Total nitrogen removal in a hybrid, membrane-aerated activated sludge process Leon S. Downing, Robert Nerenberg* Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556, USA article info Article history: Received 30 January 2008 Received in revised form 27 April 2008 Accepted 3 June 2008 Available online 24 June 2008 Keywords: Hollow-fiber membranes Biofilms Nitrification Total nitrogen removal MABR HMBP MBfR abstract The hybrid (suspended and attached growth) membrane biofilm process (HMBP) is a novel method to achieve total nitrogen removal from wastewater. Air-filled hollow-fiber membranes are incorporated into an activated sludge tank, and a nitrifying biofilm develops on the membranes, producing nitrite and nitrate. By suppressing bulk aeration, the bulk liquid becomes anoxic, and the nitrate/nitrite can be reduced with influent BOD. The key feature that distinguishes the HMBP from other membrane-aerated processes is that it is hybrid; heterotrophic bacteria are kept mainly in suspension by main- taining low bulk liquid BOD concentrations. We investigated the HMBP’s performance under a variety of BOD and ammonium loadings, and determined the dominant mecha- nisms of nitrogen removal. Suspended solids increased with the BOD loadings, maintain- ing low bulk liquid BOD concentrations. As a result, nitrification rates were insensitive to the BOD loadings, remaining at 1 gN m 2 day 1 for BOD loadings ranging from 4 to 17 gBOD m 2 day 1 . Nitrification rates decreased during short-term spikes in bulk liquid BOD concentrations. Shortcut nitrogen removal was confirmed using microsensor measurements, showing that nitrite was the dominant form of oxidized nitrogen produced by the biofilm. Fluorescence in situ hybridization (FISH) showed that ammonia oxidizing bacteria (AOB) were dominant throughout the biofilm, while nitrite oxidizing bacteria (NOB) were only present in the deeper regions of the biofilm, where the oxygen concentra- tion was above 2 mg/L. Denitrification occurred mainly in the suspended phase, instead of in the biofilm, decreasing the potential for biofouling. When influent BOD concentrations were sufficiently high, full denitrification occurred, with total nitrogen (TN) removal approaching 100%. These results suggest that the process is well-suited for achieving concurrent BOD and TN removal in activated sludge. ª 2008 Elsevier Ltd. All rights reserved. 1. Introduction The removal of total nitrogen (TN), i.e., ammonium, nitrate, and nitrite, is an increasingly important goal for municipal and industrial wastewater treatment plants. Practical and cost-effective technologies are needed, especially for older plants with limited space for expansion. TN removal typically is achieved through successive nitrification and denitrifica- tion. For nitrification, slow-growing nitrifying bacteria must be retained. Retention typically is achieved by operating at high solid retention times (SRTs), which may not be feasible for older activated sludge plants. Another approach is to add fixed or suspended biofilm attachment surfaces, which is the basis for the integrated fixed-film activated sludge (IFAS) * Corresponding author. Tel.: þ1 574 631 4098; fax: þ1 574 631 9236. E-mail address: Nerenberg.1@nd.edu (R. Nerenberg). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres 0043-1354/$ – see front matter ª 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2008.06.006 water research 42 (2008) 3697–3708