Abstract: A hydraulic conveying means, e.g. a pump, is used to create and maintain a forced through-flow of the liquid or liquid-solid mixture to be treated through a container and a forced circulation in a closed system of conduits including the container. In addition, the gas, e.g. air, is injected into the flowing liquid or liquid-solid mixture with assistance of the energy of the flowing liquid or liquid-solid mixture. The container in which mixing takes place is vertically oriented along with the delivery conduits and the gas and fine bubbled foam ejectors are concentric to the delivery conduits.