Hydrothermal carbonization (HTC) is a promising technology for wet manure treatment by converting animal manure into valuable fuels, materials, and chemicals. Among other HTC process parameters, the temperature influences HTC products the most. As various animal manures have different compositions, it is not certain how the HTC temperature influences the hydrochar and HTC process liquid. To evaluate the temperature’s effect on HTC, three different manures (poultry, swine, and dairy) were hydrothermally carbonized at three different temperatures (180, 220, and 260 °C), and solid and liquid products were characterized for their morphology, elemental compositions, and ions. The carbon contents of the hydrochar reached as high as 38.98 ± 0.36% and 40.05 ± 0.57% for poultry and swine manure, respectively, when these manures were treated at 260 °C. Ammonium showed an around 30% increase in poultry manure hydrochar with the increase in the HTC temperature. In contrast, in swine manure, it decreased by around 80%, and in dairy manure, the HTC temperature did not have any remarkable effect on the ammonium content. The process liquids from HTC of dairy manure at 220 °C showed the most balanced distribution of different ions, with 4970 ± 673 ppm of sodium, 4354 ± 437 ppm of ammonium, 2766 ± 417 ppm of potassium, 978 ± 82 ppm of magnesium, 953 ± 143 ppm of calcium, 3607 ± 16 ppm of chloride, and 39 ± 7 ppm of phosphate. These results emphasize the manure-specific effects of the HTC temperature on both solid and liquid products, indicating the need for optimized strategies to enhance HTC processes for various types of animal manures.

This study evaluates the effects of co-hydrothermal carbonization (Co-HTC) of poultry (PM), dairy (DM), and swine manure (SM) at 180, 220, and 260 °C on hydrochar yield, nutrient recovery, phytotoxicity, and heavy metal retention. Co-HTC of PM with DM (PMDM) and PM with SM (PMSM) were assessed for synergistic effects by comparing observed values to predicted additive outcomes based on individual HTC data. Hydrochar yield decreased with increasing temperature across all treatments; however, PMSM and PMDM exhibited synergistic enhancement in yield at 180 °C (synergistic coefficient: 1.13-1.23). Elemental analysis indicated that nitrogen retention was maximized in PMDM hydrochars (4.71 ± 0.61% N at 180 °C), while PMSM hydrochars exhibited superior retention of phosphorus, magnesium, and calcium. Most potassium leached into the co-HTC process liquid. Heavy metals (Zn, Cu, and Ni) were effectively immobilized, with Co-HTC resulting in lower concentrations of Mo, Pb, and Cr compared to individual HTC. Seed germination index (GI), used to assess phytotoxicity, revealed temperature- and feedstock dependent trends, with PMSM hydrochar produced at 180 °C achieving the highest GI (140.91 ± 7.05%), indicating synergistic reduction in phytotoxicity. These findings demonstrate that Co-HTC can optimize recovery of nutrients into hydrochar and enhance the agronomic and environmental quality of hydrochars through tailored feedstock interactions and process tuning.

Conventional manure management in concentrated animal feeding operations (CAFOs) poses a significant environmental challenge due to substantial greenhouse gas (GHG) emissions, particularly methane from anaerobic storage systems. Thermochemical conversion of dairy manure into hydrochar and biochar via hydrothermal carbonization (HTC) and pyrolysis, respectively, offers a promising alternative to reduce these emissions while improving carbon sequestration. In this study, dairy manure was treated using HTC (180, 220, and 260 °C) and pyrolysis (400, 600, and 800 °C) to produce hydrochars and biochars, respectively. Results show that despite increased carbon stability at higher treatment temperatures, as indicated by increasing fixed carbon content and decreasing H/C and O/C atomic ratios, the carbon sequestration potential decreased, largely due to reduced char yields. However, hydrochars had higher C/N ratios, suggesting reduced susceptibility to initial decomposition. Carbon sequestration potential peaked for biochar produced at 400 °C (32.79% ± 1.28%) and for hydrochar at 180 °C (22.11% ± 1.17%). When integrated into screening-level, model-based environmental impact assessment focusing on GHG emissions and carbon sequestration potential for two CAFOs, HTC and pyrolysis systems reduced net GHG emissions and produced modeled net carbon sequestration under the assumptions used. Compared to conventional manure management systems, which included lagoon storage, land application, or anaerobic digester, HTC at 180 °C to 260 °C resulted in net sequestration of −25 543 to −20 641 MT CO ₂ -eq/year for, while pyrolysis at 400 °C to 800 °C achieved −38 553 to −24 723 MT CO ₂ -eq/year for a 1500-head CAFO scenario.

Animal manure is an agricultural by-product that presents both nutrient value and environmental challenges, particularly through nutrient leaching and greenhouse gas (GHG) fluxes. Hydrothermal carbonization (HTC) offers a potential management strategy that converts wet manures into hydrochar, which is a more stable and nutrient-retentive amendment. This study evaluated how hydrochar derived from dairy manure, swine manure, and poultry litter, applied at 15 or 30 metric tons/ha, influences GHG fluxes and corn biomass production in a controlled greenhouse experiment using soil mesocosms with two fertilization histories (dairy-manure vs. inorganic/chemical-only). All treatments received an equal total nitrogen input through combinations of hydrochar-derived N and urea. Hydrochar treatments tended to reduce cumulative N₂O fluxes, although not always significantly, compared to the urea-only control, with the largest reductions occurring after the second urea application. Across feedstocks, hydrochar treatments tended to show higher cumulative CO₂ fluxes, although not statistically significant, relative to the control, with 15 tons/ha applications producing higher CO₂ fluxes and more corn biomass compared to 30 tons/ha. Methane fluxes exhibited no consistent treatment-level trends. Corn biomass tended to be higher in the 15 tons/ha hydrochar treatments and lower in the 30 tons/ha treatments relative to the control. Overall, findings indicate that partially replacing urea fertilizer with hydrochar, can potentially increase biomass while reducing cumulative N₂O fluxes. Generally, the increases in CO₂ fluxes from hydrochar did not fully offset the reductions in N₂O fluxes at the system level. The results suggest that hydrochar amendments can offer an agronomically efficient and environmentally beneficial nutrient-management strategy.