What is a multi-media filter?

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A multi-media filter uses one or several filtering media to pass water with high turbidity through layers of granular or non-granular materials under certain pressure, thereby effectively removing suspended impurities and clarifying the water. Common filter media include quartz sand, anthracite, and manganese sand. It is primarily used for turbidity removal in water treatment, water softening, and as a pre-treatment step for producing pure water; the turbidity of the treated water can be reduced to below 3 NTU. The meaning of filtration: In water treatment, filtration generally refers to the process in which filter media such as quartz sand and anthracite are used to trap suspended impurities in water, thereby making the water clearer. The porous material used for filtration is called filter media, and quartz sand is the most common type of filter media. Filter media come in various forms such as granular, powdery, and fibrous. Common filter media include quartz sand, anthracite, activated carbon, magnetite, tourmaline, ceramics, plastic balls, etc. A multi-media filter (filter bed) is a type of media filter that uses two or more different media as its filter layers. In industrial circulating water treatment systems, it is used to remove impurities from wastewater and adsorb oils, thereby ensuring that the water quality meets the requirements for reuse. The function of filtration is primarily to remove suspended or colloidal impurities from water; it is especially effective at eliminating tiny particles and bacteria that cannot be removed by sedimentation techniques. It also has a certain degree of effect in reducing BODs and CODs. The performance parameters are shown in the table below: The multi-media filter is primarily composed of a filter body, associated pipelines, and valves.  The filter body mainly consists of the following components: the cylindrical part ； Water distribution assembly ； Supporting components ； Backwash air pipe ； filter media ； Exhaust valve (external), etc. Criteria for selecting filter media: (1) It must have sufficient mechanical strength to prevent rapid wear and breakdown during backwashing ；  (2) Good chemical stability is required ； 　(3) It does not contain any substances harmful or toxic to human health, nor any substances that are detrimental to production and can affect it ；  (4) For the selection of filter media, those with high adsorption capacity, strong dirt retention ability, high water production rate, and good effluent quality should be preferred.   In the filter media, pebbles serve primarily as a supporting structure. During the filtration process, their high strength ensures stable spacing between them, as well as large pores, which facilitate the smooth passage of the filtered water during the backwashing process ； Similarly, during the backwashing process, backwash water and backwash air can pass through smoothly. In the standard configuration, the cobblestones are available in four sizes, and they are laid from bottom to top, starting with the larger ones and moving to the smaller ones. The relationship between the particle size of the filter media and its packing height: the ratio of the height of the filter bed to the average particle size of the filter media is 800–1,000 (according to design specifications). The particle size of the filter media is related to the filtration accuracy. 　 Multimedia filters used 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 media layer in a multi-media filter takes into account the following factors: 1. Different filter media have significant density differences, which ensures that no mixing of the layers occurs after backwashing disturbances.  2. Select the filter media based on the intended use of the produced water.  3. Regarding particle size requirements, the particle size of the lower layer of filter media should be smaller than that of the upper layer, in order to ensure the effectiveness and optimal utilization of the lower layer of filter media.   In fact, taking a three-layer filter bed as an example, the upper layer consists of filter media with the largest particle size, and is made up of lightweight materials with low density, such as anthracite and activated carbon ； The particles of the middle-layer filter media have a medium size and density; they are generally composed of quartz sand ； The lower layer of filter media consists of heavy filter materials with the smallest particle size and highest density, such as magnetite. Due to the limitations imposed by density differences, the selection of filter media for three-layer medium filters is essentially fixed. The upper layer of filter media serves for coarse filtration, while the lower layer performs fine filtration; this allows the multi-media filter bed to exert its full potential, resulting in water quality that is significantly better than that of a filter bed with a single layer of media. For drinking water, the use of filter media such as anthracite and resin is generally prohibited. Quartz sand filter: A quartz sand filter is a type of filter that uses quartz sand as its filtering medium. It can effectively remove suspended solids from water, and has a significant effect on removing pollutants such as colloids, iron, organic substances, pesticides, manganese, bacteria, and viruses from water. It boasts advantages such as low filtration resistance, large specific surface area, strong resistance to acids and alkalis, oxidation resistance, a pH operating range of 2–13, and good anti-pollution properties. Another unique feature of quartz sand filters is that, through the optimization of both the filter media and the filter design, adaptive operation of the filter is