Current processes are reviewed and a new technique is described which maintains economically a constant temperature of 40/sup 0/C in the tanks by complete isolation in winter as in summer and periodic reheating of the mass. The tanks were buried underground and had double metal walls with low density cellular concrete between them. The covers were of thick cork, permanently fixed, and coated with an impermeable substance. Reheating was necessary only once during the fermentation, whereas with tanks above ground it had to be carried out more often and more vigorously. Straw was the raw material and the products were highly profitable quantities of methane and artificial manure.