Multi-media filter

　　A multimedia filter utilizes one or more filtration media to pass turbid water under a specified pressure through a bed of granular or non‑granular materials of a certain thickness, thereby effectively removing suspended impurities and clarifying the water. Common filter media include quartz sand, anthracite, and manganese sand. These filters are primarily used for turbidity removal, water softening, and as pre‑treatment stages in producing purified water, with effluent turbidity typically reduced to below 3 NTU.

　　Structure Introduction

　　Multi-media filters are widely used in water treatment processes and typically consist of the following components: associated piping and valves. The filter vessel itself comprises: ① the cylindrical shell; ② the backwash air pipe; ③ the water distribution assembly; ④ the support structure; ⑤ the filter media; and ⑥ an external vent valve, among others.

　　Function

　　Remove impurities such as silt, suspended solids, and colloids, as well as biological contaminants like algae, thereby reducing mechanical damage and fouling of reverse osmosis membrane elements.

　　Composition of the internal filtration layer

　　The multi-media filter media within the filter consists of high-quality, uniformly sized gravel, quartz sand, magnetite, anthracite, and other materials. These media are strategically and orderly arranged inside the filter vessel according to their specific gravity and particle size: anthracite, which has a low specific gravity but relatively large particles, is placed at the top of the filter bed; quartz sand, with moderate specific gravity and small particle size, occupies the middle layer; and gravel, characterized by high specific gravity and large particles, is positioned at the bottom. This carefully engineered gradation ensures that no stratification occurs during backwashing, thereby maintaining the filter’s effective interception capacity.

　　Principle

　　A multimedia filter (also known as a mechanical filter) uses a bed composed of layered anthracite, sand, finely crushed garnet, or other materials; this is a typical configuration of a multimedia filter.

　　The top layer of the bed is composed of the lightest and coarsest-grade materials, while the heaviest and finest-grade materials are placed at the bottom. The underlying principle is depth filtration: larger particles in the water are removed in the upper layers, whereas smaller particles are captured deeper within the filter medium, thereby bringing the water quality up to the standards achieved after preliminary coarse filtration.

　　The equipment operates on a pressure‑filtration principle: as raw water flows downward through the filter media, suspended particles are retained on the filter surface by adsorption and mechanical interception. As the water moves into the middle of the filter bed, the tighter packing of sand grains creates more opportunities for collisions between the waterborne colloids, suspended solids, and the sand particle surfaces, leading to the adhesion of these impurities to the sand. Consequently, contaminants are trapped within the filter bed, resulting in clarified effluent. The suspended solids in the filtered effluent can be reduced to below 5 mg/L.

　　Characteristics of Multi-Media Filters

　　1. Porous media filters are widely used in water treatment processes; they can be employed standalone, but are most commonly used as pre-filters for advanced water purification stages such as ion exchange, electrodialysis, and reverse osmosis.

　　2. Multi-media filters are commonly used as pretreatment units for advanced water purification and can be equipped with different filter media according to process requirements.

　　3. The multi-media filter can be constructed from fiberglass-reinforced plastic, corrosion‑resistant A3 steel with anti‑corrosion coating or rubber lining, or all‑stainless steel. It is available in either fully automatic or manual operation modes; the automatic version employs imported U.S.-made controllers and pneumatic/hydraulic valves for streamlined operation and easy maintenance, making it widely used as a pretreatment unit in water‑treatment processes across various industries.

　　4. The filter media used in multi-media filters (including dual‑media filters) shall possess adequate chemical stability, and the relative densities and particle sizes of the individual media should differ to some extent. In a dual‑media filter composed of anthracite and quartz sand, the anthracite has a relative density of 1.4–1.6 and a particle size of 0.8–1.8 mm, while the quartz sand has a relative density of 2.6–2.65 and a particle size of 0.5–1.2 mm. For three‑media filters, in addition to these two media, heavier mineral materials such as manganese sand or magnetite may also be employed; these have a relative density of 4.7–5.0 and a particle size of 0.5–4 mm.

　　Filter Type Classification

　　Common filter media used in multi-media filters include anthracite, ceramic pellets, quartz sand, and activated carbon.

　　Multi-media type

　　Common multi-media filters used in water treatment include: anthracite–quartz sand–magnetite filters, activated carbon–quartz sand–magnetite filters, activated carbon–quartz sand filters, and quartz sand–ceramic filters, among others.

