Mechanistic insights into sulfate-driven performance adaptation and membrane fouling in a UASB-SBR-AXMBR system: metabolic network reconstruction and microbial community succession.

Sulfate-rich wastewater poses considerable challenges to the operational stability of biological treatment systems. This study investigated the long-term (294 days) response of a combined UASB-SBR-AXMBR process to stepwise increases in sulfate concentration from 100 to 2000 mg/L. The system maintained stable carbon and nitrogen removal performance under sulfate stress, with COD removal exceeding 90.3% and total nitrogen removal stabilizing at 85.5% via a partial nitritation-anammox (PN/A) pathway. However, high sulfate loading significantly intensified membrane fouling, with the primary driving factor likely being the co-deposition of elemental sulfur (S0) and soluble extracellular polymeric substances (S-EPS). Microbial analysis revealed persistent enrichment of Bacteroidota and Proteobacteria in the SBR, alongside a marked increase in Anammoxoglobus (from 17.1% to 51.2%) in the Anaerobic Ammonia Oxidation Membrane Bioreactor (AXMBR), underpinning system resilience. Metagenomic profiling further indicated adaptive shifts in key nitrogen-cycling genes (hao, amoA) and sulfur metabolism pathways. Notably, sulfate-reducing bacteria (SRB) outcompeted methanogens, redirecting carbon flow from methanogenesis to sulfur reduction, while niche diversification in the AXMBR expanded nitrogen removal pathways. These findings provide new mechanistic insights into the adaptive responses of integrated bioprocesses under sulfate stress and provide practical guidance for the treatment of high-sulfate industrial wastewaters such as monosodium glutamate effluent.