Dyeing and finishing textile printing and dyeing wastewater treatment equipment

Textile printing and dyeing wastewater mainly refers to the wastewater containing natural impurities, fats, and organic substances such as starch generated during the processes of raw material steaming, rinsing, bleaching, and sizing. Printing and dyeing wastewater is generated during multiple processes such as washing, printing, and sizing. It contains a large amount of organic substances such as dyes, starch, cellulose, lignin, detergents, as well as inorganic substances such as alkali, sulfides, and various salts, and is highly polluting.

1、 Characteristics of Printing and Dyeing Wastewater

The textile printing and dyeing industry is a major emitter of industrial wastewater, which mainly contains pollutants, oils, salts on textile fibers, as well as various pulp, dyes, surfactants, additives, acids and bases added during the processing.

The characteristics of wastewater are high concentration of organic matter, complex composition, deep and variable color, large pH variation, and significant changes in water quantity and quality, making it difficult to treat industrial wastewater. With the development of chemical fiber fabrics, the rise of silk like fabrics, and the increasing requirements for post printing and dyeing finishing, a large amount of difficult to degrade organic compounds such as PVA slurry, artificial silk alkali hydrolysates, new dyes, and additives have entered textile printing and dyeing wastewater, posing a serious challenge to traditional wastewater treatment processes. The COD concentration has also increased from several hundred milligrams per liter to 3000-5000 mg/l.

Pulp dyeing wastewater has high chromaticity and COD, especially with the development of printing and dyeing processes such as silk blue, silk black, ultra deep blue, and ultra deep black based on foreign markets. This type of printing and dyeing uses a large amount of sulfide dyes, printing and dyeing auxiliaries such as sodium sulfide, etc. Therefore, the wastewater contains a large amount of sulfides. This type of wastewater must be pre treated with chemicals and then undergo series treatment to ensure stable and compliant discharge. The bleaching and dyeing wastewater contains dyes, pulp, surfactants and other additives. This type of wastewater has a large amount of water, low concentration and chromaticity. If only physical and chemical treatment is used, the effluent can be between 100-200mg/l, and chromaticity can also meet the discharge requirements. However, the pollution amount is greatly increased, and the cost of sludge treatment is high, which can easily cause secondary pollution. In the case of strict environmental protection requirements, the biochemical treatment system should be fully considered. Conventional enhanced biological treatment processes can meet the treatment requirements.

2、 Chemical treatment methods

1. Coagulation method

There are mainly mixed precipitation method and mixed air flotation method, and the mixed agents used are mostly aluminum salts or iron salts. Among them, basic aluminum chloride (PAC) has better bridging adsorption performance, while ferrous sulfate has the lowest price. The number of people using polymer based coagulants is increasing abroad, and there is a trend to replace inorganic coagulants. However, in China, due to price reasons, the use of polymer based coagulants is still rare. According to reports, weakly anionic polymer coagulants have the widest range of applications, and if used in combination with aluminum sulfate, they can achieve better results. The main advantages of the mixed method are simple process flow, convenient operation and management, low equipment investment, small footprint, and high decolorization efficiency for hydrophobic dyes; The disadvantages are high operating costs, large amount of sludge and difficult dehydration, and poor treatment effect on hydrophilic dyes.

2. Oxidation method

Ozone oxidation method is widely used abroad, and Zima S.V. et al. have summarized a mathematical model for ozone decolorization of printing and dyeing wastewater. Research has shown that when the ozone dosage is 0.886gO3/g dye, the decolorization rate of light brown dye wastewater reaches 80%; The study also found that the amount of ozone required for continuous operation is higher than that required for intermittent operation, and installing baffles in the reactor can reduce ozone consumption by 16.7%. Therefore, the use of ozone oxidation for decolorization should be designed as an intermittent reactor, and the installation of baffles can be considered. Ozone oxidation method can achieve good decolorization effect on most dyes, but it has poor decolorization effect on insoluble dyes such as sulfurization, reduction, and coatings. From domestic and international operational experience and results, this method has good decolorization effect, but it consumes a lot of electricity and has certain difficulties in large-scale promotion and application. The photocatalytic treatment of printing and dyeing wastewater has a high decolorization efficiency, but equipment investment and power consumption still need to be further reduced.

