Basics of Reverse Osmosis
Reverse osmosis (RO) is a highly effective and economic process for treating contaminated water. It can treat solutions from 100 to over 50,000 milligrams per liter (mg/L). Solutions with saltiness from surface water to sea water, and even brines, can be treated by RO membrane.
Osmosis versus Reverse Osmosis
Osmosis is a naturally occurring phenomenon that occurs when a weaker saline solution migrates to a stronger saline solution. Reverse osmosis is the opposite of this process, where water molecules are forced through a semi-permeable membrane from an area of high solute concentration to an area of low solute concentration.
The process of RO is essentially the opposite of the natural process of osmosis. In osmosis, water molecules pass through a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration. The net effect is that the water level on the side with the higher concentration decreases, while the water level on the side with the lower concentration increases.
In reverse osmosis, water molecules are forced through a semi-permeable membrane from an area of high solute concentration to an area of low solute concentration. The net effect is that the water level on the side with the higher concentration decreases, while the water level on the side with the lower concentration increases.
Osmosis
Osmosis is a naturally occurring phenomenon, and it’s one of the most critical processes in nature. The process of osmosis occurs when a weaker saline solution migrates to a stronger saline solution. Some examples of osmosis are when plants take water from the soil through their roots and our kidneys absorb water from our blood stream.
The process of osmosis is shown in the figure below. Because these solutions are less concentrated, they will tend to migrate to a solution with a higher concentration. For example, if you poured water from a container with a low salt concentration into one with a higher salt concentration and then separated them by a semi-permeable membrane, the water in the lower salt concentration would flow toward the other container with more salt.
Semi-permeable membrane
A semi-permeable membrane is a membrane that allows some particles to pass through it but not others. A screen door is a perfect example. Air can flow through, but not pests or anything larger than the holes in the screen door. Another example is the material used to make Gore-Tex clothing, which includes a very thin plastic film with billions of tiny holes cut into it. The pores are large enough to allow water vapor through but small enough to prevent liquid water from passing.
Reverse osmosis process – osmosis in reverse
Reverse osmosis is simply the process of osmosis in reverse.
Osmosis does not need any energy to happen. You need to apply energy to the higher-concentration saline solution in order to reverse the process of osmosis.
The membraned used in reverse osmosis is semi‐permeable. It has tiny holes, called pores, that allow the passage of water molecules but not most of the dissolved salts, organics, or bacteria.
To force the high concentration water through the reverse osmosis membrane, you must apply pressure to the raw water. The applied pressure must be greater than the naturally occurring osmotic pressure.
The process of reverse osmosis is illustrated in the diagram below. When pressure is applied to the concentrated solution, water molecules are forced through the semi-permeable membrane. The larger contaminants are not allowed through.
How does Reverse Osmosis (RO) work?
Reverse osmosis works by using either a high-pressure pump (or the available water pressure) to increase the pressure on the contaminated water side of the RO membrane. This pressure forces the water across the semi-permeable membrane, leaving most (approximately 95% to 99%) of dissolved salts behind in the reject stream.
The amount of pressure required depends on the contaminant concentration of the feed water. The more concentrated the feed water is, the greater the pressure required to overcome osmotic pressure.
In a nutshell, when water is pumped into a reverse osmosis system, two streams of water come out: good water and bad water. The “good water” comes out of the RO system with most of the contaminants removed. It is called permeate. Another name for permeate water is product water.
Permeate is the water that flows through the RO membrane and contains minimal contaminants. Reverse osmosis system sizes are based on the permeate flow. For example, a 100 gpm reverse osmosis system will produce 100 gpm of permeate (clean) water.
The “bad water” is the water that contains all of the contaminants that could not pass through the RO membrane. This water is also known as the concentrate, reject, or brine. All three terms are used interchangeably and mean the same thing.
Flow process through RO system
As pressurized feed water enters the RO membrane (with enough pressure to overcome the osmotic pressure), the water molecules pass through the semi-permeable membrane. The contaminants are not allowed to pass through the membrane.
The contaminants are discharged through the concentrate stream. The concentrate can either go to waste or, under some conditions, be fed back into the feed water supply to be recycled through the RO system to minimize water waste.
Permeate or product water is the water that passes through the RO membrane. It typically has around 95% to 99% of the dissolved minerals removed from it.
The water flows through a RO membrane as shown in the diagram below.
An important distinction between reverse osmosis and conventional filters is the RO system employs cross filtration rather than standard dead-end filtration. With conventional filters, the contaminants are collected within the filter media.
