A multi-stage aeration apparatus includes at least two vertically oriented aeration modules. Each aeration module defines a vertically elongated aeration chamber having a top inlet and a bottom outlet. The bottom outlet of each of the aeration modules feeds a fluid stream comprising liquid and gas into the top inlet of an underlying one of the aeration modules. Each aeration module has an aeration head connected to the top inlet, such that the fluid stream passing through the top inlet into the aeration chamber must pass through the aeration head, the aeration head aerating the liquid with the gas in the fluid stream.