Rapid start-up and microbial community succession of a two-stage upflow anaerobic sludge blanket-anoxic/oxic coupled reactor system

Abstract

In this study, a two-stage upflow anaerobic sludge blanket-anoxic/oxic (UASB-A/O) coupled reactor system was developed for the rapid start-up of high-ammonium wastewater treatment, with partial denitrification-Anammox (PD/A) as the core nitrogen-removal pathway. Rapid start-up was assessed based on stable nitrogen-removal performance, sustained nitrite availability for Anammox, and enrichment of Anammox-related bacteria. The operating performance and microbial community succession were evaluated over 166 d of continuous operation under progressively increasing influent NH₄⁺-N concentrations. The front-end UASB reactor maintained a nitrogen-removal efficiency of 84% ± 1%, whereas the whole coupled system achieved a total inorganic nitrogen-removal efficiency of 98% ± 1%, indicating efficient deep nitrogen removal under high loading conditions. Microbial community analysis revealed a clear spatiotemporal succession during start-up. The community gradually shifted from mixed flora to a functionally stratified consortium. The UASB reactor and the anoxic zone were dominated by heterotrophic denitrifying bacteria, whereas the downstream aerobic zones were enriched with ammonia-oxidizing and anaerobic ammonia-oxidizing bacteria. The initially abundant non-target genus Arenimonas was gradually replaced by functional taxa, with the abundances of Thauera and Candidatus Kuenenia increasing by 19% and 3%, respectively. Functional gene analysis revealed that narG/napA, amoA/amoB/amoC/hao, and hzs/hdh were enriched, which supported the coupling of partial denitrification and Anammox. Overall, the two-stage UASB-A/O system achieved rapid start-up, near-complete nitrogen removal, and directional microbial succession, suggesting its potential for the efficient treatment of high-ammonium wastewater.

Graphical abstract

Keywords

High-ammonium wastewater / Landfill leachate / Partial denitrification-Anammox / UASB-A/O reactor system / Microbial community succession / Functional gene analysis

Highlight

	● A two-stage UASB-A/O reactor treated high-ammonium leachate.
	● Total nitrogen removal reached 98% ± 1% under high NH4+-N loading.
	● Microbial communities formed clear spatial functional zones.
	● Thauera , Nitrosomonas , and Candidatus Kuenenia were enriched.
	● Functional genes confirmed coupled PD/Anammox pathways.

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