Fate of Nitrogen, Phosphate, and Potassium during Hydrothermal Carbonization and the Potential for Nutrient Recovery

In this study, the fate of carbon and nutrient elements, nitrogen, phosphorus, and potassium, was investigated during the hydrothermal carbonization (HTC) of three model solutions with different nutrient concentrations to mimic biogas residues from silage and cattle manure. The HTC was conducted at 180 °C, 220 °C, 240 °C, and 260 °C for 3 h reaction time. ICP-OES, GC, IC, and chemical analysis methods were utilized to measure fertilizing elements in HTC products. The distribution of nutrients between HTC product phases was significantly influenced by the compositions of the initial feedstock before HTC. The incorporation of nitrogen to the hydrochar (HC) depends on the nitrogen-containing compounds during HTC; the chemical bonding was the main mechanism for the incorporation of NH4–N into the HC during HTC, the sorption of NH4–N to the HC occurred, however, to a lesser extent (0.8% to 7.4%). Most of the NO3–N stayed in the process water (PW) during HTC (78% to 87%), and the sorption (adsorption/salts precipitation) was confirmed to be the main mechanism for the recovery of NO3–N to the HC during HTC. The uptake of N to the HC is limited and depends on the availability of the carbon network in the HC, the correlation coefficient between HC formation and the recovery of bonded N to the HC showed a high linear regression coefficient R2 = 0.90–0.91. ... mehrHowever, the recovery of N to the HC via sorption showed less correlation to the HC formation during HTC (R2 = 0.35–0.54). Most of the dissolved K+ (99%) stayed in the PW during HTC. Due to the absence of metal associations in this work, most of the dissolved PO43– (99%) stayed in the PW during HTC, which proves that controlling metal cations in the feedstock before HTC has a considerable influence on the distribution of phosphate between the PW and HC.