Disclosed herein is an anaerobic digester configured to include input reactors into which livestock wastewater or food waste (hereinafter, ‘inflow’) is introduced; regions of an anaerobic digestion reactor designed for the inflow passing through an input reactor to perform methane fermentation to produce and transfer biogas simultaneously into the next region; an inlet pipe into which sludge liquid is drawn; an inlet pipe into which activated liquid is drawn; a biogas capturing device; a first heat exchange tube to allow the sludge liquid to perform heat exchange with a new inflow; regions of a subsequent treatment reactor to allow the sludge liquid whose heat exchange is completed to be introduced and to treat gas odor components generated from the sludge liquid; and a liquid-composting reactor in which the emitted sludge whose odor components have been removed is stored.