The printing and dyeing industry is a major emitter of industrial wastewater. According to incomplete statistics, the daily discharge of printing and dyeing wastewater in China is 3×10 6 to 4×10 6 m 3 . Printing and dyeing wastewater has the characteristics of large amount of water, high content of organic pollutants, deep color, large alkali, and large variation in water quality, and it is a difficult-to-treat industrial wastewater. In recent years, due to the development of chemical fiber fabrics, the rise of artificial silk, and advances in printing and dyeing finishing technologies, PVA slurries, rayon-based hydrolysates (mainly phthalates) and new types of additives are difficult to biodegrade organic materials. A large amount of wastewater has entered the printing and dyeing wastewater, its COD concentration has also increased from the original hundreds of mg/L to 2000 ~ 3000mg/L, so that the original biological treatment system COD removal rate dropped from 70% to 50% or even lower. The traditional biological treatment process has been seriously challenged; the COD removal rate of such printing and dyeing wastewater by traditional chemical precipitation and air flotation is only about 30%. Therefore, the development of economical and effective printing and dyeing wastewater treatment technology has increasingly become a topic of concern for the environmental protection industry today. ? Processing capacity (m3/d)
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1 printing and dyeing wastewater source, water quality, water volume?
1.1 Wastewater shall be discharged from the four processes of the source, printing and dyeing process, and the pretreatment stage (including singeing, desizing, scouring, bleaching and mercerizing processes) shall discharge the desizing, scouring, bleaching and mercerizing wastewaters. The dyeing process discharges the dyeing wastewater, the printing process discharges the printing wastewater and the soap liquor wastewater, and the finishing process discharges the wastewater. Printing and dyeing wastewater is the mixed wastewater of the above types of wastewater, or the integrated wastewater other than bleached wastewater. ?
1.2 Water quality and quantity of water The quality of printing and dyeing wastewater varies with the type of fiber used and the processing technology, and the composition of pollutants varies greatly. The general printing and dyeing wastewater pH value of 6 to 10, CODCr 400 ~ 1 000? mg / L, BOD5 is 100 ~ 400mg / L, SS is 100 ~ 2000mg / L, color 100-400 times. However, when the printing and dyeing process and the type of fiber used and the processing technology are changed, the quality of the wastewater will be greatly changed. For example, when the waste water contains the alkali reduction waste water produced in the process of printing and dyeing polyester filaments, the COD??Cr? of the waste water will increase to more than 2,000 to 3,000 ?mg/L, and the BOD5 will increase to 800 mg. Above /L, the pH value reaches 11.5-12, and the wastewater quality deteriorates with the increase in the amount of alkali-reducing waste water added to the dyeing of polyester filaments. When the amount of CODCr added to the alkali-reduced wastewater added exceeds the amount of CODCr in the wastewater by 20%, the biochemical treatment will be difficult to adapt. The drainage of printing and dyeing processes is generally:
(1) Desizing wastewater: The amount of water is small, but the concentration of pollutants is high. It contains all kinds of slurry, pulp decomposition, lint, starch alkali and various additives. The wastewater is alkaline and has a pH of about 12. The sizing of starch-based (eg cotton) desizing wastewater has high COD and BOD values ​​and good biodegradability, and the sizing of the desizing wastewater is mainly polyvinyl alcohol (PVA) (such as polyester/cotton warp). C OD is high and BOD is low, and the biodegradability of wastewater is poor. ?
(2) Boiled wastewater: Large amount of water, high concentration of pollutants, including cellulose, acid, wax, grease, alkali, surfactants, nitrogen compounds, etc. The wastewater is strongly alkaline, the water temperature is high, brown . ?
(3) Bleaching wastewater: The amount of water is large, but the pollution is lighter. It contains residual bleaching agents, a small amount of acetic acid, oxalic acid, sodium thiosulfate, and the like. ?
(4) mercerized wastewater: high alkali content, NaOH content of 3% to 5%, most of the printing and dyeing plant recovery of NaOH by evaporation, so the mercerized wastewater is generally discharged, after repeated use of the process, the final discharged wastewater is still strong Alkaline, BOD, COD, and SS are all high. ?
