Activity and population structure of nitrifying bacteria in an activated-sludge reactor containing polymer beads

Photo-crosslinked polymer beads were introduced into a laboratory activated-sludge unit containing municipal sewage sludge and the effect on nitrifying capacity was examined. The ammonia load started at a nitrogen-loading rate of 0.02 kg m(-3) day(-1) and was increased stepwise. It was found that the bead-containing unit could almost completely oxidize ammonia (over 95%) up to a nitrogen-loading rate of 0.216 kg m(-3) day(-1), whereas the maximum loading rate of the control unit (without polymer beads) was 0.096 kg m(-3) day(-1). The nitrifying potential of suspended and bead-associated organisms in the bead-containing unit was measured under different loading conditions. It was found that the bead-associated organisms exhibited high specific activities under high loading conditions and that the contribution of the bead-associated organisms to nitrification was greater than that of the suspended solids under these conditions. The bacterial population dynamics in the suspended solids and bead-associated organisms were analysed by denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rRNA gene fragments and by fluorescence in situ hybridization with group-specific probes. Among the known nitrifying organisms, ammonia-oxidizing beta-proteobacteria and Nitrospira-related organisms were detected by these approaches. A comparison of the activity dynamics and population dynamics, however, suggested a possibility that other organisms may also have been involved in the nitrification process under high loading conditions.