Evaluating the aqueous phase obtained from hydrothermal carbonization of municipal sludge as possible liquid fertilizer for plant growth: An analysis of heavy metals and their molecular composition

A significant amount of the aqueous phase (HTAP) (similar to 90% by mass) produced by hydrothermal carbonization (HTC) is considered a burden. This study investigated, for the first time, the possibility of using the HTAP obtained from HTC of municipal sludge (at 180 degrees C and 220 degrees C) as liquid fertilizer in agricultural applications. The HTAP is rich in nitrogen, and its heavy metal concentration is significantly lower than the limits imposed by agricultural application standards in various countries. The highest dry weight of pakchoi (Brassica chinensis L.) was obtained when the electrical conductivity of HTAP was 0.2 mS cm(-1). The HTAP has a relative abundance of 67% and 75% N-containing compounds at 180 degrees C and 220 degrees C, respectively. In hydroponics, lipid-like and aliphatic/protein-like structures, and those with low saturation, are preferentially consumed and converted into compounds with relatively high saturations, whereas S-containing compounds are difficult to remove. These findings provide new insights into the design and implementation of management strategies for HTAP.