A proposal for the classification of sludge products throughout hydrothermal treatment

As the wide variety of compounds that are solubilised and generated during hydrothermal treatments of sewage sludge cannot be defined individually, this study proposes a classification based on chemical characteristics and biodegradability. Moreover, it has deepened understanding of the role of oxygen in the reaction mechanisms and the evolution of chemical oxygen demand (COD) fractions and their components during hydrothermal treatments (180 °C, 80 bar). This information is necessary before separating specific compounds or using the hydrolysates directly as fertiliser or fermentation media. Results revealed the existence of an initial stage of sludge solubilisation (30 – 60 min), mainly produced by hydrolysis reactions and without mineralisation, followed by a second stage where oxygen promoted the conversion of solubilised compounds, mineralisation and the minimisation of the solid fraction. Total COD and total suspended solids (TSS) were reduced by 48% and 75% after 210 min of wet oxidation, compared to 28% and 50% after thermal hydrolysis. Oxygen led to a final liquid hydrolysate with fewer easily assimilable compounds (38% of the initial total COD converted into readily biodegradable), inorganic elements (17 g/kg TSSo) and refractory, inhibitory and less assimilable compounds (1.1 g O2/L of slowly and non-biodegradable COD) compared to that obtained in its absence (61%, 24 g/kg TSSo and 2.3 g O2/L, respectively). Oxidation mechanisms favoured the conversion of non-volatile hydroxy acids (NVHA) and intermediate compounds to volatile fatty acids (VFA), with VFA concentration being almost 3 times higher under an oxidising atmosphere, but with NHVFA 57% lower.