CONTROL OF NITRIFICATION EFFICIENCY IN A NEW BIOFILM REACTOR

The study of the nitrification capacity of a new gas-lift circulating bed reactor (CBR) carried out with laboratory and industrial scale pro totypes, showed a high nitrification rate (1.2-2 kgN m(-3) d(-1)) with out any nitrite accumulation. A good nitrification performance 0.5-0.6 kgN m(-3) d(-1) was maintained even when the CODs/N-NH4 ratio was inc reased up to 1.8-3.4 (laboratory CBR) and 4-10 (industrial prototype). The comparison of the apparent and intrinsic kinetics indicates that the CBR ensures high specific nitrification rates close to the maximum value. These results indicate an effective control of the attached bi omass growth and activity in this type of bioreactor. The intrinsic ki netics and diffusion limitations were studied by respirometric tests. It was demonstrated that apparent kinetics are zero-order for bulk con centrations of ammonia above 2 mgN l(-1) and half-order for lower subs trate concentrations. The diffusivity values calculated using the half -order diffusion-reaction model are in the range of 1.09 x 10(-9) to 1 .58 x 10(-9) m(2) s(-1) for ammonia nitrogen and of 1.1 x 10(-9) to 1. 37 x 10(-9) m(2) s(-1) for nitrite within the range of temperature stu died (16-28 degrees C). The application of the diffusionreaction model requires a very precise determination of the biofilm thickness or den sity. In the case of biofilms developed in heterogeneous granular surf aces, new methods have to be developed for the best estimation of the biofilm density and active cells distribution. In conclusion it can be stated that this new three-phase bioreactor ensures a high nitrificat ion rate and enhanced process stability through an effective biofilm c ontrol. The CBR is a good tool for more fundamental studies of biofilm kinetics and activity thanks to the homogeneous distribution of the f ixed biomass in the reactor. (C) 1997 IAWQ. Published by Elsevier Scie nce Ltd.