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Biofouling mitigation of polyamide reverse osmosis membranes using polymer grafting and nanoparticle coating in the treatment of municipal secondary effluent
Author(s)
Kasongo, Godwill L.
Date Issued
2026
Type
doctoral thesis
Publisher
Cape Peninsula University of Technology
Abstract
Reverse osmosis (RO) is suitable for treating municipal secondary effluent to produce high-quality water for reuse applications. However, polyamide (PA) RO membrane fouling remains the key challenge as it causes RO membrane degradation and poor performance, resulting in a significant economic impact on the overall RO process. Microbial fouling (biofouling) particularly accounts for 45% of membrane fouling. Fouling mitigation techniques, such as membrane active-layer modification, provide a viable route to tailoring membrane surface physico-chemical properties. In this research project, a novel membrane surface modification approach is initiated to mitigate biofouling of PA RO membranes. The membrane was chemically grafted with 3-Allyl-5,5dimethylhydantoin (ADMH), a titanium dioxide (TiO₂)-silver (Ag) nanocomposite, and a combined ADMH-TiO₂-Ag nanocomposite layer. Modified membrane structures were characterised using SEM-EDX, ATR-FTIR, NMR, TEM, TGA, contact angle and zeta potential. Membrane performance was evaluated for water permeability, salt rejection, flux decline, and bacterial adhesion using Escherichia coli and Staphylococcus aureus, as well as real municipal secondary effluent. Initially, the graft polymerisation of N,N′-dimethylaminoethyl methacrylate (DMAEMA) was investigated to enhance fouling resistance against secondary effluent constituentsSubsequently, 3allyl-5,5-dimethylhydantoin (ADMH) was grafted at varying concentrations to enhance antimicrobial activity against both Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. Finally, a hybrid modification combining ADMH with titanium dioxide–silver (TiO₂-Ag) nanoparticles. This initial modification increased the flux recovery ratio from 68.61% to 83.57% and reduced the flux decline ratio from 38.21% to 25.54%. The DMAEMA-modified membrane exhibited significantly higher sterilisation ratios than the unmodified membrane, demonstrating enhanced antibiofouling properties against E. coli, with a maximum sterilisation ratio of 76.8%. Pure water flux tests further confirmed the improved performance, with the flux decline decreasing from 46.0% for the unmodified membrane to 37.3% after DMAEMA modification. Subsequently, surface modification with varying concentrations of ADMH was carried out to further minimise biofouling. ADMH adhered to the membrane surface via graft polymerisation, as confirmed by SEM, FTIR, and NMR analyses. E. coli and S. aureus. The membranes were tested with E. coli and S. aureus, and, furthermore, with organic and inorganic foulant solutions, thus mimicking the microbial and fouling activity of municipal wastewater secondary effluent. Biofouling tests using E. coli and S. aureus demonstrated improved mortality ratios of 58.9% and 37.4%, respectively, along with reduced fouling decline (3.7-8.9%) and enhanced flux recovery ratios (69.2-96.9%). Although surface hydrophilicity increased with ADMH concentration, optimal fouling resistance was achieved at moderate ADMH concentration, while resistance to organic and inorganic foulants showed limited improvement, highlighting the role of foulant type in defining membrane performance. Finally, a hybrid modification involving ADMH, TiO₂, and Ag was investigated to simultaneously enhance the thin-film composite (TFC) RO membrane surface hydrophilicity and antibacterial activity, while maintaining salt rejection. The ADMH\TiO₂-Ag modified membrane maintained both thermal and mechanical stability, as confirmed by TGA analysis and a long-term filtration test. This hybrid modification produced the strongest antibacterial response, with mortality rates of 94.64% for E. coli and 90.13% for S. aureus. Contact angle was significantly reduced, indicating enhanced water affinity. The findings showed that the combination of ADMH with Ag-TiO₂ provides a synergistic effect, improving the membrane’s resistance to microbial, organic and inorganic foulants. Long-term crossflow filtration using real secondary effluent further demonstrated improved fouling resistance, with the hybrid modified membrane (M-HTA) decreasing slightly from 32 𝐿.𝑚−2ℎ−1 to 30.1 𝐿.𝑚−2ℎ−1 after 72 h, compared to 27.7 𝐿.𝑚−2ℎ−1 for the unmodified membrane. These findings support the potential of hybrid-modified membranes for long-term wastewater treatment applications. This work provides a practical, scalable membrane surface functionalization approach to improve polyamide RO membrane performance in wastewater reuse applications, effectively linking modification strategies with the fouling control requirements of real wastewater treatment applications.
Additional information
Thesis (DEng (Chemical Engineering))--Cape Peninsula University of Technology, 2026
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