With cross filtration, water passes through the filter, or crosses the filter, with two outlets:
- the filtered water goes one way
- the contaminated water goes a different route
To avoid concentrating contaminants, cross flow filtration allows water to sweep away concentrated contaminants. This allows enough turbulence to keep the membrane surface clean.
What contaminants does Reverse Osmosis Filtration Remove?
Reverse osmosis can remove 95-99 percent of the dissolved salts (ions), particles, colloids, organics, and germs from the water supply. The RO membrane rejects contaminants based on their size and charge. A properly operating RO system is capable of rejecting particles with a molecular weight greater than 200.
Similarly, the higher the ionic charge of the contaminant, the more likely it will be retained by the RO membrane. Consider two examples. A sodium ion has one charge – monovalent – and is not rejected by the RO membrane. Calcium has two charges and is rejected by the membrane.
RO systems are not effective in removing gases such as carbon dioxide (CO2) because they are not highly ionized (charged) while dissolved. CO2 also has a very low molecular weight which makes it difficult to remove.
The RO system does not get rid of gases, so the permeate water’s pH level might be slightly lower than normal depending on how much CO2 is in the feed water. The CO2 turns into carbonic acid.
Reverse osmosis is an effective way to treat water for both large and small flows of brackish, surface, or ground water. Industries that use RO water include:
- pharmaceutical
- boiler feed water
- food and beverage
- metal finishing
- semiconductor manufacturing
Reverse osmosis can effectively remove the following contaminants from drinking water.
| Contaminant | RO Removal Rate |
| Aluminum | 93-98% |
| Ammonium | 85-95% |
| Arsenic + 3 | 70-80% |
| Arsenic + 5 | 94-99% |
| Bacteria | 99+% |
| Barium | 93-98% |
| Bicarbonate | 90-95% |
| Boron | 55-60% |
| Bromide | 93-96% |
| Cadmium | 93-98% |
| Calcium | 93-98% |
| Chloride | 90-95% |
| Chromate | 90-95% |
| Chromium-3 | 94-99% |
| Chromium-6 | 94-99% |
| Copper | 94-99% |
| Cyanide | 90-95% |
| Detergents | 97% |
| Ferrocyanide | 98-99% |
| Fluoride | 90-97% |
| Hardness | 93-97% |
| Herbicides | 97% |
| Insecticides | 97% |
| Iron 2 | 93-98% |
| Lead | 94-99% |
| Magnesium | 93-98% |
| Manganese 2 | 93-98% |
| Mercury | 93-98% |
| Nickel | 93-98% |
| Nitrate3 | 85-90% |
| PFAS | 87-99% |
| Phosphate | 93-98% |
| Polyphosphate | 98-99% |
| Potassium | 90-95% |
| Pyrogen | 99+% |
| Radioactivity | 93-98% |
| Radium | 97% |
| Selenium | 93-98% |
| Selenium | 97% |
| Silica | 85-90% |
| Silicate | 95-97% |
| Silver | 93-98% |
| Sodium | 90-95% |
| Strontium | 93-98% |
| Sulfate | 93-98% |
| Sulphate | 99+% |
| Sulphite | 96-98% |
| TDS (Total Dissolved Solids) | 95-99% |
| Virus | 99+% |
| Zinc | 93-98% |
Design Calculations for Reverse Osmosis Systems
There are several computations that are used to assess the performance of an RO system as well as for design analysis. A typical RO system includes instrumentation that displays quality, flow, pressure, and occasionally other data like temperature or hours of operation.
In order to measure the performance of a reverse osmosis system, you need at least the following operation parameters:
- Feed pressure
- Permeate pressure
- Concentrate pressure
- Feed conductivity
- Permeate conductivity
- Concentrate flow
- Permeate flow
- Temperature
1 – Salt rejection percentage
The salt rejection percentage tells you how effective the RO membrane is removing the contaminants. For systems with multiple membrane, this parameter does not tell you how each individual membrane is performing – it provides and overall average of the combined system.
A properly designed RO system with membranes that are functioning normally will reject 95% to 99% of most feed water contaminants (assuming they are a certain size and charge). The following equation will show you how effective the RO membranes are at removing contaminants:
Equation 1: 
The greater the salt rejection, the more effectively the system is working. A low salt rejection rate indicates that the membranes need to be cleaned or replaced.
2 – Salt passage percentage
The salt passage percentage is simply the inverse of salt rejection described in Equation 1. This parameter relates to the amount of salt; it is expressed as a percentage that pass through the RO membrane.
The smaller the salt passage, the better the system is working. A high salt passage might indicate that the membranes need to be cleaned or replaced.
Equation 2: 
3 – Recovery percentage
Recovery is the amount of water that is being purified into usable water. The amount of water that is not sent to drain as concentrate, but rather collected as permeate or product water, may be compared to recovery.
