Tips about Multimedia filter

Multimedia filter is a process in which one or several filter media are used to pass water with high turbidity through a certain thickness of granular or non-granular materials under a certain pressure, thereby effectively removing suspended impurities and clarifying the water. The filter materials include quartz sand, anthracite, manganese sand, etc., which are mainly used for water treatment to remove turbidity, softened water, pre-treatment of pure water, etc. The turbidity of the effluent can reach below 3 degrees.

The meaning of filtration, in the process of water treatment, filtration generally refers to the process of retaining suspended impurities in the water with a filter material layer such as quartz sand and anthracite, so that the water can be clarified. The porous materials used for filtration are called filter media, and quartz sand is the most common filter media. The filter material is granular, powdery and fibrous. Commonly used filter materials are quartz sand, anthracite, activated carbon, magnetite, garnet, ceramics, plastic balls, etc. Multimedia filter (filter bed) is a medium filter that uses two or more media as the filter layer. In the industrial circulating water treatment system, it is used to remove impurities in sewage, adsorb oil, etc., so that the water quality meets the requirements of recycling. . The function of filtration is mainly to remove suspended or colloidal impurities in water, especially to effectively remove tiny particles and bacteria that cannot be removed by precipitation technology. BODs and COD also have a certain degree of removal effect.

Filter media selection basis

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

2) The chemical stability is better;

3) Does not contain harmful and toxic substances to human health, and does not contain substances that are harmful to production and affect production;

4) The selection of filter materials should try to use filter materials with large adsorption capacity, high pollution interception capacity, high water production and good effluent quality.

Why need to process backwash for Multimedia filter

1) During the filtration process, the suspended solids in the raw water are retained and adsorbed by the filter material layer and continuously accumulated in the filter material layer, so the pores of the filter layer are gradually blocked by dirt, and a filter cake is formed on the surface of the filter layer, filtering the water head. Losses keep mounting. When a certain limit is reached, the filter material needs to be cleaned, so that the filter layer can restore its working performance and continue to work.

2) Due to the increase of water head loss during filtration, the shear 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 filter material under the impact of the water flow, which will eventually cause the suspended matter in the water. As the content continues to rise, the water quality deteriorates. When impurities penetrate the filter layer, the filter loses its filtering effect. Therefore, to a certain extent, the filter material needs to be cleaned in order to restore the dirt holding capacity of the filter material layer.

3) The suspended matter in the sewage contains a large amount of organic matter. Long-term retention in the filter layer will lead to the enrichment and reproduction of bacteria and microorganisms in the filter layer, resulting in anaerobic corruption. The filter material needs to be cleaned regularly.

Backwash process

The backwashing of the filter mainly refers to that after the filter is used for a certain period, the filter material layer retains and absorbs a certain amount of sundries and stains, which reduces the quality of the effluent of the filter. The water quality deteriorates, the pressure difference between the inlet and outlet pipes increases, and at the same time, the flow rate of a single filter decreases. The principle of backwashing: the water flow reversely passes through the filter material layer, so that the filter layer expands and suspends, and the filter material layer is cleaned by the shear force of the water flow and the collision friction force of the particles, so that the dirt in the filter layer is separated and discharged with the backwash water.

Air-water combined backwash process

1) Rinse with air first, then backwash with water: first, lower the water level of the filter to 100 mm above the surface of the filter layer, let in air for a few minutes, and then backwash with water. It is suitable for filters with heavy surface contamination and light internal contamination.

Note: The corresponding valve must be closed in place; otherwise, when the water level drops below the surface of the filter layer, the upper part of the filter layer will not be infiltrated by water. During the up and down disturbance of the particles, the dirt cannot be effectively discharged, but will go deeper into the filter layer. move.

2) Combined backwashing of air and water: Air and backwashing water are simultaneously fed from the lower part of the static filter layer. The air forms large bubbles in the sand layer during the rising process, and turns into small bubbles when encountering the filter material. It has a scrubbing effect on the surface of the filter material; backwashing the water top loosens the filter layer, so that the filter material is in a suspended state, which is beneficial to the air scrubbing the filter material. The expansion effects of backwash water and backwash air are superimposed on each other, which is stronger than when they are performed alone.

Note: The backwash pressure of water is different from the backwash pressure and intensity of air. Attention should be paid to the order to prevent backwash water from entering the air pipeline.

3) After the air-water combined backwashing is completed, stop entering air, keep the same flow of backwashing water, and continue to wash for 3 min to 5 min, the air bubbles left in the filter bed can be removed.

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