Novel nitritation process using heat-shocked nitrifying bacteria entrapped in gel carriers

Nitritation (ammonium being oxidized to nitrite) is a cost-effective method for treating wastewater having high ammonium concentrations or low C/N ratios. We developed a novel nitritation process based on the observation that nitrite-oxidizing bacteria (NOB) in sewage sludge can be killed by heat shock, but ammonium-oxidizing bacteria (AOB) may survive. The effects of maximum heat-shock temperature and heat-shock duration on populations of AOB and NOB in gel carriers were measured. No NOB were detected after a heat-shock treatment higher than 60 °C for 20 min. However, the population of AOB continued to exist at above 10⁸ MPN/mL-carrier even after heat shock at 80 °C for 1 h. To evaluate the nitritation performance, continuous feeding tests were conducted using heat-shocked gel carriers treated at three temperatures. Stable nitritation was observed for 49 days when gel carriers were heat shocked at 60-90 °C for 1 h. However, because nitrate production, i.e., nitratation, was observed after 77 days, the gel carriers were heat shocked again. Consequently, nitratation stopped immediately and nitritation restarted after 14 days. These results clearly show that this technique is effective for suppressing nitratation.