Formulation and testing of a novel river nitrification model

The nitrification process in many river water quality models has been approximated by a simple first order dependency on the water column ammonia concentration, while the benthic contribution has routinely been neglected. In this study a mathematical framework was developed for sediment bed nitrification based on mass transfer theory and Monod bacterial growth kinetics. The model describes ammonia transport across the boundary layer and consumption within the biofilm to quantify the overall nitrification flux. Model results suggest that nitrification is usually controlled by the boundary layer thickness, and we estimated a nitrification velocity range between 0.14 and 0.97 m d⁻¹, assuming typical boundary thicknesses of 0.1–1.0 mm. These ranges compared favorably with reported literature values, including our own measurements. The model was applied to several river systems of different depths where nitrification rates and river depths were available. Assuming that nitrification is exclusively a benthic process, the average velocity of all the rivers evaluated was 0.85 m d⁻¹ (r² = 0.72).