Moderate thermal post-treatment of digestate to improve biomethane production from agricultural- and food waste

The aim of this study was to assess the feasibility of moderate thermal treatment (70 degrees C for one hour) of digestate in combination with post -digestion targeting residual biomethane potentials from three full-scale biogas plants digesting food waste (FW), agricultural waste (AW) and a mixture of AW and manure (AWM). Dissolved organic carbon (DOC), biomethane production, and digestate quality were investigated. For the study six laboratory -scale continuously stirred tank biogas reactors working as post -digesters, with thermally -treated and non -treated digestate were used. DOC for thermally -treated digestates increased significantly ( t -test, p < 0.05); FW-digestate (110-200 %), AW-digestate (24-92 %) and for AWM-digestate (4-73 %). Indexes for corresponding DOC quality showed lower apparent organic molecular weights and decreased aromaticity (with the exception of FW-digestate). Thermal treatment of digestate improved the biomethane production during post -digestion by 21-22 % (FW-digestate) and 9 % (AW-digestate). For AMW-digestate no clear positive effect was observed, most likely due to biogas plant operational process disturbances.