Sustainably Cultivating and Harvesting Microalgae through Sedimentation and Forward Osmosis Using Wastes

Cost-effective nutrient sources and dewatering are major obstacles to sustainable, scaled-up cultivation of microalgae. Employing waste resources as sources of nutrients offsets costs for nutrient supplies while adding value through simultaneous waste treatment. Forward osmosis (FO), using simulated reverse osmosis brine, is a low-energy membrane technology that can be employed to efficiently harvest microalgae from a dilute solution. In this study, Scenedesmus obliquus, a green microalga, was cultivated with a fertilizer plant wastewater formula and simulated coal-fired power plant flue gas and then separated through either FO, with reverse osmosis reject model water as the draw solution, or sedimentation. Microalgal batches grown with simulated wastewater removed NH₄ ⁺ within 2 days and reached nitrogen and phosphorus limitation simultaneously on Day 5. Sparging with the flue gas caused S. obliquus to produce significantly greater quantities of extracellular polymeric substances (30.7 ± 1.8 μg mL^-1), which caused flocculation and enhanced settling to an advantageous extent. Five-hour FO trials showed no statistically significant difference (p = 0.65) between water fluxes for cultures grown with simulated flue gas and CO₂-supplemented air (3.0 ± 0.1 and 3.0 ± 0.3 LMH, respectively). Reverse salt fluxes were low for all conditions and, remarkably, the rate of reverse salt flux was -1.9 ± 0.6 gMH when the FO feed was culture grown with simulated flue gas. In this work, S. obliquus was cultivated and harvested with potential waste resources.