By David H. Paul

Summary: The permeate of RO water passed through a city water system goes through various stages. How this affects dissolved substances and the bladder tank, among other factors, goes a long way in explaining the many nuances of an RO unit’s efficiency.


In 10-15 minutes, roughly the time it takes to read this article, there will be a net increase of 1,800-2,700 people on the planet, by my own rough estimate extrapolating from population projections for the year 2060. They will all need water to drink. Those of us in the drinking water production business have the potential for a great future. Companies that provide excellent products and service—backed by dedicated, knowledgeable employees—should do very well.

In the more affluent countries, there’s a continually growing perception that tap water may not be the safest available water. This perception is due, at least in part, to news reports identifying pharmaceutical drugs, health and beauty aids, and industrial and municipal waste products in our natural waters. It’s also due to effective marketing and a continuing reduction in the price of purified water.

The growth of the bottled water industry is a result of the perception that tap water may not be the safest water available. For a number of reasons, many people choose to produce purified water at home by purchasing/installing a home reverse osmosis (RO) system.

Tight filters
RO membranes are amazing. They’re regarded as the tightest “filters” made. Given the right conditions, they can greatly reduce most contaminants from feed water, turn seawater into drinking water, and make safe drinking water out of almost any feed water.
We need to know what “given the right conditions” means to really understand RO technology. The first step is to realize that they really aren’t filters, at least not in the classic sense.

Defining osmosis
While man invented reverse osmosis, he discovered osmosis. To better understand reverse osmosis, it’s beneficial to understand osmosis first. Osmosis is a natural phenomenon that’s occurring in our bodies right now.

Osmosis is the tendency of water to pass through a semi-permeable membrane into a solution of higher dissolved solids concentration. Semi-permeable means that water can pass (permeate) through the membrane, but most other things have a difficult time. For instance, the passage of fluids through the walls of our stomachs is largely ruled by osmosis. In this case, the stomach lining would be the semi-permeable membrane.

There are several theories (models) of how dissolved substances pass through a semi-permeable membrane. The one more commonly cited is the “solution-diffusion” theory. Diffusion is the phenomenon where dissolved substances pass from areas of higher concentration to areas of lower concentration due to random motion, eventually creating a uniform concentration.

Tracking movement
In other words, if we have a semi-permeable membrane that separates two water samples containing different concentrations of total dissolved solids (TDS), there will be a net movement of water molecules from the lower TDS side to the higher TDS side (see Figure 1). In Figure 1, there’s ultrapure water on one side of the membrane (essentially 0 mg/L)—keep in mind, absolute zero here is theoretical only as it’s an impossibility in real world applications—and a solution containing 300 milligrams per liter (mg/L) of TDS on the other side. Recall that mg/L is the metric equivalent of parts per million (ppm).

As a practical example, observe what happens when we drink water, coffee or other beverages primarily composed of drinking water. Drinking water typically has a TDS content of less than 500 mg/L. Many drinking water supplies have less than 200 mg/L of TDS. Our blood stream has roughly 15,000 mg/L of TDS. When we put 200-mg/L water inside of our semi-permeable, membrane-lined stomachs, water passes out of the stomach and into the blood stream due to osmosis.

Over time, due to osmosis, the level will drop on the lower TDS side. The level will rise on the higher TDS side. Without getting into complex theories, one can simply explain osmosis as essentially a numbers game. The lower TDS side has more water molecules, which are smaller, in contact with the membrane than the higher TDS side, which has both water molecules and larger dissolved solids molecules in contact with the membrane (see Figure 2). There are, therefore, more water molecules passing from the lower TDS side to the higher side.

Reverse osmosis
To make water molecules go in the opposite direction of what would normally occur due to osmosis, aka reverse osmosis, we simply have to apply force on the higher TDS side of the membrane. This is accomplished with pressure.

For residential RO units, the pressure comes from distribution pumps—located at the local, municipal water treatment plant or booster stations—which pressurize the city water lines. The city water pressure causes more water molecules to pass through the semi-permeable RO membrane, from the higher TDS city water side to the lower TDS permeate side.

Water through membranes
For most residential systems, there’s a small storage tank (bladder tank) located on the downstream side of the RO membrane to store permeate. Let’s look at what happens during operation, as far as water flow through the membrane is concerned.

