Speciation of Main Nutrients (N/P/K) in Hydrochars Produced from the Hydrothermal Carbonization of Swine Manure under Different Reaction Temperatures

Hydrothermal carbonization (HTC) has been proved to be a promising technology for swine manure (SM) treatment. Currently, there is a lack of systematic understanding of the transformation characteristics of nutrient speciation in the HTC of SM. In this study, the speciation of the main nutrients (N/P/K) in SM-derived hydrochar produced at different reaction temperatures (200–280 °C) was investigated. The recovery of P (61.0–67.1%) in hydrochars was significantly higher than that of N (23.0–39.8%) and K (25.5–30.0%), and the increase in reaction temperature promoted the recovery of P and reduced the recovery of N. After the HTC treatment, the percentage of soluble/available P was reduced from 61.6% in raw SM to 4.0–23.9% in hydrochars, while that of moderately labile/slow-release P was improved from 29.2% in raw SM feedstock to 65.5–82.7%. An obvious reduction was also found in the amounts of available N (from 51.3% in raw SM feedstock to 33.0–40.5% in hydrochars). The percentages of slow-release N and residual N in hydrochars produced at 240 °C reached the maximum and minimum values (46.4% and 18.9%), respectively. A total of 49.5–58.3% of K retained in hydrochars was residual (invalid) potassium. From the perspective of the mobility and availability of N, P and K only, it was suggested that the HTC of SM should be carried out at 220–240 °C. Compared with the original SM, it is safer and more effective to use the SM-derived hydrochar as an organic fertilizer.