BACKGROUNDAmmonium oxidizing bacteria (AOB) have been suggested to have a lower oxygen half-saturation index than that of nitrite oxidizing bacteria (NOB), i.e. K-O,K-AOB < K-O,K-NOB. However, recent research has shown the opposite. We propose that biomass floc morphology and nitrifier enrichment are the causes of the conflicting reports, thus sludge samples with low NOB enrichment were ultrasonicated to produce samples with different floc diameters and tested for their K-O,K-AOB and K-O,K-NOB. RESULTSExperimental results showed that the apparent K-O,K-NOB values stabilized at c. 0.20mg O-2 L-1 after the activated sludge sample was ultrasonicated and the medium floc diameter was reduced from c. 420 to 50 mu m. In comparison, the apparent K-O,K-AOB dropped significantly from c. 1.5 to 0.24mg O-2 L-1. Simulation results were in agreement with the experimental observations. CONCLUSIONSHigh floc diameter and low NOB enrichment were identified to be the reason for the reversion in the comparison of measured K-O,K-AOB and K-O,K-NOB. The study explains the conflicting reporting on the relative comparison of measured K-O,K-AOB and K-O,K-NOB. (c) 2017 Society of Chemical Industry