Two stage thermophilic anaerobic-aerobic mineralization-stabilization of excess activated sludge.

At the National Institute of Chemistry, Ljubljana, Slovenia, new procedure for excess activated sludge was developed and patented. The main principle of the sludge mineralization is two-step process--the first is anaerobic, and the second step is aerobic. In the first step maximal biogas production is acquired in the second step at smaller VSS the optimal VSS and COD is achieved. Anaerobic, aerobic, and combined sludge mineralization was studied. The next combinations of different successive anaerobic and aerobic mineralization have been studied: 3 + 3 (3 days of anaerobic + 3 days of aerobic digestion), 3 + 6, 5 + 5, 3 + 12, and 10 + 10 days, respectively. The best combination considering biogas production, VSS and COD reduction have been gained for the combination of 3 + 6 days, and 3 + 12 days. At 3 + 6 days digestion about 49% of VSS, and about 51% of COD reduction was gained, at 3 + 12 days about 62% of VSS, and about 57% of COD reduction was achieved. For the optimal system operation, economics, biogas production, and necessary VSS and COD reduction have to be considered.