Hydrochars derived from sewage sludge: effects of pre-treatment with water on char properties, phytotoxicity and chemical structure

Large amounts of labile compounds are adsorbed to the surface of chars produced by hydrothermal carbonization (HTC). The aim of this study was to characterize the core and adsorbed fractions of hydrochars and to gain knowledge about the possibility to remove phytotoxic compounds by washings with water. Chars were produced by HTC of sewage sludge at different temperatures (180 - 200 degrees C) and over different periods of time (4 - 8h). For comparison one pyrolysis char produced by thermocatalytic low temperature conversion (LTC) at 400 degrees C for 1h was included in the study. The chars and their feedstocks were treated varying the duration (1 x 15, 1x30 and 1x60min) and number (2 x 60 and 3x60min) of washings. Physicochemical properties, including the molecular structure of the test materials, and their effects on germination and plant growth were analysed. Element concentrations and phytotoxic effects were reduced and the number of washings had a stronger effect than their length of time. Intensive washings with water reduced the hydrochars' portion of biodegradable compounds significantly. However, also plant available nutrients were lost by washing with water, decreasing the value of hydrochars as a soil amendment.