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Biofilm density, porosity, and thickness are biofilm architecture properties that are important but often difficult to measure. In this study, wet and dry biofilm densities and biofilm porosity in shredded tire biofilters were measured using a volumetric displacement method and a new porosity equation. Methods for determining the surface area and mean thickness of biofilms attached to shredded tires were developed on the basis of the box- and ellipsoid-shape models with the data of (1)the volume calculated from the measured weight of the filter medium () or (2)the three longest lines of the filter medium measured with a digital caliper. As a benchmark, the surface area of the shredded tire particles were calculated from X-ray computed tomography (CT) scanning and compared to, and linearly correlated with, the results from the models. The ellipsoid-shape model with the modified length-width-depth data and was determined to be the best model to compute the surface area and biofilm thickness. The method of biofilm thickness measurement may be applicable in attached growth systems packed with other irregular-shaped media. (C) 2013 American Society of Civil Engineers.