The effect of varying total and volatile solids concentrations and organic loading on the performance of a temperature phased anaerobic digestion (TPAD) system treating a mixture of primary and waste activated sludge was evaluated. An optimum VS destruction of 61.5% occurred at a feed concentration of 4.8% (corresponding to a volatile solids concentration of 3.8%) in the system operated at a total detention time of 20 d. At a total solids concentration of 7.9% (VS concentration of 5.8%), the VS destruction efficiency dropped to 52.5%. At all conditions (4.4 – 7.9% total solids) the TPAD system was able to meet the requirements for Class A biosolids, including those for fecal coliform and volatile solids destruction. The effluent fecal coliform concentration never exceeded 628 MPN/g TS. Thermophilic (55oC) biomass activity tests were run on both the thermophilic sludge and the mesophilic sludge. Biomass from the thermophilic digester showed much greater activity at 55oC than at 35oC, however, significant activity was still present when the test was run at 35oC. Activity tests completed on samples from the mesophilic digester also had high activities at 55oC, sometimes equal to the activity of the thermophilic biomass. These results suggest that the bacterial consortia in the TPAD system may be temperature tolerant and not necessarily two distinct communities with two distinct temperature regimes as had been previously assumed.