Hydrothermal carbonization of kitchen waste: An analysis of solid and aqueous products and the application of hydrochar to paddy soil

Hydrothermal carbonization (HTC) technology can potentially be used to safely and sustainably utilize kitchen waste (KW). However, the characteristics of HTC solid products (hydrochar) and aqueous products (HAP) based on different types of KW have not yet been clarified. Here, four types of KW, cellulose-based (CL), skeleton-based (SK), protein-based (PT), and starch-based (ST) KW, were used for HTC at 180 °C, 220 °C, and 260 °C. The basic physicochemical properties and structures of hydrochars and HAP were analyzed, and the effects of different hydrochars on rice growth were characterized. HTC decreased the H/C and O/C of KW. All hydrochars were acidic (3.12 to 6.78) and the pH values increased with the HTC temperature, while high HTC temperature reduced the porosity of hydrochars. HTC promoted the enrichment of total carbon (up to 78.1 %), total nitrogen (up to 62.6 %), and total phosphorus (up to 171.6 %) in KW. More carbon (60.7-88.0 %) and nitrogen (up to 87.4 %) were present in the hydrochars than in the HAP. The relative content of C1s increased and O1s decreased in CL and ST hydrochars as the HTC temperature increased, while the opposite pattern was observed for SK and PT hydrochars. The dissolved organic matter (DOM) of different hydrochars and HAP were mainly humus-like substances. The biodegradability of the DOM in HAP was often higher than the corresponding hydrochar, and their DOM biodegradability increased with the HTC temperature. The content of heavy metals from different hydrochars did not exceed the relevant thresholds of fertilizer standards. Rice grain yield increased by 3.7-11.1 % in the hydrochar treatments without phosphate fertilizer addition compared with the control treatment. The results of this study provide new theoretical and empirical insights into the potential for HTC technology to be used for the recycling of KW and its products in the agricultural environment.