An unusual mode of operation of internal-loop airlift reactors is described as being of potential use in extending the range of practicable operating volumes or turndown ratios in wastewater treatment applications. Unlike normal operation, the new mode employs an inital static liquid level that is lower than the upper edge of the draft-tube or the splitting baffle. Hydrodynamics of the operation are analyzed using fundamental principles and an equation is developed for predicting the minimum required gas flow for attaining recirculation of the liquid. The equation is proven experimentally using data for air-water in split-cylinder reactors (aspect ratio = 7; volume = 0.08 m3; riser-to-downcomer cross-sectional area ratio = 2.44; − 0.03 <hc≤ 0.12, wherehcis the dimensionless clearance between the upper edge of the baffle and the static height of gas-free liquid), Effects of the clearance hc on gas holdup in the riser and downcomer, mixing behavior and liquid circulation rate are presented.