Do you know the design principle of multimedia filters?

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.

Multi-media 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.


The performance parameters are shown in the following table:


filter composition

Multimedia filter is mainly composed of filter body, supporting pipeline and valve.

The filter body mainly includes the following components: Simplified; water distribution components; support components; backwash air pipe; filter material;


Filter 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.


In the filter material, pebbles mainly play a supporting role. During the filtration process, because of its high strength, stable gaps between each other, and large pores, it is convenient for the water to pass through the filtered water smoothly in the positive washing process. Similarly, backwashing During the process, backwash water and backwash air can pass through smoothly. In the conventional configuration, the pebbles are divided into four specifications, and the paving method is from bottom to top, first large 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 to 1 000 (design specification). The particle size of the filter material is related to the filtration accuracy.​​


Multimedia filter


Multi-media filters used in water treatment, common ones are: anthracite-quartz sand-magnetite filter, activated carbon-quartz sand-magnetite filter, activated carbon-quartz sand filter, quartz sand-ceramic filter Wait.


The main factors to be considered in the design of the filter layer of the multi-media filter are:

1. Different filter materials have a large density difference to ensure that the phenomenon of mixed layers will not occur after backwashing disturbance.

2. Select the filter material according to the purpose of water production.

3. The particle size requires that the particle size of the lower filter material is smaller than the particle size of the upper filter material to ensure the effectiveness and full utilization of the lower filter material.


In fact, taking the three-layer filter bed as an example, the upper layer of filter material has the largest particle size and is composed of light filter materials with low density, such as anthracite and activated carbon; the middle layer of filter material has a medium particle size and a medium density, generally composed of quartz sand; The filter material consists of heavy filter material with the smallest particle size and the largest density, such as magnetite. Due to the limitation of density difference, the filter material selection of three-layer media filter is basically fixed. The upper filter material plays the role of coarse filtration, and the lower layer filter material plays the role of fine filtration, so that the role of the multi-media filter bed is fully exerted, and the quality of the effluent is obviously better than that of the single-layer filter material filter bed. For drinking water, the use of anthracite, resin and other filter media is generally prohibited.


Quartz sand filter


The quartz sand filter is a filter that uses quartz sand as the filter material. It can effectively remove suspended solids in water, and has obvious removal effects on colloids, iron, organic matter, pesticides, manganese, bacteria, viruses and other pollutants in water.

It has the advantages of small filtration resistance, large specific surface area, strong acid and alkali resistance, oxidation resistance, PH application range of 2-13, good pollution resistance, etc. The unique advantage of quartz sand filter is that by optimizing the filter material and filter The design of the filter realizes the self-adaptive operation of the filter, and the filter material has strong adaptability to the concentration of raw water, operating conditions, pre-treatment process, etc. Under various operating conditions, the water quality of the effluent is guaranteed, and the filter material is fully dispersed during backwashing, and the cleaning effect is good.

The sand filter has the advantages of fast filtration speed, high filtration accuracy, and large interception capacity. Widely used in electric power, electronics, beverages, tap water, petroleum, chemical industry, metallurgy, textile, papermaking, food, swimming pool, municipal engineering and other process water, domestic water, recycled water and wastewater pretreatment fields.​​

The quartz sand filter has the characteristics of simple structure, automatic control of operation, large processing flow, fewer backwash times, high filtration efficiency, low resistance, and convenient operation and maintenance.


Activated carbon filter


The filter material is activated carbon, which is used to remove color, odor, residual chlorine and organic matter. Its main mode of action is adsorption. Activated carbon is an artificial adsorbent.

Activated carbon filters are widely used in the pretreatment of domestic water and water in the food industry, chemical industry, electric power and other industries. Because activated carbon has a well-developed pore structure and huge specific surface area, it has a strong adsorption capacity for dissolved organic compounds in water, such as benzene, phenolic compounds, etc. Contaminants such as chroma, odors, surfactants, synthetic detergents and dyes are well removed. The plasma removal rate of granular activated carbon for Ag^+, Cd^2+ and CrO4^2- in water is over 85%. [3] After passing through the activated carbon filter bed, the suspended solids in the water are less than 0.1mg/L, the COD removal rate is generally 40%~50%, and the free chlorine is less than 0.1mg/L.


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.


