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 170–250mg NO2-N/L did not cause deactivation of the anammox consortium despite 2days of exposure time. Two bench-scale parallel moving bed biofilm reactors (MBBR) were operated to assess pH-associated anammox activity changes during long term treatment of anaerobically digested sludge centrate pre-treated in a suspended growth partial nitrification reactor. The pH was maintained at 6.5 in reactor R1, while it was allowed to vary naturally between 7.5 and 8.1 in reactor R2. At high nitrogen loads reactor R2 had a 61% lower volumetric specific nitrogen removal rate than reactor R1. The low pH and the associated low free ammonia (FA) concentrations were found to be critical to stable anammox activity in the MBBR. Nitrite enhanced the nitrogen removal rate in the conditions of low pH, all the way up to the investigated level of 50mg NO2-N/L. At low FA levels nitrite concentrations up to 250mg NO2-N/L did not cause inactivation of anammox consortia over a 2-days exposure time.