　　The design of the filter bed in a multimedia filter primarily takes into account the following factors:

　　1. Different filter media exhibit a significant density difference, ensuring that no layering occurs even under backwash-induced turbulence.

　　2. Select the filter media based on the intended use of the produced water.

　　3. Particle size requirements: The particle size of the lower-layer filter media shall be smaller than that of the upper-layer filter media to ensure the effectiveness and full utilization of the lower layer.

　　In fact, taking a three‑layer filter bed as an example, the top layer consists of the largest particle size and is made of low‑density, lightweight media such as anthracite or activated carbon; the middle layer has intermediate particle size and density, typically composed of quartz sand; and the bottom layer comprises the smallest particles and the highest density, usually heavy media like magnetite. Due to constraints imposed by density differences, the selection of filter media in a three‑layer filter is essentially fixed. The top layer provides coarse filtration, while the bottom layer delivers fine filtration, thereby fully leveraging the advantages of a multi‑media filter bed and yielding effluent quality that is significantly superior to that of a single‑media bed.

　　As for drinking water, the use of anthracite, resins, and other filter media is generally prohibited.

　　Quartz sand type

　　A quartz sand filter is a type of filtration device that uses quartz sand as the filtering medium. It effectively removes suspended solids from water and exhibits significant removal efficiency for colloids, iron, organic matter, pesticides, manganese, bacteria, viruses, and other contaminants. Its advantages include low filtration resistance, a large specific surface area, strong acid–alkali resistance, excellent oxidation resistance, a broad pH operating range of 2–13, and superior anti‑fouling performance. Another distinctive feature of quartz sand filters is their self‑adaptive operation, achieved through optimized filter media and design: during filtration, the filter bed automatically develops a gradient—loose at the top and denser at the bottom—which helps maintain consistent effluent quality under varying operating conditions; during backwashing, the media fully expands, ensuring thorough cleaning. Sand filters also offer rapid filtration rates, high filtration accuracy, and substantial dirt‑holding capacity. They are widely employed in the pretreatment of process water, domestic water, recirculating water, and wastewater across diverse industries, including power generation, electronics, beverage production, municipal water supply, petroleum, chemical processing, metallurgy, textiles, papermaking, food processing, swimming pools, and municipal engineering.

　　Quartz sand filter equipment features a simple structure, automated operation, high flow capacity, reduced backwash frequency, high filtration efficiency, low pressure drop, and convenient operation and maintenance.

　　Iron-manganese type

　　A dual‑media filter bed composed of refined manganese sand and quartz sand is used to remove iron and manganese from water. It is typically employed only in the pretreatment of raw water containing iron and manganese.

　　Activated carbon type

　　The filter media is activated carbon, used to remove color, taste, residual chlorine, and organic compounds. Its primary mechanism of action is adsorption; activated carbon is an artificially manufactured adsorbent. Activated carbon filters are widely employed for the pretreatment of water in domestic applications as well as in industries such as food processing, chemical manufacturing, and power generation. Due to its highly developed microporous structure and large specific surface area, activated carbon exhibits strong adsorption capacity for dissolved organic substances in water, including benzene‑type and phenolic compounds. Moreover, it effectively removes organic contaminants that are difficult to eliminate by biological or chemical methods, such as color, off‑odors, surfactants, synthetic detergents, and dyes. Granular activated carbon can achieve removal efficiencies exceeding 85% for ions like Ag⁺, Cd²⁺, and CrO₄²⁻ in water. After passing through an activated carbon filter bed, suspended solids in the water are reduced to less than 0.1 mg/L, COD removal typically ranges from 40% to 50%, and free chlorine levels fall below 0.1 mg/L.

　　Application areas

　　Pre-treatment for systems such as reverse osmosis, electrodialysis, ion exchangers, and ultrafiltration.

　　Preliminary filtration treatment of raw water for power generation, chemical processing, papermaking, beverage production, and other industries.

　　Decolorization and removal of organic matter from industrial water supplies

　　Pre-treatment of Domestic Water

　　Industrial Water Treatment

　　Swimming Pool Water Treatment

　　Circulating Water Side Filtration System

　　Turbidity and color removal from surface water and groundwater.

　　Wastewater treatment