Abstract: a wastewater treatment system having a submerged attached growth bioreactor connected to an equalization tank and a clear well. The bioreactor has a deep-bed sand filter. the wastewater flow through the filter is in two directions, i.e., down flow (forward flow) and up flow (reverse flow). The hydrostatic pressure created by the differential liquid levels within the tanks creates a forward flow from the equalization tank, into the bioreactor and out into the clear well. Reverse flow is accomplished by pumping from the clear well back up through the bioreactor. The reverse flow fills the headspace within the bioreactor, stops, and then begins to flow forward again. The flow through the bioreactor alternates between down flow and up flow. In an alternative embodiment, process air required for the oxidation of organics and nitrification is accomplished by aerating the clear well. The oxygen rich water is pumped into the filter from the clear well during the reverse flow.