Abstract
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.
Highlights
Sustainable and economical microalgal cultivation is an opportunity to treat wastes, sequester CO2, and produce biomass commodities while managing finite resources responsibly
The energy requirement for sedimentation is low, but microalgae have low settling rates; a Chlorella sp. was reported to have a settling rate of 0.1 m d−1.4 Higher settling rates are uncommon without chemical or biological additives or contaminants that cause flocculation
We hypothesize that the stress of exposure to acidic and toxic flue gas components (CO, NO2, and SO2) was compounded with the stress of ammonia toxicity in microenvironments
Summary
Sustainable and economical microalgal cultivation is an opportunity to treat wastes, sequester CO2, and produce biomass commodities while managing finite resources responsibly. The water flux values were relatively low; previous studies achieved 6.71 LMH for comparable conditions.[50] In 5-h trials, biomass concentrations increased by 40% ± 16% and 40% ± 3% for flue gas and CO2-supplemented air conditions, respectively (Figure S4). During the simulated flue gas cultivated batches, the salt content of the solution would have increased by approximately 1 g L−1 due to SO42− accumulation and 0.4 g L−1 from base addition (10 mL 1 M NaOH) These increases more than tripled the initial conductivity of the feed solution relative to the CO2supplemented air condition (3.50 ± 0.04 and 1.10 ± 0.35 mS cm−1, respectively) and decreased the salinity gradient between the feed and the draw. Biomass accumulation within the FO cell was greater for cultures grown with simulated wastewater and/or simulated flue gas, relative to control cultures, because higher concentrations of EPS drove flocculation as microalgae reached this area of lower flow. Separating microalgae from water in an energy efficient manner is key to the process of harvesting microalgal biomass for beneficial uses and a sustainable, circular economy
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