Two-Stage Thermophilic Anaerobic–Aerobic Digestion of Waste-Activated Sludge

High-rate two-stage thermophilic anaerobic–aerobic sludge digestion was studied and compared to the recent published research. The studied process is, in fact, a combination of two single-stage anaerobic and aerobic processes in the most favorable way possible. The goal was to combine the benefit of anaerobic digestion—biogas production, with the benefits of aerobic digestion—better COD and VSS removal. Several combinations were made, from 3+3 days (anaerobic 1 aerobic), 3+6 days, 5+5 days, 3+12 days to 10+10 days HRT. The best process was 3+12 days, which showed a VSS removal of 61.8% and COD removal of 57.4% in just 15 days HRT. Comparison of the processes with recently published research showed that the 3+12 process is better than most published two-stage processes. All of the two-stage processes in this paper were performed completely in the thermophilic temperature range in contrast to published two-stage processes where the second stage was always in the mesophilic temperature range. The reason for using a completely thermophilic process is in the discovery made in this study, that any further stage after a first thermophilic stage requires only 2–3% more heat for operation than if the next stage would be mesophilic. For this little additional input, the processes are worth operating in the thermophilic temperature range completely.