Abstract: A process for providing biological denitrification of a previously nitrified wastewater includes the initial step of supplying to a tank an influent including a relatively small concentration of biomass and a fraction of nitrates. The influent is supplied adjacent a bottom of the tank, and a volume of waste water is retained in the tank. This volume in the tank has a biomass concentration greater than the influent biomass concentration, and the biomass in the tank provides biological denitrification of the wastewater during movement of the wastewater through the tank. Effluent is delivered from the tank, and includes a biomass concentration substantially equal to the influent biomass concentration and is substantially free of nitrates relative to the influent, the effluent being drawn from a top of the tank. During the process, the wastewater in the tank is intermittently mixed to maintain the biomass in suspension within the tank and to assist in controlling the concentration of biomass in the tank.