Kinetic analysis of nitrifying biofilm growing on the rotating membrane disk

The authors have proposed a novel water treatment process in which nitrifying bacteria are fixed on the surface of rotating membrane disks. This biofilm-membrane process can perform strict solid-liquid separation and oxidation of ammonia nitrogen simultaneously. In this research, applicability of the conventional biofilm model (assuming the biofilm structure to be flat, homogeneous and continuous) to analysis of the biofilm developing in the proposed process was examined. A long-term operation for culturing the active nitrifying biofilm was carried out prior to kinetic investigation. By cryosectioning of the biofilm and image analysis, the thickness of the biofilm was determined to be 87 μm. From this biofilm thickness and the result of the batch ammonia consumption test, the intrinsic zero-order ammonia consumption rate of the biofilm was estimated precisely to be 930 g/m³/h. Using these parameters, the ammonia concentration profile in the biofilm was calculated by the conventional model, and the applicability of the model was examined by comparing the calculated profile with the ones measured with a microelectrode. The calculated profile was very close to the measured ones, which indicated feasibility of the conventional model to the analysis of the biofilm grown in the proposed process. The studied biofilm actually had a simple, i.e. flat, homogeneous and continuous, structure due to membrane filtration. This was the reason why the conventional model could still be employed. In the analysis of the data dealing with low concentrations of ammonia, however, first-order kinetics should be used. The first-order ammonia consumption rate constant of the studied biofilm was estimated to be 808 h^-1.