Performance and microbial structure of a nitrifying fluidized-bed reactor

The performance and composition of the microbial community have been examined in a nitrifying fluidized-bed biofilm reactor for wastewater treatment arranged as the ultimate treatment step subsequent to BOD- and biological P-elimination. The microbial composition was analyzed by means of rRNA-targeted oligonucleotide probes. An NH 4 +-N elimination of more than 95% at influent concentrations of up to 80 mg NH 4 +-N l -1 was observed during a period with 75% carrier material (v/v) and a hydraulic retention time of 1.8 h. Using a confocal laser scanning microscope it was found that the biofilm structure on the carrier material (tubes of 5 mm length) was mainly influenced by the loss processes affecting the biofilm. The microbial composition, using fluorescent in situ hybridization, was dominated by the beta-group of the Proteobacteria (up to 55% of the DAPI cell number). The ammonia-oxidizing bacteria, a subgroup of the beta-Proteobacteria, were characterized by their occurrence in clusters. Using different probes for ammonia-oxidizing bacteria, a shift in the population structure within the experimental period was observed. Nitrobacter could not be detected with probes such as NIT1, NIT2 and NIT3.