Simultaneous concentration and detoxification of lignocellulosic hydrolysates by novel membrane filtration system for bioethanol production

The presence of inhibitory compounds as well as low sugar concentration was the main challenge in lignocellulosic hydrolysate based biofuels production. The objective of this study was to develop an effective membrane filtration process for simultaneously separating inhibitors and concentrating sugars to improve the lignocellulosic hydrolysate fermentability. The nanofiltration (NF) and reverse osmosis (RO) membranes were chosen to evaluate their sugar rejection and inhibitor removal performance using real lignocellulosic hydrolysate in batch recycling mode. The influence of soluble anionic polymer addition, anionic polymer concentration and pH on sugar rejection and inhibitor removal was examined. The results implied that application of sodium tripolyphosphate during NF/RO could improve the inhibitor separation in particular formic acid and acetic acid removal was significantly enhanced. Moreover, pH adjustment for hydrolysate prior to detoxification and concentration and fermentation could be avoided. The concentration experiment indicated that NF-RO hybrid membrane improved sugar rejection but did not cause the obvious increase in inhibitor concentration. Overall, the total sugar titer at the end of concentration mode was 145.6 g/L compared to the initial titer of 38.4 g/L at a volume concentration ratio (VCR) of 4. Furthermore, diafiltration with RO after NF-RO hybrid membrane filtration achieved maximum sugar recovery and inhibitor removal, consequently led to successful ethanol fermentation using detoxified retentate. After 134 h cultivation, the ethanol maximum concentration, productivity and yield of diafiltrated retentate reached 45.16 g/L, 0.34 g/L/h and 0.44 g/g, which were comparable to that of control. These observations indicated that the novel membrane filtration system proposed could achieve simultaneous lignocellulosic hydrolysate detoxification and concentration, which would greatly reduce the number of processing steps, operating cost and energy consumption, consequently improve the production efficiency. Therefore, it could be considered as promising strategy for large scale industrial lignocellulosic bioenergy production.