Introduction to the design principles of multimedia filters

Multi-media filters utilize one or more filtering media to effectively remove suspended impurities and clarify water with higher turbidity under a certain pressure by passing it through a certain thickness of granular or non-granular material. Common filter media include quartz sand, anthracite, and manganese sand. They are mainly used for water treatment clarification, water softening, pretreatment for pure water, etc., with an effluent turbidity of less than 3 degrees.

Meaning of Filtration: In water treatment, filtration generally refers to the process of using filter layers of quartz sand, anthracite, etc., to intercept suspended impurities in water, thereby clarifying the water. The porous material used for filtration is called filter media, with quartz sand being the most common. Filter media come in granular, powdery, and fibrous forms. Commonly used filter media include quartz sand, anthracite, activated carbon, magnetite, garnet, ceramics, and plastic balls.

Multi-media filters (filter beds) use two or more media as filter layer media. In industrial circulating water treatment systems, they are used to remove impurities and adsorb oil from wastewater, ensuring the water quality meets the requirements for reuse. The main function of filtration is to remove suspended or colloidal impurities from water, especially effectively removing tiny particles and bacteria that cannot be removed by sedimentation technology. It also has a certain degree of removal effect on BOD and COD.

Multi-media filters mainly consist of a filter body, supporting pipelines, and valves.

The filter body mainly includes the following components: shell; water distribution component; support component; backwash air pipe; filter media; vent valve (external), etc.

I. Selection Criteria for Filter Media

1. Must have sufficient mechanical strength to avoid rapid wear and breakage during backwashing;

2. Must have good chemical stability;

3. Must not contain substances harmful or toxic to human health, or substances harmful or disruptive to production;

4. When selecting filter media, those with high adsorption capacity, high interception capacity, high water production, and good effluent water quality should be prioritized.

In filter media, pebbles mainly play a supporting role. During the filtration process, due to their high strength and stable spacing and large pores, they facilitate the smooth passage of filtered water during the forward washing process; similarly, during the backwashing process, backwash water and air can pass through smoothly. In conventional configurations, pebbles are divided into four specifications, and the paving method is from bottom to top, from large to small.

5. Relationship between filter media particle size and packing height

The ratio of filter bed height to the average particle size of the filter media is 800～1000 (design specification). The size of the filter media particles is related to the filtration accuracy.

II. Multi-media Filters

Commonly used multi-media filters in water treatment include: anthracite -quartz sand-magnetite filter, activated carbon-quartz sand-magnetite filter, activated carbon-quartz sand filter, quartz sand-ceramic filter, etc.

III. Factors to Consider in Multi-media Filter Layer Design:

1. Different filter media have a large density difference to ensure that no layering occurs after backwashing disturbance.

2. Select filter media based on the intended use of the treated water.

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

In fact, taking a three-layer filter bed as an example, the upper layer filter media has the largest particle size and is composed of light filter media with low density, such as anthracite and activated carbon; the middle layer filter media has a medium particle size and density, generally composed of quartz sand; the lower layer filter media has the smallest particle size and the highest density, such as magnetite. Due to density limitations, the filter media selection for three-layer media filters is basically fixed. The upper layer filter media performs coarse filtration, and the lower layer performs fine filtration, thus fully utilizing the multi-media filter bed, and the effluent water quality is significantly better than that of a single-layer filter bed. For drinking water, the use of anthracite, resin, and other filter media is generally prohibited.

IV. Quartz Sand Filters

A quartz sand filter is a filter that uses quartz sand as the filter media. It can effectively remove suspended solids from water and has a significant removal effect on colloids, iron, organic matter, pesticides, manganese, bacteria, viruses, and other pollutants in water. It has advantages such as low filtration resistance, large specific surface area, strong acid and alkali resistance, oxidation resistance, a PH range of 2-13, and good anti-pollution properties. The unique advantages of quartz sand filters also lie in the fact that through the optimization of filter media and filter design, the self-adaptive operation of the filter is achieved. The filter media has strong adaptability to the raw water concentration, operating conditions, and pretreatment processes, that is, during filtration, the filter bed automatically forms a loose-top and dense-bottom state, which is conducive to ensuring effluent water quality under various operating conditions, and the filter media is fully dispersed during backwashing, resulting in good cleaning effect. Quartz sand filters have advantages such as fast filtration speed, high filtration accuracy, and large dirt holding capacity. They are widely used in various process water, domestic water, circulating water, and wastewater pretreatment fields in the power, electronics, beverage, tap water, petroleum, chemical, metallurgical, textile, papermaking, food, swimming pool, and municipal engineering industries.

