1. The dyeing and printing industry is one of the largest water-consuming sectors in the textile industry, with water serving as a medium throughout the entire dyeing and finishing process. Dyeing and printing wastewater is characterized by large volume, high color intensity, and complex composition. It contains dyes, sizing agents, auxiliaries, oils, acids and alkalis, fiber impurities, and inorganic salts. Nitro and amino compounds in the dye structure, as well as heavy metals such as copper, chromium, zinc, and arsenic, have significant biological toxicity, severely polluting the environment.
1.1 The pollutants in dyeing and printing wastewater mainly originate from the fabric fibers themselves and the dyes and chemicals used in the processing. During the pretreatment process of dyeing and printing production, desizing wastewater, scouring wastewater, bleaching wastewater, and mercerizing wastewater are discharged. The dyeing and printing processes discharge dyeing wastewater, soaping wastewater, and printing wastewater. The finishing process discharges finishing wastewater.
1.2 Large volume and high concentration. Most of the wastewater is alkaline, with high COD and deep color.
1.3 Large fluctuations in water quality. The production processes and dyes and chemicals used in dyeing and printing plants vary depending on the type of textile and the level of management. For each factory, its products are constantly changing, resulting in frequent variations and fluctuations in the concentration of pollutants in the wastewater.
1.4 Primarily organic pollution. Besides acids and alkalis, most pollutants in the wastewater are natural or synthetic organic compounds.
1.5 Difficult to treat. Changes in dye types and the extensive use of chemical sizing agents result in wastewater containing recalcitrant organic matter with poor biodegradability. Therefore, dyeing and printing wastewater is one of the most difficult industrial wastewaters to treat.
2. Hazards of Dyeing and Printing Wastewater
2.1 Dyeing and printing wastewater contains large amounts of organic pollutants. Discharge into water bodies will deplete dissolved oxygen, disrupt the aquatic ecosystem, and endanger the survival of fish and other aquatic organisms. Organic matter settled at the bottom of the water will decompose anaerobicly, producing harmful gases such as sulfides, thus worsening the environment.
2.2 Most dyeing and printing wastewater is alkaline. If it enters farmland, it will cause soil salinization; if it enters water bodies, it will further affect the balance of the water. Sulfates in dyeing and printing wastewater can also be converted into sulfides under reducing conditions in the soil, producing hydrogen sulfide.
2.3 The color variations of dyeing and printing wastewater are diverse, severely affecting the appearance of receiving water bodies. The main factor causing the color of water is dye. Conventional biological methods are insufficient to remove color, and colored water discharged into rivers, exposed to sunlight, is even more detrimental to the growth of aquatic organisms. While chemical methods achieve better decolorization, the residues still have an impact on the water body.
2.4 Some wastewater contains toxic and harmful substances. For example, printing and engraving wastewater contains hexavalent chromium, and some dyes (such as aniline dyes) are highly toxic.
