Nutrient-driven forward osmosis coupled with microalgae cultivation for energy efficient dewatering of microalgae

In this study, a new forward osmosis (FO) coupled with microalgae cultivation process was developed as a means of energy-efficient dewatering of microalgae, adopting a microalgal culture medium as a draw solution. In so doing, the energy-intensive regeneration of draw solution is omitted, and the resulting dilute solution of nutrients is directly reused as a culture medium. Process feasibility was proved by using glucose, a common carbon source for heterotrophic microalgae cultivation, as draw solute in dewatering the microalgal culture. When glucose and microalgal suspension were used as draw solution and feed solution, respectively, membrane orientation and cross-flow velocity (CFV) were found to have substantial impacts on dewatering performance of forward osmosis: compared to active layer-facing-draw solution orientation (AL-DS) and low cross-flow velocity, active layer-facing-feed solution orientation (AL-FS) and high cross-flow velocity were more effective at reducing reversible and irreversible fouling and thereby improving water flux. In comparison with sea salt, which is considered a conventional draw solute that does not require a regeneration step, glucose showed better dewatering performance due to its lower reverse solute flux (RSF), which caused flux decline due to loss of osmotic pressure difference and cake-enhanced concentration polarization (CECP) in fouling layer. The nutrient-driven forward osmosis process coupled with microalgae cultivation led to the production of highly concentrated biomass (roughly 120 g/L), supporting the idea that this novel approach can offer a promising low-energy alternative for microalgal dewatering.