1Physical method
1.1Adsorption method
Adsorption method utilizes porous solids (such as activated carbon, adsorbent resin, etc.) to contact with dye wastewater, and makes use of adsorbent's surface activity to adsorb organic matter and metal ions in the dye wastewater and concentrate them on the surface, so as to achieve the purpose of water purification.
Activated carbon has a strong adsorption capacity for cationic dyestuffs, direct dyestuffs, acid dyestuffs, reactive dyestuffs and other water-soluble dyestuffs, but activated carbon is expensive and not easy to regenerate. The dye adsorbent made of chitosan mixed with activated carbon and cellulose has excellent adsorption capacity for reactive dyes and acid dyes, with adsorption capacity of 264 and 421 MG/G, respectively (the adsorption capacity of coconut activated carbon is less than 80 MG/G). The adsorbent has excellent dispersibility in water and can be treated by simple and inexpensive contact filtration.
Macroporous adsorption resins are polymer beads with an internal cross-linked network structure, excellent pore structure and high specific surface area. Adsorption resin can be used to remove the difficult biological treatment of aromatic sulfonates, naphthol substances. It is easy to regenerate and has good physicochemical stability. Resin adsorption has become one of the effective methods for treating dye wastewater.
1.2 Membrane Separation
Membrane separation technology applied to dye wastewater treatment is mainly ultrafiltration and reverse osmosis. It is reported that the decolorization rate of reduced dye wastewater treated with tubular and hollow fiber polysulfone ultrafiltration membranes is between 95% and 98%, the CODCR removal rate is 60% to 90%, and the dye recovery rate is more than 95%. In recent years, new membrane materials of chitosan ultrafiltration membrane and porous carbon membrane were used to treat dyeing wastewater with better results. Xia Zhining et al. studied the dye wastewater under the action of ultrasound, through the cellulose acetate membrane permeability and salt permeability, found that ultrasound in the membrane separation has obvious accelerated mass transfer and de-concentration polarization "role, there is ultrasonic action of its permeability is not ultrasonic 1.5 times, salt permeability has a greater impact on the rate of retention, respectively, for 94% and 67%. The retention rate is 94% and 67% respectively.
2 chemical method
2.1 chemical coagulation method
Chemical coagulation method mainly has precipitation and air flotation method, this method is economical and effective, but produces chemical sludge needs further treatment. Commonly used inorganic iron composite salts. In recent years the use of polymer coagulants at home and abroad is increasing. Natural polymer flocculants are mainly starch and starch derivatives, chitin derivatives and lignin derivatives 3 categories. Zeng Shulan et al. used NAOH as catalyst to react corn starch and etherifier M to produce cationic starch CST, the dosage of 7-15MG/L, the decolorization rate of acid dyes and reactive dyes reached more than 90%. Wu Bingyan et al. used lignin quaternary amine salt flocculant made by graft polymerization to treat J acid dye wastewater, and the quaternary amine ions in the flocculant and the sulfonic acid groups in the wastewater generated insoluble substances, and the dosage was 20MG/L, and the removal rate of chromaticity reached 90%.
Fang Xinlan use of shrimp, crab shells as raw materials made of chitosan used to treat printing and dyeing wastewater, CODCR removal rate of more than 85%. Natural polymer flocculant charge density is small, low molecular weight, easy to biodegradation and loss of flocculation activity. Synthetic organic polymer flocculants have large molecular weight, more functional groups in the molecular chain, good flocculation performance, low dosage, and wide PH range. Representative artificial organic polymer flocculants are PAN-DCD (dicyandiamide modified polyacrylonitrile polyelectrolyte), WX series of polymer decolorizing flocculants, PDADMA-A (dimethyldiallylammonium chloride polymer) M. 2.2 Chemical oxidation
Chemical oxidation is the use of ozone, chlorine, and its oxides to destroy the chromophores of dyes and decolorize them. Ozone oxidation method can get good decolorization effect for most dyes. FENTON reagent oxidation method, the essence of its decolorization is H2O2 and FE2+ reaction of the hydroxyl radicals generated by the dye organic matter to break the chain.FENTON reagent in addition to oxidation, but also mixed with coagulation. Studies have shown that the treatment of sodium 2-naphthalene sulfonate production wastewater with this method, first after coagulation and precipitation with FECL3, and then under the condition of PH1.5-2.5 with H2O22G/GCODCR, FE2+4G/L water, oxidation of 60MIN can remove CODCR99.6%, 95.3% of the chromaticity [19].
