Reverse Osmosis Water Filtration: Everything You Need to Know

Reverse osmosis technology improves the appearance, taste,and odor of drinking water. A reverse osmosis water filtration system uses a semipermeable membrane to remove ions, molecules, and particles from water resulting in healthy, safe water. Other benefits of an RO system include providing clean, refreshing water that's far more economical than bottled water or cheap temporary solutions like a pitcher or faucet filter. In the following article, learn all about reverse osmosis water filtration including the history behind the technology, how reverse osmosis works, and the benefits of a reverse osmosis water filtration system.

Table of Contents

• A Brief History of Reverse Osmosis Water
• What Is RO Water?
• Osmosis: A Life Function
• Semi-Permeable Membrane for Reverse Osmosis
• Comparing Osmosis and Reverse Osmosis
• How Does Reverse Osmosis Work?
• What Happens to the Contaminants during Reverse Osmosis?
• Basic Components of an RO System
• Permeate Pump
• What Contaminants Are Removed By a Reverse Osmosis Water Filtration System?
• Pre-treatment
• Fouling of RO Membrane
• Multimedia Water Filters
• Microfiltration
• Scaling in Reverse Osmosis Systems
• Antiscalants and Scale Inhibitors
• Chemical Effects on TFC Membranes
• Sodium Bisulfite Injection and Granular Activated Carbon
• Mechanical Damage on RO Systems
• Finals Thoughts

A Brief History of Reverse Osmosis and Water Filtration Technology

Seabirds have a special membrane that allows them to drink seawater. The membrane desalinates the water, allowing them to consume pure water. The salty waste is later spit out. By studying these birds, scientists were able to replicate the process in the 1970s. Their intentions? To desalinate seawater, of course! This process has come to be known as reverse osmosis, which is also referred to as RO. Since the ‘70s, humans have started applying the technology in many other ways. Reverse osmosis water filtration systems are commonly used for water treatment, as reverse osmosis removes almost all contaminants. Recently, RO water has also become increasingly popular in medical science, with particular focus on purifying water for dialysis.

Reverse Osmosis Water Filtration: Definition and How It Works

As mentioned above, reverse osmosis is the means of desalinating and deionizing water. It basically removes almost all the contaminants in water. This is achieved by passing the impure water through a special membrane, using high pressure. To truly understand RO, you should first understand another naturally occurring process: osmosis.

Osmosis: A Life Function

Osmosis is one of the most important functions of life. It’s how plants absorb water from the earth with their roots, and how our kidneys draw water from our blood. Essentially, osmosis works like this: if you divided a tank in half with a semi-permeable membrane, and then filled the container with water but added salt to one half, then the clean water would start drawing toward the salty water.

Semipermeable Membrane for Reverse Osmosis

So what is the semipermeable membrane exactly? Think of it as a screen door, but for water. In the same way that a screen door allows for air molecules to pass through but prevents flies and other pests in, the semipermeable membrane lets pure water pass through while blocking contaminants. Most reverse osmosis water filtration systems use a membrane made of cellulose acetate, polysulfonate, or polyamide. There are two parts to a reverse osmosis membrane: the active barrier (or skin) which provides submicron filtration, and a support layer.

Osmosis vs. Reverse Osmosis: Key Differences Explained

While both osmosis and reverse osmosis occur naturally, the main difference is that osmosis requires no extra energy but reverse osmosis does. Just as the seabird needs to actively swallow to force the seawater through its throat membrane to desalinate it, a reverse osmosis water filtration system uses pressure on the impure water. At the end of the process, the RO water is pure - hence why reverse osmosis water filtration systems are used often in industrial plants, medical science, and homes. However, for the water molecules to pass through the RO membrane, you need to force the water through using pressure higher than the natural osmotic pressure. In this, you can see the reasoning behind the term: RO is the process of osmosis in reverse. Instead of pure water migrating naturally toward water with a higher concentration of solubles or impurities, you have a filtration of contaminated water to create pure water.

