The kraft pulp and paper industry is currently faced with the challenge of substantially reducing the discharge of conventional and toxic pollutants to the receiving environment without compromising its competitiveness. Ultrafiltration (UF) and nanofiltration (NF) are among the various unit processes that are being investigated by researchers interested in developing alternative schemes for meeting this challenge.
The objectives of the work presented here were: (i) to review the potential uses for UF and NF in the kraft pulp and paper industry and (ii) to evaluate the performance of eleven UF and NF membranes using a caustic extraction stage effluent from a kraft mill pulping softwood with a O D55/C45 EO H D bleaching sequence. The experimental work was conducted with two flat-sheets bench-scale units fed in parallel: a low shear and a high shear unit. The most promising UF membrane gave an average permeate flux of 9 m3m2d for a volume reduction factor (VRF) of 30, under high-shear conditions at 0.69 MPa. The associated COD rejections ranged from 85%–90% at a VRF of 1 down to 70–75% at a VRF of 30. While the results demonstrated in this study show promise, further research is needed to demonstrate the cost effectiveness of both high-shear and low-shear equipment for this application as well as options for the reuse or treatment of the diluted and concentrated streams produced by the UF/NF systems.