By Peter S. Cartwright, PE
The Technology
Reverse osmosis (RO) is a water-treatment technology that utilizes atomic filtration to remove contaminants from water. The key to this removal is a special membrane that basically holds back, or rejects, almost all contaminants, including sediment, microorganisms, dissolved organics, and even salts and minerals, while allowing purified water to pass through. The rejected contaminants are swept away from the membrane surface and continuously discharged to the drain during operation.
In other words, one water stream enters the membrane device, and two streams exit. One is purified water (permeate) and the other (concentrate) contains most of the waterborne contaminants. The driving force for this separation is water pressure, usually from a pump.
Arguably the most practical and effective residential drinking-water treatment system, RO will remove over 90 percent of salts and minerals (think lead, arsenic, and sodium), and almost all microorganisms and large organic contaminants, including PFAS.
Most of the residential RO units on the market are designed to fit under kitchen sinks and consist of at least four cartridges: sediment filter, carbon filter, RO membrane, and post filter. The cartridges are typically manifolded together. The system also comes with a two- to three-gallon pressure tank to store the treated water and release it through a separate faucet mounted on the sink surface.
Countertop RO units are also available and can either be attached to the sink faucet or come with a carafe into which tap water is poured. Water from this carafe is pumped through the membrane, and the permeate is collected in another carafe. This unit does not require proximity to a faucet and can be placed anywhere an electrical outlet is available.
For institutional, commercial, and industrial uses, RO is the primary technology for treating water for such diverse applications as boiler and cooling tower feed, product rinsing, pharmaceutical and semiconductor manufacturing, and seawater desalting. RO has become the most widely used technology for removing salts from water supplies.
The Mechanism
Although RO has been a key treatment technology for over 60 years, it is still surrounded by misunderstanding and mystery. This is partially the result of a lack of agreement on the actual mechanism of salt rejection. The most widely accepted theory is that the movement of salts through the membrane is based on concentration gradient, the difference between the salts concentration on the feed side of the membrane and the permeate side.
Meanwhile, the passage of water through the membrane is a function of water pressure. The purified water wins the race. The behavior of this technology includes examples that defy this explanation, however, hence the lack of agreement.
A recent theory considers friction between salt ions, water, and the membrane polymer atoms as the mechanism—another attempt to explain the mystery.
The removal of non-ionic contaminants (nonpolar organics, microorganisms, and other suspended solids) is strictly filtration; what’s too big to pass through the membrane pores is held back. To further complicate matters, some experts claim that RO membranes don’t actually have pores.
The Shortcomings
What removal claims cannot be made about reverse osmosis?
Since the rejection of non-ionic dissolved materials is based on size exclusion, what size contaminants will not be removed? Because the size of organic molecules is related to shape as well as molecular weight, the membrane industry has settled on the conclusion that any dissolved organic molecule with a molecular weight below 150 atomic mass units, or daltons, will likely pass through the membrane.
Microorganism removal is somewhat controversial. Whereas all bacteria are large enough to be rejected, because they are viable, or alive, many experts believe they can grow through a membrane. This appears to happen only if water sits stagnant on the feed side of the membrane, which allows the bacteria to attach to the membrane and multiply. Moving water does not appear to suffer from “grow through.”
In general, because of viruses’ small size and ability to grow inside bacteria, complete virus removal is not possible with RO. Disinfection is necessary to guarantee treated water contains a minimal concentration of microorganisms.
RO System Design
Let’s say your primary interest is the design of a system to treat a water supply. How can this membrane be incorporated into a complete system to give you the quantity and quality of water you need?
Although membrane elements are available in four basic configurations (spiral, tubular, plate and frame, and hollow fiber), the overwhelming RO device configuration used in water purification is spiral. There are at least eight U.S. manufacturers of these membranes, all of high quality and competitively priced. Each manufacturer may have several models, each with special characteristics.
Once you have selected a particular spiral RO membrane, what do you need to know to design a treatment system?
It is important to understand the factors that influence RO system design. Here are some:
- Feed-water analysis.
- Membrane properties.
- Water temperature.
- Pump pressure.
- Flow rate.
- System recovery.
Your ability to adjust the feed-water chemistry is limited to the removal of slightly soluble salts such as iron and silica, but you must work with what you have. Adjusting the water temperature is usually very expensive, but this variable can affect membrane production rate, as will pump pressure and system recovery.
System Design Training
Where can you learn about RO system design? The 2023 Water Quality Association Convention & Exposition in Las Vegas will offer the Commercial/Industrial RO Sizing Workshop on April 17.
This workshop utilizes a problem-solving approach to teach the design fundamentals of RO systems. Participants sit in groups of four to six people and are provided with the calculations appropriate to develop the system design. An application is presented, and participants work together to examine the effect of influence factors on system design. As many as nine problems are presented, examining such variables as the effect of system recovery on permeate quality and the effect of temperature on permeate production flow rate. The problem offerings include seawater desalination and wastewater treatment.
A separate registration is required for this workshop. (https://convention.wqa.org/pre-convention-sessions/ro-workshop/)
Closing Thoughts
The role of acceptable quality water in sustaining life on this planet is widely recognized; however, what may be less clear are the looming crises of water quantity and quality.
Global warming has been linked to such weather-related catastrophes as hurricanes, wildfires, tornadoes, drought, and floods. These are severely affecting both the availability and quality of water supplies. More waterborne contaminants are being identified, and the increasing global population means the fixed quantity of water on this planet will become more and more contaminated.
We must employ increasingly innovative technologies to process all available sources of water—rainwater, storm water, humidity, industrial and municipal wastewater, etc.—and reverse osmosis is, and will continue to be, one of the best.
About the author
Peter Cartwright entered the water-purification and wastewater-treatment industry in 1974 and has had his own consulting engineering firm since 1980. He has a degree in chemical engineering from the University of Minnesota and is a registered professional engineer in that state. He has authored over 300 articles, written several book chapters, presented over 300 lectures in conferences around the world, and is the recipient of several patents. Cartwright is a recipient of both the Award of Merit and Lifetime Member Award from the Water Quality Association and is the technical consultant for the Canadian Water Quality Association. He was the 2016 McEllhiney Distinguished Lecturer for the National Ground Water Research and Educational Foundation and gave over 35 lectures throughout the world on groundwater contaminant mitigation. Cartwright can be reached via email at [email protected], or visit his website, www.cartwright-consulting.com.