Co-hydrothermal carbonization of biowaste and sewage sludge: hydrochar characterization and potential application in agriculture.

This study explores hydrothermal carbonization (HTC) of garden and park waste, sewage sludge and food waste as a sustainable alternative for waste management, focusing on the enhancement of hydrochar properties and phytotoxicity reduction through post-treatments such as washing and aging. Additionally, co-HTC of lignocellulosic waste mixed with the other two raw materials using several mixing ratios (1:3, 1:1, 3:1 wt:wt in dry basis) were carried out to assess potential beneficial effects on hydrochars characteristics. HTC reactions were done at 180 °C for 1 h. Pyrolysis of individual feedstocks, at 650 °C for 1 h, was also performed to compare biochar with hydrochars and co-hydrochars. Ultimate analysis reported high ash content in sewage sludge-derived chars, along with lower volatile matter and higher ash content in biochars respect to hydrochars, regardless of feedstock or mixing ratio in co-HTC. Hydrochars exhibited acidic pH, and those derived from sewage sludge and, especially, food waste showed electrical conductivity values remarkably higher than those from garden and park waste. Biochars displayed fewer surface functional groups and higher mineral concentration than hydrochars. Regarding leachates, those from food waste-derived hydrochars released higher concentrations of organic compounds, including potentially phytotoxic molecules, compared to those from lignocellulosic waste or sewage sludge. Nevertheless, phytotoxicity assays revealed that fresh hydrochars from sewage sludge and food waste inhibited tomato seed germination, whereas hydrochars from garden and park waste were not phytotoxic. Overall, co-HTC offers a promising approach for integrated waste valorization, enabling the production of hydrochars suitable for use as soil amendments.