An apparatus (100) and process (400) for the treatment of wastewater and biological nutrient removal in activated sludge systems. The process uses substantially vertically downwardly presented inlet jets for delivering the incoming wastewater and recycled activated sludge into the body of liquid in a reactor, in a vertically downward direction and at a location just below the surface of the body of liquid. An effective circulating flow pattern of liquid is thereby established, along with optional concomitant entraining, dispersion or dissolving a fluid throughout the volume of the liquid body, facilitating a universal apparatus for mixing of anaerobic, anoxic, aerobic and oxic reactors or accommodating alternating said process conditions in one reactor. When an oxygen containing gas is entrained for aerobic fermentation, optimum gas bubble size is generated for efficient reaction with the digestion bacteria throughout the volume of the liquid body. Efficient mixing and maximum utilization of the bacteria suspended in the body of liquid is thereby approached, whether the digestion is conducted aerobically, anaerobically or anoxically.