Pre-treatments and anaerobic hydrolysis as strategical key steps for resource recovery from sludge: The role of disintegration degree in metals leaching

Future sustainable technological solutions need to balance the development of sewage sludge as resource against the paramount necessity to protect human health and environment. Nutrients, high value metals, embedded energy, and the avoidance of sludge handling costs are important examples of recoverable assets. Aim of this study was to assess the feasibility of sludge pre-treatments as ultrasound and thermal hydrolysis to promote the release of metals due to organics disintegration, and to investigate the efficiency of the anaerobic process to leach out metals from municipal sludge through an enhanced hydrolysis/acidification step. A preliminary deep analysis of elements concentration (Al, As, Ca, Cd, Co, Cr, Cu, Fe, Ga, Mg, Mn, Na, Ni, Pb, Se, V and Zn) in solid residue and water phase of waste activated sludge and anaerobic digestate sampled at full scale wastewater treatment plant (WWTP) allowed to calculate partitioning coefficients and equilibrium concentrations, in order to predict metal mobility and bioavailability. The gradual sludge disintegration and organics hydrolysis significantly contribute to the leaching of toxic elements as As, Ga, Se and Zn, increasing their removal potential. A new anaerobic bioleaching approach will be presented, aiming to develop a sustainable platform to recover metals, together with methane and a clean recyclable digestate.