Temporal Variation in Maximum Cell-Specific Nitrification Rates in Municipal Wastewater Treatment Plant and Controlling Factors

The numbers of ammonia-oxidizing bacteria (AOB), Nitrospira and Nitrobacter in a municipal wastewater treatment plant were examined for five months using a real-time PCR quantification technique. The numbers of AOB and Nitrospira were in the ranges of 3.8×1010-2.0×1011 and 4.7×1010-1.6×1011 cell · l-1, respectively. Additionally, the fractional percentages against the number of eubacteria were in the ranges of 2.1-7.6 and 2.6-7.0 %, respectively. Nitrobacter was less than 1 % as common as Nitrospira. On the other hand, the maximum ammonia- and nitrite-oxidizing rates obtained from aerobic batch tests ranged from 0.08 to 0.41 and from 0.10 to 0.27 mmol-N · l-1 · hr-1, respectively. No correlation between cell number and maximum rate was observed. The maximum cell-specific ammonia- and nitrite-oxidizing rates were then estimated to be in the range of 0.53-5.6 and 1.2-5.4 fmol-N·cell-1 · hr-1, respectively. In other words, even in the same wastewater treatment plant, these maximum cell-specific rates were not unique. To explore the factors controlling the maximum cell-specific ammonia-oxidizing rate, the relationship with in situ ammonia-oxidizing activity per cell was investigated. A fairly good correlation was obtained. The result indicates that the amount of ammonia oxidized per cell controls the maximum cell-specific ammonia-oxidizing rate and is the primary contributor to the variation. Meanwhile, the maximum cell-specific nitrite-oxidizing rate responded to the increase in the maximum cell-specific ammonia-oxidizing rate when the number of Nitrospira was less than that of AOB.