A bioreactor configured to contain an aqueous suspension of biomass and media is provided. The invention includes a lift tube disposed within the vessel, the lift tube having an inlet disposed at the bottom region of the vessel and an outlet disposed at the top region of the vessel. Disposed at the bottom of the lift tube is a gas inlet positioned to feed gas into the lift tube at a location proximal to the inlet of the lift tube. An adjustable weir is disposed proximal to the outlet of the lift tube, the weir being adjustable between a lower position and a higher position. Biomass is removed from the weir and the weir height is adjustable to control the removal of biomass from the vessel. Also included as a part of the invention is a method for controlling the removal of biomass from a bioreactor as described.