Ultrafiltration for hemicelluloses recovery and purification thermomechanical pulp mill process waters

Abstract

Process waters of thermomechanical pulp (TMP) mills contain a large quantity of lignocellulosic materials that end up in the wastewater for biological treatment. The recovery of hemicellulose and lignin from these process water as value-added chemicals is beneficial for TMP mills because it reduces the organic loading to the wastewater treatment facility. The performance of three hydrophilic ultrafiltration (UF) membranes made of regenerated cellulose with different molecular weight cut-offs (5, 10, and 30 kDa) for hemicelluloses recovery and purification was evaluated in a laboratory-scale dead-end stirred cell filtration unit. The results of this study showed that 5 and 10 kDa membranes gave much better recovery and purity for hemicelluloses than 30 kDa membranes. The recovery of the hemicelluloses was above 95% for 5 kDa and around 89% for 10 kDa, with hemicellulose purity of approximately 75% and 80%, respectively. Cut-offs of 5 and 10 kDa seem to be operationally feasible for the separation of hemicelluloses, while 30 kDa cut-off membrane was unsuitable for hemicellulose recovery. An optimal cut-off of 10 kDa membrane gave the highest purity of hemicelluloses (80%).