The core principle of multi-media filters, which improve filtration accuracy through optimized filter media gradation, lies in constructing a distinct "filtration ladder" from large to small, ensuring precise interception of impurities of varying particle sizes within the filter layers. This process involves more than simply stacking filter media. Instead, it involves combining the density, particle size, shape, and chemical properties of the filter media through scientific proportioning and layered filling, creating a synergistic effect of physical interception and chemical adsorption, ultimately achieving efficient and stable filtration performance.
The primary principle of filter media gradation is "coarse at the top, fine at the bottom." In a multi-media filter, the upper layer is typically filled with coarse filter media with larger particle sizes, such as anthracite or ceramsite. These media have a higher porosity and can quickly intercept large suspended solids in the raw water, such as silt and algae. If these impurities directly enter the lower layer of fine filter media, they will accelerate filter layer clogging and shorten the filtration cycle. The coarse filter layer acts as a "pre-filtration" layer, trapping most large impurities on the surface, reducing the burden on subsequent filter layers and creating conditions for deeper filtration.
The middle filter media uses a medium with a moderate particle size, such as quartz sand. Its finer pore structure allows it to remove colloids and fine particles through screening and inertial collision. After the water passes through the coarse filter layer, the flow rate stabilizes, allowing the middle filter media to fully utilize its interception capacity, achieving a high turbidity removal rate. Quartz sand is chemically stable and resistant to acid and alkali corrosion, making it suitable for a variety of water quality scenarios and a common choice for the middle filter media in multi-media filters.
The bottom filter media requires higher pollutant interception accuracy and typically uses a smaller particle size, such as magnetite or manganese sand. Although these media have lower porosity, they can remove iron and manganese ions, heavy metals, and some microorganisms from water through deep adsorption. For example, the MnO₂ on the surface of manganese sand fixes Fe²⁺/Mn²⁺ through redox reactions, forming a stable oxide precipitate, achieving the dual effects of ion exchange and chemical adsorption. Fine filtration in the bottom filter media is a key step in improving effluent quality in multi-media filters.
Optimizing filter media gradation also requires considering density differences. Filter media of varying densities can easily intermix during backwashing, disrupting the filtration gradient. Therefore, multi-media filters typically utilize media combinations with a distinct density gradient, such as anthracite (density 1.4-1.6 g/cm³) and quartz sand (density 2.6-2.7 g/cm³). This creates a stable layered structure. During backwashing, the denser media sinks, while the less dense media floats. Combined air-water backwashing effectively restores the porosity of the filter layer and extends the filtration cycle.
The shape and surface properties of the filter media also influence filtration accuracy. Irregularly shaped filter media (such as crushed quartz sand) have a higher specific surface area than spherical media, enhancing their ability to adsorb small particles. Furthermore, media with a rough surface (such as modified zeolite) can biodegrade and remove organic matter from the water, further improving filtration efficiency. By combining filter media of varying shapes and properties, multi-media filters create a multi-dimensional filtration network for efficient impurity retention.
In practical applications, the filter media gradation of a multi-media filter needs to be dynamically adjusted based on the raw water quality. For example, when treating high-turbidity raw water, the coarse filter layer thickness can be increased to enhance pre-filtration effectiveness. When treating groundwater containing iron and manganese, manganese sand can be added to the bottom layer to improve ion removal. By flexibly adjusting the filter media ratio and layer order, the multi-media filter can adapt to the filtration needs of different scenarios, demonstrating its strong environmental adaptability.
From coarse filtration to fine filtration, from physical interception to chemical adsorption, the multi-media filter creates an efficient and stable filtration system through optimized filter media gradation. This process requires not only an understanding of the physical and chemical properties of the filter media but also the integration of hydrodynamics and backwashing processes to achieve a dynamic balance in the filter layer structure. It is this meticulous attention to detail that makes the multi-media filter an indispensable core device in the water treatment industry.
