Printing and dyeing wastewater is discharged from printing and dyeing plants which mainly process cotton, linen, chemical fibers and their blended products. Printing and dyeing wastewater consumes 100-200 tons of water per ton of textiles, 80-90% of which is wastewater. Textile printing and dyeing wastewater is one of the industrial wastewater which is difficult to treat because of its large amount of water, high content of organic pollutants, high alkalinity and great changes in water quality. It contains dyes, slurries, auxiliaries, oils, acids and bases, fiber impurities, sand and inorganic salts.
(1) Desizing wastewater, with small amount of water and high concentration of pollutants, mainly contains slurry and its decomposition products, fibre chips, acids, starch bases and enzymes, and has high turbidity. Wastewater is alkaline and its pH value is about 12. When starch size is used, BOD and COD are high and biodegradability is good; when synthetic size is used, COD is high, BOD is less than 5 mg/L, and water biodegradability is poor.
(2) Boiling wastewater, with large amount of water and high concentration of pollutants, mainly contains cellulose, fruit acid, wax, oil, alkali, surfactant, nitrogen compounds, etc. Wastewater has strong alkalinity, high water temperature, brown color, high COD and BOD, up to thousands of milligrams per liter. Chemical fibre boiling wastewater has less pollution.
(3) Bleaching wastewater, with large amount of water and light pollution, mainly contains residual bleaching agent, a small amount of acetic acid, oxalic acid, sodium thiosulfate, etc.
(4) Mercerizing wastewater has high alkali content, NaOH content is 3%-5%. Most printing and dyeing factories recover NaOH by evaporation and concentration, so mercerizing wastewater is seldom discharged. After repeated use, the final discharged wastewater is still strong alkaline, BOD, COD and SS are all higher.
(5) The quality of dyeing wastewater is changeable. Sometimes it contains toxic substances (alkali sulfide, tartrate, aniline, copper sulfate, phenol, etc.) when using various dyes, alkaline, and sometimes more than 10 PH (when using sulfur and VAT dyes), organic dyes and surfactants. Chromaticity is high, SS is low, COD is higher than BOD, biodegradability is poor.
(6) Printing wastewater, containing slurry, high BOD and COD;
(7) Wastewater from finishing process mainly contains fibre chips, resin, formaldehyde, oils and slurries with little water.
(8) Alkali deweighting wastewater: It is produced by alkali deweighting process of polyester silk-like fabric. It mainly contains terephthalic acid, ethylene glycol, etc. The content of terephthalic acid is as high as 75%. Alkali reduction wastewater not only has a high pH value (generally > 12), but also has a high concentration of organic matter. CODCr in the wastewater discharged from the alkali reduction process can reach as high as 90,000 mg/L. Polymer organic matter and some dyes are difficult to be biodegraded. This wastewater belongs to high concentration and refractory organic wastewater.
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