The hybrid membrane biofilm process (HMBP) is a novel approach to achieving total nitrogen removal from wastewater. It incorporates air-supplying, hollow-fiber membranes into a conventional activated sludge tank, where bulk aeration is suppressed. A nitrifying biofilm grows on the membranes and exports nitrite and nitrate to the anoxic bulk liquid, where suspended heterotrophic bacteria denitrify using influent BOD as an electron donor. Bench scale tests were used to assess the performance of the HMBP for a variety of nitrogen and BOD loading rates. For a nitrogen loading of 1.6 gN m−2, the nitrification flux remained at approximately 1.0 gN m−2 d−1 for BOD loadings ranging from 4 to 17 gBOD m−2 d−1 day−1. Full denitrification was achieved when sufficient BOD was available in the influent. Microsensor measurements indicated nitrite was the dominant form of oxidized nitrogen produced by the biofilm, showing that shortcut nitrogen removal was taking place. Fluorescence in-situ hybridization (FISH) tests on the biofilm revealed a unique stratification, with three distinct regions: AOB and NOB near the membrane, strictly AOB at intermediate depths, and AOB and heterotrophs at the outer edge of the biofilm.

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