Membrane bioreactors (MBRs) are highly efficient wastewater treatment systems, surpassing traditional methods. However, they suffer from membrane fouling, leading to reduced permeate flux and increased operational costs. To address these issues, self-forming dynamic membrane bioreactors (SFDMBRs) utilize cheaper, larger-pore materials like woven fabrics as membrane supports. While SFDMBRs are effective in removing organic matter (COD and DOC), their efficiency in removing nitrogenous compounds, phosphorus, and recalcitrant molecules remains limited. Previous research demonstrated that incorporating electric fields and electrochemically controlled processes into conventional MBRs (e-MBRs) enhances nutrient removal (particularly phosphorus) and reduces fouling. Electrocoagulation, electroosmosis, and electrophoresis contribute to this improvement by facilitating the removal of soluble microbial products (SMPs) and other fouling precursors. This study introduces an e-ESFDMBR system, integrating an electric field into an encapsulated SFDMBR, aiming to combine the advantages of SFDMBRs with the enhanced performance of e-MBRs. The e-ESFDMBR's performance is compared to a standard ESFDMBR, analyzing pollutant removal efficiency and fouling control. Microbiological diversity is also investigated to understand the biological processes at play within the reactor.

Existing literature extensively covers aerobic and anaerobic MBRs as efficient wastewater treatment systems. These systems excel in removing pollutants compared to conventional methods. However, membrane fouling remains a significant challenge. Researchers have explored alternative support materials like woven and non-woven fabrics to reduce costs and improve performance. Aerobic SFDMBRs demonstrate high efficiency in removing organic matter (COD and DOC), and ammonia nitrogen (NH3-N). Nevertheless, they show relatively low efficiency in removing other nitrogenous compounds, total nitrogen (TN), phosphate-phosphorus (PO43−-P), and total phosphorus (TP). Studies have shown that the addition of electric fields to conventional MBRs enhances PO43−-P removal through electrochemical oxidation and precipitation processes. Electrochemical oxidation releases ions (e.g., aluminum or iron) that precipitate phosphates, also removing recalcitrant organic pollutants and emerging contaminants. The application of electric fields has been shown to reduce membrane fouling by various electrokinetic phenomena and by reducing quorum sensing (QS) signal molecules, thus lowering fouling precursor concentrations. This prior work provides the context for the development and evaluation of the e-ESFDMBR system.

This study compared two bioreactors: a standard ESFDMBR and an e-ESFDMBR. Both reactors used a similar design and operating conditions, with the key difference being the application of an electric field in the e-ESFDMBR. The reactors were cylindrical PMMA (poly(methyl methacrylate)) chambers (19L effective volume), each housing a filtration module consisting of a Dacron mesh (30 μm pore size) sandwiched between two support meshes. Synthetic municipal wastewater was continuously fed into both reactors. Aerobic conditions were maintained using air diffusers. Permeate was continuously suctioned, with a 10-minute filtration/backwashing cycle. The e-ESFDMBR included concentric stainless steel cathode and aluminum anode electrodes, applying a current density of 0.5 mA/cm² intermittently (5 min on/20 min off). Various parameters were monitored, including permeate flux, transmembrane pressure (TMP), effluent turbidity, COD, DOC, TN, NH3-N, NO3−-N, PO43−-P, UV absorbance at 254 nm (A254), SMP, EPS, and TEP. Microbial community analysis involved culturing microorganisms from mixed liquor, ESFDM layer, and effluent samples from both reactors. Gram staining, microscopy, PCR amplification of 16S rDNA and ITS regions, sequencing, and BLASTn alignment were used to identify microbial species and evaluate microbial diversity. SEM (Scanning Electron Microscopy) was used to study the morphology of the ESFDM and cake layers. Particle size distributions were determined using laser diffraction analysis. Aluminum concentration was measured using ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy).

The e-ESFDMBR showed significantly faster SFDM formation compared to the ESFDMBR, as indicated by effluent turbidity values dropping below 1 NTU within a day versus 7 days. The e-ESFDMBR maintained consistently low turbidity throughout the experiment. The e-ESFDMBR exhibited lower TMP values and a significantly lower fouling rate (0.032 kPa/d) than the ESFDMBR (0.105 kPa/d). The e-ESFDMBR demonstrated substantial improvements in pollutant removal: PO43−-P removal exceeded 99.9%, while the ESFDMBR showed only 14.2% removal. COD and DOC removal efficiencies were high for both reactors but slightly higher in the e-ESFDMBR (99.4% and 97.5% respectively) compared to the ESFDMBR (94.8% and 93.1% respectively). The e-ESFDMBR also showed markedly improved TN (86.7% vs 38%) and NH3-N (96.0% vs 46.9%) removal. The removal of humic-like substances, indicated by A254, was also significantly better in the e-ESFDMBR. Microbial analysis revealed that the e-ESFDMBR had higher microbial counts in the mixed liquor but lower counts in the ESFDM layer. Proteobacteria was the dominant phylum in both reactors, but its abundance was reduced in the e-ESFDMBR. The e-ESFDMBR exhibited a lower overall microbial diversity. Specific microbial species associated with nitrogen and phosphorus removal were identified in both reactors. The SEM analysis revealed different morphologies of the ESFDM and cake layers in the two reactors, consistent with the observed differences in fouling and performance.

The enhanced performance of the e-ESFDMBR is attributed to the combined effects of the biological processes inherent to the SFDMBR and the electrochemical processes induced by the electric field. Electrocoagulation facilitated the removal of phosphates and organic matter through the formation of larger flocs and precipitation of aluminum phosphate. Electroosmosis and electrophoresis further contributed to fouling mitigation. The reduction in fouling precursors (SMP, EPS, and TEP) is consistent with the observed improvements in membrane filterability. The shift in the microbial community composition in the e-ESFDMBR is likely a result of the electric field's influence on microbial growth and activity. The presence of specific bacteria associated with nitrogen and phosphorus removal highlights the importance of microbial communities in the overall reactor performance. The observed lower diversity in the e-ESFDMBR did not negatively impact performance, suggesting that certain key microbial species were favoured under the conditions of the e-ESFDMBR. The results demonstrate the potential of integrating electrochemical processes into SFDMBRs for significantly improving wastewater treatment efficiency and fouling control.

The e-ESFDMBR demonstrates significant advantages over conventional ESFDMBRs in wastewater treatment, exhibiting superior removal of nutrients, recalcitrant organic matter, and reduced fouling. This is a promising advancement in wastewater treatment technology. Future studies should focus on optimizing the electric field application, detailed energy consumption analysis, and comprehensive microbial community analysis using NGS (Next-Generation Sequencing). Investigation into the longevity of electrodes and the level of aluminum in the effluent are also important aspects for future research.

This study used synthetic wastewater, which might not fully reflect the complexity of real wastewater. The intermittent application of the electric field, while efficient, could be further optimized for energy efficiency. The microbial analysis focused on culturable microorganisms, potentially overlooking significant portions of the microbial community. Further research with real wastewater and a more detailed microbial community analysis using NGS is needed to fully validate the e-ESFDMBR's applicability.