The EPA’s National Risk Management Research Laboratory (Dr. Albert Venosa) in collaboration with the University of Cincinnati (Dr. Makram Suidan) have developed a gravity-flow Biomass Concentrator Reactor (BCR) for biological water treatment. The BCR design encompasses an aeration chamber housing a high surface area porous polyethylene membrane system that retains all of the biomass within the aeration chamber. Its simple operation and low maintenance requirements may render it economically more feasible than other water treatment technologies. The water flux through the membrane relies completely on gravity with only 1 inch of hydraulic head. The BCR has been demonstrated to be effective in the biological treatment of methyl tertiary-butyl ether-contaminated groundwater or any environment where high biomass retention is desired or required for highly efficient bio-degradation to occur.