Browsing by Subject "Water purification"
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Item Assessment of fouling in native and surface-modified water purification membranes(2013-05) Miller, Daniel Joseph Lang; Freeman, B. D. (Benny D.); Paul, Donald R.Fouling is a major obstacle to the implementation of membranes in water purification applications. Hydrophilization of the membrane surface tends to mitigate fouling because hydrophobic interactions between foulants and the membrane are reduced. Polydopamine was deposited onto membranes to render their surfaces hydrophilic. The chemical structure of polydopamine, which was previously ambiguous, was investigated by many spectroscopic techniques. While previously thought to consist of covalently-linked monomers, polydopamine was found to be an aggregate of partly-oxidized dopamine units linked by strong, non-covalent secondary interactions. Polydopamine was also used as a platform for the molecular conjugation of other anti-fouling materials, such as poly(ethylene glycol), to the membrane surface. Membrane fouling was assessed by constant permeate flux crossflow filtration with an oil/water emulsion feed. The threshold flux--the flux at which the rate of fouling significantly increases--was determined by a well-established flux stepping technique. Membrane resistance evolution during fouling was compared for constant flux and constant transmembrane pressure operation using unmodified membranes. Below the threshold flux (slow fouling), good agreement in resistance evolution was found between the two operational modes; above the threshold flux, significant deviation was observed. The effect of polydopamine and polydopamine-g-poly(ethylene glycol) surface modifications was studied under constant flux crossflow fouling conditions. The surface modifications were found to increase the membrane resistance, resulting in higher transmembrane pressures in the modified membranes than in the unmodified membranes at fluxes below the threshold flux. Modified membranes were also compared to unmodified membranes with the same pure water permeance (same initial resistance). In this case, the modified membranes had lower transmembrane pressures during fouling than the unmodified membranes, suggesting that a preferred method of membrane surface modification is to begin with a membrane of higher permeance than required, and then surface-modify it to achieve the desired permeance. The efficacy of polydopamine and polydopamine-g-poly(ethylene glycol) surface modifications in reducing biofouling was also evaluated. Modified membranes showed reduced protein and bacterial adhesion in short-term tests, which are commonly used to assess biofouling propensity. However, long-term operation under hydrodynamic conditions mimicking those of an industrial module showed no benefit of the hydrophilic coatings in limiting biofouling.Item Biological treatment and biofouling in membrane treatment systems(2012-05) Vercellino, Tony; Morse, Audra; Reid, Ted W.; Hamood, Abdul N.; Song, LianfaAs the world’s population increases, the demand for water will increase accordingly. The corresponding demand for water puts a strain on the available sources of water and the technologies to reclaim water from non-potable sources. The use of membranes is quickly emerging as the prominent treatment technique for water purification. While the increase in use of membrane technology is providing the water that the world demands, operational problems such as fouling are limiting the potential of these membrane processes. Fouling due to biological growth, otherwise known as biofouling, is the foremost form of fouling that affects current membrane treatment systems. The use of covalently attached organo-selenium as a surface modification to reverse osmosis membranes was studied as a potential biofouling inhibition agent. The efficacy of the organo-selenium surface treatment was tested within a flow-cell system which exposed the membrane samples to high nutrient medias at low-flow, simulating a worst-case condition for biofouling to occur at the membrane surface. The surface treatment was also tested within a bench-scale reverse osmosis system, where the membranes were exposed to normal operating conditions for a reverse osmosis system. Within the low-flow system, the organo-selenium surface treatment was able to achieve a range of 2.01 to 3.98 logs of inhibition of total biomass. Within the RO system, the organo-selenium surface treatment was able to achieve between 2.2 and 3.8 logs of total biomass inhibition. However, when the polypropylene feed spacer also received the surface treatment, total biomass inhibition was increased to 5.9 logs.Item Fouling-resistant coating materials for water purification(2009-05) Wu, Yuan-hsuan; Freeman, B. D. (Benny D.)Membrane technology has been used in water purification for decades. However, membrane fouling remains a limiting factor. One way to control fouling is through surface modification. Several studies report that increasing surface hydrophilicity can reduce membrane fouling. Surface modification via physical coating (i.e., thin-film composite membrane) was explored in this