As the retained solid impurities increase, the coarse filter increases sharply, and the water flow gradually decreases. If not handled in time, the water flow requirements of the subsequent treatment process cannot be met. For sand filters, the pressure should be backwashed in time after rising to a certain level. During the backwashing process, some fine sand is flushed out of the filter, so sand should be added to the sand filter regularly. After repeated backwashing, the degree of crushing increases, and each backwash cannot be 100% washed. The remaining silt in the sand gradually increases, and the sand layer seems to be "knotted". At this time, the sand layer should be replaced. For non-woven fabric or PP fiber filter cartridges, it is usually difficult to flush water after the filter holes are blocked. The filter element must be replaced regularly.