Acid hydrothermal treatment of biowaste is a beneficial approach for producing hydrochar with high hydrothermal humic acid (HHA) selectivity, along with a by-product hydrothermal solution containing saccharides, furans, and aromatic compounds. Conversely, alkaline hydrothermal conditions are suitable for humification to form HHA. This study explored the recycling of hydrothermal solution as a solvent for hydrochar production, aiming to reduce salt content in wastewater and the need for acid/alkali inputs. The acidity of the recycled hydrothermal solution significantly influenced the quality of the prepared hydrochar in terms of alkaline hydrothermal humification. Insufficient solution acidity resulted in a decrease in hydrochar-derived HHA yield (65.0 wt% to 20.2 wt%). Maintaining sufficient acidity (pH 0) in the recycled hydrothermal solution not only improved the hydrochar yield (29.6 wt% to 33.1 wt%) via polymerization or condensation reactions but also maintained ab high level of hydrochar-derived HHA yield (68.2 wt% to 71.2 wt%). This can be attributed to adequate hydrolysis of biowaste and polymerization of soluble chemicals into the hydrochar. Furthermore, the total yields of hydrochar-derived and solution-derived humic substances were higher compared to values obtained without hydrothermal solution recycling. This study presents an innovative hydrothermal process for HHA production, offering cost savings and environmental benefits.

生物废物的酸水热处理是生产具有高热液腐植酸（HHA）选择性的水热炭以及含有糖类、呋喃和芳香族化合物的副产物热液溶液的有益方法。相反，碱性水热条件适合腐殖化形成HHA。本研究探索了水热溶液作为水热炭生产溶剂的回收利用，旨在减少废水中的盐含量和对酸/碱投入的需求。回收水热溶液的酸度显着影响所制备的水热炭的碱性水热腐殖化质量。溶液酸度不足导致水炭衍生的 HHA 产量下降（65.0 wt% 至 20.2 wt%）。在循环水热溶液中保持足够的酸度（pH 0）不仅通过聚合或缩合反应提高了氢炭产率（29.6 wt%至33.1 wt%），而且还保持了高水平的氢炭衍生的HHA产率（68.2 wt%至71.2重量％）。这可以归因于生物废物的充分水解和可溶性化学物质聚合成水炭。此外，与没有水热溶液循环获得的值相比，水炭衍生的和溶液衍生的腐殖质的总产率更高。这项研究提出了一种用于 HHA 生产的创新水热工艺，可节省成本并带来环境效益。2重量％至71.2重量％)。这可以归因于生物废物的充分水解和可溶性化学物质聚合成水炭。此外，与没有水热溶液循环获得的值相比，水炭衍生的和溶液衍生的腐殖质的总产率更高。这项研究提出了一种用于 HHA 生产的创新水热工艺，可节省成本并带来环境效益。2重量％至71.2重量％)。这可以归因于生物废物的充分水解和可溶性化学物质聚合成水炭。此外，与没有水热溶液循环获得的值相比，水炭衍生的和溶液衍生的腐殖质的总产率更高。这项研究提出了一种用于 HHA 生产的创新水热工艺，可节省成本并带来环境效益。