2.3 Wet air oxidation
Wet air oxidation (WAO) is the direct oxidation of organic matter in wastewater by passing air at high temperature (125-320°C) and high pressure (0.5-20 MPA) [20]. Supercritical water oxidation (SCWO) refers to the oxidation of organic matter in water when the temperature and pressure are higher than the critical temperature (374°C) and critical pressure (22.05 MPA) conditions of water. It is essentially an intensification and improvement of the wet oxidation method. The physicochemical properties of water in the supercritical state undergo large changes, the water-vapor phase interface disappears, a homogeneous oxidation system is formed, and the oxidation of organic matter reacts at an extremely fast rate.MODEL et al [21] showed that for organic wastewater with an organic carbon content of 27.33 G/L, the removal rate of organic chlorine and organic carbon was 99.99% and 99.97%, respectively, at 550 °C and within 60 S. The removal rate of organic chlorine and organic carbon was 99.99% and 99.97%, respectively. Supercritical water oxidation method compared with the traditional method, high efficiency, fast reaction speed, wide range of application, can be used for a variety of difficult to degrade organic matter; in the content of organic matter is less than 2%; can be through its own heat exchange, without external heat supply, reactor structure is simple, large treatment capacity.
2.4Photocatalytic oxidation method
Photocatalytic oxidation method is commonly used H2O2 or photosensitized semiconductor (such as TIO2, CDS, FE2O3, WO3 as a catalyst), in the ultraviolet high-energy radiation, the electrons from the valence band jumped into the conduction band, in the valence band to produce holes, thus triggering oxidation reactions. This method has high decolorization efficiency for dye wastewater, but the disadvantage is high investment and energy consumption. Zhang Guilan et al. used a new type of rotary photocatalytic reactor, and the decolorization rate of azo dyes reached 98% when TIO2 in suspension state was used under optimized conditions. Cheng Cangcang et al [23, 24] carried out photocatalytic degradation studies of organic dyes directly resistant to emerald blue GL using a fixed-bed type photoreactor and an inclined plate photoreactor, respectively, and the degradation rates were 83% and 81.4% after 60 MIN of light exposure.
3 Biochemical method
Biochemical method is characterized by low operating cost and less pollution to the environment. However, the water quality of dyestuff wastewater fluctuates greatly, with many kinds and high toxicity, and it is difficult for microorganisms with harsh requirements on temperature and PH conditions to adapt to it.
Aerobic treatment method is simple to run, the removal rate of CODCR and BOD5 is high, but the removal rate of color is not so good. And anaerobic treatment method has a higher removal rate of chromaticity for dye wastewater. Anaerobic treatment method has less sludge generation and the gas produced is methane which can be utilized as an energy source. However, when used alone, the results are not satisfactory. In the treatment of phthalocyanine blue wastewater, Huang Tianyin et al. used physical and chemical means such as gas lift, blow-off and air flotation to remove most of the NH3-N and CU2+ from the raw water and to improve its biochemical properties.
