reverse osmosis

The name Lockheed Martin invokes images of high tech aircraft, secret weapons, and other technologies that seem a whole lot more exciting than a reverse osmosis (RO) membrane.  However, Perforene™, Lockheed Martin’s latest innovation, promises to be an exciting new development for RO desalination. It’s made from graphene, an allotrope of carbon where the atoms arranged in hexagonal cells to make a sheet that is only one atom thick.  The next thinnest RO membrane is about 500 times thicker than Perforene™.  It is the almost impossible thinness of the membrane that makes it so exciting for RO; it takes about 100 times less energy to push water through the membrane when compared to the average RO membrane available commercially today.

The Perforene™ membrane was developed by placing holes that are one nanometer or less in diameter into the membrane.  These holes are small enough to trap the ions while dramatically improving the flow-through of water molecules, reducing clogging and pressure on the membrane.  Although it is only one atom thick, graphene is both stronger and more durable than almost any other material could be at this scale.

The combination of strength, high permeability, and excellent selectivity could make Perforene™ the key to cheaper desalination costs in plants across the globe.  And cheaper desalination can mean better access to fresh water in arid places such as North Africa, the Middle East or the Western United States. As the world’s population continues to grow, the demand for fresh water for drinking, agriculture, and industry will grow along with it. Perforene™ can make meeting those demands easier.

Perforene was developed and tested with the help of Sterlitech’s CF042 crossflow cell, a lab scale cross flow filtration unit designed to provide fast and accurate performance data with minimal amounts of product, expense, and time.  It enables researchers to study membrane performance in the small scale before committing resources to larger process scale systems.  Sterlitech offers the CF042 in different materials to meet different needs, in addition to the larger Sepa CF crossflow cells and the smaller HP4750 stirred cell.  Recently, we also began offering the Sepa and CF042 in Forward Osmosis (FO) configurations to further enable research into new water technologies.  Further details can be obtained by contacting us.

Toray has discontinued the production of their 80B Reverse Osmosis Membrane. But fear not, because they have also started producing a successor, called the 80E, that Sterlitech will be offering on our site.

Both the 80B and its new, functional equivalent, the 80E, are polyamide membranes that are typically used for seawater desalination. A complete list of the product numbers we are discontinuing and the product numbers of the new membranes can be found below:

Discontinued 80B Membranes;

• YM80BSP475
• YM80BSP195
• YM80BSP42
• YM80BSP18

New 80E Membranes;

• YM80ESP475
• YM80ESP195
• YM80ESP42
• YM80ESP18

Sterlitech is now carrying Dow FilmTec flat sheet membranes for reverse osmosis and nanofiltration separations. These high-performance membranes are available in Sepa CF, CF042, or HP4750 sizes, or as 12 x 12 inch sheets.

The FilmTec line of spiral wound membranes was created by the experts at Dow Water & Process Solutions for industrial, municipal and commercial water applications. While they have often been used for large industrial processes like power generation and semiconductor plants, with our precut sizes it is easier than ever to implement these membranes for laboratory scale testing with a membrane test cell. For information on how to best utilize each membrane type, consult our application tab for recommended uses on each designation.

The Silt Density Index is most frequently used to determine fouling potential prior to RO filtration. You can think of SDI as a bouncer, keeping the riff-raff out of the RO feed water. The higher the number, the greater the likelihood of fouling. The maximum SDI number allowed depends on the type of RO membrane being used; most manufacturers recommend a maximum SDI of 4 or 5.

SDI is found by calculating the rate at which a membrane filter is plugged. ASTM standard D4189-07 defines that the nominal filter for this application is a white hydrophilic MCE membrane filter, with 0.45 μm pore size and a 47 mm diameter. The reason this particular membrane is used is that it is more susceptible to plugging from colloidal material than from hard particles such as sand, therefore giving a better indication of the factors that might plug an RO membrane down the line.

