1. Sand and gravel, quartz sand filter media
1). Single-layer filter media In 1829, the Chelsea Water Supply Company in London, England, built the first slow filter tank for actual production. During the operation of the slow filter tank, the suspended impurities in the water continue to accumulate in the filter membrane, increasing the filtration resistance and gradually reducing the filtration rate. Therefore, after 2 to 3 months of operation, it needs to be regenerated and put back into operation. In 1885, Americans invented the rapid filter tank and built the first rapid filter tank in 1909, which improved the defects of the slow filter tank and greatly increased the filtration speed. At this time, the filtration is single-layer filtration, and the filter media are sand and gravel, quartz sand, etc.
2). Multi-layer filter media With the continuous improvement of filtration technology, domestic and foreign researchers have successively proposed multi-layer filter media filtration. The most commonly used multi-layer filter media in applications are double-layer filter media and triple-layer filter media. In 1940, the United States first developed a double-layer filter tank. This filter uses a light filter material with low density and large particle size in the upper layer, and a heavy filter material with high density and small particle size in the lower layer. Operation data show that the pollution holding capacity of the double-layer filter material is twice that of the traditional single-layer filter material, and the filtration rate is increased by more than 0.5 times. The research on three-layer filter material technology began in the 1960s, usually using coal, sand, and garnet as filter materials. The filtration rate of this filter tank during the initial filtration can be 3 times that of a single-layer rapid filter tank and 2 times that of a coal-sand double-layer filter tank.
In recent years, domestic researchers have also made corresponding improvements in the filter material, and a common one is a combination of activated carbon and quartz sand. Zhang Sheng and others from Tsinghua University studied the treatment of high-algae water source water by activated carbon-quartz sand double-layer deep bed filter. The results showed that the activated carbon-quartz sand double-layer deep bed filter with integrated flotation filtration had a total algae removal rate of 95.1%, a chlorophyll removal rate of 92.2%, an outlet turbidity removal rate of 54.3%, an oxygen consumption removal rate of 63.6%, and a chromaticity removal rate of 86.4%.
3). Homogeneous filter media The filter bed formed by this filter media has large, medium, and small particle sizes in each section from top to bottom, and their respective quantities are equal or close. The void ratio of the filter media in the upper and lower directions of the filter bed is the same, so that the flow state of the filter bed water is also the same. Homogeneous filter media can better overcome surface blockage, and the flocs penetrate deep into the filter bed to achieve the effect of filtering out more water and good water quality.
2. Ceramic granule filter media
The discovery of ceramsite can be traced back to 1885, but it was actually developed by S.J.Hayde in 1918. my country started to study the production and application of ceramsite in the early 1950s. Ceramsite is made of industrial waste residue, waste or discarded mineral raw materials and inferior shale. The micropores inside the ceramsite are porous, which makes the ceramsite have the characteristics of small bulk density, high strength and chemical corrosion resistance. The specific surface area of ceramsite filter material is 6 to 8 times that of quartz sand filter material, and the porosity is 1.7 to 2.2 times that of quartz sand.
In recent years, ceramsite has been widely used as a biological treatment filler for wastewater, with good wastewater treatment effect, and the ammonia nitrogen removal rate can reach more than 90%. Kato et al. used porous ceramic fillers to treat food wastewater. When the hydraulic retention time was 30.17h, the average COD removal rate exceeded 87%. At present, domestic and foreign researchers have developed new ceramsite filter materials such as enzymatic ceramsite, spherical lightweight ceramsite, and nano-modified ceramsite based on conventional ceramsite. Enzymatic ceramsite mainly uses the catalytic effect of enzymes to enhance the treatment effect; spherical lightweight ceramsite has the advantages of rough surface, moderate density, high strength, and friction resistance; nano-modified ceramsite uses nanotechnology to modify the filter material. The surface of the filter material has nanoparticles, which makes it have stronger treatment capacity.
3. Modified filter material
Considering the limited specific surface area of commonly used filter materials such as quartz sand, ceramsite, and anthracite, the negative charge on the surface under neutral conditions or uneconomical, researchers have developed modified filter materials, which have become a hot topic for extensive research. Modified filter material is to coat a layer of modifier on the surface of the carrier filter material through physical and chemical reactions, thereby changing the physical and chemical properties of the surface of the original filter material particles, which can improve the filter material's ability to intercept pollution and its adsorption capacity for certain special substances, and improve the effluent water quality. Practice has shown that modified filter materials can increase the specific surface area of the filter material and enhance its adsorption capacity, so that the enhanced adsorption and oxidation purification functions generated by the surface coating can be fully utilized in the process of contact with various organic matter, bacteria, and algae in the water. Modified filter media can not only adsorb macromolecules and colloidal organic matter, but also adsorb and oxidize a large amount of heavy metal ions and small molecular soluble organic matter in water, thereby achieving the purpose of comprehensively improving water quality.
1). Modified filter media for sewage treatment Xiangyang uses modified montmorillonite and sepiolite to adsorb organic pollutants in water. It is found that when the water contains some transition metals such as iron, chromium, manganese, copper, and platinum, the empty orbits of the transition metals attract the lone pairs of electrons of some atoms or functional groups in the organic matter, which changes the structure of the organic matter, making it very easy to break the chemical bonds of some difficult-to-degrade organic matter, thereby completely decomposing the organic matter. Professor Gao Naiyun used alumina coating to modify quartz sand to remove organic matter and turbidity, and found that the TOC and CODMn indicators in the effluent showed that the coating was better than the uncoated sand. Foreign research in this area started earlier. In 1989, Jiban K.Satpathy coated the surface of sieved quartz sand with an average geometric size of 0.7mm with iron nitrate. It was found that its surface area increased by nearly 40 times, which could effectively remove cadmium, chromium and cyanide from cadmium and chromium plating wastewater. In 1996, Arun Joshi and others from the Indian Institute of Technology used iron oxide coated sand with iron nitrate coated on the surface of quartz sand with an average geometric size of 0.49mm as filter material to make a low-cost, simple household arsenic removal device to remove trivalent and pentavalent arsenic from groundwater.
2). Modified filter media for water treatment When the research on modified filter media for sewage treatment is becoming more and more extensive, the research on water treatment has also begun. Zhou Yuexi and others found that surface modified quartz sand can effectively remove soluble phosphates in water, and the removal rate of heavy metal ions such as Cr3+ and Cu2+ is above 90%; Maeda and others in Japan coated ferric hydroxide on coral stone to obtain iron-coral stone filter material, which can effectively adsorb pentavalent arsenic in water to the surface of iron-coral stone; in the early 1990s, Professor Benjamin and his research team members began to use modified filter materials in water treatment; Stenkamp and others used iron-coated modified quartz sand filter materials for filtration tests to remove positively charged iron compound particles and negatively charged emulsion particles. The results showed that the modified filter material was superior to the original quartz sand in almost all tests. The modified filter material has the characteristics of strong electrostatic adsorption ability, short maturity period, and can reduce the amount of coagulant; Jerzy Lu Kasik et al. covered sand with iron hydroxide and aluminum hydroxide to modify the filter material to remove microorganisms in the water. The results showed that the removal rates of Escherichia coli, cholera bacteria, polio virus 1, and Escherichia coli phage MS-2 in the water were all above 99%.