Heterotrophic upcycling of hydroponic wastewater supplemented with glucose and indole-3-acetic acid into high-quality Chlorella biomass for zero-waste multiproduct microalgal biorefinery

Abstract

Wastewater from agricultural activities poses significant environmental risks and requires proper treatment before discharge. Phytoremediation using microalgae offers a compelling solution by removing contaminants and generating valuable biomass. This study aimed to optimize glucose and indole-3-acetic acid (IAA) concentrations to maximize pollutant treatment and microalgal biomass production using Chlorella sp. AARL G049 in hydroponic wastewater from lettuce cultivation without added nitrogen and phosphorus. The results showed that Chlorella sp. effectively converted pollutants in undiluted wastewater into biomass, achieving a maximum yield of 1.32 g/L (0.12 g/L/day) with 10.89 g/L of glucose and 10.15 mg/L of IAA. Pollutant removal efficiencies for chemical oxygen demand, ammonium-nitrogen, nitrate-nitrogen, and phosphate-phosphorus exceeded 92 %. An integrated zero-waste biorefinery process produced three value-added products from the microalgal biomass: functional pigments, biodiesel, and biofertilizer. The extracted pigment demonstrated significant antioxidant activity, with DPPH activity of 0.05 mg GAE/g-extract, ABTS activity of 0.31 mg TE/g-extract, and FRAP activity of 0.28 mg GAE/g-extract, as well as high-efficiency UV protection. The lipids extracted contained biodiesel-quality fatty acids with a cetane number of 54 and a high heating value of 40 KJ/kg. Additionally, the residual biomass, post-extraction, contained essential nutrients with an N-P-K ratio of 4.87–0.03–0.68 and 76 % organic matter, making it suitable for plant growth and soil fertilization. Therefore, integrating wastewater treatment with a microalgal biomass-based zero-waste biorefinery demonstrates significant potential for enhancing profitability and sustainability, promoting the sustainable development of the Food-Energy-Agriculture-Environment Nexus.