Other measures that can be derived from the SDI include the plugging factor and the Modified Fouling Index (MFI). The plugging factor expresses the level of suspended solids as a percentage of the measured SDI value to the maximum SDI value, so a 100% plugging factor would indicate that your membrane is completely plugged. The MFI incorporates cake filtration theory into its calculation of fouling potential. Since this formula is more complex than SDI, it is not as frequently used in the field.

SDI can be determined manually or automatically with a measurement kit. Got any tips or experiences measuring SDI? Let us know in the comments!

After our last post discussing how experiments with carbon nanotubes (CNT’s) might greatly improve the effectiveness of reverse osmosis desalination now comes a new report from the Institute of Physics that shows researchers are getting closer to making this a reality. Already over a billion people do not have regular access to clean water and the problem will likely get worse as the demand for drinkable water is expected to grow dramatically in the near future. With natural sources increasingly scarce, this urgent need means there is an intense global interest in any potentially viable forms of water purification.

Right now the main issues preventing RO desalination on a large-scale basis are that the membranes used to perform seawater to freshwater separation do not remove salt ions with enough efficiency and they also require great amounts of energy (and therefore expense) in order to purify the water. Jason Reese, a Professor of Thermodynamics and Fluid Mechanics at the University of Strathclyde and also the author of this report, states, “The holy grail of reverse-osmosis desalination is combining high water-transport rates with efficient salt-ion rejection.” Incredibly, these little carbon nanotubes may be able to satisfy both of these requirements for widespread adoption.

Early tests and simulations have shown that CNT membranes could have water permeability that is 20 times greater than today’s materials. Additionally, carbon nanotubes can be chemically tailored to better reject salt ions, thus improving upon the desalination process in multiple key areas.

While it is still early, these features are promising enough that scientists such as Professor Reese feel it is a very real possibility that this application of nanotechnology could be used to curtail our growing water demand.

Read more about this report here.

An increase in salinity levels at the North Reverse Osmosis Water Plant in Kill Devil Hills (yes, that’s the town name) that had been creating stress for some local officials has been explained in a recent study. Researchers from nearby Duke University found that the rising salinity levels at this coastal aquifer are the result of fossil seawater and not seawater intrusion, as had been feared. Since the well’s installation in the late 1980’s salinity has more than doubled from about 1,000 mg/L to about 2,500 mg/L. There was much cause for relief however, when researchers were able to attribute the rise to fossilized seawater and not to seawater leaking in from the coast.

According to the director of the study, Duke Professor Avner Vengosh, knowing the source of the salinity increase is important because fossil seawater raises salinity, “At a relatively slow and steady rate that is more manageable and sustainable than the rapid increase we’d see if there was modern-day seawater intrusion.” As a result of this study the community will be able to rely on this aquifer for decades to come without having to resort to more expensive seawater desalination techniques which require more energy and advanced filtration methods.

Current treatment for groundwater desalination includes the use of reverse osmosis (RO) membranes to separate dissolved salts from potable water. Even with the rising salinity level these membranes remove around 96 to 99 percent of the dissolved salts. RO membranes also remove between 16 and 42 percent of the boron and 54 to 75 percent of the arsenic from the groundwater. Additional treatment following reverse osmosis desalination continues to remove arsenic until it is within safe drinking levels (10 parts per billion, according to the EPA).

Because seawater consistently has more salt than groundwater it requires more energy to treat, and therefore the cost is higher. Per this report on desalination from the Pacific Institute, “Energy is the single largest variable cost for a desalination plant, varying from one-third to more than one-half the cost of produced water.” The report also states, “At these percentages, a 25% increase in energy cost would increase the cost of produced water by 11% (for RO plants).” In looking at these percentages, it’s easy to see why the plant was concerned about seawater intrusion. Thanks to this research, the local citizens can drink easier knowing they have a supply of healthy, affordable water for a long time to come.

Click here to learn more about this case.