BACKGROUNDResults in the literature are divergent regarding dissolved oxygen (DO) level to ensure nitrite-oxidizing bacteria (NOB) suppression in mainstream conditions. In this study, a one-stage granular partial-nitritation (PNA) was operated controlling the oxygen transfer over ammonium loading rate (OTR/ALR). The effects of extremely low DO availability on autotrophic nitrogen removal (ANR) performance and NOB suppression were assessed. RESULTSThe strategy applied in the sequencing batch reactor (SBR) in sidestream and mainstream conditions (temperature at 25 degrees C) allowed micromolar bulk liquid DO concentrations to be obtained. Nitrite production rate (NPR) by aerobic ammonium-oxidizing bacteria (AOB) was sufficient to provide nitrite to anammox bacteria (AnAOB) and sustain nitrogen removal rates (NRR) of 0.340.05 and 0.37 +/- 0.07kgNm(-3)d(-1) during sidestream and mainstream operation, respectively. Extremely low DO assisted by free ammonia (FA; 6.2mgNL(-1)) in sidestream conditions resulted in complete nitratation suppression. In mainstream conditions, Nitrospira spp. growth led to limited nitratation rates. Decreasing the OTR/ALR resulted in lower oxygen consumption by AOB than by NOB, as well as reduced nitrite consumption by AnAOB, which is likely to be due to granule structure. CONCLUSIONSLow DO availability did not compromise ANR. Micromolar DO allowed complete NOB suppression only with FA assistance (sidestream) but was sufficient to keep nitratation limited in mainstream conditions. (c) 2018 Society of Chemical Industry