Microwave assisted low-temperature hydrothermal treatment of solid anaerobic digestate for optimising hydrochar and energy recovery

With the growth of anaerobic digestion (AD) for biogas production, associated increasing digestate production may cause environmental problems if the increasing agricultural land required for digestate application is limited. An alternative is to valorise the digestate. Microwave assisted low-temperature hydrothermal treatment (MLHT; temperature 100 – 180 °C) was investigated as a post-treatment for AD of grass silage under two scenarios: 1) AD + MLHT and 2) Acid pre-treatment + AD + MLHT. Compared to the original grass silage, the digestates investigated required lower temperatures for carbonization in MLHT owing to their lower cellulose content. The higher MLHT temperatures (160 – 180 °C) led to significant increases in heating value and greater reductions in atomic ratios of O/C and H/C of hydrochar due to dehydration and decarboxylation reactions. As a result, higher temperatures contributed to higher sugar recovery, higher solid solubilization, and better quality of hydrochar. Under the MLHT at 180 °C, the hydrochar produced from digested grass silage in scenario 1 (AD + MLHT) exhibited a mass yield of 0.79 g/g total solid, a carbon content of 63.6% and an ash-free heating value of 27.6 kJ/g volatile solid; the biomethane potential from the process liquor was estimated as 68.7 ml CH₄/g total solid. Scenario 1 is preferred over scenario 2 (acid pre-treatment + AD + MLHT) as it gave a higher yield and higher heating value of hydrochar. This study suggests that MLHT is a promising method to 1) produce hydrochar with an energy value comparable to lignite coal, and 2) recover additional biomethane through process liquor recycling.