Abstract: A process of treating an animal manure slurry susceptible to aerobic composting and anaerobic microbiological degradation with the production of a fuel gas, comprises: separating the slurry into a first, solids concentrated, fraction of relatively coarse particle size, and a second, solids dilute, fraction of relatively fine particle size; carrying out on the first fraction an aerobic composting reaction which is exothermic; carrying out on the second fraction an anaerobic microbiological reaction in which a fuel gas comprising methane is produced; and utilizing heat from the exothermic reaction to promote the fuel gas producing reaction.