A UASB-anammox reactor was operated for 900 days to study its process stability. The negative effects of free ammonia (FA) and free nitrous acid (FNA) were investigated over three separate inhibitions and recoveries. The IC10, IC50 and IC90 (inhibitory concentration/a 10%, 50% and 90% activity loss) of FNA and FA responding to the NH4+-N, NO2-N and TN removal efficiency were evaluated. In the 1st inhibition, the average FNA-IC10 observed was 0.67 mu g L (1) and the FA-IC10 for TN removal was 4.85 mg L (1). In the 2nd inhibition, an FNA-IC10 of 0.44 mu g L (1) and an FA-IC10 of 3.56 were found. In the 3rd inhibition, however, both the FNA-IC10 and FA-IC10 were found to have increased, with values of 0.50 mu g L (1) and 4.42 mg L (1), respectively. A clear control region was established for multiple inhibitions and the recoveries, which followed (pH 7.5-8.5, FA below 10 mg/100 mg NH4+-N and an FNA below 0.005 mg/100 mg NO2-N) for the purpose of optimizing the operation conditions of the UASB-anammox reactor. (C) 2015 Elsevier Ltd. All rights reserved.