[High-rate Nitrogen Removal in a Two-stage Partial Nitritation-ANAMMOX Process Under Mainstream Conditions]

A two-stage partial nitritation (PN)-ANAMMOX process was successfully carried out for low-strength NH₄⁺-N (50 mg·L^-1) wastewater treatment at ambient/low temperatures. The results show that an average total nitrogen removal rate and removal efficiency above 0.6 kg·(m³·d)^-1and 80% could be maintained, respectively, at temperatures between 20℃ and 14℃. The two-stage PN-ANAMMOX process was successful both under NO₂^--N-limited and NH₄⁺-N-limited conditions. When the two-stage PN-ANAMMOX process was operated under NH₄⁺-N-limited conditions, the "limit of technology" for nitrogen removal (TN<3 mg·L^-1) could be easily achieved by following anoxic denitrification. Lowering the temperature to 12℃ resulted in the reduction of the total nitrogen removal rate to~0.5 kg·(m³·d)^-1. Due to the low temperature, the anammox reaction became the rate-limiting step for nitrogen removal, while the PN reaction was not impacted. In the temperature range of 10-20℃, the activity-temperature coefficients (θ) of the PN granules and ANAMMOX sludge were 1.056 and 1.172, respectively, suggesting that the anammox bacteria have a higher temperature sensitivity than the ammonium oxidizing bacteria (AOB). Overall, the results clearly indicate that the nitrogen removal loading rate of the two-stage PN-ANAMMOX process is mainly controlled by the activity and quantity of anammox biomass. In contrast, the process nitrogen removal efficiency mainly depends on the performance of the first-stage PN (i.e., effluent NO₂^--N/NH₄⁺-N ratio and NO₃^--N concentration) under a constant nitrogen removal loading rate (no overload). Based on these results, a hierarchically separate control strategy was proposed to obtain a high-rate nitrogen removal during the two-stage mainstream PN-ANAMMOX process.