A perforated tubular ring positioned at the bottom of a vessel containing liquid such as a bioreactor through which air or oxygen is discharged creating a swarm of bubbles that rise up through the liquid medium promoting oxygen transfer to the liquid. The size of the bubbles and their velocity determines the rate of oxygen transfer. Their size can be controlled by the number of holes, rate of flow of air or oxygen, and the location of the sparger to a rotating impeller that can disperse the bubbles.