Stable partial nitrification of domestic sewage achieved through activated sludge on exposure to nitrite

Partial nitrification is crucial for application of autotrophic nitrogen removal which is beneficial for treating carbon-limited wastewater. This study presents an alternative strategy for achieving partial nitrification of domestic wastewater treatment, by treating activated sludge of a nitrifying reactor on long-term nitrite exposure. Initially, the nitrifying reactor obtained complete nitrification. After the sludge of the reactor was treated with nitrite at concentration of 5-30 mg·L^-1 without feeding for 32 days, the nitrite accumulation ratio (NAR) above 90% was achieved in 30 days, and then the NAR stabled at 97.9% under low temperature of 16.6 °C. Further analysis showed that the activity decay rate of ammonium-oxidizing bacteria (AOB) (0.020 d^-1) was lower than that of nitrite-oxidizing bacteria (NOB) (0.035 d^-1) under nitrite stress. Meanwhile, the NOB were effectively suppressed while AOB were dominant. These observations supported the feasibility of achieving stable partial nitrification by treating sludge on long-term exposure to nitrite.