Influence of pH and fouling characterization during membrane process for lignin recovery from the process water of thermomechanical pulping

In thermo-mechanical process water, lignin presents an attractive potential for industrial applications due to the preservation of its native structure as free polymer or as part of ligno-carbohydrate complexes. In such effluent, lignin concentration is low, therefore its recovery with membrane process presents some advantages. As lignin tends to precipitate and forms lignin clusters with a pH decrease, assessing the influence of pH is relevant to understand membrane selectivity towards lignin and the related membrane productivity. It constitutes the main objective of the present study, as it was never done for thermo-mechanical process water. Two ceramic membranes with cutoff of 5 and 1 kDa were used alone or in a series and three pH were tested (4, 6.2 (native pH) and 10). For each condition, an in-depth fouling study was realized. pH 10 was not promising as a rapid flux drop was observed at low volume reduction of 40% due to the compact deposit of lignin provoking 7% of irreversible fouling and 83% of fouling removed by cleaning with acid and base. The utilization of both membranes in a series was not pertinent as lignin was partly fractionated in both retentates. Process water filtration at pH 4 with a unique 1 kDa nanofiltration step gave encouraging results with 40 and 70% of lignin and hemicelluloses respectively recovered. High flux of 33 L.h(-1).m(-2) was maintained at a volume reduction of 80%. Fouling was easily eliminated as it was composed at 70% of fouling removed with water rinsing which was largely due to hemicelluloses which tend to agglomerate at pH 4 in the membrane surrounding.