Aerated Biological Filter (BAF) is called the third generation biological filter. The filter is filled with granular filter material with smaller particle size, and aeration is carried out inside the filter. A highly active biofilm grows on the surface of the filter material. When the sewage flows through, the biooxidative degradation of the highly active biofilm on the surface of the filter material and the biological flocs between the filter materials are used to biochemically treat the sewage. Because the particle size of the filter material is small and in a compacted state, the biofilm and Due to the adsorption of biological flocs between the filter materials, the filter layer can adsorb and retain most of the suspended solids in the sewage (including shed biofilm), and there is no need to set up a sedimentation tank afterwards. As the operation time increases, the water head loss of the filter gradually increases. When the design value is reached, the filter must be backwashed to clean the trapped suspended matter and aged biofilm.