1. Main sources and properties of textile printing and dyeing wastewater
Textile printing and dyeing wastewater mainly comes from various production and processing processes. Because of the different chemical raw materials used, the pollutants produced are also different. Textile printing and dyeing enterprises generally account for 60% to 80% of the total drainage, mainly from desizing, boiling, bleaching, mercerizing, dyeing, printing and finishing processes. Desizing wastewater generally accounts for about 15% of the total textile printing and dyeing wastewater, and has a high degree of pollution. Chemical oxygen demand usually accounts for more than half of the total pollutants, and contains various pulps, fiber debris and other pollutants.
When starch slurry is used, the biodegradability of desizing wastewater is usually better. The ratio of BOD to COD in five days can reach 0.3-0.5. However, when PVA slurry is used, the biodegradability of desizing wastewater is poor and BOD5/COD value is usually lower than 0. 1. Boiling wastewater usually produces a large amount of strong alkalinity, deep color and high concentration of organic pollutants.
Bleaching wastewater is a kind of cleaner wastewater in textile printing and dyeing wastewater, which can be directly discharged or recycled. Mercerizing wastewater can be recovered and reused after evaporation and concentration, but a small amount of wastewater discharged at the end is alkaline. Dyeing wastewater has the characteristics of great water quality change, deep color and strong alkalinity. Especially when sulfur dyes and vat dyes are used, the pH value of the wastewater exceeds 10. Printing wastewater mainly includes color mixing, printing drum and screen washing water, as well as post-treatment soaping, washing, printing lining and other wastewater.
The treated wastewater mainly contains resin, formaldehyde and surfactant, and the amount of wastewater is relatively small. Thus, textile printing and dyeing wastewater has the characteristics of great changes in water quality and quantity, strong alkalinity, great changes in pH value, deep color and high concentration of organic matter. If it is discharged without effective treatment, it will cause serious pollution and harm to the ecological environment.
2. Textile printing and dyeing wastewater treatment technology
2.1 Chemical Treatment
2.1.1 Coagulation
Coagulation process has the advantages of less investment, small equipment occupation and high decolorization rate. It is widely used in wastewater treatment of small and medium-sized printing and dyeing enterprises in China. Usually, coagulation has good removal effect on hydrophobic dyes such as disperse dyes, but poor removal effect on water-soluble dyes such as reactive dyes. Therefore, choosing suitable coagulant and efficient coagulation process is the key to ensure the effect of wastewater treatment. In recent years, the development and application of multi-functional and efficient composite coagulants have greatly expanded the application scope of coagulation process, and achieved good results in organic matter removal and decolorization.
2.1.2 Chemical oxidation
Chemical oxidation uses strong oxidant chain-breaking dye molecule chromogenic groups to achieve decolorization. This method is widely used in the treatment of stable and refractory dyes, including ozone oxidation, Fenton oxidation and high temperature deep oxidation.
Ozone oxidation method. Ozone oxidation of organic compounds usually involves two ways: direct reaction and indirect reaction. The direct reaction is that ozone reacts directly with organic pollutants through cycloaddition, electrophilic or nucleophilic action, and the indirect reaction is that ozone produces hydroxyl radicals with stronger oxidation ability under the action of alkali and light. Ozone can effectively destroy the chromogenic groups of dyes, and also destroy the cyclic compounds such as benzene, naphthalene and anthracene, which constitute the chromogenic groups, so as to achieve decolorization effect. The secondary biochemical effluent of printing and dyeing wastewater was treated by ozonation. The removal rates of COD and chroma were 75% and 85% respectively.
Fenton oxidation method. Fenton oxidation process uses hydrogen peroxide (H2O2) to produce OH with high reactivity under the catalysis of ferrous ions (Fe2+). Its oxidation is not selective and can react with most organic compounds to degrade it. Fenton reagent was used to treat printing and dyeing wastewater from dyeing and sock factory. The effects of reaction time, ferrous sulfate dosage, hydrogen peroxide dosage and pH value on the chroma and COD treatment efficiency of the wastewater were investigated. It was found that under the optimum conditions, COD removal rate was more than 80%, and chroma removal rate was more than 95%.
High temperature deep oxidation method. High temperature deep oxidation method mainly includes supercritical water oxidation (SCWO), wet catalytic hydrogen peroxide oxidation (CWPO) and wet air oxidation (WAO). The reaction principle and characteristics of CWPO technology are summarized. It is pointed out that CWPO technology has good application prospects and popularization value in the treatment of printing and dyeing wastewater. Reactive brilliant blue KN-R was degraded by CWPO technology. When the initial dye concentration (mass concentration, the same below) was 200 mg/L, the decolorization rate and total organic carbon (TOC) removal rate reached 100% and 68.5% respectively under the optimal reaction conditions.
