Small wastewater treatment plants after the fixed bed process

In the fixed bed process, the purification of the wastewater is achieved by bacteria that settle on the surface of a so-called fixed bed.

Different fixed beds

Fixed beds can have different shapes. They can be designed either as a grid, or as a fixed bed body, which are held in the sewage in suspension.

Bacteria settle on the fixed bed bodies after only a short time. They form a dense biofilm. This biofilm is also called growth in fixed bed plants. The growth increases in the course of time to density.

Permanently installed fixed beds (grids) can carry more growth and are therefore more effective in their cleaning performance.

As with activated sludge plants, microorganisms absorb the pollutants in the water and convert them into water, carbon dioxide and salts.

This is the principle of biological wastewater treatment, as it is also used in SBR. A special sedimentation tank is not necessary.


To activate the purification by the bacteria, oxygen must be introduced into the fixed bed under the fixed bed body. This happens either through nozzle pipes or through so-called membrane aerators.

The injection of oxygen also creates a flow. This constant flow ensures a uniform growth density and a constant mixing of the water.

Independent bacterial regulation

The growth regulates itself independently. Due to the high supply of nutrients for the microorganisms in the wastewater they multiply constantly, the growth is too high, the flow, however, carries on the upper layers.

To a certain extent, the biofilm is stabilized by the contained filamentous bacteria, which can lead to problems and tough sludges in other plants. With fixed bed systems this risk does not exist.

Tips & Tricks

The decision for a plant type depends among other things on the required cleaning performance. With regard to specific cleaning parameters, all plant types have consistently different performance.

Video Board: Trickling filter system