Disclosed are a wastewater treatment plant and a wastewater treatment method. The wastewater treatment plant includes: a reactor including a gas outlet, a treated water outlet, and an inlet through which wastewater and gas are supplied, through which the wastewater is introduced, to perform aeration and denitrification; a sludge separation means including plural reaction unit bodies stacked inside the reactor to divide the interior of the reactor into upper and lower sides and separating sludge in a aeration process by forming a gas hold-up space for collecting the gas rising from a lower portion of the reactor; and a aeration unit introducing gas into the reactor.