Dewatering microalgae by forward osmosis

Abstract

Microalgae are known to be an excellent source of biofuels, but many issues surrounding the scale and economics of their cultivation have yet to be resolved. In particular, dewatering methods, such as centrifugation and tangential flow filtration, are prohibitively energy intensive. In this study, forward osmosis (FO) is considered as a partial dewatering method for microalgae growing on wastewater in a marine environment. Using artificial seawater as the draw solution, average dewatering rates of 2l/m2membrane/h (range 1.8–2.4l/m2h) were observed and volumes decreased by 65–85%. For a single membrane, daily dewatering rates did not significantly change in 14 consecutive experiments. Hourly dewatering rates did not gradually decrease, as might be expected; instead the dewatering rate oscillated throughout each experiment. Exposing an FO membrane in the ocean for 45days caused significant biofouling on its surface, but its dewatering rate did not change. Exposing three FO membranes in the ocean for 52days also caused significant biofouling, but in this experiment all membranes developed leaks that allowed saltwater to pass. These experiments suggest that FO may be an energy-saving step in dewatering freshwater microalgae if an appropriate draw solution is available and if conditions are controlled to prevent leakage.