Mechanistic insights into low-dose nZVI-enhanced process stability under variable industrial loads in field-scale anaerobic treatment: Interfacial evolution and strain-resolved adaptation.

Anaerobic treatment of industrial wastewater is limited by high variability and low biodegradability, which compromise process stability under varying industrial loads. A low-dose nanoscale zero-valent iron (nZVI)-augmented continuous-flow strategy was proposed and evaluated long-term in an on-site 3000 L reactor coupled to a full-scale expanded circulating granular sludge bed (ECSB). nZVI increased the mean chemical oxygen demand (COD) removal efficiency from 12.96% to 25.94% and reduced effluent fluctuation by 49%. Concurrently, sludge aggregation intensified, accompanied by protein enrichment in tightly bound extracellular polymeric substances (T-EPS) and a shift in dissolved organic matter (DOM) fluorescence toward humic-like hydrolytic intermediates. Metagenome-assembled genomes indicated a stable community core without structural replacement, alongside enriched iron-metabolism pathways. Incomplete electron-output pathways in key populations further suggest a possible contribution of nZVI-derived iron phases to conductive-material-mediated direct interspecies electron transfer (cDIET). Nonsynonymous single-nucleotide variant (SNV) trajectories and strain deconvolution further identified population-level selection in genes for iron homeostasis, oxidative stress, and electron transfer. These findings reposition nZVI from a reactive supplement to an interfacial stability regulator. More broadly, they provide field-scale evidence that refined nanomaterial dosing strategies can stabilize anaerobic treatment under real industrial loads by coupling interfacial reorganization with within-population adaptation.