Anaerobic digestion process

Claim 1

A process for the treatment of organic wastes, comprising the steps: (a) anaerobically digesting organic wastes at a temperature of 30°-75° C. and a pH of 6.5-7.5 to obtain a digested slurry and fermentation gas containing methane or hydrogen, carbon dioxide and hydrogen sulfide; (b) subjecting the digested slurry obtained in step (a) to a solid/liquid separation treatment to obtain separated digested water and digested sludge; (c) adding calcium hydroxide to a part of the separated digested water obtained in the step (b) until the water becomes alkaline; (d) contacting the alkaline digested water obtained in step (c) with the fermentation gas obtained in step (a) to absorb carbon dioxide and hydrogen sulfide in the digested water to produce a purified methane gas and thereby increase the concentration of hydrogen or methane in the gas; (e) supplying a part of the digested water obtained in step (d) to a culturing system wherein photo-synthetic algae is cultured under illumination, thereby obtaining oxygen and algae culture, said culturing system being conducted at a pH of 7.5-9.5 and at a temperature of 30°-40° C.; (f) subjecting the algae culture liquid obtained in step (e) to a solid/liquid separation treatment for obtaining algae and algae culture liquid supernatant; (g) recycling and adding part of the algae culture liquid supernatant to the alkaline digested water of step (d) for absorbing carbon dioxide and hydrogen sulfide from the fermentation gas; (h) subjecting additional alkaline algae culture liquid supernatant obtained in step (f) to an aerobic microbial treatment to reduce the BOD; and (i) recycling algae obtained in step (f) into step (a) as feedstock for fermentation.