Backwashing of multi-media filters mainly refers to the interception and adsorption of a certain amount of impurities and stains by the filter material layer after a certain period of use, which makes the water quality of the filters'effluent decrease. The main characteristics are: the water quality of the filters becomes worse after normal filtration, the pressure difference between the inlet and outlet pipelines increases, and the flow rate of a single filter decreases.
The principle of backwashing is that water flows backward through the filter material layer, which expands and suspends the filter layer. The filter material layer is cleaned by shear force of water flow and collision friction force of particles, so that the dirt in the filter layer is separated and discharged with the backwashing water.
Necessity of backwashing
(1) In the process of filtration, suspended substances in raw water are intercepted and adsorbed by the filter material layer and accumulated continuously in the filter material layer. As a result, the pore of the filter layer is gradually blocked by dirt, forming filter cake on the surface of the filter layer and increasing the loss of the filter head. When a certain limit is reached, the filter material needs to be cleaned to restore the performance of the filter layer and continue to work.
(2) As the head loss increases, the shear force of water flow on the filtrate surface becomes larger. Some of the particles move to the lower filter media under the impact of water flow, which will eventually cause the suspended matter content in the water to rise continuously and the water quality to deteriorate. When impurities pass through the filter layer, the filter loses its filtering effect. Therefore, to a certain extent, it is necessary to clean the filter material in order to restore the ability of the filter material layer to absorb contamination.
(3) Suspended matter in sewage contains a large amount of organic matter. Long-term retention in the filter layer will lead to bacterial and microbial enrichment and reproduction in the filter layer, which will lead to anaerobic corruption. It is necessary to wash the filter material regularly.
Control and determination of backwashing parameters
(1) Expansion height: When backwashing, in order to ensure that there is enough clearance between the filter particles so that the filtrate can be discharged quickly with water, the expansion rate of the filter should be larger. However, when the expansion rate is too large, the number of particles per unit volume of filter material decreases, and the chance of particle collision decreases, so it is not conducive to cleaning. The expansion rate of double-layer filter material is 40%-50%.
Note: In the production and operation, the filling height and expansion height of the filter material are inspected randomly, because in the normal backwashing process, some of the filter material will run away or wear, and need to be supplemented. The relatively stable filter layer has the following advantages: ensuring the stability of the water quality of the filter and ensuring the effect of backwashing.
(2)反洗水量和压力：一般设计要求，反洗水的强度为40 m3／(m2?h)，反洗水的压力≤0．15 MPa。
(2) Backwash water quantity and pressure: General design requirements, the strength of backwash water is 40 m3/(m2?H), and the pressure of backwash water is less than 0.15 MPa.
(3)反洗空气量和压力：反洗空气的强度为15 m ／(m/h)，反洗空气的压力≤0．15 MPa。
(3) Air volume and pressure of backwashing: the strength of backwashing air is 15 m/(m/h), and the pressure of backwashing air is less than 0.15 MPa.
Note: In the process of backwashing, the backwashing air is collected at the top of the filter, most of which should be discharged through the double-hole exhaust valve. Daily production. It is necessary to check the patency of exhaust valve frequently, which is mainly characterized by the degree of freedom of ball lifting.
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