|Abstract||The conversion of microalgal biomass into methane as a biofuel has received increased interest in the past years, as it offers the best energetic balance among the different biomass–to-biofuel scenarios for microalgae containing less than 40% lipids. In a biorefinery approach, anaerobic digestion (AD) becomes unavoidable to maximize the greenhouse gas (GHG) reduction from the net CO2 abatement associated to the algal culture. Scenedesmus sp.AMDD is a model strain that offers advantages such as high biomass yield, robustness and capacity to grow in wastewaters. In past studies, the best conversion rate of Scenedesmus into methane reached around 50% under optimal conditions including pretreatment of the biomass and long digester retention time (6-15 days). The main objective of this study was to increase the biomethane yield obtained from algal biomass, through the use of a post-treatment applied to the digestate and the return of the post-treatment product back to the digester.
Wet oxidation, a thermochemical treatment, was applied to digestate under different conditions of temperature (120 – 200ºC), oxygen pressure (0 – 105 psig) and reaction time (0-60 min). The impact of the three variables were as follow: temperature >> pressure >> reaction time. The assays performed at 200ºC showed the higher level of organic matter losses in off-gas (25%) but also the highest solubilization of the remaining solids (up to 60%).
Biomethane potential assays were performed on the suspension after wet oxidation. The best results were obtained after a post-treatment at 200ºC, 105 psig O2 and no holding time, with 202 L CH4/kg VS. In comparison, the untreated digestate produced only 69 L CH4/kg volatile solid (VS) after 6 weeks of incubation. The anaerobic digestion of Scenedesmus in a 10L high-rate digester yielded 7.5 L CH4/d. The subsequent methane produced from the treated digestate represented the equivalent of 6 L CH4/d (80% increase), for a combined yield of 400 L CH4/kg VS, obtainable within 3 days retention time. In conclusion, thermochemical post-treatment of the digestate can significantly improve the final biofuel production from microalgal biomass.|