Apparatus for manufacture of methane gas from sewage sludge or other organic waste comprises a digester tank system, a solar heating unit and conduit means to use hot air from the solar heater to heat the digester tanks. The new methane gas manufacture methods provided by the apparatus operate with water slurries of organic waste at a temperature of about 100.degree. - 140.degree. F to produce about 11 cu. ft. of gas per pound of organic waste and a dewatered residue that may be used as fertilizer.