Water Environment Federation (WEF)

Simulation of nitrification performance in a biofilm carrier system under near-steady state and dynamic operating conditions


Three kinetic models were used to simulate, under near-steady state and dynamic operating conditions, the performance of a full-scale biofilm carrier process performing tertiary nitrification. Two of the models were based on simple Monod-type kinetic relationships. One incorporated measured biomass concentrations with kinetic relationships based on the maximum specific growth rate (μm) for the nitrifying biomass. The second assumed that attached growth
nitrification is a surface-related phenomenon, and therefore independent of biomass considerations, with kinetic relationships based on the maximum substrate utilization rate (r′m). The Monod-type models were fit to dynamic operating data to determine the corresponding kinetic rates (μm and r′m). The third model, a commercially available dynamic simulation package (GPS-XTM), was calibrated with measured attached growth total solids (AGTS) concentrations and effluent data based on the user input maximum biofilm thickness (Lmax). The models were used to evaluate the relative significance of process variables (temperature, pH, DO, and biomass concentration) independently and in combination. All of the models were capable of predicting performance under near-steady state conditions, though the dynamic simulator was highly dependent on the correct assumption of the Lmax. Under dynamic operating
conditions, the models were also able to predict trends in performance reasonably well. Of the two Monod-type models, the best-fit model was that excluding biomass concentrations and including adjustments to r′m for DO only. Data from the dynamic simulation package confirmed the results from Monod-type models with the best-fit model excluding temperature effects and including only limited DO effects. This may suggest that the biofilm is highly adaptive,
increasing the mass of nitrifiers during cold temperatures and resulting in relatively high winter nitrification rates despite reduced nitrifier growth rates. The utility of the models developed in this study is dependent on the ability to predict (and in the case of GPS-X, calibrate) attached growth biomass concentrations in response to seasonal
variations and/or changes in influent NH3-N loading. For that reason, the Monod-type model that excludes biomass concentrations is anticipated to be the simplest model to use in practice as it does not require prediction of AGTS.

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