Innovative PN/A Process Optimization with Dual Intensification Strategies for Nitrogen Removal from Rare Earth Tailwater

The partial nitrification/anammox (PN/A) process represents a significant innovation in the treatment of nitrogen-containing wastewater. However, due to the relatively low ammonium concentration in rare earth tailwater (RET), the application of the PN/A process requires two critical adjustments: stable nitrite accumulation and the enhancement of anammox bacteria activity. This study evaluates a pilot-scale PN/A system for low-ammonium RET treatment, incorporating two innovative intensification strategies to enhance the stability and efficiency. The PN reactor used zeolite packing to elevate in situ free ammonia, washing out nitrite-oxidizing bacteria, and maintaining a nitrite accumulation ratio above 90%. Exogenous hydrazine (4 mg/L) added to the anammox reactor enhanced the secretion of extracellular polymeric substances and improved the sludge activity. Furthermore, the supplementary electrons provided by hydrazine influenced nitrogen metabolic pathways, thereby enhancing the reactor’s resistance to nitrogen fluctuations. Nitrosomonas (13.42%) was the dominant genus in the PN reactor and Candidatus Brocadia (15.26%) in the anammox reactor. Candidatus Brocadia and Saccharimonadales form a cooperative syntrophic microbial community. These innovations demonstrate the effectiveness of dual intensification strategies in improving PN/A system performance, addressing nitrogen management challenges in rare earth mining effluents, and showing the potential for broader applications in low-ammonium wastewater treatment.