Evaluation of draw solution for enhanced microalgae harvesting via forward osmosis

Microalgae have attracted increasing attention as a renewable resource with potential applications in biofuels, wastewater treatment, and environmental remediation. However, large-scale harvesting remains a critical challenge due to the high energy requirements and cost inefficiencies of conventional techniques such as centrifugation and chemical flocculation. This study investigates forward osmosis (FO) as a sustainable alternative for microalgae harvesting, with sodium chloride (NaCl) evaluated as the draw solution (DS). A cellulose triacetate (CTA) FO membrane was employed to concentrate Chlorella vulgaris cultures using draw solution concentrations ranging from 2M to 7M. System performance was assessed based on water flux, electrical conductivity (EC), and total solids (TS) recovery under varying operating conditions. Results showed that water flux depended strongly on DS concentration, with higher NaCl molarity enhancing osmotic pressure and initial flux. The 6M NaCl DS produced the highest water flux (12.7 L/m2·h), although performance declined over time due to dilution effects and membrane fouling. The 4M NaCl solution provided the best balance between efficiency and cost, yielding the highest TS recovery (18.4%). FO effectively concentrated biomass, with the final concentration dependent on initial culture density and operational settings. FESEM analysis confirmed the presence of fouling layers on the membrane surface, which were manageable with periodic cleaning. Overall, the results highlight the importance of draw-solution optimisation to balance concentration efficiency, operational stability, and salt diffusion in FO-based microalgae harvesting systems.