Determination of biokinetic coefficients for nutrient removal from anaerobic liquid digestate by mixed microalgae

Abstract

Anaerobic digestion is one of the most common methods used to generate energy from renewable sources. However, untreated anaerobic liquid digestate may cause eutrophication if directly discharged into the water bodies due to its high nutrient content. Microalgae can assimilate nutrients especially nitrogen and phosphorous from wastewater for their growth and produce valuable biomass. In this study, batch experiments were carried out to investigate the biokinetic coefficients for nutrient removal of mixed microalgae grown on anaerobic liquid digestate by Michaelis–Menten rate expression. The initial NH₃-N concentration was varied between 18.6 and 87.1 mg L⁻¹, while initial PO₄-P concentration was between 1.85 and 6.88 mg L⁻¹, which corresponds to 2%, 5%, 7%, and 10% dilution ratio of anaerobic digestate. According to the yield results (mg chl a mg⁻¹ nutrient), mixed microalgae uptake 10 times more nitrogen than phosphorus. Biokinetic coefficients were determined as k_N = 2.48 mg NH₃-N mg⁻¹ chl a day⁻¹, Km_N = 29.3 mg L⁻¹, Y_N = 0.45 mg chl a mg⁻¹ NH₃-N for nitrogen; and k_P = 0.21 mg PO₄-P mg⁻¹ chl a day⁻¹, Km_P = 2.94 mg L⁻¹, YP = 5.03 mg chl a mg⁻¹ PO₄-P for phosphorus. The highest chlorophyll production (39 mg L⁻¹ or 3.31 mg L⁻¹ day⁻¹) was observed at the highest dilution ratio of 10%. Moreover, the highest dilution resulted in highest biomass (1.25 g L⁻¹) despite of dark, high ammonicial, and particulate rich wastewater.