Unravelling the Role of Biochemical Compounds within the Hydrothermal Liquefaction Process of Real Sludge Mixtures

This paper aims to provide a contribution to understanding the role of sludge macro-components (lipids, proteins and carbohydrates) on the yield/quality of bio-crude obtained via hydrothermal liquefaction (HTL). This was pursued by analysing the HTL process of real sludges and the mixtures thereof at different compositions, a topic that has been explored in a very limited way in the pertinent literature. The HTL experiments were run with municipal sludge, tannery sludge and a mixture of them in 25:75, 50:50 and 75:25 weight ratios in a batch reactor at 350 °C and for different residence times. The outcomes for a single sludge showed a greater bio-crude yield for the municipal one (42.5% at 10 min), which is linked to its significant carbohydrate content. The results obtained from the sludge mixtures suggested that a carbohydrate-to-protein mass ratio of 2:1 would maximise the bio-crude yield (average value of about 38%). Moreover, LC-MS and NMR analyses highlighted that the mixed sludges contributed to the formation of a higher number of compounds after the HTL treatment, with respect to the pure municipal or tannery sludge, with an increase in amine, alcohols and aromatic compounds.