Molecular techniques were used to compare the microbial community of suspended activated sludge and attached biofilm in a biological nutrient removal process (TNCU-I) operated under various COD/total P (COD/TP) feed ratios. Analysis of restriction fragment length polymorphism of amplified 16S rRNA genes indicated that the microbial population structures were more closely related among those sludge samples taken from aerobic sludge than that taken from biofilm attached on the RBC. The use of different COD/TP feed ratios (300/2.5-300/25) had no significant effect on the change of microbial structures of sludge samples. The 16S rDNA clone library further indicated that at least eight and six different microbial populations were present in activated sludge and RBC biofilm, respectively. The phylogenetic analysis revealed that six of those eight major clones obtained from the suspended activated sludge were from the beta-subclass of the class Proteobacteria. In contrast, only one clone obtained from biofilm belonged to the beta-subclass of the Proteobacteria. This difference in the microbial population structure was possibly attributed to the growth state (suspended or attached) or carbon source (autographs or heterotrophs) of the sludge samples rather than the effect of the COD/TP ratio used.

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