Macronutrient solubilisation during hydrothermal treatment of sewage sludge

A first step in assessing the potential of the hydrothermally treated sludge as a fermentation medium or as fertiliser, among other more innovative uses, is to study in depth the compositional changes that take place during the hydrothermal treatment of secondary sewage sludge. Thus, the present study analyses, for the first time, the effects of the reaction time, temperature, pressure and atmosphere on the properties of the hydrolysates obtained and, in particular, on the fate of the four main macronutrients (C, N, P and K). Under the most aggressive operational conditions tested (200 degrees C, 50 bar, 210 min and oxidising atmosphere), 76% of total suspended solids were disintegrated and 94%, 96%, 12% and 41% of C, N, P, and K were solubilised, with the majority of these changes occurring during the first 60 min of treatment. Nevertheless, these conditions also resulted in significant mineralisation of the organic carbon (77% total Chemical Oxygen Demand reduction) and the formation of notable concentrations of ammonium (around 64 g/kg dry matter) and small amounts of nitrates and nitrites (0.14 g/kg dry matter). After long reaction times, the re-alkalinisation of the reaction medium favoured the binding of soluble P to dissolved metal ions and its subsequent precipitation. Temperature and type of atmosphere did not significantly influence the soluble K concentration, suggesting that this element mainly interacts with tightly bound extracellular polymeric substances in the raw sludge. Finally, pressure did not have significant effects on the final macronutrient composition of the treated sludge.