How Reverse Osmosis Works: A Step-by-Step Guide

A reverse osmosis water filtration system incorporates a high pressure pump to create the necessary level of force to counter the osmotic pressure. This interrupts the more natural flow of purer water toward water with higher concentrations of impurities, reversing it to push the contaminated water through the RO membrane. The water containing more impurities is referred to as feed water. The amount of pressure required by the reverse osmosis water filtration system increases in ratio to the level of concentration in the feed water. The resulting product (the desalinated water) is called RO water. However, it’s also known as permeate water because of the semi-permeable RO membrane. The semi-permeable RO membrane isn’t the only part of the reverse osmosis water filtration system, though. First, there’s the pre-filter stage, where sediment, silt, and dirt is strained from the feed water. It’s an incredibly important stage, as the sediment filter is designed to protect the more delicate RO membrane from dirt, which can severely damage the membrane. Next, the feed water is passed through a carbon filter to remove chlorine, as well as other contaminants that can negatively impact the life and performance of the semi-permeable RO membrane. The carbon filter stage also assists in improving the taste and odor of the RO water, an important function in medical and home reverse osmosis water filtration systems. The third step is the semi-permeable RO membrane. The membrane is designed to allow water molecules to pass through unhindered. Water molecules form hydrogen bonds in the membrane, fitting into the RO membrane’s matrix. Soluble salts, organics, pyrogens, and bacteria are trapped however.

Most organic substances, such as oils, pyrogens, bacteria, viruses, and other particulates, have a molecular weight over 100. The membrane traps these contaminates. Salt ions are also sieved out, but the process is a little more complicated. Dielectric interactions repel ions based on the ion’s valence. The semi-permeate membrane in reverse osmosis water filtration systems bounce ions with higher charges further away. It’s the most important step in reverse osmosis water filtration systems. Without the semi-permeable RO membrane, there would be no reverse osmosis in the filtration process! The final step in a four-stage RO system is the polishing filter. This is a second carbon filter, used to ensure that the RO water is truly odor and taste free, making it safe for medical and everyday use.

What Happens to the Contaminants during Reverse Osmosis Water Filtration?

We know that seabirds spit out the salty waste when they use reverse osmosis to filter seawater for drinking. But what happens to the contaminants that the RO membrane captures in a reverse osmosis water filtration system? While most of the feed water is forced through the membrane to become RO water, some of it gets redirected. This is the reject stream. The contaminants are discharged via the reject stream and into the drain line. This brine water is then either disposed of or (in some cases) fed back into the intake to become feed water again, to prevent wastage. A reverse osmosis water filtration system uses cross flow filtration rather than standard filtration. Standard filtration systems collect the contaminants within the filter’s housing. Cross flow filtration, on the other hand, employs two outlets for the solution (in this case water), so that the RO water passes one way (into the storage tank, through a fourth filter, and out the RO faucet) and brine water passes another, carrying the contamination away. Cross flow filtration allows the reject stream to flush away the build up of contaminants on the semi-permeable RO membrane. The motion also generates enough turbulence to keep the membrane’s surface clean, preventing degradation over time.

Key Components of a Reverse Osmosis Water Filtration System

Please note that this section refers to residential, point-of-use (POU) reverse osmosis water filtration systems. However, all RO water filtration systems have the same basic components. The main difference is not in the parts, but in the quality of those components. So what are the basic components of an RO system?

1. The cold water line valve fits onto the supply line for cold water. It connects to the inlet of the reverse osmosis water filtration system via a tube, and is the water source.
2. The feed water from the cold water line valve is first pushed through the pre-filters. As detailed above, these are the sediment filter and carbon filter. They protect the semi-permeable RO membrane from physical and chemical damage. Firstly, the sediment filter removes dirt to prevent the RO membrane from clogging, and then the carbon filter removes chlorine, which can corrode the membrane.
3. The semi-permeable RO membrane is the heart of the system removing a vast array of aesthetic and health-related contaminants.
4. Before exiting the reverse osmosis water filtration system, the permeate water runs through a final filtration. The post filter is a second carbon filter, to remove any remaining tastes or odors and “polish” the RO water.
5. After passing through all stages of purification, the RO water runs into the pressurized storage tank, which holds between 2 - 4 gallons of water at a time. Pressure is maintained by a bladder inside the tank.
6. An automatic shut off valve prevents water wastage. The shut off valve will close once the storage tank is full, so no more feed water enters the reverse osmosis water filtration system. As the storage tank’s pressure drops when RO water is drawn from the faucet, the shut of valve opens once more, allowing feed water to filter through the semi-permeated membrane.
7. Located in the outlet section of the RO system’s membrane housing, the check valve stops RO water from flowing back through the semi-permeated filtration membrane. This is a crucial component, as the RO membrane would be ruptured by backward flow.
8. Just as important is the flow restrictor, which regulates the feed water being pushed through the RO membrane. While flow controls can come in a variety of styles, they all function to maintain the optimal flow rate required. (The optimal flow rate is determined by the RO membrane’s gallon capacity.) Furthermore, it also assists in keeping steady pressure on the semi-permeated RO membrane’s inlet. Without the flow restrictor, most of the feed water would go to waste, because it would naturally take the path of least resistance - the drain line. This would result in very little RO water, and is the reason why the flow restrictor is most often found in the RO drain line tubing.
9. The RO system will have its own faucet. The kitchen sink is the most common spot for installation. While non-air gap models are undoubtedly popular, some residential areas have plumbing restrictions which enforce the need for an air gap faucet.
10. The drain line runs from the RO membrane housing’s outlet to the drain, and is used to dispose of the brine, or waste water, and impurities.

