By Michael Kim, CWS-I
In the early 1980s, RO membranes had a typical salt rejection of 96 percent at 225 psi operating pressure. The technology continued to improve over time and now typical rejection is 99.5 percent at 225 psi. In terms of percentage of salt passage (i.e., 100-percent salt rejection), the salt passage dramatically decreased from four to 0.5 percent, an 87.5-percent reduction. (Think of it as 87.5 percent fewer contaminants in your RO water than before.) And though current RO membranes are still based on polyamide chemistry, the performance of RO membranes has indeed changed a lot, and for the better. Although it is not possible to achieve 100-percent rejection, we are getting closer and closer. Membrane manufacturers have begun working on reducing operating pressures while still maintaining high rejection (i.e., low salt passage).
Factors affecting membrane performance
It is important to understand the factors that affect RO membrane performance, mainly water flux and salt rejection (or salt passage). RO membranes do not have pore sizes, such as UF membranes, but are referred to as having different water flux permeability. Those with higher water flux permeability (i.e., looser RO) will have a higher water flux but also a higher percentage of salt passage, while those with lower water flux permeability (i.e., tighter RO) will have lower water flux and smaller salt passage (higher salt rejection) as this is the nature of the physics. There are numerous factors that have a big effect on water flux and salt rejection, but operating pressure, operating temperature, feedwater concentration and percentage of recovery have the biggest influence on membrane performance.
Permeate flowrate will increase and salt rejection will improve as the operating pressure increase, as Figure 1 shows. The more pressure one applies, the more flow and better salt rejection one will achieve. The bottom line is that salt rejection improves.
As operating temperature increases, permeate flowrate will increase and salt rejection will decrease (i.e., increase in salt passage). Generally speaking, there will be three-percent increase in flowrate per one degree Celsius increase in temperature (see Figure 2). During summer months, if feedwater temperature increases, the membrane will produce more water than during the winter. If you plot permeate flowrate versus temperature, it is not exactly linear. But generally speaking, three percent is a good rule of thumb.
Feed water concentration, TDS
As feedwater concentration (TDS in ppm) increases, osmotic pressure increases. As a result, permeate flowrate and salt rejection will decrease accordingly, as seen in Figure 3. Generally speaking, for every 100 ppm TDS, it lowers net operating pressure by one psi: net operating pressure = operating pressure – osmotic pressure of feed. In theory, osmotic pressure of permeate water should be considered in calculating net driving pressure, but it is often negligible for RO membranes. Multivalent ions have approximately 0.5-psi osmotic pressure and monovalent ions one psi. If you operate at 150 psi, osmotic pressure is somewhere between 2.5 to five psi, or net driving pressure is 145 versus 147.5 psi, a negligible difference.
Percentage of recovery
Percentage of recovery is defined as permeate flowrate divided by feed flowrate times 100 percent. As the percentage of recovery increases, permeate flowrate will decrease and salt rejection will also decrease, as shown in Figure 4. Think of percentage of recovery as the concentration factor. For instance, 50-percent recovery means production of 50-percent permeate water out of incoming feedwater. By removing 50 percent of the feedwater as permeate water, you effectively increase the feed concentration by a factor of two. When feed concentration increases, permeate flow and salt rejection decrease (see Figure 3).
Figure 1. Effect of operating pressure
Figure 2. Effect of operating temperature
Figure 3. Effect of feed concentration
Figure 4. Effect of percent recovery
Emergence of LE membranes
The only other way to improve water flux and salt rejection is to come up with new membrane chemistry (such as low-energy membranes) where one can achieve the same/higher water flux at much lower operating pressures without sacrificing salt rejection or salt passage. How is the LE RO membrane different from standard RO membranes and where can it be used? Higher flux membranes have to naturally pass more salt (see Figure 5). But LE membranes can now operate at 33 to 40 percent lower operating pressure than standard RO membranes, without sacrificing flow or salt rejection (percentage of salt passage).
This is very good news for OEMs and end-users for several reasons:
- 33- to 40-percent lower operating pressure compared to standard RO membranes results in energy consumption savings for end users and possibly a lower horsepower pump selection
- hp for LE membranes and 1.0 hp for standard RO membranes) for OEMs.
- Permeate water quality is similar to standard RO membranes. There is no sacrifice in permeate water quality.
- Same or higher permeate water flow rate. Even though LE membranes operate at lower pressure, they can still produce the same/higher permeate water flow, compared to standard RO membranes.
Here is an example of a comparison between standard RO versus LE membranes:
- Assume 24/7 operation and $0.10/kWhr for electricity.
- Feed water: 500 ppm TDS
- Temperature: 20°C
- 50-percent system recovery with recirculation
- Two 4 x 40-inch standard RO membranes in series versus
- Two 4 x 40-inch LE membranes in series, operating at 18.3 gfd (gal/day/ft2).
- Permeate water quality: four ppm with standard RO membranes at 156 psi, and five ppm with LE membrane at 93 psi (only one ppm difference in permeate water quality, which is pretty negligible)
- Energy consumption: 4.2 kWh/1,000 gallons of permeate for standard RO membranes versus 2.4 kWh/1,000 gallons of permeate for LE membranes.
These LE membranes can be used in a variety of applications, such as spot-free rinse for car washes, bottled water production, water vending machines, beverage production, food/ice preparation, misting vegetables, point of entry for residential homes, etc.
The future of RO membranes is moving toward lower operating pressures, while still maintaining or improving permeate water flow and salt rejection. This is truly good news for end users and OEMs.
About the author
Michael Kim is Senior Account Manager for Dow Water & Process Solutions, headquartered in Minneapolis, MN. He is responsible for sales in the western region of the United States. Kim is a CWS-I, certified Six Sigma Black Belt and has been involved with the technical and commercial sides of the UF, NF and RO membrane technology business for 28 years. He can be reached at firstname.lastname@example.org or (949) 206-1713.
About the products/technology
For comparison, FILMTEC™ BW30-4040 and LC LE-4040 membrane elements were used in this article. For additional information on the low energy (LE) membrane products, please visit www.filmtec.com.