Coupling hydrothermal carbonisation and anaerobic digestion: sustainable cow manure valorisation

This study investigated the integration of hydrothermal carbonisation and anaerobic digestion for energy and resource recovery from cow manure. Raw cow manure was treated at 180, 220 and 260°C. The hydrochars were characterised for soil and fuel related applications, while the process waters were evaluated as anaerobic digestion substrates. Hydrothermal carbonisation predominantly retained heavy metals in the hydrochar, resulting in their limited transfer to the process waters and their derived digestates. Among the process waters, the process water at 180°C produced the highest COD-normalised methane yield of 214.1 mL CH₄/gCOD, together with 49% COD removal, and 59% TS and 73% VS destruction. The process waters at 220 and 260°C produced 150.4 and 89.1 mL CH₄/gCOD respectively. On an initial dry CM basis, the process waters at 180, 220 and 260°C produced 69, 79 and 63 L CH₄/kg dry CM respectively which are equivalent to 37, 42 and 33% of the raw CM methane yield. The integrated gross energy recoveries were 91, 94 and 84% for the 180, 220 and 260°C respectively. Hydrothermal carbonisation at 220°C followed by anaerobic digestion of the process water is therefore proposed as the most suitable condition for integrated gross energy recovery.