Environmental Impact Of Textile Printing And Dyeing Wastewater And Suggestions On Treatment Methods

　　 Characteristics of printing and dyeing wastewater :

The textile printing and dyeing industry is a major source of industrial sewage. The sewage mainly contains dirt, grease, salts on textile fibers, as well as various sizes, dyes, surfactants, auxiliaries, acids and alkalis added during processing.

The wastewater is characterized by high concentration of organic matter, complex composition, deep and changeable chroma, large change in pH, water quantity and quality, and it is difficult to treat industrial wastewater. With the development of chemical fiber fabrics, the rise of silk like fabrics and the improvement of requirements for post printing and dyeing finishing, a large number of refractory organics such as PVA size, rayon alkali hydrolysates, new dyes, auxiliaries and so on have entered textile printing and dyeing wastewater, posing a serious challenge to the traditional wastewater treatment process, and the COD concentration has also increased from hundreds of milligrams per liter to 3000~5000mg/l.

Pulp dyeing wastewater has high chroma and high COD, especially according to mercerized blue, mercerized black, ultradeep blue, ultradeep black and other printing and dyeing processes developed in the foreign market, this kind of printing and dyeing uses a large number of sulfur dyes, printing and dyeing auxiliaries, sodium sulfide, etc., so the wastewater contains a large amount of sulfide. This kind of wastewater must be pretreated with chemicals, and then subjected to a series of treatment, in order to stabilize the discharge up to standard.

Bleaching and dyeing wastewater contains dyes, slurries, surfactants and other auxiliaries. This kind of wastewater has a large amount of water, low concentration and chroma. If only physical and chemical treatment is used, the effluent will also be between 100 and 200 mg/l, and the chroma can meet the discharge requirements. However, the amount of pollution is greatly increased, and the cost of sludge treatment is high, which is easy to cause secondary pollution, In the case of strict environmental protection requirements, the biochemical treatment system should be fully considered, and the conventional enhanced biological treatment process can meet the treatment requirements.

　　 Chemical treatment:

Coagulation method:

There are mainly doubt precipitation method and doubt air flotation method. Most of the doubt mixers used are aluminum salt or iron salt, of which basic aluminum chloride (PAC) has better bridging adsorption performance, while ferrous sulfate has the lowest price. Polymeric mixers are increasingly used abroad and tend to replace inorganic mixers. However, due to price reasons, the use of polymer mixers is rare in China. It is reported that weak anionic polymer mixers are the most widely used. If they are used together with aluminum sulfate, they can play a better role. The main advantages of the confusion method are simple process flow, convenient operation and management, low equipment investment, less land occupation, and high decolorization efficiency for hydrophobic dyes; The disadvantages are high operation cost, large sludge volume, difficult dehydration and poor treatment effect on hydrophilic dyes.

　　 Oxidation method:

Ozone oxidation method is widely used abroad. Zima S.V. and others summarized the mathematical model of ozone decolorization of printing and dyeing wastewater. The research shows that the ozone consumption is 0.886gO3/g The decolorization rate of light brown dye wastewater reaches 80% when dyes are used; The study also found that the amount of ozone required for continuous operation is higher than that for intermittent operation, and the installation of baffles in the reactor can reduce the amount of ozone by 16.7%. Therefore, it is advisable to design a reactor for intermittent operation by using ozone oxidation to decolorize, and consider installing a partition board in it. The ozone oxidation method can obtain good decolorization effect for most dyes, but it can be used for vulcanization, reduction The decolorization effect of water insoluble dyes such as paint is poor. According to the operation experience and results at home and abroad, the decolorization effect of this method is good, but it is difficult to popularize and apply it on a large scale due to the high power consumption. Photooxidation has a high decolorization efficiency in treating printing and dyeing wastewater, but the equipment investment and power consumption need to be further reduced.

　 Electrolysis:

Electrolysis has a good effect on the treatment of printing and dyeing wastewater containing acid dyes, with a decolorization rate of 50%~70%, but it has a poor effect on the treatment of wastewater with deep color and high CODcr. The research on the electrochemical performance of dyes shows that the order of CODcr removal rate of various dyes in electrolytic treatment is: sulfur dyes, vat dyes>acid dyes, reactive dyes>neutral dyes, direct dyes>cationic dyes. This method is being popularized and applied.

　　 technological process:

The conventional treatment methods of printing and dyeing wastewater are generally divided into biochemical+physicochemical and physicochemical+biochemical treatment processes. However, due to the lack of hydrolytic acidification unit, the aerobic biochemical unit reaction is not thorough enough in actual operation, resulting in the high cost of subsequent physicochemical treatment. The "hydrolysis+aerobic" of hydrolysis acidification treatment is added in front of the traditional aerobic biological treatment device The serial process can hydrolyze the organic matters difficult to be degraded in the printing and dyeing wastewater to produce more biodegradable substances, and improve the biodegradability of the wastewater? So as to improve the COD removal rate of the traditional process. Many newly built printing and dyeing wastewater treatment plants in China (including domestic sewage and printing and dyeing wastewater centralized treatment) use the "hydrolysis aerobic" process developed by this process Biological treatment process? Significant environmental and economic benefits have been achieved.

