1. If the dirt trapped on the surface layer of the filter layer can't be effectively removed within a certain period, in the subsequent backwashing process, if the backwashing air is unevenly distributed, it will lead to uneven expansion height. With the scrubbing of the backwashing air, where the scrubbing momentum is small, the impurities such as oil stain on the surface of the filter material can't be effectively removed. After the next normal filtration cycle is put into operation, the local load will increase, and the impurities will sink from the surface into the interior, and the pellets will gradually increase, and at the same time, the pellets will gradually increase.
2. The filter material particles on the surface of the filter layer are fine, and there is little chance of collision with each other during backwashing, so it is not easy to clean, and the attached sand particles are easy to form small mud balls. When the backwashing is finished and the filter layer is graded again, the mud ball enters the lower filter material and moves deeper with the growth of the mud ball.
3. The oil contained in the raw water, trapped in the filter, is backwashed and the residual part accumulates over time, which is the main factor leading to the hardening of the filter material.
