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The application of free ammonia inhibition towards partial nitritation in mainstream wastewater treatment Luo, Yaqian


Partial nitritation/anammox (PN/A) is a novel pathway for nitrogen removal in wastewater treatment that offers advantages of low oxygen and organic carbon demands as well as high potential for energy recovery. However, the partial nitritation process remains a key hurdle for the widespread implementation of the PN-A process in mainstream treatment due to the difficulty in washing out nitrite-oxidizing bacteria (NOB) from active sludge. Exposing biomass to high concentrations of free ammonia (FA) has been reported as an effective strategy to achieve partial nitritation. This study examined the effectiveness of treating 20% of return activated sludge with synthetic centrate containing FA at 200 mg N/L for 24 hours to promote partial nitritation in mainstream wastewater treatment. Experimental and control bioreactors were operated under two different conditions, with or without FA treatment, respectively, after reaching similar nitrification performance. Biokinetic parameters of ammonium-oxidizing bacteria (AOB) and NOB were estimated by performing respirometric batch tests with activated sludge biomass from the two bioreactors under different operational conditions, and calibrating a process model based on oxygen mass balance. The bioreactor performance showed that the FA treatment strategy promoted the PN process, with a maximum nitrite accumulation ratio (NAR) of 41.9 ± 2.1% after treating return sludge with high FA solution for 37 days. However, this nitrite accumulation was not stable, and the NAR decreased to 10.9 ± 6.0% after 33 days, indicating that NOB were able to acclimate to the temporary exposure to a high FA concentration. The biomass yield coefficient (Y) of AOB increased during FA treatment, while the maximum specific growth rate (μmax) of AOB and NOB decreased under this condition. Microbial community analysis on activated sludge under FA treatment, and further investigations on the optimization of the FA treatment strategy combined with other NOB out-selection strategies are required to better facilitate the application of PN-A to full-scale mainstream wastewater treatment.

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