Permeate Pump for Reverse Osmosis

On average, each person will drink about 2 gallons of water a day, creating a fairly high demand for reverse osmosis water systems with a high filtration power. RO systems are therefore equipped with a semi-permeable membrane capable of filtering either 35 gallons per day or 50 gallons per day (GPD). As explained above, reverse osmosis water filtration systems use an automatic shut off valve to prevent more feed water entering the RO system once the storage tank is full (typically 50-67% of the incoming water pressure). This is because the RO water quality begins to diminish as the storage tank’s pressure increases above this point. More of the feed water becomes brine water too, instead of being put through the semi-permeable RO membrane, because the storage tank is creating back pressure. As a result, a 35 GPD reverse osmosis membrane will only filter 15 gallons a day. To isolate the RO membrane from this back pressure, a permeate pump can be installed. This allows the reverse osmosis water filtration system to operate optimally with 85% of the feed water pressure building in the storage tank. The RO membrane’s efficiency is increased as a result, and the RO water’s quality improved too. Your reverse osmosis water filtration system will also be able to store more water in the holding tank because of the increased pressure allowance. As a result, production can be more than doubled in a day, allowing a 35 GPD RO membrane to produce 35 gallons a day rather than just 15. The permeate pump works by harnessing the energy produced by the bile water, which rinses the semi-permeated RO membrane of the contaminants that build up there as part of the filtration process. This energy is then used as leverage, pushing the purified RO water into the storage tank. Because the permeate pump works on hydraulic energy (which is usually dispelled via the drain tube without being used), it doesn’t require any electricity. The permeate pump therefore isolates the semi-permeated membrane from the tank, allowing the RO membrane to perform as if it were in an atmospheric tank system.

Contaminants Removed by a Reverse Osmosis Water Filtration System

We’ve mentioned that bacteria, ionized salts, pyrogens, and even viruses are removed through reverse osmosis. But the semi-permeated RO membrane also filters other particles, colloids, and other organic solubles. The greater the contaminant’s molecular weight, the better reverse osmosis will remove it from the feed water. Water has a molecular weight of 18, whereas many contaminants rejected by the RO membrane average on 200 or more. In a similar manner, the higher the contaminant’s ionic charge, the less likely it is to pass through the RO membrane. Sodium ions are monovalent, meaning they only have one charge. Calcium also has a very low ionic charge - only two. This allows them to pass through the semi-permeated membrane. Gases such as carbon dioxide (CO2) also have a low ionic charge level when in a solution such as water, and also have extremely low molecular weights. RO water can have a slightly lowered pH level because the semi-permeated reverse osmosis membrane doesn’t remove gases. The RO water’s pH level is largely dependent on the level of carbon dioxide in your feed water, as it is converted to carbonic acid. Nevertheless, reverse osmosis water filtration systems are incredibly effective in purifying surface, ground, and brackish water, as well as saltwater. For this reason, and the heightened levels of pollution, RO water systems have become an increasingly popular addition to many households. Using reverse osmosis to filter one’s water will remove up to 99%+ of all contaminants. It’s the most reliable water filtration system known to mankind, which is why it’s been used in the pharmaceutical industry as far back as 1977. Since then, its applications have been incorporated in many other industries, including food and beverage production, metal finishing, semi-conductor manufacturing, and boiler feed water systems. Introducing a reverse osmosis water filtration system to your home and office is the natural progression of this fantastic science, offering you, your family, and/or your workplace a healthier source of water.

Reverse Osmosis Pre-Treatment: Why It's Important for Water Filtration

Just because the reverse osmosis water filtration system is the best there is, doesn’t mean it’s infallible. Proper pre-treatment is critical to prevent unnecessary degradation of the reverse osmosis water system due to fouling, scaling, RO membrane failure, and additional cleaning. Below, we explore the common issues lack of pre-treatment (or inadequate pre-treatment) creates, as well as the correct preventive measures.

