1, dye wastewater and its pollution
The pollution problem of dye wastewater is the most prominent in dye industry. In recent years, China's annual sewage discharge has reached more than 39 billion tons, of which industrial sewage accounts for 5 1%, while dye wastewater accounts for 35% of the total industrial wastewater discharge, and it is increasing year by year at the rate of 1%. Every discharge of 1t dye wastewater can cause 20t water pollution. In all industries, the COD emission of printing and dyeing textile industry ranks fourth, and the emission ratio is still increasing year by year. In the "three rivers and three lakes", the pollution of dye wastewater to Taihu Lake and Huaihe River Basin is particularly serious.
Dye wastewater mainly comes from the production enterprises of dyes and dye intermediates, and consists of dyes, sizing agents and auxiliaries discharged during dyeing and finishing. With the rapid development of printing and dyeing industry, dye wastewater has become one of the most important pollution sources in water. At present, the annual output of dyes in the world is about (8~9)x 105t. China is a big country in textile production and processing, and its textile exports rank first in the world for many years. The annual output of dyes reaches 1.5× 105t, of which about 10% ~ 15% of dyes will be directly discharged into the water with the wastewater.
Dye wastewater is one of the most difficult industrial wastewater because of its high chroma, large quantity of water, high alkalinity and complex composition. Dye is the main pollutant in dye wastewater, which contains various chromogenic groups (such as -N=N-, -N=O, etc. ) and some polar groups (-SO3Na, -OH, -NH2). The components are complex, mostly aromatic hydrocarbons and heterocyclic compounds, which are difficult to degrade, and are also important pollution sources in major waters of China.
Most organic dyes have strong chemical stability and three (carcinogenic, teratogenic and mutagenic) effects, and are typical toxic and refractory organic pollutants. In addition, dyes in wastewater will absorb light and reduce the transparency of water body, which is not conducive to the growth of aquatic organisms and microorganisms and reduces the self-purification ability of water body. At the same time, it causes visual pollution, seriously damages water, soil and ecological environment, and directly and indirectly harms human health.
2. A method for treating dye wastewater
Effective degradation and treatment technology of dyes is an important prerequisite for treating dye wastewater. Because most dyes are chemically stable and difficult to degrade, scientists all over the world attach great importance to the research on the degradation and treatment methods of dyes and dye wastewater. With the progress of science and technology and the continuous development of pollution control technology, human beings have also found many effective methods to treat dye wastewater, which can be summarized as physicochemical method, biological method and physicochemical-biological combination method.
2. 1 materialization method
2. 1. 1 coagulation and precipitation method
Coagulation and precipitation method is a stable and mature method to treat dye wastewater at present. The generally accepted mechanisms are bridging, compression of double layers, net capture and electrical neutralization. The characteristics of coagulant itself determine its sedimentation performance, and many environmental factors, including temperature, pH and Eh, may promote or inhibit its sedimentation function. In recent years, IPF (inorganic polymer flocculant) has become a hot spot in the study of coagulation behavior and mechanism. Compared with common coagulants, IPF can form a more effective flocculation form. The main research direction of coagulation method is to develop effective coagulants, especially organic-inorganic composite coagulants.
The inorganic-organic composite coagulant developed by Zhang et al. has better treatment effect on dye wastewater than polyaluminum chloride. Wu Dunhu and others studied the treatment of dye wastewater with boron mud composite coagulant. The results show that when the dosage is 0.3 ~ 0.6 g/L and the pH value is 4.0~ 1 1.5, the decolorization rate is over 92%, which is better than PAC.
2. 1.2 membrane separation method
Membrane separation technology has the advantages of simple process, low energy consumption and no pollution to the environment. Through the self-developed cellulose acetate (CA) nanofiltration membrane, Guo Mingyuan and others pointed out that CA nanofiltration membrane has obvious effect on the treatment and recovery of reactive dye wastewater. The modified chitosan ultrafiltration membrane filled with activated carbon, after proper crosslinking, has the maximum decolorization rejection rate of acid red dye wastewater of 98.8%. Feng et al. used chitosan ultrafiltration membrane to treat dye wastewater. The decolorization rate is above 95%, and the COD removal rate is about 80%. Wu Kaifen used ultrafiltration to treat indigo dye wastewater, which can realize the direct reuse of high concentration dye solution, and the permeate can be reused as neutral water. Mo such as Soma used alumina microfiltration membrane to filter insoluble dye wastewater, and the rejection rate was as high as 98%.
