Activated sludge community structure was assessed directly by oligonucleotide probes for seven treatment plants in France and two treatment plants in The Netherlands. Routine operating data also were obtained for these plants. Mathematical modeling was employed to translate the operating data into parameters that reflect community function and structure. These model-translated measures were compared to the direct probing measures of community structure. Extraction of RNA and subsequent slot-blot hybridization with probes Nso1225 (for active ammonium oxidizers) and Eub338 (for all active bacteria) confirmed that the ratio of active nitrifiers to active heterotrophs was mainly explained by the ratio of TKN:BODL removed and that probe Nso1225 provided a quantitative measure of ammonium oxidizers easily distinguishable from background. Fluorescent in situ hybridization (FISH) followed similar trends. The ratio of active nitrifiers to total VSS depended on the TKN/BODL removal ratio and the sludge age. Finally, the ng rRNA/mg VSS hybridized by probe Eub338 was strongly and inversely correlated to the sludge age and suggests that specific rRNA contents of about 11,000 ng rRNA/mg VSSactive for heterotrophic bacteria and 14,000 ng rRNA/mg VSSactive for ammonium oxidizers can be used to estimate active VSS from probing results targeted to rRNA.

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