The purpose of adopting filter in wastewater treatment is to remove the fine suspended substances, especially some fine suspended particles and colloidal substances that cannot be removed by biochemical treatment and coagulation sedimentation. At present, the filter is widely used as a three-stage treatment means to further treat the secondary treated water, or as a pretreatment of activated carbon adsorption, ion exchange, electrodialysis, reverse osmosis and membrane separation.

In addition to the removal of suspended substances, the filter also has a certain removal effect on turbidity, COD, BOD, phosphorus, heavy metals, bacteria and viruses. In urban sewage treatment, filter has become an indispensable treatment unit in water treatment and reuse system.

With the accumulation of suspended matter in the filter layer, the water resistance of the filter layer increases gradually. At this time, although the water turbidity will not change greatly, if it is not backwashed in time, the structure of the filter material layer will change due to the excessive accumulation of sludge; the filter material changes in the cross section and shape of the island, and the filter layer is compacted. At the same time, due to the increase of water resistance, the filter layer will be "broken", resulting in the short circuit of the filtered water and the poor quality of the effluent.