The degradation of 3-methylpyridine (MEP), a model heterocyclic industrial molecule, was performed in a sequential batch ozonation-biofilm process. Four process steps (bubble-column ozonation, heterotrophic biofilm. degradation, biofilm nitrification, and biofilm denitrification) were combined in different sequences. Three packed-bed biofilm reactors were started up so as to have separate, specific activities (heterotrophic, nitrification and denitrification). Batch experiments with acetate, ammonia, and nitrate proved that all reactors displayed degradation activity for all substances. Different batch sequences of these reactors were tested with the products of batch ozonation of 3MEP as the first step. The best results were obtained using a two-step process, in which the ozonation was followed by a single fluidized-bed, heterotrophic biofilm reactor. The high C/N ratio of 3MEP and the appreciable non-specific activity of this reactor made it possible to achieve all the biodegradation in the one reactor. Establishing the optimal batch ozonation time (80 min) was determined by an ozone electrode and by stopping the process when the dissolved ozone concentration rose above an initial low level. The identifiable products of 3MEP ozonation were nitrate, acetate, formate, pyruvate, oxalate and ammonium. A C-balance, compared with TOC measurement, indicated that about 50% of the carbon was in unidentified, but biogradable, ozonation products. (C) 2003 Society of Chemical Industry.