Recovery effects of the long-term cryopreserved Anammox sludge by adjusting the sludge amount

Anammox process was one of the most promising nitrogen removal technologies. This study investigated the recovery performance of Anammox sludge after 83 days of cryopreservation in two reactors (R1 and R2). Reactor R1 utilized Anammox sludge pretreated with low-substrate simulated wastewater prior to long-term cryopreservation, and successful recovery was achieved by discharging sludge under ammonia nitrogen concentrations of 100 mg/L. The total nitrogen removal efficiency (TNRE) reached 70.0% on day 48. Reactor R2 used Anammox sludge pretreated with high-substrate simulated wastewater before cryopreservation. At an ammonia nitrogen concentration of 100 mg/L, the TNRE reached 87.0% on day 18. After increasing the ammonia nitrogen concentration to 300 mg/L and discharging sludge, the TNRE reached 84.6% on day 38. When the ammonia nitrogen concentration was elevated to 500 mg/L, system performance deteriorated. Recovery was unsatisfactory when the ammonia nitrogen concentration was reduced back to 300 mg/L. Finally, adding Anammox sludge restored the TNRE to 85.6% after 35 days of operation. The results suggest that adding Anammox sludge is essential for nitrogen removal recovery in reactors under high ammonia nitrogen concentration inhibition, while sludge discharge is crucial when free ammonia (FA) is present. This study provides a simple and effective strategy for recovering the activity of Anammox sludge after long-term cryopreservation.