Main sources of printing and dyeing wastewater

Wastewater must be discharged from the four processes of printing and dyeing. The pretreatment stage (including singeing, desizing, scouring, bleaching, mercerizing and other processes) must discharge desizing wastewater, scouring wastewater, bleaching wastewater and Mercerizing wastewater, dyeing wastewater is discharged from the dyeing process, printing wastewater and soap wastewater are discharged from the printing process, and finishing wastewater is discharged from the finishing process. Printing and dyeing wastewater is mixed wastewater or comprehensive wastewater (except bleaching wastewater) of the above types of wastewater [1]. The water quality of printing and dyeing wastewater is complex, and pollutants can be divided into two categories according to their sources: one type comes from entrainments of the fiber raw materials themselves; the other type is slurries, oils, dyes, chemical additives, etc. used in the processing process. 1.2 Classification of printing and dyeing wastewater The characteristics of wastewater are mainly as follows:

1. Large water volume, high content of organic pollutants, deep color, large changes in alkalinity and pH value, and drastic changes in water quality. Due to the development of chemical fiber fabrics and advances in post-printing and dyeing finishing technology, a large amount of organic matter that is difficult to biochemically degrade, such as PVA slurry and new additives, has entered the printing and dyeing wastewater, increasing the difficulty of treatment.

2. Due to different dyes, different auxiliaries, and dyeing and finishing requirements of different fabrics, the pH value, CODCr, BOD5, color, etc. in the wastewater are also different, but their common feature is BOD5/ The CODCr values ​​are very low, generally around 20%, and the biodegradability is poor. Therefore, measures need to be taken to increase the BOD5/CODCr value to around 30% or higher to facilitate biochemical treatment.

3. The CODCr value of alkali reduction wastewater in printing and dyeing wastewater can reach more than 100,000 mg/L, and the pH is ≥ 12. Therefore, pretreatment must be carried out to recover the alkali and add acid. After the pH value is lowered and the pretreatment reaches certain requirements, it then enters the regulating tank and is treated together with other printing and dyeing wastewater in the printing and dyeing process.

4. Another characteristic of printing and dyeing wastewater is its high chroma, some of which can be as high as 4,000 times. Therefore, one of the important tasks in printing and dyeing wastewater treatment is decolorization. For this purpose, it is necessary to research and select efficient decolorizing bacteria, efficient decolorizing coagulants and treatment processes that are conducive to decolorization.

5. In the printing and dyeing industry, the use of PVA slurry and new additives has greatly increased the content of organic matter that is difficult to biodegrade in wastewater. In particular, the CODCr content caused by PVA slurry accounts for a large proportion of the total CODCr in printing and dyeing wastewater, and it is difficult for ordinary microorganisms used in water treatment to degrade this part of CODCr [3]. Therefore, it is necessary to study and screen microorganisms used to degrade PVA.

　1.3 Classification of printing and dyeing wastewater

There are eight main categories of wastewater discharged from various printing and dyeing processes, and their water quality characteristics vary greatly: 1. Desizing wastewater Desizing is the use of chemicals to remove fabrics. The slurry carried on the fiber is removed (decomposed into water-soluble decomposition products by hydrolysis or enzymes), and some impurities in the fiber itself are also removed. Desizing wastewater is organic wastewater, light yellow in color, and contains slurryDecomposition products, fiber scraps, enzymes, etc. The wastewater is alkaline with a pH value of about 12. The amount of CODCr and BOD5 accounts for about 45% of the printing and dyeing wastewater [4]. When PVA or CMC chemical slurry is used, the BOD5 of the wastewater decreases, but the CODCr is very high, making the wastewater more difficult to treat. PVA slurry is one of the main reasons for the poor treatment effect of printing and dyeing wastewater.

　2. Refining wastewater

Refining uses aqueous solutions of caustic soda and surfactants under high temperature (120°C) and alkaline (pH=10-13) conditions. Cotton fabrics are scoured to remove impurities such as grease, wax, and pectin contained in the fibers to ensure the processing quality of bleaching, dyeing, and finishing. The boiling wastewater has a large volume, high water temperature, dark brown color and strong alkaline content (alkali concentration is about 0.3%). Refining wastewater contains cellulose, fruit acid, wax, grease, alkali, surfactant, nitrogen-containing compounds and other substances. Its BOD5 and CODCr values ​​are high (up to several thousand milligrams per liter), and the concentration of pollutants is high.

3. The bleaching process of bleaching wastewater generally uses oxidants such as sodium hypochlorite, hydrogen peroxide, and sodium chlorite to remove impurities on the surface and inside of the fiber. Bleaching wastewater is characterized by large water volume, light pollution, low BOD5 and CODCr, and is relatively clean wastewater. It can be discharged directly or recycled after treatment.

4. Mercerizing wastewater mercerizing is to solution-treat the fabric in a concentrated sodium hydroxide solution to improve the tensile strength of the fiber, increase the surface gloss of the fiber, reduce the potential shrinkage of the fabric and improve the resistance to dyes. Affinity. Mercerizing wastewater is highly alkaline (containing about 3%-5% NaOH). Most printing and dyeing factories recover NaOH through evaporation and concentration, so mercerizing wastewater is generally rarely discharged. After repeated use in the process, the final wastewater is still highly alkaline, with BOD5 , CODCr and SS values ​​are all higher. 5. Dyeing wastewater The main pollutants in dyeing wastewater are dyes and auxiliaries. Since different fiber raw materials and products require the use of different dyes, auxiliaries and dyeing methods, as well as the different coloring rates, dye liquors and concentrations of various dyes, the quality of dyeing wastewater changes greatly. Dyeing wastewater is generally highly alkaline and has a large amount of water. The water quality contains slurry, dyes, additives, surfactants, etc. The color of the wastewater can be as high as thousands of times. The CODCr is much higher than BOD5, and the CODCr is generally 300-700. mg/L, BOD5/CODCr is generally less than 0.2, and biodegradability is poor.