An apparatus and method for mixing gas and liquid comprising a pipe having an enclosure positioned in-line with said pipe, wherein a sealed space is defined, at least one blower, said blower regulates the barometric pressure in said sealed space, wherein intermeshed rotating sets of discs operate on parallel shafts driven by variable speed drives, and strakes are radially mounted on the discs to carry liquid up into a mixing area and to carry air and liquid down into a mixing area resulting in a shear force that drives air into the oxygen depleted liquid. In the sealed space the barometric pressure is raised by a blower, in order to pop foam bubbles and allow for optimum mixing of air into the oxygen depleted liquid and to regulate the waterline within the sealed space, thereby preventing the escape of foam, noise and odorous gases into the local environment.