Quartz sand filter equipment is characterized by simple structure, automatic control operation, large processing flow, few backwashing times, high filtration efficiency, low resistance, and convenient operation and maintenance.

V. Activated Carbon Filters

The filter media is activated carbon, used to remove color, odor, residual chlorine, and organic matter, mainly through adsorption. Activated carbon is an artificially produced adsorbent.

Activated carbon filters are widely used in the pretreatment of water for domestic use and in the food, chemical, and power industries. Due to its developed pore structure and large specific surface area, activated carbon has a strong adsorption capacity for soluble organic matter in water, such as benzene and phenolic compounds, and also has a good removal effect on organic pollutants that are difficult to remove by biological and chemical methods, such as chromaticity, odor, surfactants, synthetic detergents, and dyes. Granular activated carbon for water The removal rate of ions such as Ag+, Cd2+, and CrO42- reaches more than 85%. After passing through the activated carbon filter bed, the suspended solids in the water are less than 0.1 mg/L, the COD removal rate is generally 40%~50%, and the free chlorine is less than 0.1 mg/L.

VI. Backwashing Process

1. Filter backwashing mainly refers to the fact that after a certain period of use, the filter material layer retains and adsorbs a certain amount of debris and stains, which causes the effluent water quality of the filter to decrease. The main characteristics are: the normal filtered water quality of the filter deteriorates, the pressure difference between the inlet and outlet pipes increases, and the flow rate of a single filter decreases.

2. Principle of Backwashing

Water flows backward through the filter material layer, causing the filter layer to expand and suspend. With the help of the shear force of the water flow and the collision and friction force of the particles, the filter material layer is cleaned, and the dirt inside the filter layer is separated and discharged with the backwash water.

3. Necessity of Backwashing

（ 1) During the filtration process, suspended solids in the raw water are intercepted and adsorbed by the filter material layer and continuously accumulate in the filter material layer. As a result, the pores of the filter layer are gradually blocked by dirt, and a filter cake is formed on the surface of the filter layer, and the head loss of the filtered water continuously increases. When it reaches a certain limit, the filter material needs to be cleaned to restore the working performance of the filter layer and continue working.

（ 2) During filtration, due to the increase in head loss, the shear force of the water flow on the dirt adsorbed on the surface of the filter material increases. Some particles move to the lower filter material under the impact of the water flow, which will eventually cause the content of suspended solids in the water to continuously increase, and the water quality deteriorates. When impurities penetrate the filter layer, the filter loses its filtration effect. Therefore, cleaning the filter material is needed to a certain extent to restore the dirt holding capacity of the filter layer.

（ 3) Suspended solids in wastewater contain a large amount of organic matter. Long-term stagnation in the filter layer will lead to the enrichment and reproduction of bacteria and microorganisms in the filter layer, causing anaerobic decay. Therefore, regular cleaning of the filter material is required.

4. Backwashing Parameter Control and Determination

（ 1) Expansion Height

During backwashing, in order to ensure that the filter material particles have sufficient gaps for the dirt to quickly be discharged from the filter layer with the water, the filter layer expansion rate should be larger. However, when the expansion rate is too large, the number of particles of the filter material per unit volume decreases, and the chance of particle collision also decreases, which is unfavorable for cleaning. For double-layer filter materials, the expansion rate is 40%----50%. Note: During production operation, randomly check the filling height and expansion height of the filter material, because during the normal backwashing process, some filter material will be lost or worn, and needs to be supplemented. A relatively stable filter layer has the following advantages: ensuring the stability of the filtered water quality and ensuring the effect of backwashing.

（ 2) Backwash Water Volume and Pressure

Generally, the design requirement is that the intensity of the backwash water is 40 m³/(㎡•h), and the backwash water pressure ≤0.15 MPa.

