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

Abstract

The in¯uence of dissolved oxygen concentrationon the nitri®cation kinetics was studied in the circulatingbed reactor (CBR). The study was partly performed atlaboratory scale with synthetic water, and partly at pilotscale with secondary ef¯uent as feed water. The nitri®-cation kinetics of the laboratory CBR as a function of theoxygen concentration can be described according to thehalf order and zero order rate equations of the diffusionreactionmodel applied to porous catalysts. When oxygenwas the rate limiting substrate, the nitri®cation rate wasclose to a half order function of the oxygen concentration.The average oxygen diffusion coef®cient estimated by ®ttingthe diffusion-reaction model to the experimental resultswas around 66% of the respective value in water. Theexperimental results showed that either the ammonia orthe oxygen concentration could be limiting for the nitri®cationkinetics. The latter occurred for an oxygen toammonia concentration ratio below 1.5±2 gO2/gN-NH 4for both laboratory and pilot scale reactors. The volumetricoxygen mass transfer coef®cient (kLa) determinedin the laboratory scale reactor was 0.017 s)1 for a super-®cial air velocity of 0.02 m s)1, and the one determined inthe pilot scale reactor was 0.040 s)1 for a super®cial airvelocity of 0.031 m s)1. The kLa for the pilot scale reactordid not change signi®cantly after bio®lm development,compared to the value measured without bio®lm.