How feedstock characteristics affect performance and stability of anaerobic digestion process: the complex role of pre-treatments

Sewage sludge processing is a key issue in wastewater treatment plants (WWTPs) due to the inefficacy of conventional treatments to produce high quality sludge to be safely used or disposed. In the framework of circular economy, anaerobic digestion has been recognized to be the most appropriate stabilization technology to approach the problem of sludge recycle, also because of energy recovery through methane production. Sewage sludge from urban wastewater treatment plants as well as biowaste produced in cities (such as food waste) may be suitable to be processed into biogas for energy recovery or to be exploited as feedstock for valuable and precious compounds. Chemico-physical characteristics of the feedstock largely influence the process stability and lead to considerably different results in biogas production and composition. Taking this into account, an accurate characterization seems to be an essential step in AD also to determine which strategy is preferred, and to help understanding, which one is optimal in any single case. In fact, the role of pre-treatments is to modify the structural and compositional properties in order to make the substrate more accessible to microbial degradation. At the same time, the composition of the feedstock for the co-digestion needs a correct optimization and balance to assess effective feasibility and the best operation parameters. To this aim, this study compares the results of long term semi-continuous tests of mesophilic anaerobic digestion of sludge, raw or sonicated, alone or mixed with food waste, and the impact of the organic load