Influence of dissolved oxygen on the nitrification kinetics in a circulating bed biofilm reactor

Abstract

The influence of dissolved oxygen concentration on the nitrification kinetics was studied in the circulating bed reactor (CBR). The study was partly performed at laboratory scale with synthetic water, and partly at pilot scale with secondary effluent as feed water. The nitrification kinetics of the laboratory CBR as a function of the oxygen concentration can be described according to the half order and zero order rate equations of the diffusion-reaction model applied to porous catalysts. When oxygen was the rate limiting substrate, the nitrification rate was close to a half order function of the oxygen concentration. The average oxygen diffusion coefficient estimated by fitting the diffusion-reaction model to the experimental results was around 66% of the respective value in water. The experimental results showed that either the ammonia or the oxygen concentration could be limiting for the nitrification kinetics. The latter occurred for an oxygen to ammonia concentration ratio below 1.5–2 gO₂/gN-NH₄ ⁺ for both laboratory and pilot scale reactors. The volumetric oxygen mass transfer coefficient (k _L a) determined in the laboratory scale reactor was 0.017 s⁻¹ for a superficial air velocity of 0.02 m s⁻¹, and the one determined in the pilot scale reactor was 0.040 s⁻¹ for a superficial air velocity of 0.031 m s⁻¹. The k _L a for the pilot scale reactor did not change significantly after biofilm development, compared to the value measured without biofilm.