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