How to select press cloth

Filter press filter cloth, the sludge produced in the process of sewage treatment, its dehydration reduction treatment is the premise and guarantee of the economic treatment of subsequent treatment. In order to reduce the moisture content and meet the requirements of sludge disposal, at present, more urban sewage treatment plants use plate and frame filter press to dehydrate the surplus sludge and reduce the moisture content of sludge. Sludge deep dewatering refers to the dewatering method of conditioning sludge, breaking cell wall, releasing binding water, adsorbed water and intracellular water, improving the dewatering property of sludge and making the moisture content of treated sludge reach below 60%. According to different sludge properties and post-treatment and disposal requirements of municipal sewage treatment plant, the filtration unit composed of filter plate, diaphragm plate and filter cloth is used to separate solid and liquid materials with the pressure of feed pump under the condition that the filter plate is pressed tightly by the oil cylinder. After feeding and bunching, the filter cake is pressed by diaphragm pressing technology. Obviously, the dewatering efficiency of high pressure filter is increased. The filter cake is hard and in block shape. The main equipment should be plate and frame filter press.

The filter cloth is the "heart" of the plate and frame filter press. The filtering effect of the plate and frame filter press mainly depends on whether the selected filter cloth is appropriate. The filtrate is clarified during the filtration process of the appropriate filter cloth, and the solid phase loss is less. Therefore, it is an important task to select filter cloth reasonably, improve filtration efficiency, reduce moisture content of dehydrated sludge and reduce production cost.

1 Type of filter press filter cloth The filter cloth of the filter press is classified and selected according to the different material, material composition and compilation methods.

1.1 Press cloth material

1.1.1 Polypropylene fiber filter cloth Polypropylene fiber is made of propylene as raw material by polymerization, melt spinning fiber, price is relatively low. Polypropylene fiber press cloth has good chemical corrosion resistance, good resistance to acid and alkali, high mechanical strength, wear resistance. The wet strength of polypropylene fiber is basically equal to the dry strength, and the strength of polypropylene fiber filter cloth basically remains unchanged during the filtration process after mud entry. The moisture absorption of polypropylene fiber filter cloth is very small, almost no moisture absorption, a large air strip under the moisture recovery rate is close to zero, but it has core suction, can pass through the capillary in the fabric to transfer water vapor, but does not have any absorption effect. The vertical surface of polypropylene fiber is flat, straight and light, and it is also convenient to peel off the sludge after the mud is compressed. However, polypropylene fiber filter cloth has poor light resistance, poor thermal stability and easy aging.

1.1.2 Polyester filter cloth polyester is a fiber made from refined terephthalic acid (PTA) or dimethyl terephthalate (DMT) and ethylene glycol (EG), which is a fiber forming polymer, polyethylene terephthalate (PET), which is produced by esterification or transesterification and polycondensation reaction. Polyester has high strength, good wear resistance, good light resistance, corrosion resistance, resistance to bleaching agents, oxidants, hydrocarbons, ketones, petroleum products and inorganic acids, resistance to dilute alkali, not afraid of mold, but hot alkali can make it decompose. Because polyester filter cloth is less hygroscopic, its wet strength and dry strength is basically the same. The elasticity is close to wool, and it can recover almost completely when it is extended by 5% ~ 6%. Wrinkle resistance is more than other fibers, that is, the fabric does not wrinkle, good dimensional stability.

1.1.3 Nylon filter cloth nylon fiber has high strength, the strength is 4 ~ 5.3 Cndtex, and the elongation is 18% ~ 45%. At 10% elongation, the elastic homocomplex rate is more than 90%. The strength of nylon is the strongest in the fiber, according to the test, the wear resistance of nylon fiber is 10 times that of cotton fiber and 50 times that of viscose. Wear resistance in a variety of fibers first. Polyamide fiber is resistant to strong alkali and weak acid. But nylon is light resistant, easy to change color brittle, so nylon fabric should not be exposed to the sun for a long time.

