Including Nitrite as an Intermediate in Simultaneous Nitrification/ Denitrification Membrane-Aerated Biofilm Reactor Models Nicholas Landes, 1 Audra Morse, 2 and W. Andrew Jackson 2, * 1 Freese and Nichols, Inc. Fort Worth, Texas. 2 Department of Civil and Environmental Engineering, Texas Tech University, Lubbock, Texas. Received: November 21, 2012 Accepted in revised form: April 15, 2013 Abstract Our understanding of biofilm function has been greatly increased with the aid of one-dimensional biofilm models. However, to date, there has been no evaluation of how the inclusion or exclusion of nitrite within nitrification/ denitrification biofilm models may impact simulated prediction of the biofilm community composition and ef- fluent water quality. As such, several variations of commonly assumed nitrification and denitrification models with a common set of kinetic parameters were simulated in a one-dimensional membrane-aerated biofilm model to identify the differences in simulation output with respect to simultaneous nitrification and denitrification. Our results indicated that the inclusion of nitrite as an intermediate affected the biofilm community composition, microbial activity, and effluent water quality. Even under circumstances when the predicted water quality results were similar, significant differences persisted in the microbial activity. For example, chemical oxygen demand (COD) removal was independent of the including nitrite as an intermediate; however, the processes that con- tributed to COD removal (i.e., aerobic oxidation vs. anoxic denitrification) varied considerably between model assumptions, and as a result, affected differences between the predicted oxygen demand. Simulations also helped to identify inefficiencies caused by nitrite looping, a phenomenon in which NO 2 - cycles between oxidation within aerobic regions and reduction within anoxic regions. Overall, results indicated that if one-dimensional biofilm models are used to inform our understanding of fundamental biofilm processes, then the manner in which nitrite is modeled must be carefully considered to avoid introducing modeling artifacts into our interpretation of simulation results. Key words: biofilm modeling; membrane-aerated biofilm; nitrite looping; simultaneous nitrification/denitrification Introduction B iofilm models are rapidly evolving to emulate the complexity of processes implicit to real biofilms. In par- ticular, nitrification and denitrification reactions within mathematical simulations have expanded from simple mod- els capable of predicting effluent ammoniacal-nitrogen (NH x ) and nitrate (NO 3 - ) concentrations in an activated sludge plant (Henze et al., 2000) to more complex models capable of simulating the fate and production of all major forms of ni- trogen (e.g., Hiatt and Grady, 2008). A variety of models have been used by researchers to simulate nitrogen dynamics in biofilms (Table 1). As a basis for the current work, all permutations of one-step and two- step nitrification and denitrification biopathways are con- sidered (Table 2). Essentially, the models are defined by the inclusion or exclusion of nitrite (NO 2 - ) within nitrification and denitrification biopathways. Inclusion of NO 2 - in- creases the number of processes modeled, thereby increasing the complexity of the model. Two different methodologies for modeling denitrification have been typically adopted by past modeling efforts. The first category, parallel denitrifi- cation, assumes that both NO 3 - and NO 2 - can be reduced to nitrogen gas (N 2 ), but NO 3 - reduction does not proceed via the intermediate product NO 2 - before terminating as N 2 (Matsumoto et al., 2007; Downing and Nerenberg, 2008; Lackner et al., 2008; Wang et al., 2009). The second category, sequential denitrification, assumes that NO 3 - reduction must proceed through NO 2 - before being reduced to N 2 (Tiedje, 1998). The degree of model complexity is often chosen based upon the bulk liquid NO 2 - concentrations. For example, if the NO 2 - concentrations in the bulk liquid are minimal, the modeler may choose to exclude NO 2 - as in the suite of acti- vated sludge models (ASM) (Henze et al., 2000); however, this approach may be ill advised since NO 2 - could be a critical component governing the rate of biological processes within the biofilm, even though it does not appear to be of any *Corresponding author: Department of Civil and Environmental Engineering, Texas Tech University, MS41023 Lubbock, TX 79409. Phone: 808-742-3523; Fax: 806-742-3449; E-mail: andrew.jackson@ ttu.edu ENVIRONMENTAL ENGINEERING SCIENCE Volume 30, Number 10, 2013 ª Mary Ann Liebert, Inc. DOI: 10.1089/ees.2012.0477 606