For establishing a sustainable society, it is crucial to reuse the organic waste as a material resource. Therefore, herein, we aim to pulverize the methane fermentation residue sludge by ball milling and high-speed agitation to directly utilize it as a liquid fertilizer. The solid particles in the sludge can be below 100 µm, corresponding to the nozzle diameter of the boom sprayer by both ball milling and high-speed agitation; when ball milling was carried out for at least 1 h, the ratio of coarse particles larger than 100 µm decreased by less than 10%. In addition, the phosphate-ion concentration in the sludge increased with a decrease in the particle size of solid in the sludge mainly due to increase in its specific surface area. Furthermore, we investigated the effects of various experimental conditions for ball milling on the pulverization efficiency. Results suggests that for grinding the soft solid particles in the sludge by ball milling, the volume ratio of the total medium balls and sludge, total volumes of the balls and sludge, and inner pot volume should be larger, while the ratio of the rotation speed to the critical rotation speed should be almost the same as those reported previously in the reports on grinding of inorganic particles to attain an effective pulverization.
Keywords: Methane fermentation residue sludge; ball mill; high-speed agitation; particle size; phosphate ion concentration.