Complete suppression of nitrite oxidizing bacteria by continuously low oxygen levels in biofilm-based mainstream partial nitritation/anammox

Mainstream nitrogen removal by partial nitritation/anammox can yield energy and cost savings for sewage treatment. Selective suppression of unwanted nitrite oxidizing bacteria (NOB) remains a key bottleneck for its scale-up. In this study, a 51L rotating biological contactor was covered to regulate the oxygen transfer and availability and was operated at 20±2°C for 900 days. Applying dissolved oxygen concentrations of <1.01 and <0.52 mg O2 L-1, for the biofilm respectively emerged above and submerged in the water, resulted in complete and long-term NOB suppression with a low nitrate production ratio of 10±4%, corresponding to the stoichiometry. Additionally, weekly free ammonia (FA) treatments (29±1 mg NH3-N L-1 for 3h) of the biofilm could speed up the NOB suppression while free nitrous acid treatments (up to 3.7±0.2 mg HNO2-N L-1 for 24h) had limited effects. This study demonstrated the potential of controlling NOB suppression in biofilm systems by continuously low oxygen levels and recurrent FA exposure under mainstream conditions, opening opportunities for resource efficient nitrogen removal.