Using Simulated Flue Gas to Rapidly Grow Nutritious Microalgae with Enhanced Settleability

Abstract

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.

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

“…Microalgae reached stationary phase at Day 5 (Figure 1 In a previous study, using 3N-BBM (125 mg L −1 N), flue gas promoted greater biomass productivity rates than CO 2supplemented air. 34 In this study, using two nitrogen species at a lesser initial concentration (47 mg L −1 N), flue gas caused lower biomass productivity rates. We hypothesize that the stress of exposure to acidic and toxic flue gas components (CO, NO 2 , and SO 2 ) was compounded with the stress of ammonia toxicity in microenvironments.…”

“…35 From biomass elemental composition analyses, the N/P molar ratio of Scenedesmus obliquus grown to stationary phase with a N-and P-rich medium and simulated power plant flue gas was 14:1. 34 Here, we show the potential for microalgal cultivation and harvest using waste resources. When S. obliquus was cultivated with simulated coal-fired power plant flue gas and simulated fertilizer plant wastewater, simultaneous N-and P-limitation was achieved in 5-day batches.…”

“…On the other hand, sufficient sulfate for microalgal growth will be rapidly accumulated from the oxidation of SO 2 in flue gas. 34 Though it is approximately 1% or less of the dried microalgal biomass, phosphorus is also an essential nutrient. Microalga N/ P ratios are wide-ranging, including within a single species, depending on environmental conditions and growth stage.…”

“…In most cases, the quantity of nitrogen supplemented by flue gases would be insignificant relative to the quantity of nitrogen required to sustain microalgal growth. On the other hand, sufficient sulfate for microalgal growth will be rapidly accumulated from the oxidation of SO 2 in flue gas …”

“…Microalga N/P ratios are wide-ranging, including within a single species, depending on environmental conditions and growth stage . From biomass elemental composition analyses, the N/P molar ratio of Scenedesmus obliquus grown to stationary phase with a N- and P-rich medium and simulated power plant flue gas was 14:1 …”