A reactor for aerobically or non-aerobically treating a fluid flowing from the bottom to the top of the reactor through a fluidized bed of granular material. The reactor includes a base having an orifice therein, through which fluid flows into the reactor and in which gas is added to the fluid. A horizontally extending plate surrounded by a supple membrane faces the base and the orifice. The plate and the membrane form a valve for preventing fluid from the reactor from entering the orifice in the event fluid flow into the reactor ceases. The gas entrains the granular material so that when the gasified fluid passes upward through the granular material a three-phase liquid-gas-granular material mixture is produced. The upper portion of the reactor includes a device for disembedding the granular material from the fluid, a device for degasing the three-phase liquid-gas-granular material mixture, and a device for separating the granular material from the fluid to produce a fluid discharge. The separation device also recycles the granular material into a fluidization zone. Also provided is a device for recycling and recuperating the fluid discharge into the fluidization zone of the reactor, thereby fluidizing the bed of granular material.