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A Hybrid Membrane-Biofilm Process for Concurrent Nitrification and Dentrification

Leon Downing, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556 and Robert Nerenberg, Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556.

The hybrid membrane biofilm process (HMBP) is a new approach to achieving total nitrogen removal from wastewater, with great potential for retrofitting into existing plants. It incorporates air-supplying hollow-fiber membranes into a conventional activated sludge tank. By using passive aeration, energy consumption may be greatly reduced. A nitrifying biofilm develops on the hollow-fiber membranes, and nitrate/nitrite produced by the aerobic biofilm is exported to the bulk liquid. Suspended, heterotrophic bacteria in the anoxic bulk liquid reduce nitrates using influent BOD as the electron donor. The SRT can be as low as 2.5 days, as the nitrifiers are attached. This process has similarities to the membrane aerated bioreactor (MABR), which also achieves total nitrogen removal with hollow-fiber membranes. However, in the MABR, both nitrification and BOD oxidation occur in the biofilm, and increased BOD loadings result in decreased nitrification rates. In the HMBP, higher BOD loadings simply result in higher suspended solids concentrations, without impacting the biofilm.

At the bench scale, the performance of the HMBP was assessed for a variety of nitrogen and BOD loading rates. The nitrification rate increased from 0.8 to 1.7 gN m-2 d-1 in response to ammonium loadings increasing from 0.8 to 2.2 gN m-2 d-1. The nitrification rate remained at approximately 1.2 gN m-2 d-1 over BOD loadings of 4 to 17 gBOD m-2 d-1 at a nitrogen loading of 1.7 gN m-2 day-1.

Liquid ion exchange microsensors were used to measure ammonium, nitrite, and nitrate gradients through the HMBP biofilm. Nitrite was shown to be the dominant form of oxidized nitrogen produced by the biofilm. This is a major benefit, as the reduction of nitrite rather than nitrate during denitrification reduces organic carbon demand by 40%. 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. Results suggest the process may be suitable for muncipal and industrial wastewater treatment.