Biological contact oxidation is a biofilm process between activated sludge process and biofilter. Its characteristic is to set fillers in the tank, aerate the sewage at the bottom of the tank to oxygenate it, and keep the sewage in a flowing state. The sewage comes into contact with the fillers immersed in the sewage, avoiding the defect of uneven contact between sewage and fillers in the biological contact oxidation tank.

The oxygen required in the biological contact oxidation method is often supplied through air blowing aeration. After the biofilm grows to a certain thickness, anaerobic metabolism occurs near the packing wall due to hypoxia. The gas produced and the scouring effect formed by aeration will cause the biofilm to fall off and promote the growth of new biofilm, forming the metabolism of biofilm. The fallen biofilm will flow out of the pool with the effluent.