3. Electrolytic method

Electrolysis has a good treatment effect on printing and dyeing wastewater containing acidic dyes, with a decolorization rate of 50% to 70%, but the treatment effect is poor for wastewater with dark colors and high CODcr. The study on the electrochemical performance of dyes shows that the order of CODcr removal rate of various dyes during electrolytic treatment is: sulfide dyes, reducing dyes>acid dyes, reactive dyes>neutral dyes, direct dyes>cationic dyes. This method is being widely applied.

3、 Process flow

The conventional treatment methods for printing and dyeing wastewater are generally divided into two categories: biochemical+physicochemical and physicochemical+biochemical treatment processes. However, due to the lack of hydrolysis and acidification units, there is a problem of incomplete aerobic biochemical unit reactions in actual operation, resulting in high subsequent physicochemical treatment costs. Adding a "hydrolysis+aerobic" series process of hydrolysis and acidification treatment before traditional aerobic biological treatment equipment can hydrolyze organic matter that is difficult to degrade in printing and dyeing wastewater into more easily biodegradable substances, improve the biodegradability of the wastewater, and thus increase the COD removal rate of the traditional process. At present, many newly built printing and dyeing wastewater treatment facilities in China (including centralized treatment of domestic sewage and printing and dyeing wastewater) adopt the "hydrolysis aerobic" biological treatment process developed by this process, which has achieved significant environmental and economic benefits.

The four processes of printing and dyeing all have wastewater discharge, including the desizing wastewater, boiling wastewater, bleaching wastewater, and silk polishing wastewater discharged from the ferrous sulfate pretreatment stage (including processes such as singeing, desizing, boiling, bleaching, and silk polishing); Dyeing wastewater discharged from the dyeing process; Printing wastewater and soap solution wastewater discharged from the printing process; The wastewater discharged from the sorting process. Printing and dyeing wastewater is a mixture of various types of wastewater mentioned above, or a comprehensive wastewater other than bleaching wastewater.

Anaerobic hydrolysis

Dyes are difficult to degrade synthetic organic compounds, and their molecular structure mainly contains electron attracting groups such as azo groups that are difficult to biodegrade. If the electron withdrawing substituents on the molecular structure can be removed to break the electron double chain, subsequent biodegradation will be easy, and the dye molecule will also lose its chromophore. The mechanism of hydrolytic acidification degradation of dye organic compounds and decolorization is to use the enzymatic action of hydrolytic acidification microorganisms to break the electronic double chain of azo groups. This biodegradation process requires the joint participation of multiple enzymes. During the hydrolysis process, bacteria belonging to the genera Pseudomonas, Aeromonas, and Rhodospirillum that grow in the hydrolyzed sludge have good decolorization ability. The decolorization ability of mixed bacterial communities is higher than that of individual bacteria. The mixed bacterial community relies on synergistic effects to achieve more complete degradation and decolorization of dyes. By using hydrolysis acidification treatment, the pH value of the original sewage can be buffered and reduced, the proportion of soluble COD in the sewage can be increased, thereby improving the COD removal rate of subsequent aerobic treatment. At the same time, it can also buffer and regulate the possible impact load effects, prevent and overcome sludge expansion or excessive growth of filamentous bacteria that may occur during the subsequent activated sludge treatment process, and enhance the stability and reliability of the treatment system operation.

Coagulation air flotation

The biodegradability of printing and dyeing wastewater is poor, and relying solely on biochemical treatment is generally difficult to meet discharge requirements. To ensure stable and compliant discharge of the final effluent, and to prevent unexpected situations in the biochemical system, a physical and chemical process is added after the biochemical system. By adding coagulants or decolorizers, residual chromaticity in wastewater can be removed. In addition, colloidal substances can be converted into suspended solids and separated from the water along with smaller and lighter suspended solids remaining in the wastewater? In addition, it can also remove some metabolic products of the bacterial cells to ensure the best treatment effect. There are two types of separation after chemical coagulation: precipitation and air flotation. Among them, pressurized dissolved air flotation has a better decolorization effect on dyeing wastewater treatment. In addition, due to the fact that the separation capacity of air flotation is about 4-5 times that of precipitation separation, it can greatly reduce the area of the separation zone, save a lot of investment, and the separation effect is stable and not affected by the external environment. Therefore, pressurized dissolved air flotation is chosen as the physical and chemical treatment measure.