Operation stability and recovery performance in an Anammox EGSB reactor after pH shock

• Long-term operation stability of Anammox EGSB reactor was investigated.
• Effect of pH shock and recovery was characterized by continuous experiment.
• FA and FNA were the main factors governing the process performance.
• The tolerance of AAOB to the substrate concentration was decreased after pH shock.

An Anaerobic ammonia oxidation (Anammox) expanded granular sludge bed (EGSB) reactor was investigated with a long-term continuous operation. The nitrogen removal performance and stability of process subjected to transient pH shock (decreased from 9.0 to 7.0 and maintaining for 24 h) were evaluated. In the steady stage, the nitrogen loading rate (NLR) and nitrogen removal rate (NRR) can reach to 10 and 8.5 kg-N m−3 d−1, respectively. However, the system had a low tolerance for transient pH shock. The deterioration of the granule sludge properties and the inhibition of specific Anammox activity (SAA) resulted in the destabilization of the EGSB after pH shock. The main factors governing the treatment performance of EGSB were the high concentration of free nitrous acid (FNA) and free ammonia (FA). However, the lower than 10% reduction of ammonia removal can be maintained when FA and FNA were lower than 15 mg l−1 and 15 μg l−1, respectively. The process was successfully recovered by controlling the FNA and FA concentration after the low pH shock.