Stability of autotrophic nitrogen removal system under four non-steady operations

•The nitritation–anammox process was performed and evaluated in four non-steady operations.•The aeration system controlled nitritation efficiency satisfactorily in the all operations.•Nitritation and anammox activities recovered after the shutdown of the system for 3 d.•The maximum nitritation and nitrogen-conversion rates were 2.3 and 9.0 kg-N/m3/d, respectively.

Stable nitrogen removal from the digester supernatant for sludge via the nitritation–anammox process under steady operations of ammonium concentration and flow rate has been often reported. In this study, the effects of four non-steady operations, intentional fluctuations of influent concentration from 890 to 650 mg-N/L and hydraulic load of the 10% increase, temporally shutdown for 3-d and maximum capacity of each reactor, were evaluated in the nitritation–anammox process treating digester supernatant for sludge. No serious effects were observed in the anammox reactor because the aeration-control system in the nitritation reactor responded and controlled the nitritation efficiency satisfactorily against intentional fluctuations and temporally shutdown. Finally, the maximum capacity of each reactor was evaluated, and the nitritation rate was found to be 2.3 kg-N/m3/d at a DO of 4.0 mg/L, and the nitrogen-conversion rate was 9.0 kg-N/m3/d.