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The biomass concentrator reactor (BCR), a gravity flow membrane bioreactor (MBR) design, was evaluated for use in treating a municipal wastewater stream. The BCR operates with less than 2.5 cm of pressure head and uses a 3 to 4 mm thick tortuous path membrane with pore size ranging from 18 to 28 m to achieve solids separation. A two-stage, aerobic/anoxic reactor was evaluated for the removal of chemical oxygen demand (COD), ammonia, total nitrogen, and solids separation. The reactor was fed 72 L/day, with a hydraulic retention time of 9.3 hours, and had a solids retention time of 20 days. The influent COD was reduced by 93%, whereas, influent ammonia was reduced below 0.1 mg/L and total nitrogen was reduced by 53.7%. A lack of readily biodegradable COD limited denitrification and thus total nitrogen removal. The reactor solids were retained completely in the reactor by the membrane for the duration of testing.