Achieving intensified nitrogen removal and vivianite recovery from sludge digester liquor in two-stage partial nitrification-electrolysis anammox system

The partial nitrification-anammox (PN-A) is a promising process to treat sludge digester liquor. However, its application was restricted by the limited total nitrogen removal efficiency and the demand for additional phosphorus removal methods. To address these issues, a novel two-stage partial nitrification-electrolysis anammox (PN-EA) process was constructed for the treatment of sludge digester liquor in this study. By incorporating a current control system and an iron electrolysis device in the anammox unit, the PN-EA system achieved significant phosphorus removal (96.58 ± 1.41%). X-ray diffraction data demonstrated that vivianite-based phosphorus accounted for 95.00% of the total solid phase in the EA unit. Additionally, Mössbauer spectroscopy further revealed that 49.27 ∼ 61.07% of the total iron in the EA samples was bound in vivianite. The intricate composition of the sludge digester liquor may potentially lead to oxidation and impurity occupation, thereby compromising the purity and recovery rate of vivianite. Furthermore, the symbiotic relationship between Candidatus Brocadia and versatile denitrifying bacteria including Denitratisoma, Arenimonas, Hydrogenopaga, and Dechloromonas was confirmed through microbial analysis, functional genes prediction, and enzyme activity analysis. The PN-EA system demonstrated the collaboration of multiple metabolic pathways, such as anammox, heterotrophic, autotrophic (hydrogen and Fe2+-driven), and electrochemical denitrification, resulting in an impressive total nitrogen removal efficiency of 92.60 ± 1.36%. This research contributes to the broader implementation of anammox process and provide new insights into the efficient removal of nitrogen and the recovery of high-quality P from nutrient-rich wastewater.