To recover sludge as fertilizer and to utilize steam as a heating source by mixing sludge and high temperature thermophilic bacteria in an enclosed fermentation unit tank to generate steam through fermentation of sludge thereby purifying the sludge through effective utilization of microorganism.

A fermentation unit tank comprises an enclosed vessel 10 for isolating the inside from outer environment during fermentation. Sludge is thrown into the vessel 10 by opening a cover 11 at the ceiling part thereof. An air supply pipe 13 provided with small holes at a constant interval is arranged in the vessel to scrawl on the bottom thereof and air is blown from the holes into the sludge to accelerate fermentation of high temperature thermophilic bacteria. When the sludge is fermented by high temperature thermophilic bacteria, temperature and pressure in the vessel are increased to vaporize moisture in the sludge thus filling the vessel with steam. Steam is exhausted through an exhaust pipe 15 to the outside and fed to the radiator of a heater.