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We examined how colloidal, biological, organic, and inorganic fouling varied in 4-year-old reverse osmosis (RO) membranes taken from lead and lag ends of pressure vessels of each of the three stages from the world’s largest potable reuse facility. Colorimetry, microscopy, spectroscopy, and microbial analysis revealed varying combined fouling extents in all cases. Bioorganics dominated all but the last element. In the first and second stages, non-siliceous multivalent inorganics accumulated more on lead elements, whereas silicon-based foulants prevailed in lag elements. The third-stage lag element suffered the heaviest inorganic and the lowest bioorganic fouling. Cleaning small swatches cut from each element with a mixture of sodium tripolyphosphate and sodium dodecylbenzenesulfonic acid fully recovered the permeability of only the lag element of the first stage, which had endured the lowest inorganic fouling. Partial permeability recovery of all of the other elements suggested that cleaning was only effective against bioorganics, and harsher cleaners and/or interstage treatment were needed to combat silicon fouling. Cleaning agents did not appear to penetrate and remove entrapped foulants, resulting in faster performance deterioration when the membranes resumed service post-cleaning. Results emphasize the importance of systematically autopsying elements from each stage and evaluating their post-cleaning characteristics to optimize RO pretreatment and cleaning protocols.

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The objective of this work was to analyze the antimicrobial and antibiofilm activities of bismuth lipophilic nanoparticles (BisBAL NPs) incorporated into chitosan-based membranes. Chitosan-based membranes were homogeneously embedded with BisBAL NPs, confirming the bismuth presence by scanning electron microscopy. The tensile strength of chitosan-based membrane alone or with BisBAL NPs showed similar results as elongation, suggesting that BisBAL NP addition did not affect membrane mechanical properties. Chitosan-based membranes complemented with 100 µM of BisBAL NPs caused a complete inhibition of biofilm formation and a 90-98% growth inhibition of six different oral pathogens. Cytotoxicity studies revealed that 80% of human gingival fibroblasts were viable after a 24-h exposure to the chitosan-based membrane with 100 µM of BisBAL NPs and collagen. Altogether, we conclude that the biological properties of chitosan-based membranes supplemented with BisBAL NPs could be a very interesting option for tissue regeneration.

Fouled nanofiltration membranes from lead and lag positions of a 2-stage pilot-scale direct potable reuse plant treating secondary municipal wastewater effluent in El Paso, Texas were thoroughly characterized after 9 months of operation to elucidate the role of silicon moieties on fouling. X-ray photoelectron (XPS), energy dispersive X-ray (EDS), and Fourier transform infrared (FTIR) spectroscopies identified silicon oxides/ silica as dominant foulants, especially on the lag element. Gas chromatography/mass spectroscopy also identified organosilicon moieties such as linear-and cyclosiloxanes on the lead element. Extensive siliceous fouling was accompanied by calcium, aluminum, and magnesium scaling as well as deposition of bioorganic materials thereby modifying membrane surfaces that resulted in irreversible productivity loss. None of the three selected cleaning agents (sodium dodecyl sulfate and NaOH, EDTA and NaOH, and HCl), either singly or in combination satisfactorily restored water permeability of the membrane. EDTA performed better in the lead element (where bioorganic fouling prevailed), and HCl was more effective in the lag element (where mineral scaling accompanied silicon oxides). This suggests the express need for novel antiscalants to reduce organosilicon/silicon oxide deposition and the necessity of harsher cleaning agents/ regimens specifically targeting silicon (e.g., hydrofluoric acid or ammonium bifluoride).