A system for biological treatment of liquid such as water, the system comprising a closed container so as to prevent gas produced in the container from leaking to the surroundings, the container comprising - a bottom, one or more side walls, and a top, - a first inlet and a first outlet arranged in the container, - at least one vertically orientated tube arranged in the container, the lower end of said tube being raised above the bottom of the container, - means comprising a first gas supply arranged at the lower end of said tube and producing gas bubbles entering into the tube through openings provided in the tube at the lower end thereof, the gas bubbles raising upwardly inside the tube for creating a first circulating flow (F 1) of liquid and microfilm carriers rising upwardly through the vertically orientated tube, and - a second gas supply arranged at the intersection between the one or more side walls of the container and the bottom of the container for introduction of gas bubbles and thus creating a second circulating flow (F 2) of liquid and microfilm carriers in the vicinity of the one or more side walls of the container towards an upper end of the container, - wherein the upwardly directed first circulating flow (F 1) through the 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 that the flow turns 180° and flow towards the bottom of the container, and - wherein 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 tube, so that two separate first and second circulating flows (F 1, F 2) are established. - a bottom, one or more side walls, and a top, - a first inlet and a first outlet arranged in the container, - at least one vertically orientated tube arranged in the container, the lower end of said tube being raised above the bottom of the container, - means comprising a first gas supply arranged at the lower end of said tube and producing gas bubbles entering into the tube through openings provided in the tube at the lower end thereof, the gas bubbles raising upwardly inside the tube for creating a first circulating flow (F 1) of liquid and microfilm carriers rising upwardly through the vertically orientated tube, and - a second gas supply arranged at the intersection between the one or more side walls of the container and the bottom of the container for introduction of gas bubbles and thus creating a second circulating flow (F 2) of liquid and microfilm carriers in the vicinity of the one or more side walls of the container towards an upper end of the container, - wherein the upwardly directed first circulating flow (F 1) through the 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 that the flow turns 180° and flow towards the bottom of the container, and - wherein 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 tube,