Membrane treatment of alkaline bleaching effluents from elementary chlorine free kraft softwood cellulose production

Abstract

This paper reports experimental results on the sequential use of ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) to fractionate alkaline extraction bleaching effluents from kraft cellulose production. The aim was to unveil the way key pollutants are distributed when subjected to sequential UF/NF/RO membrane separation processes. Alkaline bleaching effluents were obtained from a local pinewood-based mill, featuring elementary chlorine free bleaching to produce high-brightness cellulose. The experimental system was based on a laboratory-scale membrane system, DSS LabStak^® M20 Alfa Laval, using Alfa Laval UF and NF/RO membranes, operated at a constant transmembrane pressure (6 bar for UF membranes and 32 bar for NF/RO membranes), at 25°C. Results show that 78% chemical oxygen demand (COD) and total phenols, 82% adsorbable organic halogens (AOX) and 98% colour were retained by UF membranes which have molecular weight cut-off (MWCO) above 10 kDa. In all, 16% of original COD, total phenols and AOX, and the remaining 2% colour were retained by UF membranes within the 1 to 10 kDa MWCO range. Chloride ions were significantly present in all UF permeates, and RO was required to obtain a high-quality permeate with a view to water reuse. It is concluded that UF/NF/RO membranes offer a feasible option for water and chemicals recovery from alkaline bleaching effluents in kraft pulp production.

Keywords:

• sequential UF/NF/RO membrane system
• alkaline bleaching effluent
• elementary chlorine free cellulose
• water recovery
• chemicals recovery

Acknowledgements

The authors would like to thank Project Basal PFB-27 and Fondecyt 1100738 for their financial support. The authors thank the Engineering and Environmental Biotechnology Group of Environmental Science Center EULA-Chile and Natural Products Chemistry Laboratory, Natural and Oceanographic Science Faculty (University of Concepción) for permission to carry out some analytical procedures of great importance for this article.

Funding

This work was supported by the Fondecyt [grant number 1100738].