Due to membrane pollution, concentration polarization and frequent replacement, and the high price of membrane, the cost of membrane separation technology for dye wastewater treatment is too high, which greatly limits the application and promotion of membrane separation technology in dye wastewater treatment industry.
2. 1.3 catalytic oxidation method
Catalytic oxidation accelerates the decomposition of oxidant in the system through catalysis, which makes it react with organic matter in water quickly, thus leading to the oxidative degradation of organic pollutants in a short time. In order to solve the problem that the effect of advanced chemical oxidation and aerobic biological treatment on disperse dye wastewater is not ideal, Zhou Jian and others used catalytic oxidation to treat dye wastewater that could not meet the standard after internal electrolysis treatment, which not only treated anthraquinone series disperse dyes up to 2500t per day, but also reduced the chromaticity and COD value of dye wastewater that could not meet the standard after internal electrolysis treatment, greatly reducing the operating cost. ArslanLt introduced the treatment of disperse dye wastewater by Fe2+ catalytic ozonation. It is pointed out that when ozone oxidation method is used alone (dosage is 2300 mg/L), only under the condition of pH=3, the decolorization rate is only 77%, and the COD removal rate is only 1 1%. When Fe2+ flocculation, ozone oxidation and Fe2+ catalytic ozone oxidation are combined, when the dosage of Fe is 0.09 ~ 18 mmol/L and the pH value of dye wastewater is 3 ~ 13, the decolorization rate reaches 97% and the COD removal rate increases to 54%.
2. 1.4 Fenton reagent method
With Fe3+ or Fe2+ as catalyst, strong oxidation in the presence of H202 can oxidize many organic molecules, and the reaction system does not need high temperature and high pressure, the reaction conditions are not harsh, the reaction equipment is relatively simple, and the application range is wide. Chen et al.' s research on the treatment of simulated and actual dye wastewater by low-dose Fenton oxidation-coagulation method points out that this method is especially suitable for treating dye wastewater with complex components and both hydrophilic and hydrophobic dyes, with convenient operation and low operating cost. In recent years, some scholars introduced ultraviolet (uV) and oxalate into Fenton process, which greatly improved the oxidation capacity of Fenton process and made the treatment effect more remarkable. K. Swaminathan and others studied the decolorization of azo dye reactive orange -4 by light-assisted Fenton system, and concluded that the degradation ability of light-assisted Fenton system was much stronger than that of ordinary Fenton system.
The disadvantage of Fenton method is that the oxidation ability is weak, and the effluent is colored because it contains a lot of iron ions. In recent years, the immobilization technology of iron ions has become an important direction of Fenton oxidation.
2. 1.5 photooxidation method
Photooxidation is the degradation of pollutants through photochemical reactions, including catalyst-free and catalyst-free. The former is also called photochemical oxidation, and the latter is also called photocatalytic oxidation. Photodegradation usually refers to the gradual oxidation of organic matter into intermediate products with low molecular weight under the action of light, and finally produces CO2 and H2O plasma, such as PO43-, NO3-, Cl- and so on. The photodegradation process of organic matter can be divided into direct photodegradation and indirect photodegradation. Direct photodegradation refers to the further chemical reaction after organic molecules absorb light energy. Indirect photodegradation is that some substances in the surrounding environment absorb light energy to form excited States, and then induce organic pollutants to produce a series of oxidative degradation reactions, which is more effective for treating organic pollutants that are difficult to biodegrade in the environment.
2. 1.6 ozone oxidation method
Ozone has a strong oxidizing ability. In addition to dispersing dyes, it can also destroy the chromophoric or chromophoric groups of organic dyes, which has a certain decoloring effect. H.Y.Shu and others compared the degradation of eight azo dyes under O3, oxidation and UV/O3 oxidation alone. The results show that the degradation rate of organic dyes is not obviously accelerated after introducing ultraviolet light, which may be because the dye wastewater is too dark and absorbs most of the ultraviolet light. Shi et al. pointed out that direct oxidation is the main decolorization method of azo dye cationic red x-GRL.
