Science and Education Publishing American Journal of Water Resources 2333-4819 2 6 2014 12 17 Optimization of Retention Time of Microbial Community Structure of Activated Sludge Process 149 158 EN M P. Shah Industrial Waste Water Research Laboratory Division of Applied & Environmental Microbiology Enviro Technology Limited Gujarat, India AJWR2014263 10.12691/ajwr-2-6-3 2014 12 02 2014 12 11 2014 12 17 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.