Ammonia Oxidizing Bacteria community composition was analysed using fluorescence in situ hybridization (FISH) and denaturing gradient gel electrophoresis (DGGE), and the identified populations were enumerated by quantitative FISH. Potential nitrification rates were determined in batch tests and the in situ rates were calculated from mass balances of nitrogen in the plants. Increased SRT did not reduce the nitrification activity, but the number per mixed liquor suspended solids nor was community composition of AOB affected. Two dominant AOB populations related to Nitrosomonas europaea and Nitrosomonas oligotropha were identified by FISH, whereas only the latter could be detected by DGGE. The effect of a longer SRT on the activity was probably because of physiological changes in the AOB community rather than a change in community composition.

activated sludge, ammonia-oxidizing bacteria, denaturing gradient gel electrophoresis, fluorescence in situ hybridization, nitrification, nitrogen removal, solids retention time