A high efficiency reactor tank is designed to remove pollutants from domestic wastewater. Embodiments of the tank can include at least one integrally-molded internal wall that divides the tank into at least two chambers and which can be a one-piece tank with at least one top riser. The internal wall can be integrally-molded within the tank as well as separately formed and inserted into the tank. The tank can be roto-molded (i.e., rotationally-molded) using plastic or cast/made from concrete, steel, fiberglass and/or similar structural materials. The reactor tank can be applied to remove suspended solids, BOD, ammonia, nitrate, TKN, phosphorus, pathogens and other pollutants from the wastewater.