High-Rate Nitrification by CO2-Sensitive Nitrifying Bacteria
High-rate nitrification has been observed in a lab-scale activated sludge reactor using synthetic wastewater under increased (1%) pCO2. The populations found in the reactors were challenged by drastically reducing the solids retention time to cause washout of slow growing bacteria. Under conditions of increased pCO2 the experimental reactor maintained a nitrification rate that was 5 times higher than what was observed in the control reactor. This suggests that some populations of autotrophic ammonia oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) may be carbon-limited and can grow at an increased rate under elevated pCO2. Fluorescence in situ hybridization was used to determine the populations of AOB and NOB that have shown sensitivity to elevated levels of pCO2. The results from this study suggest that the NOB Nitrospira spp. are sensitive to elevated pCO2 resulting in an increase in population growth. An AOB type that displays sensitivity to elevated pCO2 was not identified by this study. These findings suggest that some autotrophic bacteria in both natural and engineered systems are carbon-limited and further investigation is warranted.