The effect of volatile fatty acids on the growth and lipid properties of two microalgae strains during batch heterotrophic cultivation

Abstract To overcome the bottlenecks of waste resource utilization and energy shortage that restrict the commercial production of microalgae biodiesel, volatile fatty acids (VFAs) derived from activated sludge were used as the sole carbon source to culture oleaginous microalgae Chlorella pyrenoidosa FACHB-1216 and Scenedesmus quadricauda FACHB-1297 under the mixotrophic and heterotrophic cultivation. Four VFAs ratios (acetic acids (AA): propionic acids (PA): butyric acids (BA)) were tested to determine the effects and mechanisms of the VFAs on the two microalgae. The highest lipid content (29.54%) and lipid production (71.10 mg L−1) were achieved by S. quadricauda at the VFAs ratio of 6: 1: 3 under heterotrophic condition, with 46.27% and 67.52% removal efficiencies of total nitrogen and phosphorus, respectively. The assimilation efficiency of AA was the highest at 73.37%, followed by that of PA and BA. For C. pyrenoidosa, VFAs promoted the rapid reproduction within 2 days under the heterotrophic condition at different initial inoculation densities. At the optimal VFA ratio, algae achieved the highest biomass concentration (0.14 ± 0.02 g L−1), with a specific growth rate of 0.91 d−1 and biomass productivity of 125.17 mg L−1 d−1. The removal rates of total nitrogen and phosphorus were 47.03% and 74.40%, respectively, and the assimilation efficiency of AA was the best (61.06%). High AA assimilation efficiency under the heterotrophic condition was beneficial for the algal growth and lipid accumulation. These results simultaneously produced microalgae-based bioenergy and recycled VFAs in anaerobically digested effluent.