In the process of denitrification in intermittently aerated reactors, the lengths of the sequential aeration and nonaeration periods within the residence time are also of significance
(34). As the induction of the different denitrification enzymes proceeds sequentially, the different intermediates of denitrification accumulate temporarily during nonaeration periods.
Oxygen inhibits the synthesis of nitrate reductase only partially,while the synthesis of nitrite reductase is completely suppressed (34). At the start of anoxic conditions after an aerobic period in intermittently aerated reactors, only the nitrate reduction takes place, leading to a temporary nitrite accumulation (26). Therefore, the length of nonaeration periods in intermittently aerated reactors is also important to achieve complete denitrification. Baumann et al. (7) assessed mRNA and the enzymes involved in denitrification in activated sludge
reactors operated with intermittent aeration and concluded that denitrifying bacteria were not able to completely synthesize the enzymes for the denitrification process in appropriate
amounts when subjected to nonaerated stages shorter than 3 h,resulting in the accumulation of nitrite during nonaeration periods. In the current study, only the reactor with a 4-h
nonaeration period showed considerable nitrite reduction during nonaeration periods.
Comparisons of amoA DGGE profiles and TKN and ammonia removal efficiencies suggest that superior ammonia and TKN removal efficiencies did not seem to be associated with specific AOB species or with higher AOB species diversity.These results seem to contradict the generally accepted concept that higher functional diversity is beneficial for performance stability (8, 11). However, this concept is necessarily related to the diversity of the microorganisms that are metabolically active in the process and not merely the species diversity of microorganisms present in the biomass. Although amoA is one of the functional genes involved in the oxidation
of ammonia, PCR amplification of environmental amoA genes does not reveal gene expression or microbial activity. Rather,amplification of the amoA gene was used only as an alternative
method for identifying AOB and hence can give an indication only of species diversity and not of functional diversity. A more appropriate approach for testing correlation of functional diversity and performance stability would involve methods that can determine which processes are catalyzed by which microorganisms and to what extents.