3.2. Start-up of the CANON MBR
The nitrogen removal performance of CANON MBR was observed for a 66-d period from start-up (Fig. 3A and B). CANON required the simultaneous activity of AerAOB and anammox, which both prefer anoxic or anaerobic conditions [20]. Although DO limitation was used from day 1 to day 18, most removed NH4+-N was still oxidized to NO3−-N, and nitrite accumulation of effluent was not observed (average concentration of NO2−-N lower than 4.1 mg/L). AerAOB enrichment is the key stage for CANON, in which, the abundance and activity of NOB that can consume nitrite should be reduced. On day 19, the influent pH was increased from 8.0 to 8.5 and the concentration of incoming NH4+-N was increased to more than 150 mg/L so that the FA concentration in the reactors sharply increased to suppress NOB without harming the AerAOB [11]. Therefore, from day 19, nitrate gradually reduced and membrane effluent displayed a significantly higher concentration of NO2−-N compared with the initial stage (average, 45 mg/L), combined with a higher NAR that approached 76.6% (on day 35) indicating growth of AerAOB and effective inhibition on NOB.