A number of advantages are claimed for dual digestion as a system for sewage sludge pasteurisation and stabilisation. In this paper, the first of a series of 4, an overview of a 4-year full-scale (45 m3 aerobic reactor and 500 m3 anaerobic digester) dual digestion research project is presented. The project was undertaken at Milnerton (Cape, South Africa) and its principal objective was to evaluate the claims made for the dual digestion system. In this paper the claims are stated; the layout and operation of the dual digestion system described; the main results obtained briefly discussed; and the claims evaluated with the aid of the results. It is concluded that most of the claims are valid, inter alia; the mesophilic anaerobic digester performance is not adversely affected by the first stage autothermal thermophilic aerobic reactor compared to conventional digestion. The 3 sequel papers focus attention on the aerobic reactor; its operation, performance, control, design and simulation of the temperature profile for both air and pure oxygen systems.