Textile Wastewater Treatment Production Process and Wastewater Generation

Textile Wastewater Treatment Production Process
Cotton textile products are mainly products produced by spinning, dyeing (or printing), finishing and other processes after mixing cotton or cotton with chemical fibers. There are pure cotton (white grey cloth, bleached cloth, dyed cloth, printed cloth) products and cotton blended products (white grey cloth, bleached cloth, dyed cloth, printed cloth). The proportion of chemical fibers in cotton blended textile products is relatively large (generally more than the number of cotton). Cotton and cotton blended textile products can be divided into two types: thin fabrics (ordinary white cloth and dyed cloth) and thick fabrics (flannel, corduroy). According to the different weaving methods, cotton and cotton blended textile products can be divided into woven products (products woven by interlacing warp and weft yarns) and knitted products (the yarns are hooked into loops by needles, and then the loops are interlocked with each other. The dyeing and printing processes are basically the same except for the slightly different pre-dyeing process.

Wastewater Source
The cotton textile industry wastewater mainly comes from the dyeing and finishing section, including desizing, scouring, bleaching, mercerizing, dyeing, printing and finishing. Weaving section wastewater discharge is less.

(1) Desizing Wastewater: The pulp on the cotton fabric and some impurities in the fiber itself must be removed before bleaching and dyeing. Desizing wastewater generally accounts for about 15% of the total wastewater, and pollutants account for about half of the total. Desizing wastewater is alkaline organic wastewater containing various pollutants such as pulp decomposition products, lint, acid and enzymes, and the wastewater is pale yellow. The pollution degree and properties of desizing wastewater vary depending on the type of slurry: in the past, natural starch slurry was mostly used, and the BOD5/CODcr value of starch slurry was 0.3 to 0.5; currently, BOD5 of chemical slurry (such as PVA) is widely used. The /CODcr value is about 0.1; in recent years, the modified starch has gradually replaced the chemical slurry. The biochemical degradability of the modified starch is very good, and the BOD5/CODcr value is 0.5-0.8.

(2) Cooking Wastewater: In order to ensure the processing quality of bleaching and dyeing and finishing, impurities such as cotton wax, grease, pectin nitrogenous compounds and other impurities in the fibers should be removed. The scouring process generally uses water solvents such as caustic soda, soap, surfactants, etc., to scour cotton fibers under the conditions of 1200C and pH value of 10-13. The amount of smelting wastewater is large and strong alkaline, with an alkali concentration of about 0.3%, the wastewater is dark brown, and both BOD and COD are as high as several thousand milligrams per liter.

(3) Bleaching Wastewater: The bleaching process generally uses sodium hypochlorite, hydrogen peroxide (hydrogen peroxide), sodium chlorite and other oxidants to remove the colored impurities on the surface and inside of the fiber to bleach the fabric. Since hydrogen peroxide is almost completely decomposed in the bleaching wastewater, and most of the chlorine in chlorine-containing bleaching agents such as sodium hypochlorite and sodium chlorite is decomposed during the bleaching process, the bleaching wastewater is characterized by a large amount, but a small degree of pollution, BOD5 and CODcr are low, basically belong to clean wastewater, can be directly discharged or recycled.

(4) Mercerizing Wastewater: The mercerizing treatment is to soak the fabric in concentrated sodium hydroxide lye, the purpose is to improve the tensile strength of the fiber, increase the surface gloss of the fiber, reduce the potential shrinkage rate of the fabric, and increase the affinity with dyes. Mercerizing wastewater contains 3% to 5% of sodium hydroxide, which is generally recovered by multi-effect evaporation and concentrated, and then first used for mercerizing application, and then used to prepare cooking liquid, waste lye and desizing. Therefore, the mercerizing wastewater is actually rarely discharged. It is reused many times in the process. Although it is recycled by alkali, the alkalinity is still strong, and the BOD is low (but still higher than that of domestic sewage). The degree of pollution depends on the processing of bleached cloth. or natural color cloth. When processing bleached cloth, the fabric is first bleached and then mercerized, and the degree of pollution is low; when processing uncolored cloth, it is directly mercerized after desizing, so that the fiber impurities that originally entered the scouring wastewater are transferred to the mercerized wastewater, which increases the pollution degree accordingly.

(5) Dyeing Wastewater: Dyeing wastewater is characterized by large changes in water quality and deep color, and the main pollution sources are dyes and auxiliaries. Different fiber raw materials require different dyes, auxiliaries and dyeing methods. In addition, the dyeing rate of dyes, the concentration of dye liquor, and the dyeing equipment and scale are different, and the quality of wastewater varies greatly. Generally, the alkalinity of dyeing wastewater is strong, especially when sulfur dyes and vat dyes are used, the pH value is as high as 10 or more. The BOD of the dye itself is lower, but the COD is much higher. Many substances in dyeing wastewater are not easy to be decomposed by biology, and the COD removal rate of biological treatment of printing and dyeing wastewater is only 60% to 70%, and the decolorization rate is only about 50%.

(6) Printing Wastewater: The pollutants in printing waste water mainly come from the washing water of toning, printing drum and printing screen, as well as the waste water of post-processing soaping, water washing and printing interlining. The pollution level of printing wastewater is very high. In addition, reactive dyes use a large amount of urea, which increases the ammonia nitrogen content of printing wastewater.

(7) Finishing Wastewater: The finishing wastewater contains a variety of resins, formaldehyde, surfactants, etc., but the amount of wastewater is small.

https://www.biocell-enviro.com/