Importance of the operating pH in maintaining the stability of anoxic ammonium oxidation (anammox) activity in moving bed biofilm reactors

► Two parallel moving bed biofilm reactors were operated to assess pH-associated anammox activity changes. ► It was observed that the reactor operating under pH of about 7.5-8.1 exhibited 61% lower NRR than a reactor set at pH of 6.5. ► It was found that high nitrite was not necessarily the cause for the reactor destabilization. ► Free ammonia was shown to be an important stability parameter in anammox systems. ► Nitrite as high as 170-250 mg NO2-N/L did not cause deactivation of the anammox consortium despite 2 days of exposure time.