Aerobic digestion of primary and secondary sludges was studied in airlift bioreactors at mesophilic and thermophilic temperatures. The experimental studies were conducted with a laboratory U-shaped airlift reactor (operating volume = 23 1) and in a pilot U-shaped airlift reactor with an operating volume of 1150 1. In the laboratory reactor, at a lower temperature (6°C) and with a concentrated feed (3 to 4% solids) consisting of primary and secondary municipal sludge, a 30% VSS reduction was achieved with a hydraulic retention time of 2.5 days. A VSS loading rate of 8.2 kg VSS per m3 per day was achieved. This loading is comparable to that obtained in a pure-oxygen sparged mixed reactor.
In the pilot plant reactor at mesophilic temperatures (31 to 33°C), a VSS loading rate of 7.9 kg VSS per m3 per day and a VSS reduction of 33% were achieved with a hydraulic retention time of four days.