(5) Dyeing wastewater: The amount of water is relatively large, and the water quality differs depending on the dyestuffs used. It contains pulp, dyes, auxiliaries, surfactants, etc. It is generally strongly alkaline, with high chroma, COD is higher than BOD. Many are less biodegradable. ?
(6) Printing waste water: Large amount of water, in addition to waste water during the printing process, it also includes soaping and washing wastewater after printing. The concentration of pollutants is relatively high. It contains pulp, dyes, auxiliaries, etc., BOD, COD are all relatively high. ?
(7) Disposal of waste water: The amount of water is small, including lint, resin, oil agent, slurry, etc. ?
(8) Alkali-reduced wastewater: It is produced by the reduction process of polyester silk fibroin. It mainly contains polyester hydrolysate terephthalic acid, ethylene glycol, etc. The content of terephthalic acid is as high as 75%. Alkali-reduced wastewater not only has a high pH (generally >12), but also has a high organic concentration. The CODCr discharged from the alkali reduction process can reach up to 90,000 mg/L. Polymer organics and some dyes are difficult to biodegrade. Wastewater is a high concentration of refractory organic wastewater. ?
2 Printing and dyeing wastewater treatment methods
At present, domestic methods for the treatment of printing and dyeing wastewater are mainly based on biochemical methods, and some of them also use chemical methods in series. This is basically the case in foreign countries. Due to the development of chemical fiber fabrics in recent years and advances in finishing techniques after printing and dyeing, PVA slurries, new types of additives, and other biodegradable organic materials that are difficult to biodegrade have entered the printing and dyeing wastewater, adding to the difficulty of processing. Most of the original biological treatment systems have been reduced from the original 70% COD removal rate to about 50% or even lower. The removal of chromaticity is a major problem in the treatment of printing and dyeing wastewater. The old biochemical methods have been unsatisfactory in terms of decolorization. In addition, COD caused by chemical pulps such as PVA accounts for a large proportion of the total COD of printing and dyeing wastewater, but because they are difficult to be used by ordinary microorganisms, the removal rate is only 20% to 30%.
In response to these problems, in recent years, some research work has been carried out at home and abroad, mainly new biological treatment processes and the exploration and application of highly efficient specialized bacteria and new chemical agents. Among them are: anaerobic-aerobic biological treatment process, selection and application of high-efficiency decolorizing bacteria and PVA degrading bacteria, and development of high-efficiency decolorizing coagulants. The following is a review of the three aspects of physical, chemical, and biological methods, introducing current methods for the treatment of printing and dyeing wastewater, and the status of research. ?
2.1 physical method of printing and dyeing wastewater treatment - adsorption method? In the physical treatment method is the most widely used adsorption method, this method is to be activated carbon, clay and other porous substances, powder or particles mixed with wastewater, or let the wastewater through the particles The bed consists of a filter bed, so that pollutants in the wastewater are adsorbed on the surface of the porous material or removed by filtration. At present, foreign countries mainly use activated carbon adsorption (mostly used for tertiary treatment), this method is very effective for the removal of dissolved organic matter in water, but it can not remove colloidal and hydrophobic dyes in water, and it only for cationic dyes, direct dyes, Water-soluble dyes such as acid dyes and reactive dyes have good adsorption properties. Saito T. et al.'s research shows that the adsorption rate, BOD removal rate, and COD removal rate of activated carbon are 93%, 92%, and 63%, respectively. The activated carbon adsorption capacity can reach 500mg COD/g carbon. If the wastewater is first aerated, it will Accelerate the adsorption rate. However, if wastewater BOD5 is >200 mg/L, this method is not economical.
Adsorption used in a variety of adsorbents, the project need to consider the selectivity of the dye adsorbent, should be based on the quality of wastewater to select the adsorbent. Studies have shown that in the printing and dyeing wastewater with pH=12, the removal efficiency of anionic dyes can reach 95% to 100% with silica polymer (methyl oxygen) as adsorbent.
Kaolin is also an adsorbent. Studies have shown that kaolin can effectively adsorb yellow, direct dyes in wastewater after long-chain organic cation treatment. In addition, active diatomite and coal slag are also used to treat traditional printing and dyeing wastewater. The cost is low and the decolorization effect is good. The disadvantage is that the sludge production is large and further processing is difficult. ?
