Two experiments were performed in order to investigate the anoxic and the aerobic degradation of filtrated organic matter in a biofilter. In submerged lab: scale reactors with Biocarbone media as filter material, accumulated particulate organic matter from pre-settled wastewater served as the only carbon source for anoxic and aerobic degradation respectively. In order to utilise the accumulated organic matter, the bacteria in the biofilm had to produce extracellular hydrolytic enzymes for the hydrolysis process. In the first experiment with anoxic degradation, a significant denitrification occurred. The maximum nitrate removal rate, with particulate organic matter as carbon source, was found to I g NO3-N/(.)(-)(m(2)d), declining exponentially as tau(ANO3-N) = 1.04 e(-4t) g NO3-N/(m(2)d) (t = days). A significant release of soluble organic matter (< 0.45 mu m) and production of inorganic carbon to the bulk liquid was observed as an indication of hydrolysis taking place. The second experiment was designed as a series of on-line OUR batch experiments in a biofilm reactor with recirculation, in order to investigate further the degradation of particulate organic matter. After the biofilm had been starved in order to remove the original organic matter, particulate organic matter was added and the degradation kinetics recorded. The initial removal rate was high, r(A.O2) = 2.1 g O-2/(m(2)d) though fast declining towards endogenous respiration. The respiration ratio between removed oxygen and produced carbon dioxide was declining from 1.3 to 1.0 g O-2/g CO2 during the degradation of organic particulates. The respiration ratio during endogenous respiration was determined to be 0.7 g O-2/g CO2 indicating a thorough mineralisation of biomass. (C) 1997 IAWQ. Published by Elsevier Science Ltd.