2.1.3 Photocatalytic Oxidation
Photocatalytic oxidation refers to the process in which the semiconductor photocatalyst is stimulated under light conditions, and the generated hole electrons react with water molecules to form OH, which can remove organic pollutants by oxidation. The commonly used semiconductor catalysts are titanium dioxide, zinc oxide, zinc sulfide, iron oxide and copper oxide. In theory, photocatalytic oxidation can mineralize organic pollutants in wastewater to ensure sufficient reaction time and appropriate reaction conditions, but it is difficult to achieve this goal under actual conditions. The photocatalytic oxidation method is still in the research stage and has not been put into practice on a large scale in China.
2.1.4 Electrochemical method
The electrochemical method is a method of degrading and removing organic pollutants from wastewater by redox reaction of anode and cathode. It has the advantages of simple process and obvious treatment effect. Rhodamine B was degraded by electrofenton. The effects of different reaction conditions on the removal efficiency were investigated. It was found that the removal rate of Rhodamine B with initial concentration of 10 mg/L reached 97.5% under the optimal reaction conditions.
2.2 Biological Treatment
2.2.1 Aerobic Treatment
Aerobic treatment includes activated sludge process and biofilm process. Activated sludge process is widely used in printing and dyeing wastewater treatment because of its good removal effect of organic matter and partial chroma, good quality of treated effluent and low cost. However, sludge bulking is easy to occur, which is generally applicable to the situation of low water volume. For printing and dyeing wastewater with good biodegradability, the removal rate of BOD5 can generally reach 80% by aerobic treatment. Nevertheless, with the extensive use of synthetic dyes and slurry additives, the single aerobic treatment of printing and dyeing wastewater has been unable to achieve better COD and chroma removal effect.
2.2.2 Anaerobic Treatment
Anaerobic treatment can be used not only to treat high concentration organic wastewater, but also to treat medium and low concentration organic wastewater. Although the removal effect of organic matter is not as good as that of aerobic treatment, it can improve the biodegradability of wastewater and the effect of subsequent biological treatment. For printing and dyeing wastewater, anaerobic biological treatment can break the azo, anthraquinone and triphenylalkyl groups in dyes, but the treatment effect of aromatic amines, the intermediate of reactive dyes, is very poor. In practical printing and dyeing wastewater treatment projects, anaerobic treatment method is usually combined with aerobic treatment method.
2.2.3 Anaerobic-Aerobic Treatment
Anaerobic-aerobic treatment mainly decomposes macromolecular refractory organic matter into biodegradable small molecular organic matter through anaerobic biological treatment process, which improves the biodegradability of wastewater and is conducive to the efficient removal of organic matter by subsequent aerobic biological treatment. Anaerobic-aerobic treatment is the most widely used treatment method for textile printing and dyeing wastewater treatment. It has the characteristics of high COD removal rate, good decolorization effect, stable operation and strong impact load resistance. Azo dye wastewater was treated by anaerobic bio-rotating disc-aerobic movable bed membrane bioreactor, and good removal effect was obtained.
2.2.4 Bioaugmentation Technology
Bioaugmentation technology is based on microbial composition and environmental conditions in wastewater treatment system, adding bacterial microorganisms with specific functions to improve wastewater treatment efficiency. The added microorganisms can be domesticated, enriched, screened and cultured by the microorganisms in the original treatment system, and can also be added with exogenous microorganisms with high degradability. The technology can improve the removal efficiency of refractory organic matter in printing and dyeing wastewater and achieve good decolorization effect. The methyl orange wastewater was degraded by cultured white rot fungi. It was found that the decolorization rate of Methyl Orange Wastewater with initial concentration of 10-50 mg/L reached 55.8%-72.6% after one week.
3 Conclusion
To sum up, textile printing and dyeing wastewater has the characteristics of large amount of wastewater, complex water quality and great changes in water quality and water quantity. Its treatment is relatively complex. The treatment methods include physical method, chemical method and biochemical method. According to the raw materials, product varieties and processing methods of fabrics, it is necessary to know the dosage of medicines, dyes and auxiliaries in each process, analyze the characteristics of wastewater components, and select the optimal treatment technology through technical and economic comparison.
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