Aerobic thermophilic pretreatment of mixed sludge for pathogen reduction and Nocardia control

Abstract

Pilot scale experiments have been conducted to investigate the use of aerobic thermophilic pretreatment (ATP) of mixed (waste activated and primary) sludge before mesophilic anaerobic digestion to meet Class A sludge pathogen control requirements. The ability of ATP to control Nocardia present in the waste activated sludge, and hence prevent foaming in the subsequent anaerobic digestion, was also investigated. Aerobic thermophilic pretreatment of mixed sludge before anaerobic digestion was able to produce sludge that can meet the following Class A pathogen reduction requirements: fecal coliform density <1 000 MPN/g total solids or Salmonella sp. density <3 MPN/4 g total solids; time and temperature requirements (1.8 hr at 63°C); enteric virus density <1 PFU/4 g total solids; and vector attraction requirement (>38% volatile solids reduction). ATP was able to control Nocardia to below detectable levels, whereas the Nocardia levels in the control anaerobic digester were ≥105 intersections/g VSS when both were fed mixed sludge containing Nocardia filament counts in excess of 105 intersections/g VSS. The ATP system was also able to produce higher volatile solids (VS) reduction, higher methane content in sludge gas, and lower sludge supernatant COD when compared with those achieved in the control anaerobic digester.