When first installed and placed in operation, the bladder tank has zero gauge pressure. Let’s assume the city water gauge pressure is 60 pounds per square inch, or psi (4 bar). Immediately upon startup, water molecules only pass from the feed side to the permeate side because there’s no permeate on the downstream side.

As the bladder tank fills and the pressure in the tank increases, there are increasing numbers of water molecules colliding with the membrane on the permeate side, and therefore trying to go back to the feed water side. When the bladder tank is filled, there’s essentially the same pressure in the tank as in the city water line. At this point, there’s an equal number of water molecules passing through the membrane in both directions. Osmosis and reverse osmosis are equal.

As soon as permeate leaves the faucet, the pressure in the bladder tank drops and reverse osmosis resumes until the pressure in the tank again rises to equal the city water pressure.

What would happen if the bladder tank was fully pressurized and the city water line lost pressure for some reason? We would have more pressure on the permeate side than the feed water side. There would be a net movement of water from the permeate side to the feed water side. The pressure in the bladder tank would decrease as water passed from the bladder tank back into the feed water side. This is why many residential systems have a check valve.

Water passage vs. TDS passage
So far we’ve only discussed the passage of water through the membrane. Next is a discussion of the passage of TDS through the membrane.

RO membranes were invented to remove dissolved substances. We can’t see pores in an RO membrane, even with our most powerful microscopes. Therefore, in theory, an RO membrane—if perfect—won’t allow any suspended particles to pass. Suspended particles are organic or inorganic particles greater than 0.01 µm. As a reference, many viruses are in the 0.01- to 0.1-µm range and many bacteria are in the 0.2 to 10 µm range. Meanwhile, a human hair is roughly 100 µm in diameter.

TDS is composed of two categories of contaminants—charged and uncharged. Some contaminants have one, two or three positive or negative charges. These are called ions. Examples include positively charged ions like hydrogen (H+), sodium (Na+), calcium (Ca+2), magnesium (Mg+2), copper (Cu+2), lead (Pb+2) and aluminum (Al+3). Negatively charged ions include hydroxide (OH-), chloride (Cl-), bicarbonate (HCO3-), nitrate (NO3-), carbonate

(CO3-2), sulfate (SO4-2) and phosphate (PO4-3).
The uncharged contaminants are mainly organic and certain silica molecules. Silica refers to silicon-containing compounds. Organic compounds include most carbon-containing compounds.

Getting a charge
RO membranes reject dissolved charged substances based on their charge. The more charges an ion possesses, the more it’s rejected. RO membranes reject dissolved uncharged substances based on size. The larger the uncharged substance, the more it’s rejected.

In this model, it’s relatively difficult—but not impossible—for many dissolved substances to pass through the membrane. Yet, it’s relatively easy for water molecules to pass through. Given a certain amount of time, with higher city water pressure and lower bladder tank pressure, a lot of water will pass through the membrane while only a small amount of dissolved substances will pass through. Permeate in the bladder tank will have a much lower TDS than the city water.

When the bladder tank fills, however, and there’s no longer a net movement of water molecules, the dissolved substances that can pass through the membrane continue doing so due to diffusion. This is why the conductivity (measurement of ions in water) of permeate in the bladder tank goes up over time. Residential RO units, therefore, will provide the lowest TDS water if the bladder tank is emptied frequently.

Conclusion
RO permeate is produced in residential RO units due to the pressure of the city water forcing water molecules through the RO semi-permeable membrane. Many of the dissolved substances in the city water can pass through the membrane, albeit relatively slowly over time. When the bladder tank is full and not in use, certain dissolved substances continue to pass through the membrane, causing the conductivity of the permeate to rise. The lowest conductivity permeate will be produced in units where the bladder tank is emptied frequently. It’s a small effect with frequent use of permeate (hopefully, in normal operation). Given enough time, though, without use of permeate, its conductivity will become the same as the feed water, i.e., when equilibrium is reached.

About the author
David H. Paul is president and CEO of David H. Paul Inc. (DHP), of Farmington, N.M. Paul, who has bachelor’s and master’s degrees in biology and microbiology, respectively, has over 25 years of experience in the water treatment industry as a practitioner, researcher, consultant and trainer. He’s the author of more than 100 published articles on membrane treatment and has created a 4,000-page training program for on-campus, correspondence degree and non-degree programs. He can be reached at (877) 711-4347, (505) 327-2934 (fax), email: [email protected] or website: www.dhptraining.com

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