(525d) Mechanisms and Control of Irreversible Fouling in Commercial and Nano-Structured Ro/Nf Membranes

Much of the world's population, particularly in arid regions, is experiencing water shortages. These shortages can be alleviated with the treatment and use of brackish and ocean waters. Largely untapped brackish and ocean waters can provide a drought-proof water supply. They can be treated by membrane desalination, i.e., nanofiltration (NF) and reverse osmosis (RO) membrane processes; however, cost and environmental impact are the major factors inhibiting widespread application of membrane desalination technology. Operating cost of membrane desalination processes largely result from energy consumption due to high-pressure pumping requirements. Environmental impacts are traditionally associated with feed water intake and concentrate disposal issues. The higher the operating pressure, the more energy consumed. The main energy source for most membrane desalination processes is combustion of fossil fuels, which is suspected to be the leading cause of global warming. So, an additional concern for membrane desalination processes is minimizing the energy consumption and fossil fuel combustion.

At elevated feed water pressures, polymeric membranes are damaged internally by physical compaction ? ?internal fouling.? These elevated pressures are required for RO and NF membrane processes and increase with time due to fouling which occurs on the membrane surface. This surface fouling and the resulting higher operating pressures leads to more physical compaction. Although surface deposits that lead to elevated operating pressure can be removed by physical and chemical cleaning methods, internal fouling can not be reversed. Irreversible fouling of NF or RO membranes leads to higher long-term operating pressures, and thus, higher operating cost and more fossil fuel consumption. An improved understanding of the mechanisms that cause physical membrane compaction can help membrane material researchers to develop membrane compaction mitigation methods. The ultimate goal of our research is to reduce operating costs and environmental impact of membrane desalination processes through minimization of both short term (surface) and long term (internal) fouling of NF and RO membranes.

Irreversible, internal fouling of NF/RO composite membranes by physical compaction is of major concern in membrane processes because of the sponge-like morphology of the porous supports on which they are cast. While some membranes are cast on surfaces with large macrovoids, other membranes have support structures that are relatively free of macrovoids. Our hypothesis is that membrane compaction occurs when macrovoids collapse in the porous support layer due to excessive applied pressures. The pressure drop causes a reduction in size of the support layer voids, thereby, reducing the permeability through the entire membrane cross-section. Therefore, our proposition is that support structures with large macrovoids will tend to compact more than support structures without macrovoids. Further, we are exploring methods of creating nano-structured membranes with increased resistance to compaction.

A laboratory scale, crossflow membrane filtration system was constructed to evaluate compaction mechanisms in commercial and nano-structured NF/RO thin film composite membranes. Membranes were compacted for 24 hours at varying pressures while the temperature was kept constant at 25 ºC. The flux was measured as a function of pressure by a digital chromatography flow meter. Pressures ranging from 0 to 600 psi were tested. DI water was used with varying concentrations of MgSO₄ to supply the necessary osmotic pressure. Using a standard Darcy resistance model, the apparent membrane resistance was determined for each membrane across the appropriate range of applied pressures. The relationship between pressure and membrane resistance was unique to each membrane. Cross-section SEM images were taken of both compacted and uncompacted membranes to determine the physical change in the sub-structure of the membrane. Using both the SEM images and the experimentally determined membrane resistances in combination with the Kozeny-Carmen model, the true mechanisms through which RO/NF membranes are physically compacted and irreversibly fouled can be determined. At the conference we will present results of compaction tests for commercial and ?compaction resistant? nano-structured NF/RO membranes.