Measurement of Significant VSS Destruction in Aerobic Digestion

A series of aerobic batch tests are performed on digested biosolids and waste activated sludge from Cairns and Newcastle, Australia. The objective is to determine the potential for compliance with the stability criterion outlined in US EPA Part 503 regulations and the New South Wales Biosolids Guidelines which state that no further than 15% VS reduction should occur in a stabilized biosolids product over 30 days of batch digestion at 20°C. The tests found that actual VSS reduction in an aerobic digester was far greater than predicted (more than 27% as opposed to 17%) and that further VSS reduction of about 40-50% was achieved in this biosolids product in a batch digester over 25 days. The results suggest that endogenous residue and unbiodegradable VSS may be digested aerobically if sufficient time is provided for acclimatization and that such time is available in the 30 day batch test nominated in the above biosolids regulations and guidelines, bringing into question the value of this test.