Assessment of pre-treatments to mesophilic-thermophilic anaerobic digestion

The legislative standards in many European countries and in the USA on sludge use in agriculture require effective removal of pathogens during stabilization processes. Great attention is also paid nowadays to organic micropollutants like PCBs, surfactants (LAS), hydrocarbons, polynuclear aromatic hydrocarbons (PAH), halorganics (AOX) and pharmaceuticals whose removal is not effective in conventional sludge processing. Anaerobic thermophilic digestion was considered suitable for producing class A biosolids since 30 years (Rimkus et al. 1982) considering that more effective removal of pathogens seems to be attained in thermophilic digestion in comparison with mesophilic conventional.A research project was funded in 2011 under FP7 by the European Commission with the acronym ROUTES (Novel processing routes for effective sewage sludge management) to assess whether innovative techniques might be useful for producing a sludge of very good quality introducing also in Europe the concept of biosolids instead of sludge (undesirable waste to be disposed as soon as possible).In this paper the preliminary results of this research project are presented together with a benchmarking of two innovative techniques, in term of the energy balance, in comparison with a conventional one. This study evidenced that the energy balance (energy production-energy requirement) is hardly achieved by a conventional scheme while separate processing of primary and secondary sludge might increase such chances. Secondary sludge processing by thermal disintegration + thermophilic digestion seems better than sonication, and mesophilic + thermophilic digestion. However, in the former case an additional heat source would be required.