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Due to the difficulty in the treatment of wastewater from dry-process acrylic fibres, it is extremely difficult at current technology level to achieve emission standards for wastewater COD of 100 mg/L in the biochemical phase. As a result, the State Environmental Protection Administration issued a Notice on the issuance of the “Comprehensive Emission Standard for Wastewater (GB8978-1996) in the revised COD Standard Value List for Petrochemical Industry†at the end of 1999. The COD discharge standard for dry-process acrylic wastewater was adjusted to: The discharge standard is 160mg/L, the secondary emission standard is 250mg/L, and the tertiary emission standard is 500mg/L.
The treatment of acrylic waste water has attracted the attention of many domestic environmental protection workers. This article summarizes the research status of acrylic fiber waste water and provides feasible solutions and operation basis for the acrylic fiber waste water treatment plant.
Dry acrylic fiber wastewater production and characteristics of the use of two-step dry spinning technology for the production of acrylic fiber, acrylic fiber from various parts of the production process, due to the process of adding dimethylformamide (D MF), acrylonitrile (AN) , EDTA, nonylphenol polyoxyethylene ether and other raw materials, in the polymerization reaction and then generate different molecular weight polyacrylonitrile, so the acrylic fiber wastewater pollutants mainly sulfate, AN, DMF, EDTA, sodium propionitrile sulfonate, organic Amines, oils and polyacrylonitrile oligomers.
According to the contaminants existing in the acrylic fiber waste water, the acrylic waste water can be mainly summarized as having the following four characteristics: First, more than 20 kinds of raw materials are added in the production. At the same time, various high molecular weight polymers and by-products are generated in the polymerization reaction. Therefore, waste water There are many contaminants, high molecular weight polymers that are difficult to biodegrade and hard to settling naturally. Second, sulfuric acid is added during production, and the reaction by-product sodium propionitrile sulphonate is anaerobicly hydrolyzed to produce sulfate. Therefore, the high concentration of sulfate is also Become the main pollutants in wastewater; Third, wastewater contains organic amines and ammonia nitrogen, which requires the system has the ability to denitrify nitrogen; Fourth, wastewater contains 100 to 150ppm of E DTA and 50 to 70ppm of nonylphenol polyoxyethylene Ether, these two substances have long been considered to be difficult to biodegrade, directly affecting the discharge of acrylic fiber wastewater.
The existing acrylic fiber factory wastewater treatment process and existing problems There are five sets of dry acrylic fiber production facilities of the same type in the country, namely Liaoning Fushun Petrochemical Company Acrylic Fiber Factory, Zhejiang Jinyi Acrylic Fiber Factory, Hebei Qinhuangdao Acrylic Fiber Factory, Guangdong Maoming Acrylic Fiber Factory and Shandong Qilu Petrochemical Company Acrylic Fiber Factory. The five acrylic fiber plant sewage treatment plants are anaerobic - aerobic - biological activated carbon treatment process, there are different degrees of non-compliance.
The main problems that restrict the normal operation of the sewage treatment plant are:
Dry powder impact. Dry powder is a monomer of high polymer, oligomer or mixture, the largest molecular, difficult to be microbial degradation, and the oligomer has a strong blocking, the anaerobic pool and the aerobic tank of soft filler, so that the microorganism The membrane was destroyed and the treatment efficiency in the sewage treatment plant drastically decreased.
The adverse effects of sulphate on anaerobic. Sulfate-reducing bacteria and methanogens compete for matrix production, reducing the ability of microorganisms to degrade organic matter and forming an impact on anaerobic treatment.
Sludge reflux problem. In the initial design of the sewage treatment plant, the problem of ammonia nitrogen was not considered. The sludge in the secondary sedimentation tank was returned to the anaerobic tank. The sludge retention time of the aerobic tank could not be guaranteed, and the system lost the ability to remove ammonia nitrogen.
No consideration was given to clean water diversion. Domestic sewage has good biochemical characteristics. After entering the anaerobic tank, it increases the sewage load of the anaerobic tank on the one hand, and weakens the degradation of non-biodegradable materials by bacteria on the other hand.
Dry Acrylic Wastewater Treatment Research Status Dry acrylic fiber wastewater has become a major problem for environmental protection because it contains difficult biodegradable substances and high concentrations of sulfate. The Institute of Domestic Research conducted a four-year study on acrylic fiber wastewater, using three different technologies, namely SBR, traditional active pollution, and bio-contact oxidation, and the COD can be reduced to 280 mg/L during half-load operation. COD can be used at full load. When it fell to about 400 mg/L, it was considered that the waste water contained non-degradable substances and was difficult to accumulate, so it could not be confirmed.
