Unravelling the optimal strategy on the stable mineralized anammox granular systems in treating low-strength wastewater

Effective retention of anammox bacteria (AnAOB) is the prerequisite for the application of anammox process, especially in treating low-strength wastewater. The employment of anammox granules coupled with phosphorus minerals has gained extensive attention as retaining biomass. To achieve the stability of mineralized anammox system for low-strength wastewater treatment, two switching strategies (cascade strategy and plunging strategy) were compared in terms of nitrogen removal performance, physicochemical properties and microbial community. Results showed cascade strategy significantly implemented the nitrogen removal rate of 0.86 ± 0.05 kg N/m3/d at hydraulic retention time of 1.6 h, with higher phosphorus removal via the hydroxyapatite formation. The cross-section observation indicated mineralized anammox granules exhibited a unique spatially heterogeneous structure with distinct zoning. Moreover, the granules displayed dense structure, excellent settleability, superior mechanical strength and outstanding biomass retention based on cascade strategy. Microbial analysis revealed the dominant populations were shifted from Ca. Kuenenia to Ca. Brocadia after switching to low-strength conditions. Further investigation is warranted to explore effective methods for phosphorus recovery from hydroxyapatite. Overall, the study provided a reference for promoting and understanding the synergistic nitrogen removal and phosphorus recovery in actual applications.