Recovery mechanisms of deteriorated nitrogen removal in aerobic granular sludge with/without salinity stress:
Zihan Sheng1, Jie Xu1, Shichang Liu1
1School of Environmental and Municipal Engineering, Qingdao University of Technology, Qingdao, 266580, China.
Abstract:
The deterioration of pollutant removal in aerobic granular sludge (AGS) during long-term operation represents one of the bottlenecks in AGS biotechnology. This study investigated the recovery process of nitrogen removal in long-term operating AGS reactors under different salinity by adjusting aeration intensity, sludge age and granule size. Results demonstrated that biomass of small granules determined nitrification capacity of AGS and saline AGS (SAGS). Salinity induced the specific enrichment of Nitrosomonas in smaller granules, leading to significant alterations in granule size requirements for full NH4+-N removal (salt-free system: 2-4 mm granules; saline system: 2-3 mm granules). Refluxing sludge after crushing oversized granules proved to be efficient for enhancing nitrification. Excellent TN removal was achieved after recovering (R1: 83.52 ± 1.43 %, R2: 82.89 ± 1.87 %). Restructuring of denitrification pathways occurred alongside enhanced nitrification. Redundancy analysis (RDA) revealed that the increased small granules boosted conversion of organic substrates, establishing the competitive advantage of heterotrophic denitrifiers (Candidatus_Competibacter, Thauera, Denitromonas) over autotrophic denitrifiers (Hydrogenophaga, Pseudarcobacter). This work provides new insights into mechanism of nitrogen removal in mature AGS with/without salinity stress and highlights the importance of selecting appropriate repairing strategies for enhancing nitrification.
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