High ammonium nitrification was studied in a sequencing batch reactor (SBR) with two different oxygen supply methods: controlled oxygen supply rates (dissolved oxygen [DO] in the reactor controlled between 2 and 3 mg/L) and uncontrolled oxygen supply rates (DO in the reactor uncontrolled and changed ambiently with oxygen use rate). Experimental results showed that oxygen supply methods have significant effects on SBR performance for high ammonium nitrification. With the controlled oxygen supply method, ammonium and nitrite oxidations took place simultaneously. With the uncontrolled oxygen supply method, ammonium oxidation proceeded smoothly during the low DO period of a cycle; however, nitrite oxidation was at first completely inhibited at low‐DO but recovered during the high DO period of the same cycle. Ammonium and nitrite were oxidized sequentially during a cycle in SBR. Ammonium oxidizers developed an ability to endure the fluctuation of DO, but nitrite oxidizers did not. Extensive starvation under high DO concentrations plays an important role in ammonium oxidation under low DO conditions.
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