Impacts of Reynolds number on nitrification biofilm kinetics

Abstract

Nutrient mass flux is often a limiting factor determining the efficiency of a fixed film bio-reactor. The relationship between total ammonia nitrogen (TAN) removal rate and the Reynolds number ( Re) in a steady-state nitrification fixed biofilm system was investigated in this study through measurement of the ammonia removal rate in a long plastic tubular reactor and analysis of mass transport across a fixed biofilm. The results indicate that hydraulic condition is an important factor limiting TAN removal rate. For a fixed substrate concentration, TAN removal rate at Re=66 710 was about five times of that at Re=1668. These results suggest that the performance of nitrification biofilters could be significantly improved by increasing the Reynolds number of the flow over the biofilm surface. An empirical relationship was developed to obtain an estimate of the Sherwood number ( Sh=0.13 Re 0.63 Sc 1/3, Sc is Schmidt number) for the Reynolds number ranging from 1668 to 66 710. The thickness of the boundary layer (or water film) can be determined, and simulation can be performed to better understand the hydrodynamics of fixed biofilm nitrification based on the Sherwood number equation.