Water reclamation in the textile industry: Nanofiltration of dye baths for wool dyeing

The textile industry is a large water consumer: dyeing, rinsing, and follow-up treatment of textiles use large amounts of freshwater. As regulations become more and more stringent and the cost of freshwater increases, reclamation of wastewater becomes more and more attractive. This paper explores the possibility of using nanofiltration to improve the wastewater quality to the standards that are used for the dyeing of wool. Four different samples from the wastewater treatment sequence of a textile factory were filtrated with three different nanofiltration membranes (NF70, UTC-20, and NTR 7450). The samples were a used and untreated metal complex dye bath, a used and untreated acid dye bath, a sample from the storage tank (containing a diluted mixture of the previous baths), and the effluent of the biological treatment. Nanofiltration was possible for all samples, but the biologically treated dye baths showed a more efficient color removal. For direct nanofiltration of used dye baths, two membrane passages would be needed to provide the required permeate quality. Flux decline due to adsorption of organic material on the membrane decreased the membrane capacity by up to 73%, but the process water flux reached a stable value in all experiments. The flux decline was less important for the biologically treated water. The effect of flux decline was only partly reversible; the effect of osmotic pressure on the process water flux is fully reversible. It was found that flux decline is largely concentration-dependent: higher concentrations of organic compounds always caused lower process water fluxes.