Survival strategies of Nitrospira in a stable nitritation-denitritation system treating low-strength fermented wastewater

Mainstream nitritation and denitritation can save aeration energy and carbon sources, but it is difficult to eliminate nitrite-oxidizing bacteria. Except for nitrite oxidation, Nitrospira may benefit from mixotrophic life-styles. In our previous study, stable NOB suppression was achieved in a multi-cycle sequencing batch reactor (MCSBR) treating fermented low-strength wastewater. However, abundant Nitrospira was always detected, which was contrary to the negligible nitrate accumulation. Batch tests indicated that only acetate could be consumed by the enriched Nitrospira within six hours. In another batch test within six days, nitrate was fully or partially denitrified to nitrite after adding formate or acetate, while Nitrospira decreased 86.5 % and 28.7 % respectively. After adding formate, two hydrogenotrophic denitrifiers Hydrogenophaga and Methyloversatilis highly prolifer-ated, but only Hydrogenophaga was also found in the MCSBR. Therefore, Nitrospira might still grow through nitrite oxidation, while the generated nitrate was timely reduced by Hydrogenophaga in the presence of simple organic carbons, or directly survived by heterotrophic acetate metabolism.