Nitrite reduction by a mixed culture under conditions relevant to shortcut biological nitrogen removal

Dissimilative reduction of nitrite by nitrite-acclimated cells was investigated in a batch reactor under various environmental conditions that can be encountered in shortcut biological nitrogen removal (SBNR: ammonia to nitrite and nitrite to nitrogen gas). The maximum specific nitrite reduction rate was as much as 4.3 times faster than the rate of nitrate reduction when individually tested, but the reaction was inhibited in the presence of nitrate when the initial nitrate concentration was greater than approximately 25 mg-N/l or the initial NO3--N/NO2--N ratio was larger than 0.5. Nitrite reduction was also inhibited by nitrite itself when the concentration was higher than that to which the cells had been acclimated. Therefore, it was desirable to avoid excessively high nitrite and nitrate concentrations in a denitrification reactor. Nitrite reduction, however, was not affected by an alkaline pH (in the range of 7-9) or a high concentration of FA (in the range of 16-39 mg/l), which can be common in SBNR processes. The chemical oxygen demand (COD) requirement for nitrite reduction was approximately 22-38% lower than that for nitrate reduction, demonstrating that the SBNR process can be economical. The specific consumption, measured as the ratio of COD consumed to nitrogen removed, was affected by the availability of COD and the physiological state of the cells. The ratio increased when the cells grew rapidly and were storing carbon and electrons.