Abstract: Coal pulverized by a mill or crusher is used as a floating agent which is charged into a floating machine, together with wastewater, for example, from a wastewater treatment facility or homes which produce organic substances such as human wastes. The pulverized coal and wastewater are mixed and uniformly stirred by a stirrer in the form of an impeller or a cyclone separator while being aerated. The produced floating floc is discharged from an upper portion of a first floating machine, whereas sedimented materials are discharged from a lower portion of the first floating machine. The same process is repeated in other floating machines connected in series to the above-described first floating machine. The collected floating flocs are dewatered to produce solid cakes which are used as a fuel. The water which has been extracted to form the solid cake, is purified and discharged.