Partial nitrification/denitrification can be attributed to the slow response of nitrite oxidizing bacteria to periodic anoxic disturbances.

This work aims to assess and model the behavior of both ammonium (AOB) and nitrite (NOB) oxidizing bacteria during the transition from completely anoxic to aerobic conditions. An enhanced aerobically grown culture containing AOB and NOB was subjected to anoxic conditions of varying durations from 1.5 to 12 h before its exposure to aerobic conditions. Experiments were carried out in both continuously stirred tank reactor (CSTR) and batch type reactors. Although the AOB did not exhibit any impact in their performance following the anoxic disturbance, the NOB were seriously inhibited presenting a period of reduced growth rate, which was proportional to the duration of the disturbance. This finding proves the previously postulated mechanism (NOB inhibition under periodic aerobic/anoxic operation) for achieving nitrogen removal via the partial nitrification/denitrification (PND) process as demonstrated in lab- and pilot-scale operating conditions. A mathematical model was developed to describe with sufficient accuracy the performance of AOB and NOB under aerobic, anoxic, and transient conditions in both CSTR and batch type systems. The model is able to describe the inhibitory effect of anoxic exposure to NOB by assuming enzyme deactivation (under anoxic conditions) and reactivation (adjustment of the NOB enzymatic mechanism) under aerobic conditions. The presented kinetic model is quite simple and general and therefore may be used for predicting the performance of mixed growth biological systems operating via the PND process.