Dynamic model of a nitrifying fixed bed column: Simulation of the biomass distribution of Nitrosomonas and Nitrobacter and of transient behaviour of the column

A dynamic model for a fixed bed nitrifying column with recirculation of the liquid and gas phases was developed. Liquid RTD experiments demonstrated t hat the liquid phase was perfectly mixed inside the column. Heterogeneity o f biomass distribution on the solid phase (beads) was represented by an N-t anks in series model, and a back-mixing term was set to account for the wel l-mixed liquid phase throughout the column. In autotrophic conditions, comp etition for oxygen is the cause of the spatial segregation of the two speci es. Nitrosomonas is concentrated on beads at the bottom of the bed whereas Nitrobacter is more widely distributed. This is consistent with biomass dis tribution results reported by Cox et al. [17] in a nitrifying fixed bed col umn. Nitrification takes place at the bottom of the column, always in oxyge n gas-liquid mass transfer limiting conditions. Nevertheless, considering t he whole process, nitrification is complete (>98% of NH3 oxidised) and ther e is no oxygen limitation (the outlet dissolved oxygen concentration is not limiting). The dynamic behaviour of the column, in conditions set up to av oid biofilm diffusion limitation, was simulated for different NH3-load vari ations and oxygen shutdowns. The simulated behaviour of the column can be c ompared to results reported by Bazin et al. [16]. This confirms that the ou tput transient nitrite peaks are higher when changes in the process conditi ons produce a rearrangement of biomass distribution in the fixed bed.