Post-anaerobic treatability and residual biogas potential of digestate

Abstract

Anaerobic digestion is a well-established method of stabilizing various organic wastes. However, effective and low-cost methods need to be developed and applied for digestate management to realize the full benefits of anaerobic digestion. The post-treatment of the digestate does not only reduce the greenhouse gas emissions due to storage or land application of digestates but also leads to an increase in the recovery of methane. The objective of this study was to quantify anaerobic treatability and biogas formation from the digestates of three full-scale anaerobic digesters fed with fruit processing and brewery wastewater and animal manure and food waste. Biochemical methane potential assays were conducted to determine the residual biogas production and yields achievable in the digestate samples investigated. The methane yields for the digestates investigated ranged between 0.055 and 0.147 l CH4/g VS which agree well with the relevant values from the literature. Anaerobic treatability assessment of digestates indicated chemical oxygen demand (COD), total solids (TS), and volatile solids (VS) removals of up to 62, 35, and 41%, respectively. Very low removals were observed for nitrogen and phosphorus as expected. The total potential energy yield and electricity production based on anaerobic digestion of the digestates of the three full-scale digesters are calculated as 35.3 GJ/day and 3.4 MWh, respectively. Moreover, a significant reduction in the greenhouse gas production will be achieved by anaerobically digesting the digestates of the three full-scale anaerobic digesters investigated in this study. If these findings are projected for all 127 biogas systems in Michigan, the significance of digestate treatment and valorization becomes clearer.