What is Reverse Osmosis?

Reverse Osmosis (RO) is a popular water filtration process that can remove up to 99.99% of specific contaminants. RO is utilized in recycling and wastewater treatment, as well as consumer water filtration systems, whereby a semi-permeable membrane captures the smallest particles in your drinking water .

Normal tap water does not have a filter and often contains unwanted and harmful substances and chemicals, which makes dangerous drinking water.

RO is employed worldwide, from large-scale plants filtering millions of gallons per day, to residential homes filtering drinking water for individual families.  RO is one of the most commonly used methods of producing clean water.

RO uses a semi-permeable membrane to separate water molecules from other unwanted substances. It applies pressure to overcome osmotic pressure that favors even distributions. RO can remove dissolved or suspended chemicals as well as biological substances (principally bacteria); retaining the solid and unwanted chemicals on one side of the membrane and allowing the purified water to pass through to the other side. It relies on the relative sizes of the various molecules to decide what passes through the  “selective” membranes; rejecting large molecules, while accepting smaller molecules (such as water).

RO is most commonly known for its use in drinking water purification from seawater, removing the salt and other effluent materials from the water molecules.

History

The process of osmosis through semi-permeable membranes was first observed in 1748 by Jean-Antoine Nollet. For 200 years thereafter, osmosis was only a laboratory phenomenon. In 1950, the University of California at Los Angeles (UCLA) first investigated osmotic desalination (removing salt from sea water). Researchers at both UCLA and the University of Florida desalinated seawater in the mid-1950s, but the flux was too low to be commercially viable. Sidney Loeb at UCLA and Srinivasa Sourirajan at the National Research Council of Canada, Ottawa, found techniques for making asymmetric membranes characterized by an effectively thin “skin” layer supported atop a highly porous and much thicker substrate region. John Cadotte, of Filmtec corporation, discovered that membranes with particularly high flux and low salt passage rates could be made by interfacial polymerization of m-phenylene diamine and trimesoyl chloride. Cadotte’s patent on this process was the subject of litigation and expired. Almost all commercial RO membranes are now made by this method. By 2019, approximately 16,000 desalination plants operated around the world, producing over 95 million cubic meters (25 billion US gallons) per day. Around half of this capacity was in the Middle East and North Africa region.

Osmosis

In (forward) osmosis, a solvent moves from an area of low solute concentration (high water potential), through a membrane, to an area of high solute concentration (low water potential). The driving force for the movement of the solvent is the reduction in the Gibbs free energy of the system in which the difference in solvent concentration between the sides of a membrane is reduced. This is called osmotic pressure. It reduces as the solvent moves into the more concentrated solution. Reverse osmosis occurs when external pressure is used to “reverse” the natural flow of pure solvent. The process is similar to other membrane technology applications.

RO differs from filtration in that the flow of the water is reversed.  As the water passes through the membrane, solid materials are left behind. The predominant removal mechanism in membrane filtration is straining, or size exclusion, where the pores are 0.01 micrometers or larger, so the process can theoretically achieve perfect efficiency regardless of factors such as the solution’s pressure and concentration. RO instead involves solvent diffusion across a membrane that is either nonporous or uses nanofiltration with pores 0.001 micrometers in size. The predominant removal mechanism is from differences in solubility or diffusivity, and the process is dependent on pressure, solute concentration, and other conditions.

RO requires pressure between 30–250 psi (pounds pe square inch) for fresh and brackish water, and 600–1200 psi for seawater. Seawater has around 390 psi natural osmotic pressure that must be overcome.

Membrane pore sizes vary from 0.1 to 5,000 nm. Particle filtration removes particles of 1 µm or larger. Microfiltration removes particles of 50 nm or larger. Ultrafiltration removes particles of roughly 3 nm or larger. Nanofiltration removes particles of 1 nm or larger. RO is in the final category of membrane filtration, hyperfiltration, and removes particles larger than 0.1 nm.

What does reverse osmosis remove?

So, what does RO remove from your drinking water? The effectiveness of each RO system varies; some may actually remove more contaminants than others. The differences are they types and quantities of contaminants that are removed or reduce, i.e. the effectiveness of the RO system. Generally, a good RO system is highly effective at removing most:

• • Lead, arsenic, and fluoride
  • Bacteria, viruses, and cysts
  • Pesticides, herbicides, and fungicides
  • Chlorine and chloramine
  • Microplastics, PFOA/PFOS, VOCs
  • Minerals
  • Sediment

Reverse osmosis is considered the best available technology by the EPA when it comes to removing uranium, radium, and other radionuclides. It is also the only type of filter capable of removing small contaminants like fluoride. The EPA states that RO filters “are effective in eliminating all disease-causing organisms and most chemical contaminants.”

Reverse Osmosis v. Regular Water Filters

If you are considering both an RO system and a regular water filter, there are a several major differences that you will need to know.

Contaminant Reduction. RO systems are the only proven method of water filtration on the market today that remove fluoride. Because fluoride is such a microscopic contaminant, it takes powerful filtration technology to remove it (RO’s semi-permeable membrane technology). ROs also remove contaminants that most water filters cannot such as arsenic, total dissolved solids (TDS), and nitrate (a tasteless compound that is harmful for babies if ingested).

Installation. Reverse osmosis systems take a little more time to install because, unlike direct connect or countertop filters, ROs have permanent fixtures. Most ROs require a dedicated faucet to be installed which takes a little more time.

Water Flow. Many RO systems use a water tank to store filtered water so that it is immediately available when needed. Depending on how much water is used in the home, low water pressure may result for a few minutes while the water tank is being refilled.

Benefits of Reverse Osmosis

RO systems have several substantial and positive health benefits.

Helps Reduce Sodium Intake. Homes that use water softeners to remove minerals from hard water will produce water that is great for cleaning, bathing, and laundry, but not so great for drinking. Most water softeners replace hard minerals with sodium, causing a salty water taste. Adding a reverse osmosis water filter will produce softer water without the added sodium and accompanying salty taste.

Helps Prevent Dementia. A recent study found that dialysis patients could prevent dementia (a comorbidity that occurred in 18 out of 258 patients) by simply using RO filtered water. With no other treatment, scientists were able to improve the condition in 7 out of 9 previously exposed patients and prevent dementia in those whose water was treated from the start of the study.

Helps Prevent Gastrointestinal Illness. Another study tracked the gastrointestinal health of 1,400 families and found 14% more gastrointestinal illness in families drinking tap water than in those who were drinking water purified with reverse osmosis. The study noted that “14-40% of gastrointestinal illnesses are attributable to tap water meeting current standards and that the water distribution system appears to be partly responsible for these illnesses.”

If you would like to know more about reverse osmosis systems, please contact Nevada Clean Water at (702) 321-6475 and schedule a free in-home water purity test.

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Written by Robert Victor, February 22, 2022
Excerpts taken from Rachel Carollo, What is Reverse Osmosis: Explained in Simple Terms