The treatment of industrial effluents containing sodium hydroxide to enable the reuse of chemicals and water

Abstract

An economically viable treatment sequence has been developed and piloted at two textile factories for the recovery and reuse of water, chemicals and heat energy from sodium hydroxide effluent produced during the scouring of cotton fibre. The treatment sequence involves pretreatment of the scour effluent by neutralisation, using an acidic gas, cross-flow microfiltration and charged membrane ultrafiltration (also called nanofiltration). The sodium hydroxide is then recovered in an electrochemical membrane cell with the simultaneous evolution of acidic gas which is recycled within the treatment process. Two possible configurations of the treatment process, where the acidic gas is either chlorine or carbon dioxide, have been discussed. Pilot plant results have been presented for both systems. The carbon dioxide system was the preferred route and is discussed in detail. The pretreatment sequence neutralised the scour effluent, lowered its chemical oxygen demand by 86% and removed 65% of both the calcium and the organics and 50% of the magnesium. The sodium hydroxide (100 to 200 g/l) and depleted brine solution (total solids 500 mg/l) from the electrochemical membrane cell were of suitable quality for reuse in the factory process. The electrochemical membrane cell produced sodium hydroxide at 62% current efficiency at an electrical power consumption of 4 000 kWh/ton 100% NaOH. The effect of electrolyte, in particular, anolyte flow rate, temperature and concentration on the limiting current density and power consumption has been investigated. Some design data for a full scale treatment plant has been presented. The operation of an acceptable background concentration closed-loop recycle wash system in the scour process was found to reduce the required membrane area by 82%. Minimum dissolution of the precious metal oxide anode coating occurred and long anode life was predicted. Serious electromembrane fouling, with increased resistance, was not apparent.