The resistance of sand filter increases sharply with the increase of solid impurities retained, and the water flow decreases gradually. If it is not treated in time, it cannot meet the water flow requirements of the subsequent treatment process. For the sand filter, when the pressure rises to a certain degree, it shall be back washed in time. During backwashing, some fine sand is flushed out of the filter, so the sand filter should be added with sand regularly. After repeated backwashing, the degree of crushing of sand increases. At the same time, it is impossible to wash the sand 100% each time. The remaining silt in the sand gradually increases, and the sand layer will appear "hardening". At this time, the sand layer should be replaced.