Comparison of both mesophilic (35ยฐC) and thermophilic (55ยฐC) anaerobic digestions of the organic fraction of municipal refuses in pilot digesters designed to process in a semisolid phase at total solids concentrations of ca. 25% shows that the average gas production is 20-25% higher in thermophilic conditions than in mesophilic conditions even for a retention time of 10 days. These results and the data recorded during long periods of experimentation indicate that the process allows to increase the net energy production and to improve the economical balance of an industrial plant. 5 m'/m' day has been recorded with 60% CH, content and 50% volatile solids (VS) reduction, when a loading rate of 12 kg VS/m3 day and a hydraulic retention time (HRT) of 15 days were applied.
The development of this "semisolid" process allows us to eliminate the limiting factors due to dilution, water handling, and sludge processing. With regards to HRT, most authors concluded that thermophilic digestion (50-65ยฐC) makes shorter HRT and higher loading rates possible. Greater conversion efficiencies could be obtained: Cooney2 obtained an average gas production which was 50% higher under thermophilic conditions compared to