Oxygen consumption and organics decomposition in drainage systems with attached biofilm were investigated using a batch reactor. In order to distinguish the relative contribution to these processes of suspended biomass and attached biofilm parallel experiments were performed. The reactors with low and high glucose concentrations simulated lightly polluted rivers and sewers, resp. The dynamic behaviour was different for the system dominated by biofilm and that with only suspended biomass, due to different controlling mechanisms. The biofilm played a major role in the biodegradation only when the suspended biomass concentration was below a critical value. The ratio of wetted area on which the biofilm grows over the water volume (WA/V) is closely correlated with the biofilm oxygen uptake rate. The relative importance of the suspended biomass and the biofilm did not depend on the initial glucose concentration.