research to prevent membrane fouling. Before making thin-film composite membranes, it was important to study structure/property relations in a series of potential coating materials. This research aims to contribute to a better fundamental understanding of the structure/property relations which govern water transport, rejection of model foulants (i.e., emulsified oil droplet or protein), and fouling characteristics in hydrogels based on poly(ethylene glycol) diacrylate (PEGDA) and N-vinyl-2-pyrrolidone (NVP). Crosslinked poly(ethylene glycol) (PEG) free-standing films were prepared by UV-induced photopolymerization of PEGDA crosslinker in the presence of varying amounts of water or monofunctional poly(ethylene glycol) acrylate (PEGA). The crosslinked PEGDA films exhibited polymerization induced phase separation (PIPS) when the water content of the prepolymerization mixture was greater than 60 wt%. Visible light absorbance measurements, water uptake, water permeability, and salt kinetic desorption experiments were used to characterize the structure of these phase-separated, crosslinked hydrogels. The films with PIPS exhibited a porous morphology in cryogenic scanning electron microscope (CryoSEM) studies. Dead-end filtration experiments using deionized water and bovine serum albumin (BSA) solutions were performed to explore the fundamental transport and fouling properties of these materials. The total flux of pure water through the films after prior exposure to BSA solution was nearly equal to that of the as-prepared material, indicating that these PEGDA films resist fouling by BSA under the conditions studied. Crosslinked NVP free-standing films were prepared by UV-induced photopolymerization in the presence of water, with NVP as the monomer and N,N’-methylenebisacrylamide (MBAA) as the crosslinker. A series of crosslinked films were polymerized at various prepolymerization water contents, NVP/MBAA ratios and at various levels of UV light intensity in the polymerization. Like PEGDA, the NVP films also underwent phase-separation during polymerization. The influence of monomer/ crosslinker ratio, prepolymerization water content, and UV intensities on membrane morphology and water transport was characterized with CryoSEM, bio-atomic force microscope (Bio-AFM) and dead-end filtration. Molecular weight cutoff (MWCO) measurements were used to characterize the sieving property of crosslinked NVP films polymerized at different UV intensities. UV intensity was found to have an impact on the interconnectivity of crosslinked membranes. Finally, tests of fouling resistance to protein solution (bovine serum albumin) and oily water emulsion were performed. The NVP crosslinked films had good protein and oily water fouling resistance. Overall, both crosslinked PEGDA and NVP films exhibit fouling resistance to oily water emulsions or protein solution. NVP films had more porous structure and higher water permeability than did PEGDA films, while the more compact structure of PEGDA films led to better rejection of model foulants (e.g., protein) than in NVP films. Based on different applications (e.g., oil/water separation, protein filtration), different coating materials must be chosen according to the membrane morphology, transport property, and rejection of model foulants to achieve the highest water flux and foulant rejection in membranes used for water purification.Item Salt solubility measurements in partially disulfonated poly(arylene ether sulfone) for reverse osmosis water purification applications(2010-05) Passaniti, Linda Kimberly; Paul, Donald R.; Freeman, Benny D.Partially disulfonated poly(arylene ether sulfone) (BPS) membranes have shown great promise as robust, chlorine tolerant alternatives to the current polyamide materials as reverse osmosis desalination membranes for water purification. The random copolymers are synthesized by direct polymerization of a disulfonated monomer (3,3’-disulfonato-4,4’-dichlorodiphenyl sulfone (SDCDPS)) and other monomers (4,4’-dichlorodiphenyl sulfone (DCDPS) and 4,4’-biphenol (BP)). The sulfonation of the materials adds necessary hydrophilic character and adjusting the percent sulfonation of the material changes the water and salt uptake of the material. Additionally, sulfonation causes the membranes to be charged, making them ion exchangers in which anions are partially excluded from the membrane, thus affecting the partitioning of salt in the membrane. The amount of sodium chloride present in the membrane after equilibration with external soaking solutions of varying concentrations of sodium chloride was measured by measuring the amount of individual ions, i.e., the sodium cation and chloride anion, separately. One area in which this work is unique is that it sought to measure the concentrations of the ions independently of one another. The analysis of sodium and chloride has shown the concentration of sodium in the membrane to be significantly greater than that of chloride, where the uptake of chloride is the limiting factor in the uptake of sodium chloride. The trends in the concentrations as well as in the partition coefficients of the ions are consistent with Donnan Exclusion.