Biogas Production from Waste Microalgal Biomass Obtained from Nutrient Removal of Domestic Wastewater

In this study, a semi-continuous photobioreactor was operated for the investigation of nutrient removal efficiency of a unialgal culture, Chlorella vulgarıs. Maximum nitrogen and phosphorous removal efficiencies of 99.6 and 91.2 % were achieved in the photobioreactor. The microalgal slurry obtained from the effluent of the photobioreactor was subjected to biochemical methane potential assay, after application of heat, autoclave, and thermochemical pretreatments to improve anaerobic digestibility and biogas production. Evaluation of pretreatment options indicated that heat pretreatment is the most efficient method in terms of enhancing anaerobic digestibility, at the chemical oxygen demand (COD) loading of 19 ± 0.5 g L−1. This method increased the methane yield by 83.0 %, from 223 to 408 mL CH4 g VSadded−1, compared to untreated microalgal slurry reactor with the same COD value. Among reactors with 35 ± 1.5 g L−1 initial COD concentration, autoclave-pretreated microalgal slurry was found to yield the highest methane value of 356 mL CH4 g VSadded−1, which was 43.0 % higher than the value observed in the reactor fed with untreated microalgal slurry. The thermochemical pretreatment caused production of inhibitory compounds and resulted in lower biomethane production and COD treatment values, compared to untreated microalgae. Outcomes of this study reveal that coupled micro-algal and anaerobic biotechnology could be a sustainable alternative for integrated nutrient removal and biofuel production applications.