Estimating minimum fixed biomass concentration and active thickness of nitrifying biofilm

The objective of this study was to develop an approach to estimate the minimum fixed biomass concentration (M(min)) and corresponding active thickness of a nitrifying biofilm. By analyzing the effect of the accumulation of fixed biomass on the specific removal rate of ammonium-N (q(obs)), it was found that q(obs) attained its maximum at a fixed biomass concentration of around 35 mu g COD of biomass/cm(2). This concentration was not sensitive to variation in the influent ammonium-N concentration (S-o), ranging from 16-82.5 mg/L. This suggests that a minimum fixed biomass concentration (M(min)) is needed to maximize the specific ammonium-N removal on a unit biomass basis. The active thickness (Th-a)(min) corresponding to M(min) was calculated. Results showed that (Th-a)(min) was very thin, and decreased from 21.9 to 8.1 mu m, with the increase of the volumetric density of biofilm. Comparison between (Th-a)(min) and the active thickness determined from reaction-diffusion theory was made. This study showed that there exists a minimum fixed biomass concentration and active thickness, above which little improvement in effluent quality was produced by further increase of fixed biomass concentration for a constant influent, Consequently, the concept of the minimum fixed biomass concentration can be therefore embodied in modeling of the biofilm process.