Model of Ammonia and Carbon Oxidation in Biofilms

The oxidation of organic carbon and ammonium nitrogen in aerobic biofilms is analyzed mathematically. Monod intrinsic kinetics and Fickian diffusion are used for organic carbon uptake. Zero‐order intrinsic kinetics are assumed for oxygen and nitrogen uptake. The resulting nonlinear equation for organic carbon uptake is integrated by approximation, yielding an explicit solution for the concentration profile. Nitrification is assumed to occur only in a biofilm layer below the depth at which heterotrophic and nitrifying growth rates are equal. Using parameters from the literature, the model predicted average flux for organic carbon and ammonium nitrogen; these are compared to observed fluxes. Equations and charts are presented for the organic carbon and the ammonium nitrogen fluxes at the biofilm surface as functions of the surface concentrations of ammonium nitrogen, dissolved oxygen, and organic carbon. An example is presented for the design of a fixed‐film reactor for sewage treatment.