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The control of NOB challenges the implementation of partial nitritation/anammox (PN/A) at main line. The mechanisms leading to long-term persistence or successful control of NOB were studied in hybrid PN/A systems, where biofilms and suspended biomass coexist. A hybrid MBBR was run during 565 days in SBR mode on aerobically pre-treated MWW at 15 C and different DO setpoints. Selective NOB washout from the flocs was successfully achieved and their re-growth prevented under prolonged operation at low DO. NOB could not be washed out from the biofilm in the long-term, but their activity was consistently controlled by DO setpoint, and fully suppressed by operation at limiting DO. High nitrogen removal efficiency (88%) and volumetric nitrogen removal rates during aeration (79 mgN∙ L-1∙ d-1) were achieved along with the best effluent quality reported so far for mainstream PN/A (1.9 mgNH4-N∙ L-1, 0.3 mgNO2-N∙ L-1, and 0.5 mgNO3-N∙ L-1). A dynamic mathematical model of the hybrid system was developed assuming perfect biomass segregation with AOB and NOB in suspension and AMX in the biofilm. Experimental evidence and simulation results identified the DO setpoint, controlled removal of suspended sludge, and anammox activity in the biofilm as the main parameters controlling NOB washout. Differently from sole biofilm systems, a lower oxygen affinity constant of AOB over NOB is not required. NOB washout is in principle possible even when KO2, NOB/KO2, AOB= 0.14. This study strongly supports the inherent flexibility of hybrid systems for NOB control towards robust mainstream PN/A.