Removal of inhibitors from birch pretreatment liquor by nanofiltration: Mechanisms of separation and influence of operational variables and mode

Lignocellulosic biomass has attracted considerable attention as an alternative feedstock for the production of fuels, energy and chemicals, due to its renewability, abundance and reduced cost. Pretreatment and hydrolysation of lignocellulose releases sugars that are subsequently converted by fermentation. However, by-products such as aliphatic acids and furans could be generated during the upstream processes, which could inhibit enzymes and fermenting microorganisms. In addition, fermentation of low-concentrated sugars would lead to low products concentration and, consequently, to higher recovery and purification costs. Therefore, it is beneficial to reduce the concentration of the inhibitors and to concentrate the fermentable sugars in the liquor, prior to the fermentation process. This study was focused on the identification of the operating conditions to effectively remove aliphatic acids and furans from birch pretreatment liquor by nanofiltration. Two commercial NF membranes (NF90 and TS40) were employed. Effects of main operating parameters such as pH, feed concentration, temperature, pressure and tangential velocity on the separation performances were investigated, in both dead-end and cross-flow modes. The membrane performances were compared in terms of retention of sugars and inhibitors, permeate flux and permeability loss. It was found that NF90 membrane was more suitable than TS40 for simultaneous sugars concentration and beneficial removal of the inhibitors. Better separation performances were achieved at pH 1.5, 8 bar, room temperature and 3000 rpm. Dilution of 5 times of the liquor promoted the rejection of the sugars and kept low the retention of the inhibitory compounds. Water permeability loss was recovered by flushing with NaOH and water. The protocol and the operating conditions proposed in this study are suitable to perform the filtration process not only on laboratory scale, but also on industrial scale.