Evaluation of the removal of pathogens included in the Proposal for a European Directive on spreading of sludge on land during autothermal thermophilic aerobic digestion (ATAD)

The European Union is promoting a new legislation through the “Proposal for a Directive of the European Parliament and of the Council on spreading of sludge on land”, with special interest on human pathogens. Autothermal thermophilic aerobic digestion (ATAD) is described in this Proposal as an advanced treatment capable of producing sanitized sludge. A one-stage ATAD digester with effective volume of 15-m3 was started up and studied over 19 months for its ability to stabilize and sanitize municipal sludge. Salmonella spp., Escherichia coli, and Clostridium perfringens spores were cultivated and pathogenity genes invA and cpa PCR-amplified. Volatile solids removal was 38.0% and the pathogen content significantly decreased by completely eliminating Salmonella spp. and E. coli but not Clostridium perfringens spores (9.6 × 103 spores mL−1). To completely achieve the disinfection of the sludge, a mesophilic stage was introduced after the ATAD treatment. Denaturing gradient gel electrophoresis analysis showed differences in the structures of the bacterial and fungal communities between thermophilic, mesophilic and raw sludge. The results demonstrated that the ATAD technology had the capability to produce sludge suitable for agricultural application when the operational parameters were stable and a mesophilic stage was introduced.

► Pilot-scale autothermal thermophilic aerobic digester was studied over 19 months.
► Removal of pathogens included in the future European legislation was evaluated.
► Escherichia coli and Salmonella spp. were not detected in ATAD-digested sludge.
► Clostridium perfringens spores were eradicated following a mesophilic stage.
► Qualified organic amendment was obtained after mesophilic–thermophilic treatment.