The invention relates to a device for mixing and distributing a gas and a liquid, arranged upstream from a granular bed. The device includes a substantially horizontal lower plate, said lower plate being equipped with a plurality of risers evenly distributed over the surface of said plate, each riser having an upper part and a lower end communicating with the underside of said plate, the upper part of the risers being provided with at least one lateral orifice. A substantially horizontal upper plate is provided above which the gas and the liquid are supplied, wherein said upper plate rests on risers extending upwardly from the lower plate.