Protein recovery from solubilized sludge by hydrothermal treatments.

New alternatives for sludge management have been developed in recent years, with hydrothermal treatments being one of the most attractive ones. Even though many studies have been made on the application of hydrothermal treatments as pre-treatment or end-line technologies for sludge stabilisation and/or minimization, there is a lack of knowledge about the products generated during the process and its characteristics. This information is a crucial step for the assessment of the recovery of valuable products of the sludge, mainly proteins, humic acids and carbohydrates, which can considerably improve the economic balance of the hydrothermal treatment. This work assesses, for the first time, the potential of hydrothermally hydrolysed sludge as renewable source for proteins recovery. For this purpose, firstly, the concentrations and properties of the main soluble biopolymers generated during the hydrothermal treatment, either in presence (wet oxidation, WO) or absence (thermal hydrolysis, TH) of oxygen, were measured, determining the reaction time necessary for a maximum solubilisation. Peak concentrations of 7.7g/l (0.291g/gVSSo) of proteins for WO and 7.2g/l (0.272g/gVSSo) for TH, were achieved at 87min of experiment. Afterwards, different separation methods, usually applied at industrial scale, were assessed for the separation of protein from the hydrolysed sludge, in terms of protein recovery and selectivity. Ammonium sulphate addition was found to be the best separation method, achieving 87% and 86% of protein recovery for TH and WO samples respectively, and the highest selectivity. Although further studies are required in order to achieve complete protein purification, a new perspective in sludge management is now open, by recovering valuable compounds.