（ 3) Backwash Air Volume and Pressure

The intensity of the backwash air is 15 m/(m•h), and the backwash air pressure ≤0.15 MPa. Note: During the backwashing process, the backwash air is collected at the top of the filter, and most of it should be discharged through the double-hole exhaust valve. The patency of the exhaust valve should be checked frequently in daily production, mainly based on the degree of freedom of the valve ball rising and falling.

VII. Air-Water Combined Backwashing

1. Air washing first, then water backwashing: First, lower the filter pool water level to 100 mm above the filter layer surface, introduce air for several minutes, and then backwash with water. Suitable for filter pools with heavy surface pollution and light internal pollution.

Note: The corresponding valves 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 is not wetted, and during the process of particle up and down disturbance, the dirt cannot be effectively discharged, but will move to the deeper part of the filter layer.

2. Combined air and water backwashing: Air and backwash water are simultaneously sent from the bottom of the stationary filter layer. During the upward process, the air forms large bubbles in the sand layer, and when it encounters the filter material, it becomes small bubbles, and at the same time, it produces a scrubbing effect on the surface of the filter material; the backwash water loosens the filter layer, making the filter material in a suspended state, which is conducive to the air scrubbing the filter material. The expansion effects of backwash water and backwash air are superimposed, which is stronger than when performed individually.

Note: The backwash pressure of water and the backwash pressure and intensity of air are different, and the order should be noted to avoid backwash water entering the air pipe.

3. After the air-water combined backwashing is completed, stop the air inflow, maintain the same flow rate of backwash water, and continue washing for 3 min~5 min to remove the bubbles remaining in the filter bed.

Note: Pay attention to the status of the top double-hole exhaust valve.

VIII. Analysis of Filter Material Clogging

1. If the dirt intercepted on the surface of the filter layer cannot be effectively removed within a certain period, in the subsequent backwashing process, uneven distribution of backwash air will lead to uneven expansion height. With the rubbing of the backwash air, the oil stains and other impurities on the surface of the filter material in areas with less rubbing cannot be effectively removed. After entering the next normal water filtration cycle, the local load increases, and impurities will sink from the surface to the inside, the agglomerates gradually increase, and at the same time extend to the filling depth of the filter, until the entire filter fails.

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

2. The surface filter material particles of the filter layer are small, and the chance of collision during backwashing is small, 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 backwashing is completed and the filter layer is redistributed, the mud balls enter the lower filter material, and as the mud balls grow, they continuously move deeper.

3. The oil contained in the raw water is intercepted in the filter, and the remaining part after backwashing accumulates over time, which is the main factor leading to the clogging of the filter material in the filter. When to perform backwashing can be determined according to the characteristics of the raw water quality and the requirements of the effluent water quality, using standards such as limiting head loss, effluent water quality, or filtration time.

IX. Precautions for Filter Processing and Acceptance

1. The parallelism tolerance between the outlet tank and filter plate shall not exceed 2 mm.

2. The levelness and unevenness of the filter plate are both less than ±1.5 mm. The integral processing of the filter plate structure is optimal. When the cylinder diameter is large, or subject to constraints in raw materials, transportation, etc., it can also be formed by splicing two petals.

3. Reasonable treatment of the joints of the filter plate and the cylinder is particularly important for the air backwashing process.

（ 1) In order to eliminate the radial gap between the filter plate and the cylinder caused by errors in filter plate processing and cylinder rolling, arc ring plates are generally used for segmented welding. The contact parts must be fully welded.

（ 2) The radial gap between the central pipe and the filter plate is treated in the same way.

Note: The above measures ensure that filtration and backwashing can only be connected through the filter cap or pipe gap. At the same time, it also ensures the uniform distribution of backwashing and filtration channels.

4. The radial error of the through holes processed on the filter plate is ±1.5 mm. The increase in the matching size between the filter cap guide rod and the filter plate through hole is not conducive to the installation or fixing of the filter cap. The processing of through holes must use mechanical equipment.

5. The material of the filter cap, nylon is the best, ABS is the second best. Because the filter material added to the upper part exerts a great compressive load on the filter cap, high strength is required to avoid deformation. Elastic rubber pads should be added to the contact surfaces (upper and lower surfaces) of the filter cap and filter plate