Coupled UASB–GDM system with electrospun nanofiber membranes for decentralized wastewater treatment

This study evaluates the performance and biofilm-mediated flux stabilization of a coupled up-flow anaerobic sludge blanket (UASB) reactor and gravity-driven membrane (GDM) system equipped with electrospun nanofiber membranes (ESN) for decentralized wastewater treatment. The system was operated under fully anaerobic conditions at 37 °C over three different hydraulic retention times (HRTs: 24, 16, and 8 h) while maintaining a constant organic loading rate of 1.5 kg COD·m⁻³·d⁻¹. Two ESN types were tested: a bare polymeric (B-ESN) and a nanoclay-composite (C-ESN). The integrated system achieved high organic matter removal across all stages, with COD removal exceeding 99% at 24 h HRT and decreasing slightly at shorter HRTs (97–95% for C-ESN and 95–92% for B-ESN). Nutrient removal followed similar trends, with the C-ESN consistently outperforming B-ESN, achieving up to 78% TN and 74% TP reduction at HRT 24 h; given the readily biodegradable feed and high COD:N conditions, the observed decreases in aqueous TN and TP are most plausibly explained by partitioning into retained biomass/biofilm-associated solids, with sorption and mineral deposition acting as secondary sinks. Flux analysis revealed rapid initial decline followed by self-regulated stabilization between 2.0–3.8 L·m⁻²·h⁻¹ under a constant hydrostatic pressure of 50 mbar, without backwashing or chemical cleaning. X-ray diffraction confirmed localized struvite and calcium phosphate accumulation within the biofilm, consistent with time-dependent mineral deposition rather than bulk precipitation as the dominant nutrient removal mechanism. Biofilm characterization and 16S rRNA sequencing showed that Methanobacteriaceae and Methanosaetaceae dominated both the UASB and membrane biofilms, facilitating organic matter degradation and stable flux through balanced fouling biodegradation dynamics. The coupled UASB–GDM system demonstrates a low-energy, self-sustaining treatment configuration suitable for decentralized and small-community wastewater applications, with potential for non-potable reuse and future resource recovery integration.