Autothermal thermophilic aerobic digestion (ATAD) is a microbial fermentation process characterized as a tertiary treatment of waste material carried out in jacketed reactors. Heat is generated which selects a thermoduric microbial population. The process results in a stabilised, pasteurised sludge suitable for land application as a fertiliser. The microbial population biodegrades sludge contents, are unique in terms of diversity and have biotechnological potential as enzymes and proteins associated with the microbial population are thermostable.