Improvement of membrane surface antimicrobial properties to enhance resistance to fouling in the treatment of municipal wastewater

Abstract

The increase in water scarcity has become a worldwide issue due to population growth and growing pollution. Therefore, implementing wastewater management strategies and wastewater treatment techniques has been on the rise to create environmental sustainability and reduce freshwater consumption. Municipal wastewater effluent is used for tertiary treatment to produce water quality suitable for various reuse applications. Effective tertiary treatments such as reverse osmosis (RO) membranes are additional to secondary treatments for further reduction of bacteria, organics, and inorganics in municipal wastewater. However, fouling significantly hinders reverse osmosis membrane applications as it lowers membrane performance by causing the permeate flux decline, reducing the permeate quality and shortening the membrane life span. Microbial fouling is a significant contributor, accounting for more than 45% of membrane fouling. In this study, the surface modification of a thin film RO membrane was investigated by graft polymerization of 3-allyl-5, 5-dimethylhydantoin (ADMH) to improve the membrane’s antimicrobial properties. The modifying agent ADMH was synthesized at four different concentrations and grafted onto a lowpressure RO membrane, producing modified RO membranes with varying concentrations of ADMH. The synthesis of ADMH was confirmed by Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) and NMR (Nuclear Magnetic Resonance) were used to characterize the surface of modified membranes. The anti-microbial tests used two types of bacteria, E. coli (Gramnegative) and S. aureus (Gram-positive) contaminants. The membranes were further tested with humic acid and sodium bicarbonate, depicting municipal wastewater organic and inorganic foulant models, respectively. The FTIR and NMR peaks characteristics confirmed the presence of ADMH bonds, such as the C=O bond at 3107 𝑐𝑚−1 and (=CHCH2) bond at 42.17 ppm, respectively, on the surface of the membrane. The appearance of a new layer on the membrane surface shown in the cross-sectional view of the membrane. The antimicrobial tests with E. coli and S. aureus bacteria revealed 45.23%, 58.93%, 48.48% and 33.76% enhanced mortality ratio compared to the unmodified membrane for the M0.2mol/L, M0.4mol/L, M0.6mol/L and M0.8mol/L respectively. In contrast, the antimicrobial tests with S. aureus bacteria revealed an improved mortality ratio of 6.71%, 37.42%, 22.89%, and 2.44% for the aforementioned membranes, respectively. Additionally, it was found that membranes M0.4mol/L and M0.6mol/L exhibited a flux decline ratio (FDR) of 12.68% and 8.91% against E. coli, respectively, while the unmodified membrane FDR was and FRR values of 94.27% and 96.88%, respectively. Also, the same membranes had FDR values of 30.21% and 23.79% and FRR values of 59.70% and 70.15% against S. aureus. Overall, this study demonstrated the significant impact of ADMH concentration against biofouling on reverse osmosis membranes. The ADMH exhibited qualities that contributed to improving the antimicrobial properties of reverse osmosis membranes while enhancing the membrane permeability and salt rejection, and slightly maintaining its resistance to organic and inorganic fouling. This study is an excellent step towards future studies in developing fouling mitigation methods to treat municipal wastewater.