Hydrothermal carbonisation of anaerobic digestate for hydro-char production and nutrient recovery

This study investigates the potential of hydrothermal carbonisation (HTC) for fractionating anaerobic digestate of sewage sludge into carbon-rich hydrochar and nutrient-rich aqueous phase (AP). AP is subsequently used to supplement cultures of the alkali halophilic microalgae D. tertiolecta (CCAP 19/30), to convert sodium bicarbonate into sodium carbonate solution as part of an integrated biogas purification system. HTC at 200°C gave the highest hydrochar yields (78%) and solid carbon retentions (75%), indicating high carbon capture potential. In contrast, the essential growth nutrients nitrogen, phosphorus and sulphur were partially solubilised, resulting in HTC-AP concentrations between 11 times (S) and 50 times (P) higher than those in artificial growth medium. Trace nutrient concentrations in the AP were 10 to 80 times higher compared to the artificial medium, with minimal heavy metal solubilisation. Dunaliella tertiolecta grew successfully and without inhibition at HTC-AP concentrations up to 2% (produced at 200°C). AP-supplemented cultures achieved higher cell concentrations (up to 10.0 x 10⁶ cells mL^-1), biomass content (maximum of 1.14 ± 0.06 g L^-1) and bicarbonate-to-carbonate conversion (83% and 80%, for 1% and 2% of HTC-AP) than the control cultures. Therefore, HTC-AP appears to be a suitable artificial growth medium substitute for cultivating alkali-halophilic microalgae to regenerate carbonate and produce algae biomass, providing an added-value product.