The need for backwashing


(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 parameter control and determination


(1) Swelling height: During backwashing, in order to ensure that the filter material particles have enough gaps so that the dirt can be quickly discharged from the filter layer with water, the expansion rate of the filter layer should be larger. However, when the expansion rate is too large, the number of particles in the filter material per unit volume decreases, and the chance of particle collision is also reduced, so it is not good for cleaning. Double layer filter material, expansion rate is 40%—-50%. Note: During the production operation, the filling height and expansion height of the filter material are randomly checked, because during the normal backwashing process, there will be some loss or wear of the filter material, which needs to be replenished. The relatively stable filter layer has the following advantages: ensuring the stability of the filtered water quality and the effect of backwashing.

(2) Quantity and pressure of backwashing water: In general design requirements, the strength of backwashing water is 40 m3/(m2•h), and the pressure of backwashing water is ≤0.15 MPa.

(3) Backwash air volume and pressure: the strength of backwash air is 15 m/(m •h), and the pressure of backwash air is ≤0.15 MPa. Note: During the backwashing process, the incoming backwashing air is collected at the top of the filter, and most of it should be discharged through the double-hole exhaust valve. in daily production. It is necessary to check the patency of the exhaust valve frequently, which is mainly characterized by the degree of freedom of the valve ball up and down.


Gas-water combined backwash


(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.

Remarks: You can pay attention to the status of the double-hole exhaust valve at the top.


Analysis of the Causes of Filter Material Hardening

(1) If the dirt trapped on the upper surface of the filter layer cannot be effectively removed within a certain period, in the subsequent backwashing process, if the distribution of the backwashing air is not uniform, the expansion height will be uneven. The rubbing of the washing air, where the rubbing momentum is small, the impurities such as oil stains on the surface of the filter material cannot be effectively removed. After the next normal water filtration cycle is put into use, the local load increases, the impurities will sink from the surface into the interior, and the pellets will gradually increase. large, and at the same time extend into the filling depth of the filter until the entire filter fails.

Remarks: In actual operation, the phenomenon of uneven backwash air often occurs, mainly due to the perforation of the bottom air distribution pipe, the blockage or damage of the local filter cap, or the deformation of the grid tube spacing.

(2) The filter material particles on the surface of the filter layer are small, there are few chances of collision with each other during backwashing, and the momentum is small, so it is not easy to clean. The attached sand particles are easy to form small mud balls. When the filter layer is re-graded after backwashing, the mud balls enter the lower layer of filter material and move to the depths as the mud balls grow.

(3) The oil contained in the raw water is trapped in the filter. After backwashing and the residual part, it accumulates over time, which is the main factor leading to the hardening of the filter material. When to carry out backwashing can be determined according to the water quality characteristics of the raw water and the requirements of the effluent quality, using criteria such as limited head loss, effluent quality or filtration time.


Precautions for filter processing and acceptance procedures


(1) The parallel tolerance between the water outlet and the filter plate is required to be no more than 2 mm.

(2) The levelness and unevenness of the filter plate are both less than ±1.5 mm. The structure of the filter plate adopts the best overall processing. When the diameter of the cylinder is large, or restricted by raw materials, transportation, etc., two-lobed splicing can also be used.

(3) The reasonable treatment of the joint parts of the filter plate and the cylinder is particularly important for the air backwashing link.

①In order to eliminate the radial gap between the filter plate and the cylinder caused by errors in the processing of the filter plate and the rolling of the cylinder, the arc ring plate is generally welded segment by segment. The contact parts must be fully welded.

②The treatment method of radial clearance of central pipe and filter plate is the same as above.

Remarks: The above measures ensure that filtration and backwashing can only be communicated through the gap between the filter cap or the exhaust pipe. At the same time, the distribution uniformity of the backwashing and filtering channels is also guaranteed.

(4) The radial error of the through holes machined on the filter plate is ±1.5 mm. The increase in the size of the fit between the guide rod of the filter cap and the through hole of the filter plate is not conducive to the installation or fixation of the filter cap. The machining of through holes must be done mechanically.​​

(5) The material of the filter cap, nylon is the best, followed by ABS. Due to the filter material added in the upper part, the extrusion load on the filter cap is extremely large, and the strength is required to be high to avoid deformation. The contact surfaces (upper and lower surfaces) of the filter cap and the filter plate shall be provided with elastic rubber pads.

Post time: Jun-20-2022