A compost aerator for aerating a compost/liquid mixture. The aerator has a aerator pump located downstream from a liquid inlet. The aerator pump has a plurality of impellers mounted on a rotor which spins within a circular channel though which the liquid flows. The liquid is propelled through the channel by the impellers and, as the area of the channel decreases downstream from the impellers caused by the increase in diameter of the rotor, the velocity of the liquid increases. Air enters the channel upstream from the impellers and is drawn with the liquid through the channel. The diameter of the rotor then decreases significantly thereby causing the area of the liquid carrying channel to increase suddenly downstream of the widest portion of the rotor. Turbulence occurs in this area and bubbles are formed which are released from the outlet of the compost aerator into the compost/liquid mixture to perform aeration for bacterial breakdown.