A continuous flow multistage aerobic wastewater sludge treatment system
and process comprising at least two stages connected in series, each
stage comprising one or more covered tanks, tank segments or
compartments, each having an enclosed separate gas headspace, wherein the
gas headspaces of each stage are connected in a specified sequence that
may be identical to or different from the staging sequence followed by
the sludge liquid. An oxygen containing gas stream, such as air or an
oxygen enriched gas stream, is supplied to the headspace of each stage,
and a means for aerating the sludge in one or more of the tanks, tank
segments or compartments is provided to enhance the transfer of oxygen
from the gas headspace into the sludge liquid for use by bacteria in
digestion of the sludge. At least one stage of the sludge digestion
system is operated at a thermophilic temperature above 50.degree. C., and
the flow of oxygen containing gas through the system can be co-current,
countercurrent, or a mixture of both with respect to the flow of the
liquid. Embodiments of the invention include use of the aerobic digestion
system prior to anaerobic digestion.