Pilot plant data and computer simulation show significant process benefits from the separation of aeration and mixing in open tank aerobic digestion. Open tank operation results in surface heat losses which cancel the heat gain of the exothermic organism decay reaction and drive digestion temperatures toward ambient atmospheric conditions. Diffused aeration produces a further evaporative heat loss through vapor enrichment of the diffused air. Mechanical mixers, however, elevate digestion temperatures through heat input from mixing and lower required air flow rates. Solids destruction rates are increased by higher digestion temperatures and increased oxygen transfer efficiency. For this reason, mechanical mixers should be included in the design of aerobic digesters and may relieve overload conditions at existing wastewater facilities. Also, the correlation between temperature history and solids destruction indicates that time-temperature data may be a valid indicator of sludge stability.