Reuse of untreated reactive dyeing wastewater for reactive coloration of cotton

Textile wastewater contains a wide range of hazardous dyes and chemicals released during pretreatment, dyeing, printing, and finishing processes, leading to significant environmental and health concerns. Among textile fibers, cotton is the most widely used, and more than half of it is dyed with reactive dyes. However, nearly 60% of these dyes remain unexhausted in the dye bath, along with substantial amounts of residual chemicals such as salts and alkalis. Although the unused reactive dyes become hydrolyzed, the remaining chemicals and water still offer considerable potential for reuse, which could reduce freshwater consumption as well as chemical usage and ETP operating costs. To evaluate this potential, untreated reactive dyeing wastewater was directly reused for cotton coloration. Wastewater collected from textile mills was assessed, and its dyeing fusibility was examined through three approaches: (i) cotton was dyed using the collected untreated wastewater with the addition of fresh dyes and chemicals according to the standard recipe to replace freshwater; (ii) only fresh dyes were added to the untreated wastewater, and dyeing was performed without auxiliary chemicals to determine the effectiveness of the residual chemicals; and (iii) based on the performance of these residual chemicals, the amounts of dyes and auxiliaries were further optimized. The dyed samples were then analyzed for shade characteristics and serviceability properties. Based on the color yield and fastness results, the study concludes that reactive dyeing wastewater can be directly reused for reactive coloration. However, repeated reuse may adversely affect dyeing levelness.