Effect of varying hydrothermal temperature, time, and sludge pH on sludge solubilisation

In recent years, hydrothermal treatment has been considered as among the most promising option for sludge solubilisation and carbon recovery in terms of sludge management. In this study, the effect of different individual hydrothermal operating conditions like temperature (110–250 °C), sludge pH (6–13) and reaction time (0.5–3 h) were varied to understand their influence on sludge solubilisation. The most effective hydrothermal conditions (severity factor of 9.7) were found to be at 200 °C, sludge pH of 12 and reaction time of 1 h which solubilised about 1743 mg/g and 131 mg/g of COD and carbohydrates respectively into the aqueous phase. Also, gas chromatography- mass spectrometry (GC–MS) analysis was done that identified the organic compounds in the treated liquid phase to be mainly carboxylic acids, phenols, esters, and their derivatives. Although further studies are required to efficiently separate and recover the different organic compounds present, this work provides more insights for future valorisation of the organic rich hydrothermally treated liquid phase.