the degradation of the organic contaminants in the wastewater.

Moving and fluidized bed filters are based on a combination of the activated sludge and the fixed bed technology. The biological wastewater purification is accomplished by microorganisms which float in the activation tank and form activated sludge flocs. The microorganisms are also attached to small moving plastic particles which funtion as carrier material. The bacteria need oxygen for the degradation of the organic contaminants in the wastewater. The oxygen is provided through high pressure aeration systems which are fixed to the bottom of the bed.

Rotating disk filters are based on a combination of the activated sludge and the fixed bed technology. The biological wastewater purification is accomplished by microorganisms which float in the activation tank and form activated sludge flocs. They are also attached to plastic disks, where they form a biofilm. The disks are fixed onto a shaft. The lower half is immersed in pretreated wastewater which flows horizontally through a container and the upper half is surrounded by air. The bacteria need oxygen for the degradation of the organic contaminants in the wastewater.

Activated sludge plants are based on the activated sludge process. The biological wastewater purification is here accomplished by microorganisms which float in the activation tank and form activated sludge flocs. The bacteria need oxygen for the degradation of the organic contaminants in the wastewater. The oxygen is added through technical aeration systems, which furthermore provide the physical mechanism for mixing the sewage and sludge.

Constructed wetlands are planted soil filters. The soil body consists of a sand and gravel mixture, which must guarantee a sufficient permeability for the water, and is isolated from the natural subsoil. The biological purification of the wastewater is accomplished by the microorganisms living in the soil, which form a biofilm in the soil body. Depending on the type of construction, the pretreated wastewater flows horizontally or vertically through the filter. The bacteria need oxygen for the degradation of the organic contaminants in the wastewater.

In ponds, the biological wastewater purification is accomplished by microorganisms that are free-floating or attached to the sides of the pond or the soil. The bacteria need oxygen for the degradation of the organic contaminants in the wastewater. One distinguishes between aerated ponds and ponds that are not aerated. The oxygen uptake in unaerated ponds takes place through the water surface, while technical aeration systems provide the oxygen in aerated ponds. For the treatment of domestic wastewater, unaerated ponds are mainly used.