Reuse of Textile Spent Reactive Anthraquinone Dyebaths Following Biological Decolorization Process Under Hypersaline Conditions

The textile industry widely uses reactive anthraquinone dyes, which exhibit strong resistance to color removal and generate substantial volumes of wastewater containing significant quantities of residual dye requiring treatment prior to discharge. As part of a study aimed at reusing rather than discharging spent reactive anthraquinone dyebaths, Reactive Blue 4 (RB4) dye was used in dyeing cotton, and the generated spent dyebaths were biologically decolorized using a fluidized bed reactor (FBR) operated under hypersaline conditions at a salt concentration of 100 g NaCl/L, which is typically found in commercial spent reactive dyebaths. Across five consecutive runs, the FBR achieved a mean decolorization efficiency of 91.2 ± 2.8% within a 6 h incubation period. The quality of cotton dyed with the treated and reused spent dyebaths was evaluated through shade reproducibility and color consistency assessments. Five repetitive dyeings using the biologically decolorized dyebaths showed that the ΔEcmc fabric color difference values were 0.58~0.80, which were lower than the industry-accepted value of 1.0. This study demonstrates that biologically decolorized spent dyebaths can be effectively reused, offering substantial reductions in water and salt consumption and improving the economic and environmental sustainability of the reactive dyeing process.