Pedrouso Fuentes, AlbaMorales, NicolásRodelas González, BelénCorrea Galeote, DavidVal del Río, ÁngelesCampos, José LuisVázquez Padín, José RamónMosquera Corral, Anuska2023-06-062023-06-062023Separation and Purification Technology 317 (2023) 123851http://hdl.handle.net/10347/30645Two-stage partial nitritation (PN) and anammox (AMX) systems showed promising results for applying autotrophic nitrogen removal under mainstream conditions. In this study, a pilot-scale (600 L per reactor) two-stage PN/AMX system was installed in a municipal wastewater treatment plant (WWTP) provided with a high-rate activated sludge (HRAS) system for organic carbon removal. The PN/AMX system was operated without temperature control (ranging from 11 to 28 °C) and was subjected to the same variations in wastewater characteristics as the WWTP (22 to 63 mg NH4+- N/L). The developed strategy is simple, does not require the addition of chemicals and is characterised by short start-up periods. The PN process was established by applying a high hydraulic load and maintained by in situ accumulated free nitrous acid (FNA) of 0.015–0.2 mg HNO2-N/L. Based on pH value, a controlled aeration strategy was applied to achieve the target nitrite to ammonium ratio in the effluent (1.1 g NO2–-N/g NH4+-N) to feed the AMX reactor. Although NOB were not fully washed out from the system, nitrite accumulation remained (>99 %) stable with no evidence of NOB activity. In the AMX reactor, an overall nitrogen removal efficiency of 80 % was achieved. Regarding effluent quality, 12 ± 3 mg TN/L was obtained, but 5 mg NO3–-N/L was already in the HRAS effluent. The relative abundance of NOB showed a strong negative correlation with the FNA concentration, providing a good strategy for establishing PN under mainstream conditionseng© 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/http://creativecommons.org/licenses/by-nc-nd/4.0/Ammonium oxidising bacteria (AOB)AnammoxMunicipal wastewater treatmentNitrite oxidising bacteria (NOB) suppressionPN/AMX pilot plantjournal article10.1016/j.seppur.2023.1238511383-5866open access