Recirculation of solid digestate to enhance energy efficiency of biogas plants: Strategies, conditions and impacts

Highlights: • Three solid digestate (SD) recirculation strategies were defined. • Additional post-treatments to increase SD methane yield do not appear necessary. • Direct SD recirculation can increase biogas production and digester solid content. • Under certain conditions, direct SD recirculation increases biogas plant efficiency. • Direct SD recirculation can be a relevant digestate management practice. This article aims at providing insight into the recirculation of (post-treated) solid digestate (SD) within digesters. Such a practice has been further identified as promising for anaerobically digesting recalcitrant feedstocks and for improving the energy efficiency of continuously stirred tank reactor (CSTR) agricultural biogas plants. Firstly, implementation strategies and the potential impact of SD direct recirculation on five CSTR biogas plants were investigated. According to the selected strategy, results indicated that plant methane production could rise by 0.6 to 6.3% or that a potential feedstock shortage of 64 to 1431 tons/year could be compensated. Secondly, the relevance of additional post-treatments for improving these initial results was evaluated. Thermo-chemical post-treatments successfully increased SD biodegradability by 30 to 46% although their costs were not compensated by additional methane production. Short-term aerobic post-treatments failed in increasing SD biodegradability (up to 21% loss in biomethane potential). Hence, at full scale, a quick and direct recirculation of SD excluding any post-treatment appears to be the optimal condition to apply. Finally, conditions for the full-scale implementation of direct SD recirculation were theoretically studied. This practice has proved to increase the solid retention time by 11 to 38% and the plant total solid content by 6 to 20%. Thus, the critical point for its implementation should be the capacity of the plant mixing system to handle such an increase in solids. The relevance of SD recirculation needs to be determined on a case-by-case basis. Consequently, for the first time, this article provides a framework where the conditions can be identified for direct SD recirculation to be a relevant digestate management practice. Overall, this article demonstrates how direct SD recirculation can be a simple and low-cost mean for improving agricultural CSTR biogas plant efficiency. It also highlights the importance of achieving efficient digestate management in the biogas sector in order to reduce the costs of biogas production.