The multi-media filter uses more than two kinds of media as the filter layer (filter bed). In the industrial circulating water treatment system, it is used to remove impurities in sewage, absorb oil, suspended or colloidal impurities, and can effectively remove tiny particles. and bacteria etc.

The filter materials commonly used in multimedia filters include quartz sand, anthracite, manganese sand, etc., which are mainly used for 

water treatment and turbidity removal, and can also be used for softening water and pre-treatment of pure water. The turbidity of the 

effluent can reach below 3 degrees.

The composition of the multimedia filter

The multi-media filter is mainly composed of a filter body, supporting pipelines and valves, mainly including the following components: tank 

body, water distribution component, support component, backwash air pipe, filter material, exhaust valve (external), etc.

The principle of multimedia filter

The filtration principle of the multi-media filter: when the raw water passes through the filter material from top to bottom, the suspended 

matter in the water is intercepted by the surface of the filter layer due to adsorption and mechanical resistance; when the water flows into 

the middle of the filter layer, due to the sand particles in the filter layer The closer the arrangement, the more opportunities for the particles 

in the water to collide with the sand particles, so that the surface of the flocs, suspended solids and sand particles in the water adhere to 

each other, and the impurities in the water are trapped in the filter material layer, thus obtaining clarified water quality.

The principle of backwashing of the filter: the water flow reverses (from bottom to top) through the filter layer to make the filter layer swell 

and suspend, and the filter layer is cleaned by the shear force of the water flow and the collision friction of the particles, so that the dirt in 

the filter layer The material is separated and discharged with the backwash water. The backwashing is controlled by the parameter setting 

of the inlet and outlet pressure difference, which is generally 2 to 3 days, depending on the turbidity of the raw water.

Filter material selection basis and filter layer design

The main factors to be considered in the selection of filter media are:

(1) It must have sufficient mechanical strength to avoid rapid wear and tear during backwashing;

(2) Chemical stability is better;

(3) Does not contain harmful and toxic substances to human health;

(4) The selection of filter materials should try to use filter materials with strong adsorption capacity, large sewage interception capacity, high 

water production and good effluent quality.

In the filter material, the pebbles mainly play a supporting role. During the filtration process, the strength is high, the spacing gap is stable, 

and the pores are large, so that the water flow can pass through smoothly in the washing process. (The backwash process is the same)

In the conventional configuration, pebbles are divided into four specifications, and the bedding method is from bottom to top, first big and 

then small.

The relationship between the particle size of the filter material and the filling height: the ratio of the height of the filter bed to the average 

particle size of the filter material is 800-1000 (design specification).

The particle size of the filter material is related to the filtration accuracy: the particle size requires that the particle size of the lower filter 

material is smaller than that of the upper filter material, so as to ensure that the lower filter material can be fully utilized.

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