DENITRIFICATION IN A METHANOL-FED FIXED-BED REACTOR .2. COMPOSITION AND ECOLOGY OF THE BACTERIAL COMMUNITY IN THE BIOFILMS

In a methanol-fed post-denitrification biofilm system, the density and structure of the denitrifying bacterial community were investigated. Denitrification was operated in a sand filter succeeding a municipal t wo-stage wastewater treatment with full nitrification. The quantitativ e and qualitative analysis of the community structure revealed distinc t taxonomic groups to be involved in denitrification. Autecological da ta on these bacteria indicate potential roles in the filter system whi ch help to control operation. Members of the genera Hydrogenophaga and Comamonas of the beta-subclass of Proteobacteria seem to proliferate at both aerobic and anoxic conditions with non-Cl-carbon sources being available, i.e. in the upper layers of the filter bed with biomass be ing washed in from the secondary clarifiers. It is likely that a high number of the beta-subclass bacteria is washed in and does not belong to the autochthonous flora of the denitrifying biofilms. In contrast, members of the genus Paracoccus of the alpha-subclass are likely to co nstitute an important part of the autochthonous denitrifiers. They see m to be favoured at aerobic conditions with Cl-sources being available , as is the case in the surface layers of the biofilms. Moreover, meth ylotrophic members of the genus Hyphomicrobium are supposed to belong to the autochthonous flora. They are favoured by anoxic conditions and , therefore, are supposed to proliferate throughout the biofilms. The assumptions are verified by in-situ hybridization techniques. Both par acocci and hyphomicrobia are sensitive to pH drops in the filter bed, e.g. by acid production in thick biofilms, which should strictly be av oided. For details on physico-chemical and biomass parameters of the f ilter cells in situ see Part 1. (C) 1997 Elsevier Science Ltd.