2.2 Printing and dyeing wastewater chemical treatment?
2.2.1 Coagulation method There are mainly coagulation and sedimentation methods and coagulation gas floatation methods. Most of the coagulants used are mainly aluminum salts or iron salts, among which the bridge adsorption of basic aluminum chloride (PAC). The performance is better, and the price of ferrous sulfate is the lowest. In recent years, the use of polymer coagulants abroad has been increasing, and there is a trend to replace inorganic coagulants, but in the domestic market due to price reasons, the use of polymer coagulants are rare. It has been reported that weak anionic polymer coagulants have the widest range of applications, and when used in combination with aluminum sulfate, better results can be achieved. The main advantages of the coagulation method are simple process flow, convenient operation and management, less equipment investment, less floor space, and high decolorization efficiency of hydrophobic dyes; the disadvantages are high operating costs, large amount of sludge, and dehydration difficulties. Water-based dyes have a poor treatment effect. ?
2.2.2 Oxidation method Ozone oxidation method is widely used abroad. Zima SV et al. summarized the mathematical model of ozone decolorization of printing and dyeing wastewater. Studies have shown that when the amount of ozone is 0.886gO3/g dye, the bleaching rate of light brown dye wastewater is up to 80%; the study also found that the ozone required for continuous operation is higher than the amount of ozone required for intermittent operation, and the separator is installed in the reactor. Can reduce the amount of ozone 16.7%. Therefore, the use of ozone oxidation decolorization should be designed as a batch reactor, and may consider installing a partition in it.
Ozone oxidation method can obtain good decolorization effect for most dyes, but it has poor decolorization effect for dyes that are insoluble in water such as vulcanization, reduction, and coating. From the experience and results of domestic and foreign operations, the method has good decolorization effect, but it consumes more power, and there are certain difficulties in large-scale promotion and application.
The photo-oxidation method has a high decolorization efficiency for the treatment of printing and dyeing wastewater, but the equipment investment and power consumption have yet to be further reduced. ?
2.2.3 electrolytic method? Table 1 Application of domestic biological treatment process of printing and dyeing wastewater
Electrolysis has good treatment effect on the treatment of printing and dyeing wastewater containing acid dyes, and the decolorization rate is 50% to 70%, but the treatment effect is poor for deep color and high CODCr wastewater. The electrochemical properties of the dyestuffs show that the order of the CODCr removal rate of various dyestuffs during electrolysis is: sulfur dyes, vat dyes> acid dyes, reactive dyes> neutral dyes, direct dyes> cationic dyes. At present, this method is being promoted and applied. ?
2.3 Biological treatment of printing and dyeing wastewater? Since the 1970s, domestic printing and dyeing wastewater has been dominated by biological treatment, accounting for more than 80%, and aerobic biological treatment methods account for the vast majority. From the current situation, China's printing and dyeing wastewater biological treatment method in the surface accelerated aeration and contact oxidation majority. In addition, the blast aeration activated sludge process, jet aeration activated sludge process, biological rotors, etc. also have applications, biological fluidized bed is still in experimental application stage. However, due to the fact that the removal rate of color to the organism is not high, it is generally around 50%. Therefore, when the chromaticity requirement of the effluent is high, it needs to be supplemented with physical or chemical treatment. Table 1 shows the application of domestic biological treatment process of printing and dyeing wastewater. ?
The effect of aerobic biological treatment on BOD removal is obvious, generally up to 80%, but the removal rate of color and COD is not high, especially the widespread application of chemical pulp, surfactants, solvents and berberine reduction techniques such as PVA Not only make the COD of printing and dyeing wastewater reach 2000-3000?mg/L, but also the BOD/COD from the original 0.4-0.5 to 0.2 or less, the difficulty of simple aerobic biological treatment is getting more and more difficult, and the water is difficult to reach the standard; The high operating costs of aerobic processes and the disposal or disposal of excess sludge have historically been a problem that has not been solved in the area of ​​wastewater treatment. According to statistics, the cost of sludge treatment or disposal accounts for 50% to 70% of the total cost of the sewage treatment plant (foreign countries), and it also accounts for about 40% in China. Due to the above reasons, the anaerobic biological treatment technology of printing and dyeing wastewater has begun to attract people's attention. It has become increasingly important to explore new technologies for high efficiency, low consumption, and investment in printing and dyeing wastewater.
