A container for biological treatment of liquid such as water, the container comprising - a bottom, one or more side walls, an upper end and - a liquid inlet for inletting liquid to be treated into the container, - at least one vertically orientated tube having a lower end having openings and being arranged in the container, the lower end of said tube being raised above the bottom of the container, - a first gas supply arranged at the lower end of said tube for producing gas bubbles entering into the tube through the openings provided in the tube at the lower end thereof producing gas bubbles for creating a first circulating flow (F 1) of liquids and bio-film carriers through the vertically orientated tube during use, - a second gas supply arranged at the intersection between the side wall(s) of the container and the bottom of the container for creating a separate second circulating flow (F 2) of liquids and bio-film carriers to rise upwardly in the vicinity of the one or more side walls during use, whereby the container is configured so that, during use, the first circulating flow (F 1) is upwardly directed through the vertically arranged tube and the upwardly directed second circulating flow (F 2) in the vicinity of the wall of the container will, when they approach the top of the container, be diverted towards each other and the interaction between the two flows will result in the flow being turned 180° and flow towards the bottom of the container, and at the bottom of the container, the flow is split into a flow towards the wall of the container and a flow going into the vertically arranged tube; thereby two separate first and second circulating flows (F 1, F 2) are established, and - wherein the first gas supply and the second gas supply are configured to be controlled individually, the container comprising an outflow channel extending along a perimeter of an upper part of the container wall, the outflow channel having ∘ a fluid penetrable and bio-film carrier stopping wall section facing inwardly so that fluids may flow from the interior of the container, through the fluid penetrable and bio-film carrier stopping wall section and into the outflow channel, and ∘ an outlet (12, 32) for leading fluids out from the outflow channel. - a bottom, one or more side walls, an upper end and - a liquid inlet for inletting liquid to be treated into the container, - at least one vertically orientated tube having a lower end having openings and being arranged in the container, the lower end of said tube being raised above the bottom of the container, - a first gas supply arranged at the lower end of said tube for producing gas bubbles entering into the tube through the openings provided in the tube at the lower end thereof producing gas bubbles for creating a first circulating flow (F 1) of liquids and bio-film carriers through the vertically orientated tube during use, - a second gas supply arranged at the intersection between the side wall(s) of the container and the bottom of the container for creating a separate second circulating flow (F 2) of liquids and bio-film carriers to rise upwardly in the vicinity of the one or more side walls during use, - wherein the first gas supply and the second gas supply are configured to be controlled individually, the container comprising an outflow channel extending along a perimeter of an upper part of the container wall, the outflow channel having ∘ a fluid penetrable and bio-film carrier stopping wall section facing inwardly so that fluids may flow from the interior of the container, through the fluid penetrable and bio-film carrier stopping wall section and into the outflow channel, and ∘ an outlet (12, 32) for leading fluids out from the outflow channel. ∘ a fluid penetrable and bio-film carrier stopping wall section facing inwardly so that fluids may flow from the interior of the container, through the fluid penetrable and bio-film carrier stopping wall section and into the outflow channel, and ∘ an outlet (12, 32) for leading fluids out from the outflow channel.