The higher the recovery rate, the less water you are sending to drain as concentrate, and thus saving more permeate water. However, if the RO design is unable to recover from a large amount of data loss, it might cause larger difficulties due to scaling and fouling.
Design software is used to establish the recovery for an RO system, taking into account a variety of factors such as feed water chemistry and RO pre-treatment before the RO system. Consequently, the RO’s exact recovery rate hinges on its intended design. By calculating the recovery, you may quickly assess if the system is operating outside of the designed parameters.
The equation for recovery is presented below. It is expressed as a percentage.
Equation 3: 
Consider the following example. A reverse osmosis system has recovery rate of 75%. This means that for every 100 gallons of feed water that enter the RO system, it is recovering 75 gallons as usable permeate water. This also means that 25 gallons of brine are being produced and will be dumped down the drain.
Recovery rates for industrial RO systems range from 50% to 85%, depending on the feed water qualities and other design factors. Residential RO systems have much lower recovery rates.
4 – Concentration Factor
The concentration factor is an important equation for RO system design. The more water you recover as permeate (the higher the % recovery), the more concentrated salts and contaminants you collect in the concentrate stream. This can lead to higher potential for scaling on the surface of the RO membrane when the concentration factor is too high for the system design and feed water composition.
Equation 4: 
The concept behind a water softener is no different than that of a boiler or cooling tower. Both have purified water exiting the system (steam) and end up leaving a concentrated solution behind. As the degree of concentration increases, the solubility limits may be exceeded and precipitate on the surface of the equipment as scale.
Consider this example. Your RO system has a feed flow of 100 gpm and a permeate flow of 75 gpm. For this system, the recovery is (75/100) x 100 = 75%. To determine the concentration factor, the calculation would be 1 ÷ (1-‐ 75%) = 4.
A concentration factor of 4 means that the water going to the concentrate stream will be four times more concentrated than the feed water. If the feed water in this example was 500 ppm, then the concentrate stream would be 2,000 ppm. This high concentration can cause problems such as scale buildup on pipes and equipment.
5 – Flux rate
Flux is the rate at which water passes through a reverse osmosis membrane. This is measured in gallons per square foot per day (GFD) or liters per square meter per hour (l/m2/hr). A higher flux means more water is passing through the RO membrane, so it’s important to ensure that the system operates within a certain range to keep the water flowing properly.
Equation 5: 
6 – Mass balance for reverse osmosis water
A Mass Balance equation is used to help ensure that your flow and quality instrumentation is reading properly. If your instrumentation isn’t reading correctly, then the performance data you’re collecting is useless.
Before performing a Mass Balance calculation on an RO system, you will need to gather the following data:
- Feed flow (gpm)
- Permeate flow (gpm)
- Concentrate flow (gpm)
- Feed conductivity (μS)
- Permeate conductivity (μS)
- Concentrate conductivity (μS)
The mass balance equation is:
Equation 6: 
Single pass versus double pass reverse osmosis system
A “pass” can be thought of as a standalone RO system. The difference between a single pass RO system and a double pass RO system has to do with the number of times the contaminated water is passed through a membrane.
With a double pass reverse osmosis process, the permeate from the first pass (first RO) becomes the feed water to the second pass (or second RO). This process produces a much higher quality permeate because it has essentially gone through two RO systems.
Reverse Osmosis Pretreatment
Proper pretreatment of the raw water is critical to prevent fouling, scaling and costly premature RO membrane failure. Pretreatment consists of both mechanical and chemical treatments.
Read my article on replacing a reverse osmosis membrane.
The following sections discuss the most common problems RO systems experience due to lack of proper pretreatment.
1 – Fouling
Fouling happens when unwanted particles build up on the membrane surface and end up clogging it. While you may not be able to see them, there are many contaminants present in municipal feed water that can quickly damage an RO system if ingested.
The pressure drop across the RO system is increased, and the permeate flow is reduced, as a result of fouling in the front end. Eventually, the RO membranes will need to be cleaned or replaced, which drives up operating costs.
Even when your pretreatment and cleaning regimen is as thorough as possible, fouling will eventually occur to some extent, owing to the extremely tiny pore size of an RO membrane. However, by putting in place the right pretreatment, you can significantly decrease the frequency with which fouling issues must be addressed.
The following factors can contribute to fouling:
- Particulate or colloidal mater such as dirt, silt, and clay
- Organic compounds such as humic acid and fulvic acid
- Microorganisms
- Breakthrough of filter media upstream of the RO unit
Bacteria are one of the most prevalent fouling issues, since RO membranes in use today cannot withstand strong disinfectants such as chlorine. As a result, bacteria can grow and proliferate on the outer surface of a membrane. These microorganisms may produce biofilms that cover the membrane surface and result in heavy fouling.
