NOVELTY - Low-quality water or underground water are processed by injection of bubbles of air, oxygen, oxygen mixed solution, air mixed solution, water containing dissolved ozone, ozone mixed solution, air compressed solution, ozone compressed water, oxygen dissolved solution, oxygen pressurized solution or ozone, and recycled to obtain solid fuel or biofuel. The air mixed solution is obtained by dissolving and mixing air in water. The air compressed solution is obtained by mixing water and air, and compressing mixture.    USE - Flotation and recycling of lightweight material, garbage, and combustible and organic materials in underground water used for producing biodiesel, bioalcohol, solid fuel, biofuel, biogas and methane gas.    ADVANTAGE - The method efficiently and economically removes lightweight material, garbage, and combustible and organic materials in underground water.    DETAILED DESCRIPTION - Low-quality water or underground water comprising precipitate, precipitate soil, deposit, deposit soil, microorganism, nutrient, surfactant, supercritical material and dirt are processed by injection of bubbles of air, oxygen, oxygen mixed solution, air mixed solution, water containing dissolved ozone, ozone mixed solution, air compressed solution, ozone compressed water, oxygen dissolved solution, oxygen pressurized solution or ozone, and recycled to obtain solid fuel or biofuel. The air mixed solution is obtained by dissolving and mixing air in water. The air compressed solution is obtained by mixing water and air, and compressing mixture. The oxygen dissolved solution is obtained by dissolving oxygen in water. The oxygen mixed solution is obtained by mixing oxygen with water. The oxygen pressurized solution is obtained by mixing and pressurizing oxygen in water. The water containing dissolved ozone is obtained by dissolving ozone in water. The nutrient is material comprising microorganism with nutritive value. The surfactant is material which absorbs and decreases the surface tension. The supercritical material is a material which is classified to liquid and gas, when reaching high pressure and high temperature. The supercritical material comprises discrete material and constituent particles containing general-sized, microscopic or nano-sized compound. The general size is more than the micrometer and more than the nanometer, and the microscopic size is less than the micrometer. The nano size is less than the nanometer. The content of each blending element is 1-99 wt.%.