How do multi-media filters achieve gradient filtration of suspended solids of different particle sizes through media gradation design?
Publish Time: 2026-01-24
In water treatment systems, multi-media filters, as key pretreatment units, play a crucial role in removing suspended solids, reducing turbidity, and protecting subsequent advanced treatment equipment. Their core advantage lies not only in the use of multiple filter media but also in their scientific "media gradation design"—that is, by rationally selecting the types, particle sizes, densities, and filling sequences of filter media, a gradient structure is constructed in the filter bed from top to bottom, from coarse to fine and from light to heavy. This achieves efficient, stratified interception of suspended impurities of different particle sizes, ultimately ensuring that the effluent turbidity is stably controlled below 3 degrees.
1. Filter Media Selection and Functional Division: Constructing a Multi-Layer Interception System
Multi-media filters typically use a combination of 2-3 types of filter media, most commonly anthracite, silica sand, and fine silica sand or garnet. Anthracite has a low density, large particle size, and high porosity, primarily intercepting larger flocs, algae, and coarse suspended solids in water. Quartz sand has a higher density and moderate particle size, responsible for capturing medium-sized impurities. The bottom layer can use even finer or heavier filter media for further filtration of tiny particles. This "coarse on top, fine on bottom; light on top, heavy on bottom" structure avoids the problem of surface clogging common in traditional single-layer sand filters, significantly extending the operating cycle.
2. Reverse Matching of Density and Particle Size: Key to Preventing Backwash Mixing
The core principle of filter media gradation is "lower density, larger particle size; higher density, smaller particle size." For example, although anthracite has a larger particle size than quartz sand, its lower density allows it to float on top during backwashing, while quartz sand settles faster, naturally stratifying the media. This design ensures that the filter media automatically restores its original gradation after each backwash, preventing a decrease in filtration efficiency due to mixing. Improper gradation will cause fine sand to sink and coarse sand to float after backwashing, disrupting the gradient filtration structure and causing "short circuits" or penetration.
3. Gradient Filtration Mechanism: From Surface Retention to Deep Adsorption
As water flows downwards through the filter bed, it first contacts the anthracite layer, where large particles are mechanically removed. Medium-sized particles are carried deeper by the water flow and captured in the quartz sand layer through inertial impaction, gravity settling, and adsorption. Tiny colloids are adsorbed deep within the filter media through van der Waals forces and electrochemical interactions. This three-stage interception mechanism—"surface coarse filtration—middle fine filtration—bottom polishing"—ensures that pollutants are evenly distributed throughout the entire filter layer thickness, rather than concentrated on the surface, greatly improving dirt-holding capacity and filtration efficiency.
4. Process Adaptability: Customized Grading Schemes Based on Water Quality Requirements
The filter media combination can be flexibly adjusted for different water sources. For example, when treating groundwater containing iron and manganese, manganese sand can be added to the bottom layer to simultaneously remove iron and manganese through catalytic oxidation; in high-algae water bodies, the thickness of the anthracite layer can be increased to enhance floc retention; and as a pretreatment for reverse osmosis, this customizability makes multi-media filters widely applicable to water treatment pretreatment stages in industries such as power, chemical, food, and pharmaceutical.
5. Structural and Operational Guarantees: Supporting High-Efficiency Gradient Filtration
The filter body is often made of corrosion-resistant carbon steel, FRP, or stainless steel, offering pressure and corrosion resistance; coupled with an automatic control system, it ensures that the filter media is cleaned regularly and does not become ineffective. A reasonable water distribution system ensures uniform water flow distribution, avoiding localized penetration caused by uneven flow, further ensuring the stability of the gradient filtration effect.
In summary, the multi-media filter, through precise filter media gradation design, organically combines physical sieving, deep filtration, and adsorption to form a highly efficient, stable, and adaptive gradient filtration system. It is not only the "gatekeeper" of turbidity removal but also the cornerstone of the reliable operation of the entire water treatment chain, fully embodying the engineering wisdom of "structure as function."
