Operation of a Methanol Denitrification Reactor Fed with NO3- and NO2- as Electron Acceptors and Analysis of Denitrifying Bacterial Genes

We compared the denitrification rates and bacterial denitrification genes (nirK and nirS) between denitrification processes using NO2- and NO3-. We operated a sequencing batch reactor (SBR) for denitrification using activated sludge. The SBR was first fed with NO3- (Run 1), and the electron acceptor was then changed to NO2- (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 NO2-. 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.