Because of the low solubility of ozone in water, how to improve the solubility of ozone in aqueous solution more effectively has become the focus and key to study ozone oxidation technology. In addition, the use of ozone will produce some by-products, especially aldehydes such as formaldehyde and acetaldehyde in carbonyl compounds. Because these substances have acute and chronic toxicity and certain carcinogenicity, teratogenicity and mutagenicity, it is easy to cause secondary pollution. In addition, the cost of ozone generator is relatively high, so it is not economical to use it alone.
2. 1.7 ultrasonic oxidation method
With the development of ultrasonic chemistry, ultrasonic oxidation is considered as a clean and promising effective technology to treat water pollution. The high temperature and high pressure produced by acoustic cavitation effect under the action of ultrasonic waves dissociate water vapor in cavitation bubbles from other gases to generate free radicals, thus triggering ultrasonic chemical reactions. N.Ince et al.' s research on the influence of pH and dye molecular structure on ultrasonic degradation efficiency shows that pH has an important influence on dye degradation, and the degradation degree increases with the decrease of pH; The smaller the molecular weight, the simpler the structure, and the easier it is for dye molecules with azo o-hydroxyl substituents to be degraded. G. tezcanli-gtiyer and others have just found that hydroxyl radicals attack the chromophore of dyes first, and the decolorization process of dyes is faster than the destruction process of aromatic rings. J. Ge and others also pointed out that the introduction of ultrasonic wave can effectively accelerate the degradation of dyes and improve the mineralization rate.
2. 1.8 electrochemical method
Electrochemical treatment technology has developed rapidly in recent years, and the synergistic effect of oxidation, photocatalytic oxidation or catalytic oxidation has been added on the original basis, which has solved the limitations of micro-electrolysis technology well. Zhou Guangyuan's research on the treatment of salt dye wastewater shows that the generation of residual chlorine plays a key role in decoloration and COD removal. After electrolysis 1 h, the decolorization rate can reach 85% and the COD removal rate can reach 99.8%. When Zhang Tingxi and others used internal electrolysis-catalytic oxidation-oxidation pond method to treat dye wastewater, the removal rate and decoloration rate of COD were above 95%. Qi Menglan et al. used the combined process of micro-electrolysis, catalytic oxidation and fly ash adsorption to treat reactive dye wastewater. The decolorization rate reached 99.9% and the COD removal rate was above 95%.
At present, electrochemical methods are mainly used to remove organic pollutants with biological toxicity. One of the most attractive features of this method is that it can give full play to the unique electrocatalytic performance of electrochemical methods and selectively degrade organic pollutants to some extent. In addition, electrochemical method has a good synergistic effect with other treatment methods and can be combined to achieve ideal treatment effect. However, the equipment for completely degrading organic pollutants in water by electrochemical method is too expensive and consumes a lot of energy.
2.2 Biological methods
Biological treatment is to separate and oxidize dyes through flocculation, adsorption and biodegradation of biological bacteria. Bioflocculation and biosorption will not cause chemical changes of dyes. The process of biodegradation is to oxidize or reduce dye molecules through the action of microbial enzymes, destroying chromophores and unsaturated bonds of dyes. Through a series of processes, such as oxidation, reduction, hydrolysis and combination, dye molecules are finally degraded into simple inorganic substances or transformed into nutrients or protoplasm needed by various microorganisms. There are three biological treatment methods: aerobic treatment, anaerobic treatment and anaerobic-aerobic combined treatment.
In view of the fact that traditional biological treatment methods can not effectively treat organic dyes in textile printing and dyeing wastewater, in recent years, some scholars have vigorously developed anaerobic-aerobic combined technology and achieved unexpected results. Studies have shown that many aerobic biological methods can not oxidize or degrade organic dyes to a limited extent by applying aerobic and anaerobic methods at the same time, but can achieve different degrees of degradation by anaerobic methods.
