Effects of water recirculation on microalgae assemblage and corresponding sustainability of the photobioreactor cultivation system

Abstract

An algae photobioreactor (APB) was used to determine the effect of water recirculation on the growth of algal assemblage. CO2 in the flue gas from a power plant was the carbon source. Boiler water was used as the source water. The results showed that microalgae cultivation under recirculation conditions was stable over a period of four months. Biomass productivities during the 1st through 4th months of recirculation (0.26, 0.23, 0.20, and 0.18 g L−1 d−1, respectively) were not significantly different than freshwater (0.22 g L −1 d −1). Furthermore, the relationship between eukaryotic and bacterial domains in the assemblage remained consistent throughout the four months of recirculation (80.7, 87.1, 83.1, and 82.1%, respectively, and 19.2, 12.8, 16.9, and 17.8%, respectively). This was not significantly different than the abundance of each domain in the control freshwater cultivation (83.7% eukaryotic and 16.2% bacterial). A 1 m3 photobioreactor was then envisioned for a mass, energy, and exergy analysis to evaluate the water recirculation on sustainability of the culture system. The mass balance analysis concluded that 98% reduction in water usage, 25% reduction in nitrogen, and 12.5% reduction in phosphorus could be achieved during cultivation operating under recirculation for one year, while maintaining biomass productivity of 1.2 kg wet algal biomass and sequestration of 0.4 kg CO2 per day. The exergy balance analysis concluded that without considering solar irradiation, the culture with water recirculation greatly enhanced the rational exergy efficiency, which represents a more sustainable cultivation system for CO2 capture and utilization.