The hydrodynamic behaviour of the fixed film wastewater treatment processes is a significant factor affecting performance. This behaviour can be studied with the help of the residence time distribution (RTD), using a tracer, here the lithium. The method was applied to 8 full scale trickling filters. To explain the obtained RTDs, a model simulating the slow mass exchanges of tracer between the liquid film and the bacterial slime is considered. This model successfully relates the significant tailing observed. This model yields a mobile and immobile volume after being fitted to the experimental RTD curve. The mobile volume is compared to the drainage volume, whereas the immobile volume may be a relevant parameter in order to assess the biomass volume, which is a key parameter for the trickling filter operation. In more, characteristic kinetic parameters of the biofilm process have been established in the case of the conventional filling media. The RTD method has also been applied to two upflow biofilters used for tertiary nitrification, namely the ‘Biofor' and ‘Biostyr' processes. The RTDs have been determined at several points of the horizontal surface in order to assess the flow homogeneity. Samples were taken at the top and bottom of the biofilter media, so as to separate the influence of the bottom volume in which the effluent is distributed, from the biofilter media alone. Inside the later, hydrodynamic parameters such as the active volume and the dispersion have been computed, assuming that the flow is similar to a dispersed plug flow. It was concluded that the differences in residence time observed at the biofilter surface from one location to the other can partly be explained by the uneven effluent distribution at the filter bottom. The method also allowed the discovery of temporary clogging of the filling media. The possible consequences on performance have been assessed with a biofilm kinetic model. Results suggests that detrimental consequences can be feared only in the case of a dramatic increase of the influent substrate concentration.