After anaerobic treatment, all the indicators can reach the primary standard of comprehensive sewage discharge standard, CODCR removal rate of 90.0%, BOD5 removal rate of 88.9%, NH3-N removal rate of 99.1%, CU2+ removal rate of 99.7%. Due to the recent development of dyestuffs in the direction of resistance to decomposition and biodegradation, it is difficult to achieve satisfactory results with one process alone. Nowadays, the treatment process is developing towards anaerobic- aerobic combined treatment process. Yan Qingsong et al [26] used an anaerobic- aerobic process for dye wastewater. The anaerobic section adopts UASB process with medium temperature digestion, retention time 48H, CODCR removal up to 55%, effluent BOD5/CODCR value increased from 0.1 to 0.42, and granular sludge is formed in the system, which has good settling performance. The aerobic section adopts contact oxidation method, and after domestication, the degradation ability of sludge to wastewater is gradually improved. HIGHSOLUTIONBACTERIA is a method to treat dye wastewater by using composite microorganisms, and the strains have been developed to more than 100 kinds, such as denitrifying alkali-producing bacteria, denitrifying Thiobacillus, oxidizing Thiobacillus, etc. It can be formulated for different wastewaters and can be used to treat different types of wastewaters. It can be highly targeted for different wastewater by formulating different bacterial groups to decompose different pollutants. Highly efficient microorganisms decompose organic matter into SO2, H2O and many small organic molecules that have no effect on water quality. Using H.S.B technology to treat the wastewater of disperse dyes and acid dyes (CODCR concentration of 2000-2500MG/L) produced by a dye factory in Wuxi, the effluent CODCR was less than 100MG/L, and the average removal rate was 92.68%. Aniline removal was 94%, phenol was 93%, ammonia nitrogen was 92%, and all the coloration was below 50 times [27]. In order to increase the concentration of dominant bacterial species in the biological treatment unit and improve the treatment efficiency of dye wastewater, free bacteria are usually immobilized by chemical or physical means to maintain their biological activity and increase their utilization. Studies have shown that high efficiency decolorizing bacterial colonies immobilized on activated sludge increase decolorizing enzyme activity by 70%.
4Electrochemical method
Electrochemical method of wastewater treatment, in essence, indirectly or directly using electrolysis, the toxic substances in the dye wastewater into non-toxic substances. In recent years, due to the development of the electric power industry, the supply of electricity is sufficient and the cost of treatment has been greatly reduced, the electrochemical method has gradually become a very competitive wastewater treatment method. Electrochemical purification of dye wastewater can be divided into internal electrolysis, electrocoagulation and electrofloatation, and electrocatalytic oxidation according to the way of electrode reaction.
The most widely used internal electrolysis method is the iron chip carbon method. Jin Jianyong used iron filings internal electrolysis method to decolorize 11 kinds of dye wastewater of 5 categories. The study shows that the decolorization rate is above 96% for wastewater with medium color and concentration; adding additives can make the CODCR removal rate of wastewater above 70%. The advantage of the internal electrolysis method is the use of waste in the premise of not consuming energy to remove a variety of pollutants and color, the disadvantage is the slow reaction rate, the reaction column is easy to clog, the high concentration of wastewater treatment effect is poor.
Under the action of external voltage, the use of soluble anode (iron or aluminum) to produce a large number of cations, colloidal wastewater coagulation, while precipitating a large number of hydrogen microbubbles on the cathode, with flocs adhering together with the upward floating. This method is called electrical coagulation and electrofloatation. Compared with the chemical coagulation method, its material loss is about half less, less sludge, and no bulky dosing measures. The disadvantage is that electrical energy consumption and material consumption is too large.
Electrocatalytic oxidation is a direct degradation of organic matter through the anodic reaction, or through the anodic reaction produced by hydroxyl radicals, ozone and other oxidants to degrade organic matter. The advantage of electrocatalytic oxidation is that the organic matter is oxidized completely without secondary pollution. However, the real application of this method in the industrial treatment of wastewater depends on the cheap and efficient catalytic electrode with high oxygen precipitation potential. At the same time, the structure of electrode and electrolyzer also plays an important role in reducing energy consumption. Jia Jinping et al. studied the degradation of a variety of simulated printing and dyeing wastewater by a composite electrode of activated carbon fiber electrode and iron, with good results.
5 Conclusion
Dye production process is complex, the amount of wastewater is large and difficult to treat, and the cost of pollution control is very high. The sulfur-containing wastewater discharged during sulfide alkali reduction is difficult to meet the discharge standard by other methods except for the expensive wet oxidation method. In recent years, the hydrogenation reduction method has been adopted to completely eliminate the pollution of sulfide. The mercury-catalyzed sulfonation method for the production of amino anthraquinone has been changed to the nitration reduction method, completely eliminating mercury pollution. The research and application of various new technologies have greatly improved the efficiency of dyestuff wastewater treatment and reduced the treatment cost. However, it is more important to treat the symptoms than the root cause, and it is equally important to research and develop economically reasonable clean production process and develop highly efficient and economical wastewater treatment process. It is a long-term solution to reduce the sewage discharge fundamentally.
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