The study of the nitrification capacity of a new gas-lift circulating bed reactor (CBR) carried out with laboratory and industrial scale prototypes, showed a high nitrification rate (1.2-2 kgN m−3 d−1) without 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 increased 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 biomass growth and activity in this type of bioreactor.

The intrinsic kinetics and diffusion limitations were studied by respirometric tests. It was demonstrated that apparent kinetics are zero-order for bulk concentrations of ammonia above 2 mgN l−1 and half-order for lower substrate concentrations. The diffusivity values calculated using the half-order diffusion-reaction model are in the range of 1.09 × 10−9 to 1.58 × 10−9 m2 s−1 for ammonia nitrogen and of 1.1 × 10−9 to 1.37 × 10−9 m2 s−1 for nitrite within the range of temperature studied (16-28°C). The application of the diffusion-reaction model requires a very precise determination of the biofilm thickness or density. In the case of biofilms developed in heterogeneous granular surfaces, 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 nitrification rate and enhanced process stability through an effective biofilm control. The CBR is a good tool for more fundamental studies of biofilm kinetics and activity thanks to the homogeneous distribution of the fixed biomass in the reactor.