Microalgae (Scenedesmus obliquus) dewatering using forward osmosis membrane: Influence of draw solution chemistry

Forward osmosis (FO) is gaining increasing interests for its potential applications in biofuel generation. In this study, bench-scale experiments were conducted to investigate the FO performance for microalgae dewatering which is one of the technical challenges in algal biofuel production. The filtration performance was assessed by analyzing permeate water flux and algal biomass concentration in the feed solution. Compared to the active layer facing draw solution (AL-DS) orientation (>45% flux reduction), active layer facing feed solution (AL-FS) was more efficient (<15% flux reduction) due to the lower membrane fouling and higher cleaning efficiency (>90% water flux recovery after deionized water flushing). In the AL-FS orientation, FO performance strongly depended on the draw solution chemistry with NaCl exhibiting the best results. When Ca2+-containing solution was used as draw solution, microalgae responded to the back diffusion of calciumions by an extensive excretion of carbohydrates, accelerating the formation of algal flocs, thus enhancing the rate and extent of flux decline and reducing the algae dewatering efficiency. However, most of the flux decline was reversible by simple hydraulic flushing without any chemical cleaning reagents and air scouring. In addition, substantial adsorption of algal biomass was observed on feed spacer. This study has the implication for Scenedesmus obliquus dewatering using FO technology. Selection of AL-FS orientation, Ca2+-free draw solutions and prevention of microalgae adhesion onto feed spacer may significantly improve the efficiency and productivity of the dewatering process. (C) 2016 Elsevier B.V. All rights reserved.