Suppression of nitrite-oxidizing bacteria under the combined conditions of high free ammonia and low dissolved oxygen concentrations for mainstream partial nitritation

Abstract

Abstract The possibility of nitrite-oxidizing bacteria (NOB) suppression in mainstream wastewater (COD/N of 6.0, ammonia of approximately 50 mg N L−1) was investigated by controlling the ratio of dissolved oxygen to total ammoniacal nitrogen (DO/TAN) in a continuous suspended sludge reactor without prior COD removal. It was found that a combination of high free ammonia (FA) of approximately 20 mg N L−1 and low dissolved oxygen (DO) of 0.3 ± 0.1 mg L−1 (DO/TAN ratio of 0.003) established in the start-up period was an efficient way to inhibit the growth of NOB. The activity of NOB was more inhibited as FA concentration increased, particularly at the initial ammonia concentration of 175 mg N L−1 (FA of about 20 mg N L−1) Then, the influent ammonia was lower to 50 ± 2 mg N L − 1 (FA of 4.9 ± 0.2 mg N L−1), a typical nitrogen concentration of mainstream wastewater. After four months of operation, mainstream partial nitritation (PN) became stable with nitrite accumulation ratio over 87% and COD removal of above 94% (effluent COD of 18.4 mg L−1). Besides, the effluent NO 2 − -N/NH3-N ratio was 1.22 ± 0.12, which was suitable for the subsequent anaerobic ammonium oxidation (anammox) reaction. The relative abundance of ammonia-oxidizing bacteria (AOB) (e.g., Nitrosomonas) was 15 times higher than NOB. The abundance (0.3%) of Nitrospira (k-strategist NOB) was insignificant, while Nitrobacter (r-strategist NOB) and Nitrotoga (NOB) were not detected during the steady state of PN under low nitrogen load because NOB was suppressed and washed out of the reactor by the condition of extremely low DO/TAN ratio in the start-up period. Thus, it was noted that NOB suppression for stable PN could be accomplished by providing high FA and low DO concentrations, especially in the early stage.