An anaerobic ammonium oxidation (anammox) process was operated for 900 days in a UASB reactor. The nitrogen removal rate (NRR) reached 3.6 g NL-1d-1 in the steady operation stage and red granules with a diameter of 0.5-3.5 mm were formed. The inhibition processes caused by a high substrate concentration, lack of K+ and overloading were investigated in a long-term experiment. All the inhibition processes led to the excess of substrate that further inhibited the anammox reaction. In the substrate-based inhibition processes, ammonium, nitrite and unsuitable pH were shown to synergistically inhibit the anammox activity. Free nitrous acid (FNA) was considered the most toxic factor and should be kept below 1.5 μg L-1 to maintain stable operation over the long-term. The SAA (specific anammox activity) tests also indicated that nitrite played a major role in the substrate-based inhibitory processes. After three inhibition episodes, the substrate tolerance ability was significantly improved in the batch tests. While all the inhibition processes were able to be reversed by various compensation methods, restoration was time consuming. The consumption ratio of NO2- to NH4+ (RS) was lower than the reported ratio (1.32) and varied with changes in the operational conditions.
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