Microalgae biomass dewatering by forward osmosis: Review and critical challenges

The massive outdoor culture of microalgae rarely reaches biomass concentrations higher than 1 g/L. The dewatering step is known to be a significant barrier in the large-scale production of microalgae-based commodities and bioproducts. High cost and energy consumption, along with low effectiveness, are still key constraints for commonly used dewatering operations. The potential use of forward osmosis as a membrane-based technology for algae dewatering employing osmotic pressure instead of external hydraulic pressure has several advantages such as low energy use and remarkable algae cell recovery. Nevertheless, the main drawbacks of forward osmosis are membrane fouling, concentration polarization phenomena, and reverse solutes diffusion. A lot of research is being carried out to overcome these limitations by developing new membranes and optimizing the process design. This article aims to review the main studies performed to date on microalgae biomass dewatering with the focus on forward osmosis and critical challenges, particularly fouling phenomenon. The forward osmosis performance evaluation, with respect to water flux behavior and microalgae dewatering efficiency, has been discussed in detail as an integral part of this review. When one appraises the achievements of the research carried out up until now, it becomes clear that we are still far from developing forward osmosis for microalgae dewatering at the industrial scale, and novel solutions are needed to enhance its technological readiness level. Some of the remaining challenges include: improving module design and arrangements to mitigate concentration polarization, lab-scale evaluation of different available draw solutions in relation to the properties and composition of the feed solution (i.e. the pH, microalgae species, etc.) for good dewatering process design, the membrane cleaning method and the pilot-scale validation of forward osmosis for dewatering microalgae, all of which are necessary to evaluate the economic and methodological feasibility of the process in a real setting.