MODELING THE EFFECT OF OXYGEN CONCENTRATION ON NITRITE ACCUMULATION IN A BIOFILM AIRLIFT SUSPENSION REACTOR

For an integrated nitrification-denitrification process, nitrite forma tion in the aerobic stage leads to big savings. Recently experimental observations (Garrido ei nl., 1996) have shown that it is possible to obtain full ammonium conversion with approximately 50 % nitrate and 50 % nitrite in the effluent of a biofilm air-lift suspension reactor. W ith oxygen concentrations between 1 and 2 mg/l a maximum nitrite accum ulation of 50 % was reached. Here we give a simple diffusion-reaction model describing these results. All the kinetic and mass transfer para meters were taken from the literature, except the mass transfer coeffi cient around the biofilm surface, which was fitted. The proposed model describes very well the measured data, despite the assumptions made. Using this model the influence of operational parameters was evaluated in order to establish ways to affect the NO2- concentration. None of these (surface loading, k(i), pH) had a significant effect on the maxi mal NO2- accumulation. Controlling the oxygen concentration seems to b e the most practical method to obtain optimal nitrification to nitrite in BAS reactors. (C) 1997 IAWQ. Published by Elsevier Science Ltd.