Fate of Na & Cl in kitchen waste during hydrothermal carbonization

Hydrothermal carbonization (HTC) is a sustainable approach to recover nutrients from kitchen waste (KW) to realize the applications of hydrochar as fuel, soil amendment and so on. However, the salt content of KW can be as high as 5 wt%, which brings potential risks for product utilization. Thus, the migration path of endogenous (from ingredients) and exogenous (from condiments) Na & Cl in KW during HTC process was investigated. Results showed that the endogenous and total Na element in KW was 0.03–2.98 wt% and 2.97–4.82 wt%, respectively. The main form of sodium was water-soluble (48–82%). The KW feedstocks and cooking methods had little effect on the migration of Na & Cl. During HTC treatment, all sodium element was transformed into water-soluble state, subsequently about 97% was dissolved into the liquid phase. The content of Na element in hydrochar was only below 0.05 wt%, so its negative effect can be negligible. As for the fate of chlorine, the endogenous and total Cl element in KW was 0.31–1.62 wt% and 1.82–3.19 wt%, respectively, and the main form was inorganic Cl accounting for 83–91%. During HTC treatment, about 40–75% of endogenous organochlorine was converted into inorganic Cl by breaking C-Cl bonds, and all inorganic Cl was leached into the liquid phase. In this pattern, partial organochlorine remained in hydrochar with a content of 0.21–0.32 wt%, which need to be further reduced. Methods of adding Na2CO3 solution or HTC-pyrolysis were effective, and approximately 90% of the organic Cl can be removed. Based on the above findings, this study can provide theoretical guidance for the value-added conversion of solid waste with high moisture and high salinity.