Design principle & Operation process of Multigrade filter

The Multigrade filter is a process of clarifying water by using one or several kinds of filter media to remove suspended solids from the water with high turbidity under a certain pressure. The commonly used filter material is quartz sand, anthracite, manganese sand, etc.

The Multigrade filter is mainly composed of a filter body, auxiliary pipes and valve.

The filter body mainly contains the following components: a simplified body, a water distribution component, a support component, a back-washing pipe, a filter material and an exhaust external valve.

The selection basis of filter materials:

1. It should has sufficient mechanical strength to avoid the breakage during the back-washing process.

2. Good chemical stability and it should not contain or produce substances which is harmful to human health or affect the production.

3. It should be chosen for the material with high adsorption capacity, sewage interception capacity.

Main factors need to be considered for the layer design of multigrade filter:

1. Different filter materials have a large density difference, so needs to ensures that the mixed layer does not occur after back-washing.

2. Select the filter material according to the output water use.

3. It requires that the particle size of the lower layer filter material should be smaller than the particle size of the upper layer filter material to ensure the effectiveness and full utilization of the lower layer filter material.

In fact, taking a three-layer filter as an example, the upper filter material has the largest particle size and is composed of a light filter material with a small density, such as anthracite and activated carbon; the middle filter material has a medium particle size and a medium density, generally composed of quartz sand; The lower filter material consists of a heavy filter material with the smallest particle size and the highest density, such as magnetite.

Due to the density difference, the filter selection of the three-layer media filter is basically fixed. The upper filter material acts as a coarse filter, and the lower filter material acts as a fine filter, thus fully exerting the function of the multigrade filter, so the output water quality will be significantly better than that of the single-layer filter material.

Back-washing process:

After the filter be used for a certain period of time, the filter layer intercepts and adsorbs a certain amount of impurities and dirt, which will cause the lower output water quality. Main characteristic is that the pressure difference between the inlet and outlet pipes increases.

The principle of back-washing is that the water flow reversely passes through the filter layer, so that the filter layer expands and suspends, and the filter layer is cleaned by the shear force of the water flow and the collision friction of the particles to remove the impurities and dirt in the filter layer and discharge with the backwash water.

The necessity of back-washing:

1. During the filtration process, the suspended solids in the raw water are trapped and adsorbed by the filter layer and continuously accumulate in the filter layer, so that the pores of the filter layer are gradually blocked by the dirt. When the certain limit is reached, the filter material needs to be cleaned to restore the working performance of the filter layer and continue to work.

2. Due to the increase of head loss during the filtration, the shearing force of the water flow on the dirt adsorbed on the surface of the filter material becomes larger, and some of the particles move to the lower layer filter material under the impact of the water flow, and finally the suspended matter in the water increased. When the impurities penetrate the filter layer, the filter will lose the filtration effect. Therefore when the certain limit is reached, the filter material needs to be cleaned in order to restore the dirt holding capacity of the filter layer.

3. The suspended solids in the sewage contains a large amount of organic matter. The long-term retention in the filter layer will lead to the enrichment and reproduction of bacterial microorganisms in the filter layer, causing anaerobic spoilage, so the filter material should be cleaned regularly.

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