Fouling of RO Membrane

The semi-permeable RO membrane can get clogged if contaminants build up on the surface and aren’t flushed properly by the bile water system. This is known as ‘fouling’. Municipal feed water, although filtered to some degree, are full of contaminants that are naked to the human eye and even harmless when consumed. But they can be large enough to congest the reverse osmosis water filtration system. This often happens in the system’s front end, resulting in a gradual pressure drop and impaired RO water flow. This in turn results in increased operating costs, culminating in a need to have your reverse osmosis water system professionally serviced or parts replaced. No matter how thorough the pre-treatment process and cleaning measures you employ, fouling is inevitable considering the minute pore size of the RO membrane. Proper pre-treament will minimize fouling incidents, and increase your reverse osmosis water filtration system’s life. You can determine whether your RO feed water has a high potential for fouling by performing analytical tests. The most common preventive measures to protect your RO water system is multimedia filters and microfiltration, although in some situations cartridge filtration will be enough.

Chemical Effects on TFC Membranes

The thin film composite (TFC) membranes used in modern reverse osmosis aren’t very tolerant of chlorine and chloramines, which are oxidizers. They essentially burn the RO membrane, connecting pores and forming larger holes. This is irreparable, and will require replacement of the membrane. Chemical attacks increase the number of contaminants that pass through the reverse osmosis filter, resulting in low-quality RO water. Without a biocide, microorganisms begin growing on the RO membrane.

Multimedia Water Filters

Multimedia filters typically contain three layers: anthracite coal, sand and garnet, and gravel at the bottom as a foundation layer. These three media are used for their varying size and density. The arrangement of these layers allow for the largest contaminants to be removed first, and smaller particles filtered out lower down. In this way, the entire bed acts as a highly efficient filter. Well-operated and maintained multimedia filters will remove contaminants as small as 15-20 microns. Multimedia filters that incorporate coagulation as well are even more effective, filtering particles as small as 5-10 microns - which is about 10% the width of a human hair.

Microfiltration

Microfiltration typically has a pore size of 0.1-10µm, and is highly effective in filtering colloidal and bacteria. The ‘hollow fiber’ membrane configuration is the most commonly used. Feed water is pumped through the filters, collecting from within the fibers. When used in potable water systems, microfiltration usually operates in a ‘dead-end’ flow, where all of the feed water is filtered (no cross filtration). This requires periodic backwashing to clear the filter cake that forms as a result.

Sodium Bisulfite Injection and Granular Activated Carbon

Sodium bisulfite is a biocide. Adding the proper dosage to your feed water will assist in removing residual chlorine. Granular activated carbons have two uses: they remove organic contaminates, and reduce residual disinfectants like chlorine from your feed water. Granular activated carbon media is comprised of coal, wood, or nutshells, and create a chemical reaction that transfers electrons to the contaminates from the surface. As a result, the chlorine and chloramines are altered into a chloride ion, which is not an oxidizer and will not cause chemical damage to your semi-permeable reverse osmosis water filtration membrane.

Understanding Scaling in Reverse Osmosis Systems and How to Prevent It

Certain dissolved, inorganic particles increase in concentration, which can cause scaling when the compounds exceed their limitation on solubility. ‘Scaling’ refers to the thin, scale-like film of precipitation that forms on the RO membrane. Calcium carbonate is the most common compound to cause scaling. This results in pressure dropping in your reverse osmosis water filtration system, allowing more salt and other contaminates to pass through the semi-permeable membrane. In turn, your RO water quality drops.

Anti-scalants and Scale Inhibitors

As the name implies, anti-scalants and scale inhibitors are chemicals added to the feed water to reduce its scaling potential. They increase the solubility limits, allowing for inorganic compounds to be concentrated more than without these chemicals. By doing so, your reverse osmosis water filtration system achieves a better recovery rate, and can still function with higher concentrations of scalable contaminants. The scale inhibitors and anti-scalants interrupt scale formation and the growth of microscopic crystals. Water softeners that exchange scale forming ions for non-scalable ions are also used to prevent scaling in your reverse osmosis water system.    

Mechanical Damage in Reverse Osmosis Systems: Causes and Prevention

Plumbing controls before and after the reverse osmosis water filtration system should always be implemented as part of your pre-treatment scheme. A lack of proper plumbing in the feed water system will result in mechanical damage to the RO membrane. Too much back pressure will do the same. Variable frequency drive motors should be used in conjunction with the reverse osmosis water filtration system pump, and check valves and pressure release valves will relieve excessive back pressure. One of the best ways to prevent mechanical damage on both fronts is to make use of a permeate pump, which we mentioned and discussed earlier.

Final Thoughts on Reverse Osmosis Water Filtration

A reverse osmosis water filtration system is the most advanced and effective means of decontaminating water for industrial, pharmaceutical, and household water supplies. Annual cleaning, maintenance, pre-treatment measures, and timely replacement ensures a long lifespan for your reverse osmosis water filtration system. It's also hugely important to sanitize water filtration equipment. RO filters that are properly cared for can last as long as 10-15 years.