In 1997, the Qilu Petrochemical Research Institute conducted biochemical experiments with six different combinations of anaerobic, anoxic and aerobic processes and various advanced treatment experiments. Among the six processes, the anaerobic-aerobic biological treatment process was the best. COD 280 ~ 320mg / L, concluded that dry acrylic fiber wastewater does contain substances that are difficult to biodegrade, the traits are mainly EDTA and organic sulfonates, so it is necessary to strengthen the pretreatment to improve the biochemical system processing efficiency. Some scholars have conducted experimental studies on coagulation, air flotation, ozone oxidation, ultrafiltration pretreatment of acrylic wastewater, and obtained the following main conclusions:
PAC and CP-937 are the best flocculants for the coagulation and sedimentation process. The optimal dosage of PAC is 60-100 mg/L, and the dosage of CP-937 is 1 mg/L. The removal rate of COD is 10%-20. %, the effluent turbidity is less than 10mg/L.
The optimal dosage of PAC for pressure dissolved air flotation process is 20-50 mg/L, CP-937 dosage is 1 mg/L, COD removal rate is between 10% and 35%, and the effluent turbidity is lower than 10 mg. / L.
Ozone oxidation pretreatment, when the ozone concentration was 0.87g/L, 1.73g/L, 2. 6g/L respectively, the removal rate of C OD of acrylic wastewater was 2.12%, 3.13%, 4.94%, respectively. The effect of ozone oxidation on the COD removal rate of acrylic fiber is not high. The ultrafiltration pretreatment test adopts an ultrafiltration device. The experimental results show that the COD removal rate of the acrylic fiber wastewater by ultrafiltration is 9.9%, and the efficiency is low.
According to the results of pretreatment, the coagulation-two-phase anaerobic-anoxic-aerobic process flow was determined. The test results showed that the effluent COD was 220-260 mg/L and the BOD was 510-15 mg/L.
For acrylic fiber wastewater containing macromolecular non-biodegradable substances, the COD concentration is 1500 to 2000 m g/L. Theoretically, anaerobic biological treatment should be used. In an anaerobic process, acrylic fiber contains 300 to 1200 mg/L of wastewater. Sulfate, and because of the discontinuity of production, the sulfate concentration in the acrylic fiber waste water fluctuates greatly, resulting in the anaerobic reactor can not run normally and stable, such as the use of two-phase anaerobic can effectively solve the adverse effects of sulphate on anaerobic.
As the acrylic waste water quality fluctuates greatly and there are difficult to degrade biodegradable substances, the secondary treatment uses the AB process (B-stage uses two-phase anaerobic-anoxic-aerobic process). Section A consists of an aeration adsorption tank and a sedimentation tank. Sewage enters section A. Suspended matter and colloids in the sewage form a suspension-microorganism coexistence with the contaminants from the return stream. Flocculation and adsorption occur between them. The activated sludge has a strong adsorption capacity. The refractory organics in the sewage and nitrogen, phosphorus, etc. The adsorption of plant nutrients by the sludge is mainly removed by the removal of excess sludge. This section has a strong ability to withstand impact loads and has a buffering effect on pH and toxic substances. A more thorough removal of suspended and colloidal organics in the A-stage greatly increases the amount of COD removed by the non-biodegradable route throughout the process, thereby reducing operating and investment costs.
Air floatation-AB process test method and test results effluent COD concentration 150-180mg/L. The ammonia nitrogen in the effluent of the biochemical system could not be detected, and the concentration of nitrate nitrogen was less than 50 mg/L. The current research results show that there are difficult biodegradable substances in the acrylic fiber waste water. If the air floatation-AB process is adopted, the acrylic fiber waste water treatment can meet the national first-level industry emission standards.
Problems and Countermeasures of Acrylic Wastewater Treatment
The dry acrylic fiber technology is a patented technology of DuPont Co., USA. The acrylic fiber produced by this technology is favored by the market because of its variety and superior quality. However, this method produces many pollutants that are discharged, and the amount of sewage is large and difficult to handle, and the environment is formed. The harm is relatively large. For this reason, the world's major acrylic fiber production line has been transferred to developing countries.