Design principle analysis 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.

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 layers. 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. BOD and COD are also removed to some extent.

I. Filter Components

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

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

II. Selection Criteria for Filter Media

1. Sufficient mechanical strength is required to prevent rapid wear and breakage during backwashing;

2. Good chemical stability;

3. Free of substances harmful or toxic to human health, and free of substances harmful or affecting 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.

III. 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.

IV. Multi-media Filters

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

The design of the filter layer in a multi-media filter mainly considers:

1. Different filter media have a large density difference to ensure that no layer mixing 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, resulting in significantly better effluent water quality than single-layer filter beds. For drinking water, the use of anthracite, resin, and other filter media is generally prohibited.

V. 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 effect on removing colloidal substances, 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 advantage of quartz sand filters lies in the self-adaptive operation achieved through optimized filter media and filter design. The filter media has strong adaptability to raw water concentration, operating conditions, and pretreatment processes, i.e., during filtration, the filter bed automatically forms a loose-top and dense-bottom state, which helps ensure effluent water quality under various operating conditions. During backwashing, the filter media is fully dispersed, resulting in good cleaning efficiency. Sand filters have advantages such as fast filtration speed, high filtration accuracy, and large dirt-holding capacity. They are widely used in various fields such as power, electronics, beverages, tap water, petroleum, chemicals, metallurgy, textiles, papermaking, food, swimming pools, and municipal engineering for various process water, domestic water, circulating water, and wastewater pretreatment.

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

VI. Activated Carbon Filters

The filter media is activated carbon, used to remove color, odor, residual chlorine, and organic matter. Its main function is adsorption, and activated carbon is a man-made adsorbent.

Activated carbon filters are widely used in the pretreatment of water for domestic water supply, food processing, chemical industry, and power generation. Due to its developed pore structure and large specific surface area, activated carbon has a strong adsorption capacity for dissolved organic matter in water, such as benzene and phenolic compounds. It also effectively removes organic pollutants that are difficult to remove by biological and chemical methods, such as chroma, odor, surfactants, synthetic detergents, and dyes. The removal rate of Ag+, Cd2+, and CrO42- ions in water by granular activated carbon 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.

VII. Backwashing Process

Filter backwashing mainly refers to the process where, after a certain period of use, the filter media layer retains and adsorbs a certain amount of impurities and stains, resulting in a decline in the effluent water quality. This is mainly characterized by: deterioration of the normal filtered water quality, increased pressure difference between the inlet and outlet pipes, and reduced flow rate of the single filter.

Backwashing principle: The water flows backward through the filter media layer, causing the filter layer to expand and suspend. The shear force of the water flow and the collision and friction force of the particles clean the filter media layer, allowing the impurities within the filter layer to detach and be discharged with the backwash water.

VIII. Necessity of Backwashing

1. During the filtration process, suspended solids in the raw water are retained and adsorbed by the filter media layer and continuously accumulate in the filter media layer. As a result, the pores of the filter layer are gradually blocked, and a filter cake is formed on the surface of the filter layer, causing the head loss of the filtration to continuously increase. When it reaches a certain limit, the filter media needs to be cleaned to restore its working performance and continue operation.

2. During filtration, due to the increased head loss, the shear force of the water flow on the impurities adsorbed on the surface of the filter media increases. Some particles move to the lower layer of the filter media under the impact of the water flow, eventually causing the suspended solids content in the water to continuously increase and the water quality to deteriorate. When impurities penetrate the filter layer, the filter loses its filtration effect. Therefore, to a certain extent, it is necessary to clean the filter media to restore the fouling capacity of the filter media layer.

3. Suspended solids in wastewater contain a large amount of organic matter. Long-term retention in the filter layer can lead to the enrichment and reproduction of bacteria and microorganisms in the filter layer, causing anaerobic decay. Therefore, regular cleaning of the filter media is necessary.

IX. Backwashing Parameter Control and Determination

1. Expansion height: During backwashing, in order to ensure that the filter media particles have sufficient gaps for impurities to be quickly 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 per unit volume of filter media decreases, and the chance of particle collision also decreases, which is unfavorable for cleaning. For double-layer filter media, the expansion rate is 40% to 50%.

Note: During production operation, the filling height and expansion height of the filter media should be checked randomly, because some filter media may be lost or worn during normal backwashing, and need to be replenished. A relatively stable filter layer has the following advantages: ensuring stable filtered water quality and ensuring the effectiveness of backwashing.

2. Backwash water volume and pressure: The general design requirement is that the intensity of the backwash water is 40 m3/(m2·h), and the pressure of the backwash water is ≤0.15 MPa.

3. Backwash air volume and pressure: The intensity of the backwash air is 15 m3/(m2·h), and the pressure of the backwash air is ≤0.15 MPa.

Note: During backwashing, the backwash air enters the top of the filter, and most of it should be discharged through the double-hole vent valve. In daily production, the patency of the vent valve should be checked frequently, mainly based on the degree of freedom of the valve ball rising and falling.

X. 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. This is suitable for filter pools with heavy surface pollution and light internal pollution.

Note: The corresponding valves must be closed properly; otherwise, when the water level drops below the surface of the filter layer, the upper part of the filter layer will not be wetted, and during the process of particle movement up and down, the impurities cannot be effectively discharged, but will instead move to the deeper part of the filter layer.

2. Combined air and water backwashing: Air and backwash water are simultaneously introduced from the bottom of the stationary filter layer. During the upward movement, the air forms large bubbles in the sand layer, which become small bubbles when encountering the filter media, simultaneously producing a scrubbing effect on the surface of the filter media; the backwash water loosens the filter layer, making the filter media in a suspended state, which is conducive to the air scrubbing the filter media. The expansion effects of the 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 introducing air, maintain the same flow rate of backwash water, and continue rinsing for 3 to 5 minutes to remove the remaining bubbles in the filter bed.

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

XI. Analysis of Filter Media Clogging

1. If the impurities retained on the surface of the filter layer cannot be effectively removed within a certain period, and the distribution of backwash air is uneven during subsequent backwashing, the expansion height will be uneven. With the scrubbing of the backwash air, in places with less scrubbing, the oil stains and other impurities on the surface of the filter media cannot be effectively removed. After 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 simultaneously extend to the filling depth of the filter, until the entire filter fails.

Note: In actual operation, uneven backwashing air is a common phenomenon, mainly caused by perforation of the bottom air distribution pipe, blockage or damage of local filter caps, or deformation of the grid spacing.

2. The surface filter media particles of the filter layer are fine, and the chances of collision during backwashing are few, the momentum is small, so it is not easy to clean. Adhering sand particles are easy to form small mud balls. When the backwashing is finished and the filter layer is re-graded, the mud balls enter the lower filter media, and move deeper as the mud balls grow.

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 consolidation of the filter media 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 the standards of limiting head loss, effluent water quality or filtration time.

Twelve. Precautions for filter processing and acceptance procedures

1. The parallel tolerance between the outlet trough 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 is optimal with integral processing. When the diameter of the cylinder is large, or subject to restrictions 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 the processing of the filter plate and the rolling of the cylinder, arc ring plates are generally welded section by section. The contact parts must be fully welded.

(2) The treatment method of the radial gap between the central pipe and the filter plate is the same as above.

Note: The above measures ensure that filtration and backwashing can only be connected through the filter cap or the gap between the pipes. At the same time, it also ensures the uniformity of the 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 is best nylon, and ABS is second. Because the filter material added to the upper part has a great compressive load on the filter cap, the strength is required to be high to avoid deformation. Elastic rubber pads should be added to the contact surface (upper and lower surfaces) of the filter cap and the filter plate.