Previously, using maximum feed concentrations of 500 mg/L, an immobilized cell reactor (ICR) with established biofilm was shown to enhance the biodegradation rate of a candidate naphthenic acid up to two orders of magnitude when compared to a freely suspended cell, well mixed tank reactor, reaching biodegradation rates of 22000 mg/L day with 100% biodegradation efficiencies [Paslawski et al., J. Chem. Technol. Biotechnol. 2009a]. In this work, dispersion in the ICRs was found to be negligible and a plug flow model was developed and shown to represent the immobilized cell biodegradation data well. The effectiveness coefficient (percent of dry biofilm containing active biomass) was predicted to be approximately 55% with an effective thickness of 0.15 mm.