We compared the denitrification rates and bacterial denitrification genes (
nirK and
nirS) between denitrification processes using NO
2- and NO
3-. We operated a sequencing batch reactor (SBR) for denitrification using activated sludge. The SBR was first fed with NO
3- (Run 1), and the electron acceptor was then changed to NO
2- (Run 2). Methanol was fed as the major electron donor through out the operational period (64 days). Denitrification rates (mg-N · g-MLSS
-1 · h
-1) were measured at regular intervals. Results showed that the maximum denitrification rates were more than 40 mg-N · g-MLSS
-1 · h
-1 irrespective of electron acceptor type. However, it takes 12 days to reach the maximum denitrification rate after changing the electron acceptor to NO
2-. The results of the cloning analysis of
nirK and
nirS implied that the lag time was attributable to bacterial population shifts, because the
nirK and
nirS detected at Run 1 were phylogenetically different from those at Run 2. However, the changes in the total copy numbers of
nirK or
nirS could not explain the changes in the denitrifying rates.
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