POPULATION-DYNAMICS AND BIOFILM COMPOSITION IN A NEW 3-PHASE CIRCULATING BED REACTOR

The biofilm characteristics of a novel three-phase reactor, the circul ating bed reactor (CBR), were studied using industrial prototype fed w ith primary and secondary settled effluent in conditions of tertiary N and secondary C+N nitrification. The results showed a high nitrificat ion rate close to the intrinsic values for N and C+N conditions: up to 2 and 0.6 kgN-NH4 m(-3) d(-1), or 1.88+/-0.26 and 0.22+/-0.07 gN g(-1 ) PR d(-1), respectively. The application of an integrated approach fo r biofilm analysis enabled the better understanding of biofilm dynamic s. The biofilm remained relatively thin, below 100 mu m, indicating an effective control of the biofilm development. Protein, measured by th e conventional colometric method and pyrolysis-GC-MS, was the major fr action accounting for up to 35% of the biomass dry weight and 58% of t he biopolymer content. The polysaccharide's fraction remained very low (<3%). The ribosomal RNA probes analysis confirmed the predominance o f bacterial cells in the CBR biofilm (80-86% of bacteria versus the un iversal probe) showing a high proportion of nitrifying bacteria accoun ting for up to 50% and 27% in the N and C+N removal respectively. Nitr osomonas predominated in tertiary nitrification whereas carbon input l ed to the appearance of other ammonia oxidizers. This particular compo sition was characterized by a high state of oxidation of the biomass, expressed by the low COD/DW ratio of about 0.85. In conclusion, it can be stated that this new three-phase bioreactor ensures a high nitrifi cation rate through an effective biofilm control promoting the develop ment of bacterial cells, especially nitrifying bacteria, and minimizin g exopolysaccharides production. (C) 1998 IAWQ. Published by Elsevier Science Ltd.