Investigation of lipid, carbohydrate and protein production from <i>Chlorella vulgaris</i> in controlled environment minkery wastewater

Abstract

In an effort to incorporate a novel bioremediation technology into minkery wastewater treatment systems, this study aimed to investigate the effect of light cycles on the simultaneous production of Chlorella vulgaris biomass, lipid, carbohydrate, and protein in controlled environment minkery wastewater. Diluted minkery cage-washing wastewater used in the experiments, which were designed as a completely randomised design with a single factor (light cycle) experiment. The single factor had four levels: six-day continuous light (photoautotrophic), 48-h light/24-h dark (mixotrophic), 24-h light/48-h dark (mixotrophic), and six-day continuous dark (heterotrophic) cycle, which were randomly assigned to the 2.25-litre vertical column controlled environment photobioreactors (PBR), each equipped with an independent cool-white fluorescent light (8 W, 6,700 K) operated at room temperature. The pH in each PBR was monitored and controlled using independent pH meters and air pumps. During six-day cultivations, continuous light and 48-h light/24-h dark cycles achieved the largest biomass (82.50 mg L−1 day−1 and 79.50 mg L−1 day−1, respectively) and protein productivities (42.62 mg L−1 day−1 and 38.79 mg L−1 day−1, respectively) out of Chlorella vulgaris in minkery wastewater; however, using 48-h light/24-h dark cycle instead of continuous light cycle would reduce the energy cost of cultivation. The light cycles of continuous light, 48-h light/24-h dark, and 24-h light/48-h dark achieved the highest lipid (15.06 mg L−1 day−1, 16.03 mg L−1 day−1, 12.82 mg L−1 day−1, respectively) and carbohydrate (11.15 mg L−1 day−1, 11.89 mg L−1 day−1, 8.08 mg L−1 day−1, respectively) productivities, and there was no statistically significant difference in between; however, using 24-h light/48-h light cycle would again reduce the energy cost of cultivation compared to the other two cycles. Overall, for the cultivation of Chlorella vulgaris in minkery wastewater, the most appropriate light cycle for the production of microalgae biomass and crude protein seems to be the mixotrophic growth under 48-h light/24-h dark cycle, while the mixotrophic growth of 24-h light/48-h dark cycle was the most appropriate system for the production of lipid and carbohydrate. Providing a supplemental organic carbon source, preferably from another waste stream, has a potential to change the story with respect to the competitiveness of continuous dark (heterotrophic) cycle, and studying other microalgae strains may make minkery wastewater even more competitive compared to traditional culture mediums.