Influence of domestic solvents on the hydrothermal carbonization of cow dung: Surface morphology and thermal behavior of hydrochar and the fate of nutrients in process water

The aim of this study is to demonstrate the effect of domestic solvents such as water, human urine, and cow urine on the hydrothermal carbonization (HTC) process of cow dung for improving the properties of hydrochar (HC) and process water (PW) in terms of its combustion behavior, surface morphology, and nutrient loading. The physiochemical properties, i.e., the surface area, morphology, functional groups, thermal behaviors of HC, and nutrient profiles of PW, were characterized. The highest HHV (higher heating value) (18.7 MJ/kg) was observed for water as a solvent for HTC-produced HC (HC_W), at 200 °C for 2 h. All the HC demonstrated high specific surface areas (9.1–12 m²/g) compared to raw cow dung (1.7 m²/g) which indicated its improved adsorbent properties. The results from kinetics analysis indicated the minimum activation energy (~ 99 kJ/mol) required for the HC_W combustion. The nutrient profiles of HTC_PW indicated that the chemical oxygen demand, nitrogen, phosphorus, and potassium concentrations were significantly increased in comparison to raw solvents. The highest total ammoniacal nitrogen, total phosphate, and total potassium contents were observed for human and cow urine-based HTC-generated PW (PW_HU and PW_CU) (6.7, 1.6, and 6.5 g/L; and 3.0, 0.9, and 7.5 g/L, respectively). The PW and HC that were produced have the potential to be utilized in other resource recovery techniques, including nutrient recovery, wastewater treatment, and energy generation.