achieved; the filter media exhibits strong adaptability to factors such as the concentration of the raw water, operating conditions, and pre-treatment processes. That is, during filtration, the filter bed automatically assumes a structure that is loose at the top and dense at the bottom, which helps to ensure the quality of the treated water under various operating conditions. During backwashing, the filter media spread out fully, resulting in effective cleaning. Sand filters have advantages such as fast filtration speed, high filtration precision, and large dirt retention capacity. It is widely used in the pre-treatment of process water, domestic water, recycled water, and wastewater in various fields such as power, electronics, beverages, tap water, petroleum, chemicals, metallurgy, textiles, papermaking, food processing, swimming pools, and municipal engineering. 　Quartz sand filter equipment features a simple structure, automatic operation control, high treatment capacity, few backwashing cycles, high filtration efficiency, low resistance, and ease of operation and maintenance. The filter media of an activated carbon filter is activated carbon, which is used to remove color, taste, residual chlorine, and organic substances. Its primary mechanism of action is adsorption; activated carbon is an artificially produced adsorbent.   Activated carbon filters are widely used for the pretreatment of water in domestic use, as well as in industries such as the food industry, chemicals, and power generation. Due to its highly developed pore structure and large specific surface area, activated carbon possesses a strong adsorption capacity for soluble organic substances in water, such as benzenes and phenolic compounds. It also exhibits good removal efficiency for organic pollutants that are difficult to eliminate using biological or chemical methods, including color, unpleasant odors, surfactants, synthetic detergents, and dyes. Granular activated carbon can achieve a removal rate of over 85% for ions such as Ag^+, Cd^2+, and CrO4^2- in water. 　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 level is less than 0.1 mg/L. The backwashing of filters in the backwashing process refers to the process of cleaning such filters after they have been in use for a certain period of time. During this time, the filter media accumulates and absorbs a certain amount of debris and contaminants, which leads to a decline in the quality of the water flowing out of the filter. This is indicated by an improvement in the quality of the water before filtration, an increase in the pressure difference between the inlet and outlet pipes, and a reduction in the flow rate of each individual filter. Principle of backwashing: Water flows in the reverse direction through the filter media layer, causing it to expand and become suspended. The shear force of the water flow along with the collision and friction forces between particles are used to clean the filter media layer, removing contaminants from it so that they can be carried away by the backwash water. The necessity of backwashing (1) During the filtration process, suspended solids and other impurities in the raw water are trapped and adsorbed by the filter media layer, accumulating there continuously. As a result, the pores in the filter layer become gradually blocked by these contaminants, a filter cake forms on the surface of the layer, and the head loss for filtration increases steadily. When a certain limit is reached, the filter media needs to be cleaned in order to restore the filtering layer’s performance and allow it to continue functioning. (2) During filtration, as the head loss increases, the shear force exerted by the water flow on the contaminants adsorbed on the surface of the filter media grows. Some of these particles are carried downward to the lower layers of filter media by the force of the water flow, which ultimately leads to an increase in the concentration of suspended solids in the water and a deterioration in water quality. Once impurities penetrate through the filter layer, the filter loses its filtering capacity. Therefore, to a certain extent, it is necessary to clean the filter media in order to restore the dirt-holding capacity of the filter layer.  (3) The suspended solids in wastewater contain large amounts of organic matter; their prolonged retention in the filter layer leads to the accumulation and proliferation of bacterial microorganisms, resulting in anaerobic conditions. Therefore, the filter media need to be cleaned regularly. Control and determination of backwashing parameters: (1) Expansion height: During backwashing, in order to ensure that there is sufficient space between the filter media particles for contaminants to be quickly washed out of the filter layer along with water, the expansion rate of the filter layer should be relatively high. However, when the expansion rate is too high, the number of filter media particles per unit volume decreases, and the chances of particle collisions also reduce, which is not favorable for cleaning. Double-layer filter media, with an expansion rate of 40%–50%. Note: During operation, the filling height and expansion height of the filter media are checked periodically, as some of the filter media may be lost or worn during normal backwashing, and thus need to be replenished. A relatively stable filter layer has the following advantages: it ensures the stability of the filtered water quality and guarantees the effectiveness of backwashing.  (2) Backwash water volume and pressure: According to general design requirements, the intensity of the backwash water should be 40 m3/(m2•h), with the backwash water pressure ≤ 0.15 MPa. (3) Backwash air volume and pressure: The intensity of the backwash air is 15 m³/(m•h), and the pressure of the backwash air ≤ 0.15 MPa. Note: During the backwashing process, the air introduced for backwashing gathers at the top of the filter, and most of it should be discharged through the dual-port exhaust valve. In daily production. The openness of the exhaust valve needs to be checked regularly, as indicated primarily by the degree of freedom with which the valve ball moves up and down. Air-water combined backwashing: First, flush with air and then with water. Initially, lower the water level in the filter to 100 mm above the surface of the filter media; blow air in for a few minutes, and then carry out backwashing with water. Suitable for filters with heavy surface contamination but light internal contamination. Note: The corresponding valves must be closed completely ； Otherwise, when the water level drops below the surface of the filter layer, the upper part of the filter layer remains dry; during the upward and downward movement of the particles, contaminants cannot be effectively removed and instead move deeper into the filter layer.  (2) Combined air and water backwashing: Air and backwash water are introduced simultaneously from the bottom of the stationary filter bed; as the air rises, it forms large bubbles within the sand layer, which then turn into smaller bubbles upon contact with the filter media, thereby scrubbing the surface of the filter media ； The backwash water loosens the filter media, keeping them in a suspended state, which facilitates air scrubbing of the filter media. The expansion effects of backwash water and backwash air are additive to each other, resulting in a stronger effect than when they occur individually.   Note: The backwash pressure and intensity for water are different from those for air; it is important to pay attention to the sequence to prevent backwash water from entering the air pipeline. (3) After the combined air-water backwash is completed, stop the inflow of air. Keep the flow rate of the backwash water the same and continue flushing for 3 to 5 minutes to remove any air bubbles remaining in the filter bed. Note: Pay attention to the status of the dual-hole exhaust valve at the top. Analysis of the reasons for filter media caking (1): The dirt trapped on the surface layer of the filter medium, if not removed effectively over a certain period of time, will lead to uneven expansion during subsequent backwashing processes due to uneven distribution of the backwashing air. As the backwashing air moves through the filter medium, in areas where the movement is minimal, impurities such as oil on the surface of the filter medium cannot be removed effectively. Once the next normal filtration cycle begins, the local load increases, and these impurities sink from the surface inward; the clumps formed gradually grow larger and extend deeper into the filter, until the entire filter becomes ineffective.  Note: In actual operation, uneven distribution of backwash air frequently occurs. This is mainly caused by perforations in the bottom air distribution pipes, blockage or damage to certain filter caps, or deformation of the spacing between grid pipes. (2) The surface filter media in the filter layer are fine-grained; during backwashing, there are few opportunities for them to collide with each other, and their momentum is low, so they are not easily cleaned thoroughly. The attached sand particles tend to form small clumps of mud. When backwashing is completed and the filter media is realocated, the mud balls enter the lower layer of filter material, moving deeper as they grow larger. (3) The oil contained in the raw water gets trapped in the filter; after backwashing, some of it remains. Over time, this accumulation is the main factor causing the filter media to become caked. The timing for backwashing can be determined based on the characteristics of the raw water quality and the requirements for the quality of the treated water, by using criteria such as specified head loss, treated water quality, or filtration time. Precautions for filter processing and acceptance procedures (1) The parallelism tolerance between the water outlet tank 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. For the structure of the filter plate, overall machining is the most optimal approach. When the diameter of the cylinder is large, or due to constraints such as raw materials and transportation, it is also possible to use two halves joined together for formation.   (3) Proper treatment of the joints between the filter plate and the cylinder is particularly important in the air backwashing process.   ①To eliminate the radial gap between the filter plate and the cylinder caused by errors in filter plate processing and cylinder rolling, arc-shaped ring plates are generally welded section by section. Full welding must be used at the contact points.  ②The method for handling 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 occur through the gap between the filter cap or the discharge pipe. At the same time, it also ensures uniform distribution of the backwashing and filtration channels.   (4) The radial error of the through-holes machined on the filter plate is ±1.5 mm. An increase in the fit dimension between the filter cap guide rod and the through-hole in the filter plate makes it difficult to install or secure the filter cap. The machining of through-holes must be carried out using mechanical equipment. 　 (5) For the material of the filter cap, nylon is the best choice, followed by ABS. Due to the filter media added at the upper part, the compressive load on the filter cap is extremely high; therefore, it requires high strength to prevent deformation. Elastic rubber pads must be placed on the contact surfaces (upper and lower surfaces) between the filter cap and the filter plate.