In the coming decades, global freshwater shortages will pose a significant hazard for humanity. Not only is clean, fresh water critical for essential human life, but our ever expanding industrial and agricultural footprint further increases the demand for freshwater. A combination of factors including population growth, the destruction of natural reservoirs, industrial pollution and the impacts of climate change will see water as one of the most important commodities of the 21st century. Reverse osmosis (RO) water treatment utilizes hydraulic pressure and a semipermeable membrane to separate 90-99% of the total dissolved solids (TDS) from the feed water supply.1 Critically, reverse osmosis allows for the desalinization of brackish water and sea water; salt-laden water represents 97.5% of all water on earth, thus reverse osmosis treatment greatly increases the potential freshwater supply. Challenges with reverse osmosis include high energy costs and the deleterious effects of scaling that occur over the operating cycle. Scaling is a particular problem requiring chemical pre-treatment as well as costly chemical cleaning processes to extend the life of the membranes. Figure 1 (in the downloaded file) illustrates a schematic cut-away of a spiral-wound-element (SWE) reverse osmosis membrane. The design and construction of the feed channel spacer as well as the semipermeable membrane itself is critical to producing an efficient and long-lasting solution for water treatment.
The use of reverse osmosis treatment has tripled since 2000 with over 16,000 water treatment plants in operation worldwide.2 Reverse osmosis now represents 65% of the desalinization market while consuming 90% less energy than desalinization via distillation. Osmosis is a spontaneous process wherein two solutions differing in concentration and separated by a semipermeable membrane experience diffusion of solvent molecules from the solution of lower solute concentration to the solution of higher solute concentration. Reverse osmosis utilizes hydraulic pressure on the feed solution to overcome the osmotic potential leading to the purification and diffusion of water molecules across the semipermeable membrane.
Fouling of the membrane is one of the key challenges of operating reverse osmosis treatment plants. Alleviation of fouling may require expensive chemical pre-treatment as well as costly cleaning procedures. Factors that contribute to fouling of the membrane include the concentration of the solution at the liquid-membrane interface as well as the design of the feed spacer. Constructed from a simple diamond-shaped mesh, the feed spacer separates the membrane leaves and guides the water flow as it diffuses through the membrane. The researcher, Jeremy Walker, as part of a doctoral thesis in civil engineering at Wayne State University, conducted a series of studies to improve the design of the feed channel spacer used in reverse osmosis membranes.
The goal of these studies sought to design a feed spacer with reduced fouling and enhanced de-scaling properties during membrane cleaning2. Master Bond UV15TK, a UV-curable epoxy with a relatively high modulus, was chosen for use as a 3-D printing material to directly print micromixer elements of an optimal design onto the semipermeable membrane itself. Fluid dynamic modeling was performed to determine the optimal geometry of the chevron pattern. Optimal flow velocity and turbulent mixing decreases the tendency for scale formation, crystallization of solute species, and decreases the effects of fouling. Figure 2 (in the downloaded file) shows a standard industrial mesh feed spacer and the optimized 3-D printed micromixer elements comprised of UV-cured Master Bond UV15TK epoxy. Studies compared performance between the 3-D printed micromixer elements and the industry standard mesh with respect to fouling tendency and reverse-flow de-scaling.
1E. Maynard, C. Whapham, in Decontamination in Hospitals and Healthcare (Second Edition), 2020. URL: https://www.sciencedirect.com/topics/engineering/reverse-osmosis-membrane. Accessed: 08/20/2022.
2J. Walker. 2018. ‘Analysis of Micromixers to Minimize Scaling Effects on Reverse Osmosis Membranes.’ Doctor of Philosophy in Civil Engineering. Wayne State University. Detroit, Michigan. URL:
https://digitalcommons.wayne.edu/cgi/viewcontent.cgi?article=3131&contex... Accessed: 08/20/2022.