How Does A Multi-media Filter Work?

Multimedia filters are commonly used pretreatment equipment in the field of water treatment. The core is to remove suspended solids (SS), colloids, turbidity and some organic matter and microorganisms in raw water through the synergistic effect of multiple layers of filter materials with different characteristics, combined with the mechanisms of physical interception, adsorption, precipitation, etc., and provide qualified water quality for subsequent deep treatment (such as reverse osmosis, ion exchange). Its working principle can be decomposed into two parts: "filter layer design logic" and "specific purification mechanism", which are as follows: 1. Core premise: "gradient design" logic of filter material layer The filter material of multi-media filter is not a single material, but is loaded in a layered manner according to the gradient of ** "coarse/light, fine/heavy" (common combinations: upper anthracite, middle quartz sand, lower garnet/magnetite, and the bottom is pebble support layer). This design is the key to achieving efficient filtration. There are two reasons: layered interception, extending the filtration cycle: When raw water flows from top to bottom, the upper coarse particulate filter material (such as anthracite, particle size 0.8-1.8mm) First intercept the larger suspended particles in the water to avoid rapid blockage of the lower fine filter material; the lower fine particle filter material (such as quartz sand, particle size 0.5-1.2mm) Then intercept the fine suspended substances and colloids to achieve step-by-step purification from "coarse to fine", greatly improving the overall utilization rate and filtration cycle of the filter material. The density difference anti-mix layer ensures stability of layering: The filter material density is matched by "low upper layer and high lower layer" (such as anthracite density 1.4-1.6g/cm³, quartz sand 2.6-2.7g/cm³, garnet 4.0-4.3g/cm³). Even when backwashing (filter material is loose), it can naturally settle back to the original layer due to density differences to avoid the filtration effect being reduced due to mixing different filter materials. Support layer function: The bottom pebble support layer (particle size 2-5mm) does not participate in filtration, and is only used to support the upper filter material to prevent the fine filter material from flowing with the water flow, and ensure uniform distribution of backwash water. 2. Specific purification mechanism: The four core functions are coordinated. When raw water passes through the gradient filter layer, purification is mainly achieved through the following four mechanisms, and different mechanisms contribute differently to different filter layers: 1. Physical interception (the main mechanism) Principle: "Irregular pore channels" form between filter material particles (such as the anthracene seam has large pores and small pores of quartz sand layer). When the diameter of suspended particles and colloidal particles in the raw water is greater than the pore diameter, they will be directly blocked on the surface or pores of the filter material and cannot penetrate downward with the water. Features: Upper filter material (anthracite): intercepts large particles (such as silt and algae) with a value of ≥10μm; Lower filter material (quartz sand): intercepts small suspended substances and colloids (such as clay particles and tiny organic matter); intercepts the effect is positively correlated with the particle size and porosity of the filter material - the finer the filter material and the denser the pores, the higher the interception accuracy, but the greater the water flow resistance. 2. Adsorption (assisted purification) Principle: The surface of the filter material (especially anthracite and quartz sand) has a certain "surface energy" and adsorption active sites, and can absorb fine colloid particles, organic molecules (such as humic acid) and some microorganisms (such as bacteria) in water through van der Waals force and electrostatic gravity. Features: Colloidal particles are usually negatively charged, while the surface of the filter material is mostly negatively charged in neutral water. It needs to be used as a "bridge" to enhance the adsorption effect; the adsorption amount increases with the increase of the surface area of ​​the filter material (fine filter material has a larger specific surface area and a more significant adsorption contribution). 3. Precipitation and bridge building (strengthening interception) Principle: Although some fine particles can penetrate the pores of the filter material, when the water flow velocity slows down (the water flow in the filter material layer is laminar), it will settle on the surface of the filter material due to gravity; at the same time, the intercepted particles will gradually "pile", forming a "particle bridge" between the pores of the filter material, further reducing the pore size and intercepting smaller impurities. Note: Excessive bridge formation will cause blockage of pores in the filter material layer, which is manifested as an increase in the pressure difference between the inlet and outlet of the filter and a decrease in water production. At this time, the "backwash" operation is required to clear the intercept. 4. Contact flocculation (removal of colloids) Principle: For colloid particles with strong stability (such as clay colloids), it is difficult to remove by simply intercepting. The colloid needs to be "destabilized" (losing charge repulsion) through pretreatment (such as adding polymer aluminum chloride PAC and aluminum sulfate) to form tiny flocculants; when these flocculants come into contact with the surface of the filter material in the filter material layer, they will further accumulate to form larger flocs, intercepted or precipitated by the filter material pores. Application scenario: When the turbidity of raw water is high (such as surface water, well water), multi-media filters are often combined with the "coagulation" process to greatly increase the turbidity removal rate through contact flocculation (can be reduced from 100NTU to below 1NTU). 3. Operation cycle: filtration and backwashing cycle. The multi-media filter does not operate permanently. It needs to maintain efficiency through the "filtration - backwashing" cycle, further reflecting the integrity of its working principle: Filtration stage: Raw water enters from the water inlet at the top of the filter, and passes through the filter material layer from top to bottom after being evenly distributed by the water distributor. Impurities are intercepted and adsorbed, and the purified water flows out from the bottom outlet (water production). In this stage, the inlet and outlet pressure difference gradually increases (the initial pressure difference is 0.02-0.05MPa, and it rises to 0.15-0.2MPa after blockage). Backwashing stage: When the pressure difference reaches the set value (or the filtering cycle ends), backwashing starts: backwashing water enters water from the bottom and passes through the filter material layer from bottom to top, making the filter material particles loose and suspended, and the intercepted impurities are discharged from the top sewage outlet with the backwashing water; some filters will also cooperate with "air scrubbing" (compressed air is introduced from the bottom) to enhance the peeling effect of impurities on the surface of the filter material. After backwashing, the filter material is re-settled and layered, restored the filtration capacity and entered the next cycle.