Cotton and polyester dyeing using nanofiltered wastewater

Cotton and polyester dyeing using effluents treated by nanofiltration was evaluated on laboratory and pilot scale. Wastewaters were collected separately from reactive and direct dyeing (for cotton) and from disperse dyeing (for polyester). For reactive and direct dyeing, experiments were carried out with both NaCl and Na2SO4.Flux decline due to fouling was observed during nanofiltration. Additionally, retention of ions, SO42− in particular, led to lower fluxes or a higher pressure to be applied. After nanofiltration, not all permeates were colourless and in most cases the COD was above the limit for good quality process water. Considerable differences in pH and conductivity were observed between the different fractions.For reactive dyeing, reuse tests were carried out with fresh water for wetting and dyeing, and 50% permeate for rinsing. Large differences in colour and deviations in fastness to laundering and rubbing were observed when NaCl was used. However, when Na2SO4 was used, colour and fastness figures were similar to the reference, in spite of the presence of organic matter.For direct dyeing, a 75/25 mixture of permeate and process water was used for dyeing and rinsing. In case of NaCl, fastness to laundering and wet rubbing was not satisfactory; the colour difference was, however, limited. In case of Na2SO4, all results were satisfactory. Since NaCl is hardly rejected by nanofiltration membranes, while Na2SO4 is well rejected, it was concluded that the conductivity of the permeate determines the possibility of reusing the water for direct and reactive cotton dyeing, while the remaining colour and COD have a minor influence.For disperse dyeing, 100% permeate was used for wetting, dyeing and rinsing. This corresponds to a ‘worst case scenario’. However, no differences in colour and fastness to laundering and rubbing were observed. The components causing colour in the permeate apparently do not disturb the dyeing process.