A tank is disclosed as having a tub therein that divides the volume of the tank into an anoxic chamber inside the tub and an equalization chamber outside of the tub. The anoxic chamber anoxically treats wastewater while the equalization chamber equalizes fluctuations in wastewater influent as well as provides a holding space for wastewater if power to the tank is cut off. The tank can be part of a system for treating wastewater, such as one with an anaerobic tank having an anaerobic chamber for anaerobically treating wastewater, an aerobic/filtration tank having a filtration sub-tank therein that divides the volume of the tank into a filtration chamber inside the sub-tank and an aerobic chamber outside of the sub-tank. The aerobic chamber can aerobically treat wastewater while the filtration chamber has a membrane unit for filtering wastewater.