The wastewater treatment system includes at least one fluidized bed reactor, such as a vessel containing media to which biofilms may attach. The vessel has an inlet and an outlet adjacent a bottom, and an overflow outlet adjacent a top, of the vessel. An element for admitting wastewater into the vessel and for agitating wastewater therein with sufficient energy to scour biofilm from the media, and for admitting additional wastewater into the vessel following a predetermined time to a level sufficient to cause water and at least some of the scoured biofilm to exit via the overflow outlet. Wastewater is drainable through the bottom outlet after a predetermined time for permitting aeration of the media. These fill and drain cycles effect a "tidal" nitrification and denitrification of the wastewater, with, for example, ammonia being converted ultimately to nitrogen gas.