This study evaluated methane production and nutrient recovery from industrially produced, lipid extracted algal biomass (LEA) of Auxenochlorella protothecoides using semi-continuous anaerobic digestion (AD) at different organic loading rates (OLRs) and hydraulic retention times (HRTs). It was shown, that AD can improve biofuel production efficiency and sustainability, especially for scaled processes, through up to 30% increase in energy generation (up to 0.25 L of methane per g of LEA volatile solids) and partial nutrient recovery and recycling. The nutrient recycling with the AD effluent may reduce the cost of the supplied fertilizers by up to 45%. However, methane production was limited to nearly 50% of theoretical maxima potentially due to biomass recalcitrance and inhibition effects from the residual solvent in the LEA. Therefore, further AD optimization is required to maximize methane yield and nutrient recovery as well as investigation and elimination of inhibition from solvent residues.
In 2017, the United States Environmental Protection Agency (EPA) reported that Americans generated over 268 million tons of municipal solid waste (MSW). The majority (52%) of this waste ends up in landfills, which are the third largest source of anthropogenic methane emissions. Improvements in terms of waste management and energy production could be solved by integrating MSW processing with hydrothermal carbonization (HTC) and anaerobic digestion (AD) for converting organic carbon of MSW to fuels. The objectives of this study were to (a) investigate HTC experiments at varying temperatures and residence times (b) evaluate aqueous phase and solids properties, and (c) perform AD bench scale bottle test on the aqueous phase. A mixture of different feedstock representing MSW was used. HTC at 280 °C and 10 min yielded the highest total organic carbon (TOC) of 8.16 g/L with biogas yields of 222 mL biogas/g TOC. Results showed that AD of the aqueous phase from a mixed MSW feedstock is feasible. The integrated approach shows organic carbon recovery of 58% (hydrochar and biogas). This study is the first of its kind to investigate varying temperature and times for a heterogeneous feedstock (mixed MSW), and specifically evaluating HTC MSW aqueous phase anaerobic biodegradability.
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