The invention concerns an arrangement (1) for the processing and addition of a gas (2), or a mixture of gases, to a fluid (3). The arrangement comprises at least one vessel (4) that is intended to be at least partially submerged in the fluid (3), the vessel (4) of which comprises at least one inner compartment (5) provided with at least one inlet (10) for the input flow of fluid into the vessel (4) and at least one outlet (11) for the output flow of fluid from the vessel. The arrangement (1) comprises at least one flow-creation arrangement (14) through which an output flow of fluid (3) from the vessel (4) takes place during the formation of at least one vortex (27) in the inner chamber (5). What is unique for the present arrangement is that the inlet (10) is constituted by at least one input flow channel that is located in the lower part of the vessel (4) and that the input flow channel is directed essentially in the tangential direction of the vessel (4) obliquely upwards within the interval 91 to 125 degrees relative to the axial direction of the vessel. What is furthermore unique is that the flow-creation arrangement (14) is constituted by an impeller (16) and that the impeller (16) is located at the outer surface of the vessel at a distance from the bottom of the vessel. The present invention comprises also a method for the use of the arrangement.Linvention porte sur un agencement (1) pour le traitement et lajout dun gaz (2), ou dun mélange de gaz, à un fluide (3). Lagencement comprend au moins un récipient (4) qui est destiné à être au moins en partie immergé dans le fluide (3), le récipient (4) comprenant au moins un compartiment interne (5) doté dau moins une entrée (10) pour le flux dentrée de fluide dans le récipient (4) et dau moins une sortie (11) pour le flux de sortie de fluide du récipient. Lagencement (1) comprend au moins un agencement de création de flux (14) grâce auquel un flux de sortie de fluide (3) du récipient (4) a lieu pendant la formation d