The main anaerobic treatment structures are anaerobic tanks. Fukunaga N. et al. modified the traditional digester tanks and filled the tanks with fixed microorganisms, mainly obligate Alcaligenes. The azo gene, triphenylmethane gene, and mononitrogen gene polymer in the dye can all be decomposed anaerobicly, usually at medium temperature (37°C), hydraulic retention time 6 h, and the color of wastewater mainly containing methyl red dye. Can be completely removed. Some studies have shown that anaerobic treatment of silk printing and dyeing wastewater, HRT = 1.0 ~ 1.1d, COD removal rate of 74% ~ 82%, decolorization rate were: black 51%, purple 94%, Rose Red 96%, 30% eggplant purple 55%. The medium and long-term operation results of direct treatment of high-concentration dye wastewater with UASB and pipeline anaerobic digesters showed that the color and COD removal rates in the wastewater stabilized at 80% and 90%, respectively.
In order to explore new technologies for efficient and low-cost, low-cost printing and dyeing wastewater treatment, a large number of experimental studies have been conducted on the combination of anaerobic and aerobic processes in recent years, and great success has been achieved. Anaerobic treatment combined with aerobic treatment at this time is not conventional anaerobic digestion. Its hydraulic retention time (HRT) is generally 3 to 5 hours, and only hydrolysis and acidification occur. The proposed process is mainly aimed at some macromolecular substances with poor biodegradability in printing and dyeing wastewater. It is expected that they will be hydrolyzed and acidified in the anaerobic stage to become smaller molecules, thereby improving the biodegradability of wastewater. Aerobic treatment creates conditions. Using this process, the PVA and dye processing problems are better solved. Another major feature of this process is that the excess sludge produced by the aerobic section is returned to the anaerobic section, and the anaerobic section has a longer solid retention time (SRT), which is favorable for the anaerobic digestion of the sludge and thus significantly Reduced the amount of residual activated sludge in the entire system. Therefore, the anaerobic stage in the anaerobic and aerobic system has a dual role: First, the wastewater is pretreated to improve its biodegradability, and some organic compounds are adsorbed and degraded. Second, the excess sludge of the system is digested.
Using this process, two processes are currently being developed: anaerobic, aerobic, biochar contact, anaerobic, and aerobic biological carousel. Both processes have their own characteristics in terms of equipment and process. ?
2.3.1 Anaerobic? Aerobic? Biological carbon contact oxidation process? The main design parameters are as follows: conditioning pond: HRT 8 ~ 10h; anaerobic pond: HRT 3 ~ 5h; aerobic pond: HRT 6 ~ 8h; biochar pool: HRT 1 ~ 2h.
Tests and practical applications show that anaerobic-aerobic biochar process can achieve the CODr of 800-1000μm/L dyeing wastewater with the above operating parameters. The treatment effect can completely meet the national emission standards, and further processing Can also be reused, the system's sludge tends to balance itself. At present, a number of production plants have adopted this process. The longest operating time is more than five years. The treatment effect is stable, and sludge has never been discharged, and no excessive growth of sludge in the anaerobic tank has been found. ?
2.3.2 Anaerobic? Aerobic biological carousel? An anaerobic biological carousel is connected in series with an aerobic biological carousel for the treatment of printing and dyeing wastewater. Good results have also been achieved. In this process, anaerobic and aerobic sludge separation and return devices are used, and the remaining sludge in the whole system is returned to the anaerobic biological carousel. The first is to increase the biomass, thus reducing the total hydraulic residence time, and the second is to digest the excess activated sludge inside the system. The process is also characterized by both fixed growth and suspended growth. It is also possible to further increase the COD removal rate and decolorization rate by adding flocculants to the turntable. The removal rate of COD, color, etc. in this process has reached over 70%. Appropriate dosing of micro-flocculant, measured CODCr, color removal rate can be increased by 15% to 20%. Further increasing the suspended sludge concentration in the anaerobic tank can also increase the decolorization rate and COD removal rate. However, the corrosion of the metal components of the turntable in this process requires further study and resolution. ?
