The sewage biochemical treatment process often adopts the contact oxidation process for treatment, and the biofilm carrier filler is the core part of the contact oxidation process, which directly affects the treatment effect, oxygenation performance, infrastructure investment, operation cycle and cost. Therefore, the selection of fillers is particularly important for sewage treatment processes, and factors such as the type, specifications, and materials of fillers, the performance of products, molding processes, and costs are all limiting factors.
1. Overview
Almost all organic polluted wastewater and sewage can be treated using the contact oxidation process. For many years, this process has been widely adopted by design departments due to its unique advantages such as high efficiency and energy saving, small footprint, resistance to impact loads, and convenient operation and management, and has been well received and favored by users. The biofilm carrier filler is the core part of the contact oxidation process, which directly affects the treatment effect, oxygenation performance, infrastructure investment, operating cycle, and cost.
The selection of fillers for sewage treatment processes mainly includes determining the types, specifications, and materials of fillers, the performance of products, molding processes, and costs. The selected filler should meet the requirements of the production process while minimizing equipment investment and operating costs.
2. Selection of fillers
2.1 Selection of filler types
The selection of filler types should consider the requirements of the separation process, usually taking into account the following aspects:
(1) Mass transfer efficiency should be high. Generally speaking, the mass transfer efficiency of regular packing is higher than that of bulk packing.
(2) The flux needs to be large. On the premise of ensuring high mass transfer efficiency, packing materials with high flash point gas velocity or gas-phase kinetic energy factor should be selected.
(3) The pressure drop of the packing layer should be low.
(4) The filling material has strong anti fouling and blocking performance, and is easy to disassemble, assemble, and maintain.
2.2 Selection of Packing Specifications
Filler specification refers to the nominal size or specific surface area of the filler.
2.2.1 Selection of Bulk Packing Specifications
The commonly used bulk packing materials for industrial towers mainly include several specifications such as DN16, DN25, DN38, DN50, DN76, etc. The smaller the size of similar fillers, the higher the separation efficiency, but the resistance increases, the flux decreases, and the cost of fillers also increases significantly. However, the application of large-sized fillers in small-diameter towers can result in poor liquid separation and severe wall flow, which reduces the separation efficiency of the tower. Therefore, there should be a regulation on the ratio of tower diameter to packing size, and generally the ratio of tower diameter to nominal packing diameter D/d should be greater than 8.
2.2.2 Selection of Specifications for Regular Packing
There are many ways to represent the models and specifications of commonly used regular packing materials in industry. In China, specific surface area is commonly used to represent them, mainly including 125, 150, 250, 350, 500, 700 and other specifications. For the same type of regular packing, the larger the surface area, the higher the mass transfer efficiency, but the resistance increases, the flux decreases, and the packing cost also increases significantly. When selecting, comprehensive consideration should be given to separation requirements, flux requirements, site conditions, material properties, equipment investment, operating costs, etc., so that the selected filler can meet both technical requirements and economic rationality.
2.3 Selection of Packing Materials
The materials of fillers are divided into three categories: ceramics, metals, and plastics.
(1) Ceramic fillers. Ceramic fillers have excellent corrosion resistance and heat resistance. They are inexpensive and have good surface wetting properties. However, their biggest drawback is their brittleness and fragility. It is widely used in processes such as gas absorption, gas washing, and liquid extraction.
(2) Metal fillers. Metal fillers can be made of various materials, and corrosion is the main consideration when selecting. Carbon steel fillers have low cost and good surface wetting properties, and should be prioritized for use in non corrosive or low corrosive systems; Stainless steel fillers have strong corrosion resistance and are generally able to withstand corrosion from common systems other than Cl -, but they are expensive and have poor surface wetting properties.
(3) Plastic fillers. The materials of plastic fillers mainly include polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC), and polypropylene is generally used in China. Plastic fillers have good corrosion resistance and can withstand the corrosion of general inorganic acids, alkalis, and organic solvents. It has good temperature resistance and can be used for a long time below 100 ℃.
3. Considering the performance of fillers
The performance of fillers mainly includes particle size and distribution, oil absorption value, filling amount, relative density, thixotropy, filler price, etc., all of which will affect the selection of fillers.
3.1 Particle size and distribution
There are two requirements for the particle size of fillers: one is the average particle size; The second is the particle size distribution Generally, the average particle size should be around 5um, and the maximum particle size should not exceed 20um. The surface of the particles should be smooth. Particles exceeding 20um will have adverse effects on the performance of the product.
3.2 Oil absorption value
Oil absorption value, also known as resin adsorption capacity, is an index indicating the amount of resin absorbed by fillers. In practical applications, most fillers use the oil absorption value as an indicator to roughly predict the demand for resin by fillers. Fillers with the same particle size have higher oil absorption values with voids compared to fillers without voids.
3.3 Filling amount
The filling amount refers to the amount of filler added, which can be calculated according to the following formula:
Filling amount=Filling volume/(Filling volume+Resin volume) × 100%
Filler is a very cheap raw material that can significantly reduce the cost of molding materials and their products. Therefore, people often hope to add as much filler as possible to molding materials to increase the filling rate of fillers.
3.4 Tensionality
Thixotropy is a physical phenomenon in which the viscosity of a material significantly decreases when it is subjected to oscillation, and returns to its original viscosity when the oscillation stops. Materials that are sensitive to thixotropy can experience low viscosity and significant material loss under molding pressure, and may even cause separation between the resin and the reinforcing material. When the filler content is high, a moderate degree of thixotropy should be generated.
3.5 Special Performance
The addition of some fillers can improve the physical properties of the molding compound. Hydrated alumina can endow molding materials with self extinguishing properties and anti leakage components; Barium sulfate can improve the corrosion resistance of molding materials; Talc powder can improve the arc resistance of molding materials.
3.6 Matching of fillers
During use, two or more fillers can be mixed to complement each other's strengths and weaknesses, in order to achieve a more ideal effect. This combination can be a combination of different types of fillers, or a combination of different varieties and fineness, or a combination of different oil absorption values.
4. Considering from the perspective of sewage treatment technology
The selection of fillers mainly depends on the nature of the sewage, the type of structure or equipment, and the selection of the removal object. The main selection objects are fillers with large specific surface area, light weight, low cost, easy to obtain materials, long service life, and reusability. You can roughly choose according to the following rules:
(1) Packing in sedimentation tank: Generally, corrugated plates (pipes) and honeycomb packing are selected. This type of filler is beneficial for the adhesion and sedimentation of sludge, resulting in better sedimentation effect.
(2) Filling materials in anaerobic tanks: Semi soft filling materials, soft filling materials, composite filling materials, modified soft filling materials, and porous spherical filling materials are commonly used. Sometimes irregular fillers such as pebbles, gravel, etc. are also chosen.
(3) Packing in aerobic tank: Generally, packing materials with large specific surface area and easy attachment of biofilm are selected, including semi soft packing, soft packing, composite packing, modified soft packing, porous spherical packing, activated carbon, etc.
(4) Filter material inside the filter: The filter material used inside the filter is also a type of filler, which intercepts suspended solids in sewage. It mainly uses irregular mineral fillers, activated carbon fillers, fiber filter materials, etc.
The sewage treatment process is complex and varied, but it is mainly considered from the aspects of long service life, good charging performance, low power consumption, fast start-up and membrane hanging, easy membrane detachment and renewal, resistance to high load impact, significant treatment effect, simple operation and management, no blockage, no agglomeration, and low price. When applied under different process water quality conditions, various anaerobic, facultative, aerobic and other wastewater treatment processes can be applied by adjusting the density of fillers and different assembly forms.