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Favorable microalgal nutrition from waste resources and improved harvesting methods would offset costs for a process that could be scaled up to treat pollution and produce valuable animal feed in lieu of soy protein. Co-benefits include avoidance of carbon dioxide emissions, which may provide an additional revenue stream when carbon markets begin to flourish. To sustainably achieve these goals at scale, barriers to microalgal production such as tolerance for waste streams and dramatic improvement in dewatering and settleability of the microalgae must be overcome. Presently, it is largely assumed that nutritious microalgae, including Scenedesmus obliquus , would be inhibited by SO x and NO x in flue gases and settle slowly as discrete particles. Studies conducted with a 2 L photobioreactor, sparged with simulated coal-fired power plant flue gas, demonstrated that both biomass productivity and settling rates were increased. The average maximum biomass productivity was 700 ± 40 mg L –1 d –1 , which significantly exceeded that of the control culture (510 ± 40 mg L –1 d –1 ). Thirty-minute trials of modeled bulk settling showed rapid coagulation, likely facilitated by extracellular polymeric substances, and compaction when the cultures were grown with simulated emissions. Control cultures, not exposed to the additional toxicants in flue gas, settled as discrete particles and did not show any settling progress within 30 min. Of the SO 2 sparged into the cultivation system, (111 ± 4)% was captured as either SO 4 2– in the medium or fixed in the S. obliquus biomass. The stress of simulated-emissions exposure decreased the S. obliquus protein contents and altered the amino acid profiles but did not decrease the fraction of methionine, a valuable amino acid in animal feed.

High-protein microalgae are a promising alternative to soy for more rapidly and sustainably produced protein-rich animal feed. However, there are still significant barriers to be overcome in growing nutritious microalgae, recovering nutrients from wastewater, and fixing CO 2 from flue gas in full-scale sustainable operations. Currently, it is generally assumed that nutritious microalgae, including Scenedesmus obliquus, are inhibited by CO 2 levels characteristic of industrial flue gases. Experiments in a 2 L photobioreactor with the ability to control CO 2 concentrations and pH demonstrated that the inhibition of S. obliquus was not important until 10% CO 2 and was not prohibitively reduced even at 35% CO 2 . The rate of growth exceeded all values in the literature for S. obliquus at concentrations greater than 2.5% CO 2 , and the amino acid content of the microalgae was equal or superior to that of soy. A substrate inhibition model indicated that CO 2 levels comparable to flue gases do not substantially inhibit S. obliquus growth, with careful pH control. The model indicated maximum biomass productivity of 640 ± 100 mg L −1 d −1 at 4.5% CO 2 (K m of 0.8 ± 0.4% CO 2 , K i of 26 ± 9% CO 2 , and v max of 860 ± 120 mg L −1 d −1 ), which exceeds previously measured biomass productivity values at inhibitory CO 2 concentrations. Protein contents of S. obliquus and soy were comparable.

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