Carbon beds, especially if they are GAC carbon beds and softener beds, may develop an under-drain leak. If the media fouls the RO system, there is not enough post-filtration in place.
Mechanical filtration is utilized to avoid fouling an RO system. The most commonly used methods to prevent fouling are the use of multi-media filters (MMF), microfiltration (MF), or cartridge filtration.
2- Scaling
If certain dissolved compounds become more concentrated, then they can cause scaling to occur. This results in higher pressure drops across the system as well increased salt passage rates and lower permeate flows due do less water being able go through these membranes without getting stuck on its surface or accumulating somewhere along its path like calcium carbonate does when present at high levels.
Hard water reverse osmosis systems require more frequent cleaning and maintenance than those used with soft water.
3 – Oxidation
Modern thin-film composite membranes are easily damaged by chlorine or chloramine. Oxidizers such as this will “burn” holes in the membrane pores and cause irreparable damage. This can result in a higher permeate flow and reduced contaminant rejection. Consider placing a carbon filter upstream to a standard system reverse osmosis process for enhanced membrane performance.
4 – Mechanical damage
Pre and post plumbing and controls should be in place to prevent mechanical damage to the membranes during hard starts or if backpressure is too high. Variable frequency drive motors can help with hard starts, while check valves/pressure relief valves can help avoid excessive backpressure on the RO unit.
Pretreatment Solutions
The solutions listed below can help reduce fouling, scaling and chemical damage for RO systems.
1 – Particle filter
A particle filter is used to help prevent fouling of an RO system. A filter typically contains layers of media consisting of non-woven polymer fibers. Some filters might include three or more layers of these materials with different micron removal ratings.
Usually, the outer layer in a spirally wrapped filter is coarser than the inner layers. This arrangement allows the largest dirt particles to be removed near outer layer with the smaller dirt particles being retained deeper in the media. This allows much longer filter run times before the cartridge has to be replaced. Stage the filter upstream to your RO filter for maximum performance and reduce maintenance costs.
2 – Granular activated carbon (GAC)
GAC is a substance used to remove both organic and residual disinfectants from water. GAC media can be created from coal, nutshells or wood.
Carbon has a chemical reaction with residual chlorine or chloramines, in which electrons are transferred from the surface of the GAC to the remaining chlorine or chloramine molecules. The chlorine or chloramines becomes a chloride ion, which is no longer an oxidizer. It is extremely effective at removing these disinfectants and their byproducts from tap water.
One of the disadvantages about using a GAC before your RO unit is that it will remove chlorine quickly at its very topmost layer, leaving behind none for biocide purposes. This will leave the GAC bed with no biocide to kill microorganisms, leaving room for new organisms to grow. Carbons systems reverse osmosis combination can be very effective.
Whole-House Versus Under-Sink Configurations
Reverse osmosis systems can be installed as either a whole-house or an under-sink model.
Whole-house reverse osmosis systems
Whole-house RO systems are designed to work in conjunction with your existing water supply. They’re typically installed where the main water line enters your home. All of the water in your home will then go through the RO system before it’s distributed to the various fixtures.
These systems are also known as point-of-entry (POE) systems. This approach lends itself quite well to this treatment technology.
While whole-house RO systems are more expensive, they’re also more convenient. You won’t have to worry about carrying around pitchers of filtered water or installing extra faucets in your home.
Read my article – The Definitive Guide to Whole House Reverse Osmosis Filter Systems
Under-sink reverse osmosis systems
Under-sink RO systems are designed to be installed in specific areas where you need purified water the most, such as your kitchen sink. They’re small and take up very little counter space. These systems usually come with their own faucet that’s installed next to your regular kitchen faucet.
Under-sink RO systems are less expensive and can be installed by almost anyone. If you’re handy with tools, you should be able to install one in about an hour or two.
Read my article – Complete Guide to Under-sink Reverse Osmosis Filters
Reverse Osmosis Membrane Cleaning
The membranes in your RO system need to be cleaned periodically. If the pressure drops or salt passageways change by 15%, it’s time for an inspection and cleaning session.
You can either do this yourself (if equipped) or have them taken care of by a professional. This is typically required about once each year.
Read my article on RO system maintenance.
Conclusion
Reverse osmosis is currently considered one of the most economic and effective process for water desalination. It is usually best to use this method with solutions that contain between 100 and 50,000 milligrams of salt per liter. RO membranes can treat a wide range of substances, including solutions with salt levels ranging from surface water to sea water, as well as brines.