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Sludge age or mean cell residence time (MCRT) plays a crucial role in design and operation of wastewater treatment plants. The change in performance, for example micropollutant removal, associated with changes in MCRT is often attributed to changes in microbial diversity. We operated four identical laboratory-scale sequencing batch reactors (two test and two control) in parallel for 212 days. Sludge age was decreased gradually (from 10.4to 2.6 days) in experimental reactors whereas it was kept constant (10.4 days) in control reactors. The reactor performance and biomass changed in a manner consistent with our understanding of the effect of sludge age on a reactors performance: the effluent quality and biomass declined with decreasing MCRT. The composition of the bacterial and ammonia-oxidizing bacterial communities in four reactors was analyzed using denaturing gradient gel electrophoresis (DGGE), and similarities in band patterns were measured using the Dice coefficient. The overall similarity between the communities in reactors run at different sludge ages was indistinguishable from the similarity in communities in reactors run at identical sludge ages. This was true for both the general bacterial communities and putative AOB communities. The number of detectable bands in DGGE profiles was also unaffected by sludge age (p approximately 0.5 in both cases). Initially, the detectable diversity of activated sludge communities in all four reactors clustered with time, regardless of their designation or sludge age; however, these clusters were only weakly supported by bootstrap analysis. However, after 135 days, a sludge age specific clustering was observed in the bacterial community but not the putative ammonia-oxidizing bacterial community. The mean self-similarity of each reactor decreased, variance increased, and the number of detectable bands in DGGE profiles decreased over time in all reactors. The changes observed with time are consistent with ecological drift. Sludge age has a subtler and slower effect than we anticipated. However, we postulate that sludge age may be more evident in the taxa occurring below the detection limit of DGGE. New sequencing technology may help us address this hypothesis.

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