Steam explosion pretreatment of separated dairy cattle manure: Mass balances and effect on biomethane potential

Manure is a renewable feedstock, whose theoretical potential for biogas production is scarcely deployed due to modest methane yields that prevent economic feasible operation of anaerobic digestion plants. Steam explosion pretreatment has the potential to improve the digestibility of manure, however it is energy intensive, and the optimal conditions depend on the feedstock. In this work, the solid and the liquid fraction of separated dairy cattle manure were pretreated between 130 and 210 °C for 5 to 40 min by steam explosion to individually determine the optimal conditions for each fraction. Additionally, mass balances for volatile solids (VS), cellulose, hemicellulose and proteins were performed to better understand the effects of the pretreatment. For the manure solids, a pretreatment at 130 °C for 20 min was most effective, the biomethane potential (BMP) improved by 40 %. In contrast, the BMP of the liquid fraction could not be improved at any pretreatment condition. The mass balances showed that at more severe conditions up to 18 % of the VS were lost by decomposition and/or evaporation, with the proteins being the most thermolabile fraction. Based on the observation, that a pretreatment of the liquid phase can be omitted, a heat integrated plant concept is suggested where the necessary heat input is only as large as in conventional anaerobic digestion. Taken together, this work underlined the benefits of steam explosion pretreatment of manure and identified the prevention of VS loss as a promising avenue for further improving the process.