Analysis of mesophilic and thermophilic anaerobic digestion process applied to pressed biowaste during transient/stressed conditions and co-digestion with waste active sludge

The anaerobic digestion and co-digestion with waste activated sludge of liquid pressed biowaste from kerbside collection was studied both in mesophilic and thermophilic conditions. The research focused on the aspects related to a comparison between mesophilic and thermophilic conditions, also considering the processes stability during “non steady state” conditions and in specific perturbation in loading rate. The second part of the research was specifically dedicated to the study of the co-digestion with waste activated sludge in mesophilic and thermophilic conditions. Continuous operation results indicated that anaerobic digestion of biowaste juice was viable at all operating conditions tested, with the greatest specific gas production of 0.94 m3/kg TVSfed at an organic load rate of 4.7 kg TVS/mrd in thermophilic conditions while it was around 0.79 m3/kg TVSfed in mesophilic ones. No significant unbalanced and critical conditions were observed during perturbed conditions, especially when higher OLRs were applied, suggesting the possibility of either increase the OLR in existing anaerobic reactors or reducing the design volumes of new reactors. The co-digestion experimentation was designed in terms of OLR referring to the daily specific production of sludge and biowaste per inhabitant equivalent. The OLR was lowered to 1.7 kgTVS/ mrd (half due to biowaste and half due to sludge). Observed biogas yields were 0.63 m3/kg TVSfed, in thermophilic conditions, and 0.56 m3/kg TVSfed in mesophilic conditions, methane being 60% of biogas in both cases. The obtained results were generally consistent with the highest values reported in literature because of the high biodegradability of the pressed semi-liquid biowaste. Finally, the contents of heavy metals and pathogens of fed substrate and both effluent digestates, in the two experimentations, were analyzed to verify the quality of the resulting materials. These showed low levels for both the parameters thus indicating the good quality of digestate and its possible use for agronomic purposes.