A Steady state kinetic model for ammonia oxidation (nitrification ) in a rotating biological disk (RBD) reactor was developed. The model can be described as a process of molecular diffusion with a simultaneous zero-order nitrification reaction within the biofilm. The model states that the nitrification rate per unit disk area is expressed as the product of an overall rate constant and an equilibrium bulk ammonia concentration. The overall rate constant is composed of two coefficients. One describes the process of ammonia transport to the biofilm surface and the other describes the process with a simultaneous zero-order nitrification reaction within the biofilm. At extremely low ammonia concentration, a bulk first-order reaction occurs, because the process of ammonia transport to the biofilm surfaca is rate-limiting.
As bulk ammonia concentrations increase, half-order and zero-order reactions occur with respect to the biofilm surface ammonia concentrations. These reactions depend on the penetration thickness of ammonia and oxygen within the biofilm. If the penetration thickness of oxygen is larger than that of ammonia, a half-order reaction occurs. In the opposite case, a zero-order reaction occurs. The transition from halforder to zero-order occurs at an equilibrium ammonia concentration and depends on the operational conditions.
A series of nitrification experiments using two completely-mixed flow reactors war carried out to verify the proposed model. Experimental variables were temperature, disk rotating velocity, partial pressure of oxygen in the gaseous phase, hydraulic loading and influent ammonia concentration. All experimental results were adequately explained by the proposed model.
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