Denitrification in a methanol-fed fixed-bed reactor. Part 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 two-stage wastewater treatment with full nitrification. The quantitative and qualitative analysis of the community structure revealed distinct taxonomic groups to be involved in denitrification. Autecological data on these bacteria indicate potential roles in the filter system which help to control operation. Members of the genera Hydrogenophaga and Comamonas of the β-subclass of Proteobacteria seem to proliferate at both aerobic and anoxic conditions with non-C1-carbon sources being available, i.e. in the upper layers of the filter bed with biomass being washed in from the secondary clarifiers. It is likely that a high number of the β-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 α-subclass are likely to constitute an important part of the autochthonous denitrifiers. They seem to be favoured at aerobic conditions with C1-sources being available, as is the case in the surface layers of the biofilms. Moreover, methylotrophic 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 paracocci and hyphomicrobia are sensitive to pH drops in the filter bed, e.g. by acid production in thick biofilms, which should strictly be avoided. For details on physico-chemical and biomass parameters of the filter cells in situ see Part 1.