3 Alkali reduction wastewater treatment method
According to information, the mature technology for treating alkali reduction wastewater is still blank in the country. In the study of the treatment of this waste water, chemical methods and chemical removal of terephthalic acid are generally used. However, there are still many problems.
The theoretical basis for the chemical treatment of alkali reduction wastewater is: after the alkali reduction wastewater is neutralized with acid to make the pH value reach 4-6, the terephthalic acid is precipitated, and the alkali reduction wastewater of terephthalic acid is removed and then simulated with polyester. The wastewater of silk printing and dyeing wastewater is mixed with other processes such as refining, printing and dyeing. The pH of integrated wastewater is generally less than 11 and the CODCr does not exceed 1400?mg/L. In this case, the biochemical method is used for treatment, and then the materialized water is treated as effluent. National emission standards can be met.
Generally, the process of alkali reduction wastewater treatment is: alkali reduction wastewater → conditioning pond → neutralization pond → PE filter → effluent and other wastewater mixed for further biochemical treatment.
The use of chemical methods to precipitate terephthalic acid as an alkali reduction wastewater pretreatment technology, and then use biotechnology to treat integrated wastewater is an effective method to treat high-density polyester silk printing and dyeing wastewater, and is currently the main way to treat this type of wastewater. The actual application of the Shantou Special Economic Zone Xinchang Textile Printing and Dyeing Factory Co., Ltd. shows that in the case of high concentration of raw water and a large fluctuation range, the discharged water can meet the national water quality discharge standards.
The wastewater of this plant adopts this method to govern the investment of ?5500? yuan/m3 of waste water; it occupies an area of ​​0.61m2/m3 of waste water; and the electricity charge is 0.44 yuan/m3 of waste water; the medicine fee is 0.9 yuan/m3.
The use of chemical treatment of alkali decontamination wastewater treatment effect is good, but there are still some problems: (1) The optimal pH of the pretreatment process is in the range of 4 to 6, while the pH of the alkali decontamination wastewater is 12 to 14, to reduce the pH value requires the consumption of a certain amount of acid, so that the operating costs increase, which is an issue that needs to be solved. (2) The white powder of terephthalic acid produced by pretreatment has industrial recycling value, but the market needs to be opened up. ?
4 Conclusions and problems
Printing and dyeing wastewater is a kind of wastewater with large amount of water, high chroma, and complex composition, and its water quality has a wide range of variation. In cities with better sewers and sewage treatment plants, wastewater is first pretreated at the factory and treated centrally after reaching the urban sewer discharge standards. Wastewater after pretreatment and discharge can improve the quality of sewage, reduce the treatment load of the municipal wastewater treatment plant, and at the same time facilitate the adoption of different pretreatment methods according to different wastewater quality. In the final treatment of printing and dyeing wastewater, the removal of organic matter is generally based on biological methods. For anaerobic printing and dyeing wastewater, anaerobic (hydrolysis) is used. The combination of aerobic treatment is more appropriate. For printing and dyeing wastewater that is easily biodegradable, Use a biological treatment. The removal of chromaticity is generally based on physicochemical methods. Electrolysis, chemical flocculation, and ozone oxidation can be used for large-scale, high-processing plants. For small-scale plants, slag filtration can be used.
From the current status of wastewater treatment technology in China's dye industry, despite years of hard work, a number of practical technologies have been obtained and many problems have been solved, but there has been no substantial breakthrough in general, and in particular, the product structure and factory layout are unreasonable. The existence of factors has aggravated the difficulty of wastewater treatment. Therefore, it is believed that the fundamental solution to the waste water problem lies in the process reforms, and the use of advanced production processes to reduce or eliminate waste water. There have been many successful examples in this respect, such as the production of aniline and o-toluidine, which changed the reduction of iron powder to hydrogenation and reduction, completely eliminating the pollution of iron mud water, and used hydrogenation reduction instead of sulfuric acid alkali reduction for amino anisole. The production completely eliminates the sulfur-containing waste water.
Prevention and control of the pollution of printing and dyeing wastewater are two aspects that complement each other. If both preventative measures are adopted and various methods are used to actively treat and recycle water after treatment, this will not only reduce water consumption, but also effectively Reduce the pollution of printing and dyeing wastewater to the environment.