PURPOSE: To utilize water hyacinth efficiently as a resource, and to improve the water quality of a fresh-water lake, etc., by squeezing the water hyacinth, carrying out the methane fermentation of the juice, and drying the solid component using the methane gas as the fuel to obtain a feed.

CONSTITUTION: Mowed water hyacinth is cut to about 5W10cm length with the crusher 6, and squeezed with a continuous press 8. The pressed cake 10 is charged to the rotary drier 13, dried to a water-content of 5W7% with hot air supplied from the burner 15 using the recovered methane gas as the fuel, and the dried product is used as a feed. The juice of water hyacinth collected in the receptor 9 is transferred to the high-efficiency methane fermentation tank 20 to effect the methane fermentation. The separated liquid obtained after the fermentation is made harmless e.g. by aerobic treatment and discharged, and the methane gas generated from the fermentation tank 20 and the precipitation tank 23 is used as a fuel.