　　 The four processes of printing and dyeing process have wastewater discharge:

Desizing wastewater discharged from ferrous sulfate pretreatment stage (including singeing, desizing, boiling, bleaching, mercerizing and other processes)

Cooking wastewater, bleaching wastewater, mercerizing wastewater; Dyeing wastewater discharged from dyeing process; Printing wastewater discharged from printing process

Soap liquor wastewater; The finishing wastewater discharged from the finishing process.

Printing and dyeing wastewater is the mixed wastewater of the above kinds of wastewater, or the comprehensive wastewater except bleaching wastewater.

　　 Anaerobic hydrolysis:

Dye is a kind of refractory synthetic organic matter, and its molecular structure mainly contains azo group, which is difficult to be biodegradable. If the electron attracting substituents on the molecular structure can be removed to disconnect the electronic double chains, the subsequent biodegradation will be easy, and the dye molecules will also lose the chromophore group. The mechanism of degradation of dye organics and decolorization by hydrolytic acidification is to break the electronic double chain of azo groups by the enzymatic action of hydrolytic acidification microorganisms. This biodegradation process requires the joint participation of a variety of enzymes. During the hydrolysis process, Pseudomonas, Aeromonas and Rhodospirillum bacteria growing in the hydrolysis sludge have good decolorization ability. The decolorization ability of mixed bacteria is higher than that of individual bacteria, and the mixed bacteria rely on synergistic action to make the dye degradation more complete and decolorization more thorough. Hydrolysis acidification treatment can buffer and reduce the pH value of the raw sewage, increase the proportion of soluble COD in the sewage, so as to improve the COD removal rate of subsequent aerobic treatment. At the same time, it can also buffer and adjust the impact of possible impact load, prevent and overcome the possible sludge bulking or excessive growth of filamentous bacteria in the subsequent activated sludge treatment process, Enhance the operation stability and reliability of the processing system.

　　 Coagulation flotation:

Printing and dyeing wastewater has poor biodegradability, and it is difficult to meet the discharge requirements only by biochemical treatment. In order to ensure the stable discharge of final effluent up to standard, and to prevent the occurrence of accidents in the biochemical system, a stage of physicochemical process is added after the biochemical system. By adding coagulant or decolorant, the residual chroma in the wastewater can be removed. In addition, colloidal substances can be converted into suspended solids, which can be separated from the water together with the smaller and lighter suspended solids remaining in the wastewater. In addition, some metabolites of bacteria can be removed to ensure the best treatment effect. The separation after dosing and coagulation includes sedimentation and air flotation. The pressurized dissolved air flotation has a better decolorization effect on the treatment of dyeing wastewater. In addition, since the separation capacity of air flotation is about 4-5 times that of precipitation separation, it can greatly reduce the area of the separation area, save a lot of investment, and the separation effect is stable and not affected by the external environment, so the pressurized dissolved air flotation method is selected as the physical and chemical treatment measure.

With the development of textile printing and dyeing industry, the impact of printing and dyeing wastewater on the environment is increasing, and the research and development achievements of printing and dyeing wastewater treatment are also constantly transformed and applied.

Characteristics of various textile printing and dyeing wastewater;

(1) Cotton textile printing and dyeing wastewater Cotton textile printing and dyeing wastewater (including pretreatment process, dyeing or printing and finishing process) is organic wastewater, mainly composed of synthetic organics and some natural organics, and contains a certain amount of refractory substances.

(2) Wool textile printing and dyeing wastewater Acid dyes are mainly used in the dyeing process of wool textile products. The dye uptake is high, and the chroma of dyeing wastewater is relatively low. Wool textile dyeing wastewater has good biodegradability and is suitable for biochemical treatment.

(3) Silk textile printing and dyeing wastewater Real silk printing and dyeing wastewater is neutral organic wastewater with good biodegradability and relatively low organic content.

(4) Hemp textile printing and dyeing wastewater Hemp textile fiber is cellulose fiber, which produces degumming wastewater and printing and dyeing wastewater during its processing. The degumming wastewater in the production process of linen textile products is high concentration organic wastewater, which is easy to be biodegradable. The printing and dyeing wastewater discharged during the processing of hemp textile products is similar to that of cotton textile, but the chroma is slightly lower.

In a word, all kinds of printing and dyeing wastewater generally belong to organic wastewater, and the colors and pollutants contained therein mainly include natural organic substances (wax, gum, hemicellulose, grease, etc. contained in natural fibers) and artificial organic substances (dyes, additives, size, etc.); Because the dyeing solution and printing solution are electrolyte solutions in the dyeing and printing process of fabrics with different fiber materials, in order to better print and dye to different fabrics, they need to be carried out under different pH conditions, so the pH values of wastewater discharged in the printing and dyeing process are different. Different fiber fabrics use different dyes and dye uptake in printing and dyeing processes, and the color of wastewater discharged is also different.