A novel membrane-aerated biofilm reactor configuration was tested in which substrate was delivered to the biofilm via advective as well as diffusive transport. Ammonium-rich synthetic wastewater was pumped across a fabric of hollow-fiber membranes and biofilm grew around and between the membrane fibers to form a continuous structure. High ammonium removal rates (10-24 g N m(-2) d(-1)) were achieved in our reactors compared to the conventional biofilm processes, which rely mostly on diffusive transport for substrate delivery. The removal rates were in good agreement with our mathematical model predictions. Low pressure drops (<0.05 atm) were maintained across the biofilm throughout the course of study. The major problem with this novel configuration appeared to be the short-circuiting due to non-uniform structure of the biofilm formed on membrane fabric. Further research is needed to address the problem of short-circuiting by studying the effect of operational parameters and membrane design on the behavior of system. (C) 2008 Elsevier B.V. All rights reserved.