The chemical name of Vylon filter cloth is polyvinyl alcohol. Its strength is lower than that of polyester, and its strength is only 3.52 ~ 5.72 Cndtex. Elongation of fracture is 12% ~ 25%. The elasticity is poor, the shape of the fabric is poor, the wear resistance is good, and the durability is 1 ~ 2 times that of pure cotton. But one of the biggest advantages is that it can withstand the use of strong alkali, and good moisture absorption, easy to combine with rubber in one, is a good material used in the rubber industry, its disadvantage is low temperature resistance, temperature up to 100 °C shrinkage, no acid resistance.

1.2 Structure of press cloth materials According to the structure of press cloth materials, it can be divided into single filament, compound filament long fiber and short fiber. Monofilament is generally synthetic fiber drawn into 0. 16 ~ 0. 32 dtex single filament, the filter cloth woven by it has the advantages of smooth surface, uniform clearance, small specific resistance, quick water penetration, not easy to plug, easy cleaning and sludge stripping performance. However, the particle diameter is larger, the solid content of filtrate is higher and the recovery rate is lower. Complex silk is the warp/weft line, composed of a number of fibers woven into a silk. The composite filament fiber is woven into the press cloth with good tensile strength and better particle retention performance than the monofilament, but the stripping performance of the sludge is a little worse. Multifilament staple fiber is made of natural cotton wool fiber or synthetic staple fiber multi-twist, it has fluff fiber, showing good particle retention performance, good sealing, but the disadvantage is easy to plug pores, cleaning and sludge stripping performance is poor.

1.3 Press cloth weaving method According to the press cloth weaving method, the press cloth can also be divided into three types: plain weave, twill weave and satin weave. Plain filter cloth compact structure, small pores, so good particle retention, filtrate clarification, long service life, the price is cheaper. Disadvantages are larger than resistance, easy to plug, unloading slag performance is poor. Satin woven fabric is the largest pore, smaller than resistance, not easy to block, sludge stripping performance is good. But the particle retention ability is low, the filtrate is turbid and the recovery rate is low. The properties of twill filter cloth are between plain and satin, but it has strong anti-friction ability, high filtration speed and longest life, so it is widely used.

Urban sewage treatment plant sludge production volume is large, the work intensity is high, the press cloth repeated extrusion often used, need to have a certain tensile strength, no expansion, good dimensional stability, high mechanical strength requirements for the press cloth; In the process of sludge conditioning and conditioning, PAM, iron salt, aluminum salt or lime and other agents are often needed to be added. The properties of the sludge are more complex. The sludge after conditioning is alkaline, and the filter cloth should be able to have a certain corrosion resistance. The sludge in urban sewage treatment plant is mainly composed of organic matter particles, which are fine and need filter cloth with relatively small pores. The filtrate after filtration should be clear, which is conducive to improving the recovery rate of sludge. The water permeability of the filter cloth is also good, the filtration rate is fast, and the moisture in the sludge can be discharged quickly, improving the working efficiency of the plate frame machine. Press cloth should not be blocked; After dewatering the filter cloth, the sludge stripping property is good, and the mud cake can be automatically stripped off after the filter plate of the plate frame press is opened. The press cloth is easy to be washed and regenerated, extending the service cycle and reducing the use cost. Therefore, in the selection of plate and frame filter press cloth, to be able to meet the following requirements: high strength, corrosion resistance, filtrate clean, fast water penetration, not easy to plug, easy to peel, long life.

4 Conclusion With the growth of urban population, the continuous improvement of municipal service facilities, the innovation of sewage treatment technology and the deepening of treatment degree, the amount of sludge generated in the process of sewage treatment will be greatly increased, and the reduction of sludge dehydration is imperative. As a more commonly used treatment equipment, plate and frame filter press has a broad development prospect. The selection of plate and frame press cloth is very important, and it is necessary to consider several factors such as filtrate, water content of mud cake and use cost. When summarizing the characteristics of sludge properties and treatment process in municipal sewage treatment plant, two kinds of commonly used press cloth are selected for experimental study. The results show that polyester monofilament twill filter cloth can get lower water content. However, the filtrate is turbidness and the recovery rate of sludge dehydration is low, while the water content of polypropylene filament long fiber twill filter cloth is high, but the recovery rate of filtrate is good. Each sewage treatment plant can choose suitable filter cloth according to its own situation, so as to reduce the water content and improve the efficiency of use.