As a practical water pollution treatment technology, microbial treatment of dye wastewater has been developed and studied for many years. The mechanism of microbial decolorization and degradation is very complex and diverse, and many degradation processes and reaction mechanisms are still unclear, which need to be discussed continuously.
White rot fungi, represented by Phanerochaete chrysosporium, have become effective weapons to control various pollutants because of their low consumption, high efficiency, broad spectrum and strong applicability to various toxic, harmful, refractory and persistent xenobiotics in the environment. Fungal technology developed in recent years has been called innovative environmental biotechnology by many scholars. It may be due to the role of lignin peroxidase and manganese peroxidase produced in the secondary metabolism stage that many white rot fungi have broad-spectrum decolorization and degradation ability for dyes. Culture conditions have great influence on decolorization and degradation activity of white rot fungi. Conneely and others think that white rot fungi have a strong biosorption effect on some dye wastewater, such as rimoxazole green blue G 133, phthalocyanine dye, ivezole green blue and Heiligong blue. And decolorize and degrade the dye through the metabolism of extracellular enzymes.
One of the development directions of using microorganisms to treat dye wastewater is to breed and cultivate high-efficiency degradation engineering bacteria. Decolorization and degradation of organic dyes by microorganisms used to focus on facultative anaerobic bacteria, such as Bacillus, Pseudomonas and some photosynthetic bacteria. In recent years, many new varieties have been gradually screened out. Some scholars use Pseudomonas to treat various printing and dyeing industrial wastewater. The results showed that the decolorization rate of methyl orange and B 15 dyes by Pseudomonas oleophilus was over 80%, and Pseudomonas oleophilus showed strong tolerance to high concentration dye environment.
In the early 1980s, immobilized microorganism technology became the research focus of organic industrial wastewater treatment at home and abroad. This technology fixes the microorganisms that can degrade dyes on the surface of a specific carrier, thus improving the microbial degradation efficiency. There are many immobilization methods of microorganisms, single and mixed. Related research shows that mixed bacteria have better decolorization and degradation ability. With the development of immobilized decolorizing bacteria carrier technology, the reaction time of decolorization and degradation has been greatly shortened.
Bioaugmentation technology is to add microorganisms with specific functions to the biological treatment system to improve the treatment performance of the original treatment system and remove refractory organic matter. The main ways to implement bioaugmentation technology are: adding highly degradable microorganisms; Adding genetically engineered bacteria (GEM); Optimize the nutrient supply of the existing treatment system, and stimulate the growth of microorganisms or improve their vitality by adding substrates or similar substances.
Membrane bioreactor is also a new sewage treatment technology developed in recent years. It was first used in the fermentation industry. In 1980s, membrane bioreactor technology attracted great attention in academic circles. Membrane technology can intercept organic matter and reduce the organic matter contained in effluent. Maximize the use of oxygen through bubble-free aeration and membrane bioreactor. In recent years, membrane bioreactor has been successfully used to treat waterway sewage, fecal sewage and landfill leachate, and has been used to treat dye wastewater. Many scholars believe that enzyme-containing membrane bioreactor will be an important direction to treat dye wastewater in the future. Due to the high cost of membrane preparation and easy blockage, the comprehensive promotion of membrane bioreactor technology in the field of water treatment is limited.
Although the biological method has made great progress, with the decrease of biodegradability of dye wastewater and the strict requirements of microorganisms on nutrients, pH value, temperature and other conditions, it is difficult for the biological method to adapt to the actual situation of large fluctuation of dye wastewater quality, many kinds of dyes and great toxicity in practical application. Such as high efficiency and immobilization of microorganisms. Many experts and scholars devote themselves to the screening of high-efficiency degrading bacteria and the construction of genetically engineered bacteria, so as to make use of the rich resources in nature to serve mankind. However, the practice shows that the newly developed high-efficiency bacteria may not fully achieve the expected strengthening effect when applied to dye wastewater treatment. In addition, microorganisms themselves also have safety problems. Efficient bacteria and genetically engineered bacteria may flow into the natural environment, threatening the natural environment and ecological balance. Therefore, the application of these biological methods must undergo strict environmental safety inspection and evaluation in advance. At the same time, the degradation mechanism of dyes by microorganisms and the metabolism mechanism of microorganisms also need further study and discussion.