The performance of two Anammox upflow biofilm filters (UBF) was investigated using synthetic wastewater. Results showed that the Anammox UBFs became unstable under high nitrogen loading rate. Performance deterioration was observed when the nitrogen loading rate exceeded the maximum removal capacity of the reactors. Effluent pH was increased to 8.50~9.05 because of Anammox reaction, which were higher than the optimum pH range of Anammox bacteria, i.e. 6.70~8.30, resulting in direct inhibition of Anammox bacteria and subsequent destabilization of the two Anammox UBFs. Unfortunately, deficiency of free nitrous acid [only (1.9±4.3)×10-5-(2.2±2.5)×10-5 mg•L-1] and excess of free ammonia (as high as 178.1 mg•L-1), both of which were the substrates of Aanammox bacteria, were induced by high pH, resulting in starvation effect of free nitrous acid and toxicity of free ammonia to Anammox bacteria, strengthening instability of the Anammox UBFs. Moreover, the performance of the bioreactors can not recover automatically. Tap water was used necessarily to rinse the bioreactors to wash out the residues. After that, performance recovery can be reached quickly by applying low nitrogen loading rate.
