Control of nitrification efficiency in a new biofilm reactor

Abstract

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 mˉ³ dˉ¹) without any nitrite accumulation. A good nitrification performance 0.5-0.6 kgN mˉ³dˉ¹ 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 indicates an effective control of the attached biomass growth and activity in this type of bioreactor. The intinsic 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 lˉ¹ and half-order for lower substrate concentrations. The diffusivity values calculated using the half-order diffusion-reaction model are in the range of 1.09x10ˉ9 to 1.58x10ˉ9 m² sˉ¹ for ammonia nitrogen and of 1.1x10ˉ9 to 1.37x10ˉ9 m² sˉ¹ for nitrite within the range of tempperature studied (16-28ºC). 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.