Replacing chloride anions in dyeing enables cheaper effluent concentration and recycling

Abstract

Reverse osmosis followed by distillation is commonly used to recycle fresh water and concentrated brine in the textile dyeing industry to achieve zero liquid discharge. The chemistry of industrial effluents is governed by the choice of chemicals used in various processing steps. This study experimentally demonstrates that replacing chloride with sulfate anions in the dyeing process baths can lead to significant energy savings in effluent treatment by enabling the use of NF membranes for brine concentration. The impact of electrolyte choice and concentration on fabric color is evaluated in a dyeing unit. While achieving similar fabric color, thermodynamic minimum energy consumption for sulfate brine concentration is around 50% lower as a result of its lower osmotic pressure. Practically, we show that while an RO membrane can concentrate the sulfate brine up to 60 g/kg at 60 bar applied pressure, nanofiltration membranes can concentrate the same to around 99 g/kg at the same pressure as a result of its higher permeability. Commercially acceptable color reproducibility was achieved for 3-5 cycles for different shades of dyeing when reusing NF brine in the dyebath. Ultrafiltration treatment of the concentrated brine reduces color deviation by 50-80% and is a promising method for brine polishing.