Hydrothermal carbonization of anaerobic digestate and manure from a dairy farm on energy recovery and the fate of nutrients

[Display omitted] •Raw and anaerobically digested manure were hydrothermally carbonized.•Using whey for HTC increased the net energy gain and available nutrients.•P recovery was higher for the coupled AD-HTC process compared to single-step HTC.•Net energy gain was higher from HTC compared to the coupled AD-HTC process. Hydrothermal carbonization (HTC) of raw and anaerobically digested (AD) manure with either water or whey was studied, with the goal of recovering energy and nutrients. Specifically, the impacts of HTC reaction temperature (180–240 °C), solid feedstock, and type of liquid on hydrochar quality and aqueous phase properties were tested. Of the hydrochars produced, the calorific value of whey-based hydrochar was the highest, (19.4 and 16.0 MJ/kg for manure and digestate, respectively). Overall, the net energy gain was higher for HTC of manure with whey (7.4–8.3 MJ/kg dry feedstock) and water (4.4–5.1 MJ/kg) compared to the combined AD-HTC process with whey (4.4–5.3 MJ/kg) and water (2.3–2.9 MJ/kg). Digestate-derived hydrochar contained up to 1.8% P, higher than manure-derived hydrochar (≤1.5%). Using whey as a liquid for HTC increased the aqueous-phase N-P-K concentrations up to 3,200, 410, and 7,900 mg/L, respectively, suggesting its potential use as a liquid fertilizer.