he specifications of the Safe Sludge Matrix mean that there is a need to examine treatment processes which achieve a degree of digestion. There are a number of processes which operate in the thermophilic range. This Study examines one such option, an autothermal thermophilic aerobic digester (ATAD) which uses a high aspect ratio design and a hyperbolic stirrer. Mixing times were found to range from 4 to 10 minutes and oxygenation capacities from 10 to 30 kg h(-1). Using a hydraulic retention time of 7 days, autothermal conditions did occur with a specific heat yield of 15.68 MJ kg(-1) oxygen. A comparison of a range of air:liquid ratios (v/v) showed that a ratio of 0.5:1 Would be recommended, This gave a reduction in volatile solids of 35%.