Enhancing low-carbon wastewaters with flue gas for the optimal cultivation of Desmodesmus multivariabilis

Abstract

Phycovalorization of wastewater into algal biomass and extracellular polysaccharides (EPS) is an attractive alternative for producing raw materials for biofuels. Industrial flue gas has also been demonstrated as a potential substrate for growing microalgae. Combining wastewater (WW) and flue gas in a single process would provide an attractive process for valorisation of both waste-streams. This study investigated enhancing the potential of low carbon wastewaters with flue gas and mapping the interaction between the organic carbon present in the wastewater and the carbon dioxide introduced with the flue gas, with the aim of achieving optimal biomass, fatty acid methyl esters (FAME) and EPS yields. Desmodesmus multivariabilis, isolated from a local eutrophic reservoir in South Africa was used in the study. Wastewater media from a local wastewater treatment works and synthesized flue gas (13 % CO2) were used as feed. Experiments were performed in triplicate. The ratio of organic: inorganic carbon was manipulated by varying the CO2 concentration. In the control experiment, BBM media with no organic carbon was used instead of wastewater. CO2 uptake was calculated by means of performing a carbon balance. The experimental results indicated a point of optimal efficiency where all production parameters (Biomass Production, EPS Production, Nitrogen Uptake) were at maximum value, and the input ratio was at minimum value (Input ratio = mg TOC0/mg CO2 uptake). EPS production only occurred at low input ratios and organic substrate utilization conversely only occurred at high input ratios, indicating a switch between autotrophic and mixotrophic metabolism. The results evidenced an association between higher productivity and autotrophic metabolism.