What is Reverse Osmosis?
Reverse Osmosis technology removes most contaminants from water by pushing the water through a semi-permeable membrane under pressure. This article provides an overview of Reverse Osmosis (RO) technology and its applications.
This article covers the following topics:
- Understanding Osmosis and Reverse Osmosis
- How does Reverse Osmosis (RO) work?
- What contaminants does Reverse Osmosis (RO) remove?
- RO systems performance and design calculations
- Understanding the difference between passes and stages in a Reverse Osmosis (RO)system
- Pre-treatment for Reverse Osmosis (RO)
- Pre-treatment Solutions for Reverse Osmosis (RO)
- Reverse Osmosis (RO) performance trending and data normalization
- Reverse Osmosis (RO) membrane cleaning
- Summary
Understanding Osmosis and Reverse Osmosis
Reverse osmosis, often abbreviated as RO, removes a significant portion of dissolved solids and other contaminants from water by forcing it through a semi-permeable membrane.
Osmosis
To understand RO’s purpose and process, first you must understand the naturally occurring process of Osmosis.
Osmosis is a naturally occurring phenomenon, and one of the most important processes in nature, where a weaker saline solution will tend to migrate to a strong saline solution. For example, when plant roots absorb water from the soil, or our kidneys absorb water from our blood.
The diagram to the right shows how osmosis works.
A less concentrated solution has a natural tendency to migrate to a solution with a higher concentration.
For example, if you had a container full of water with low salt concentration and another container full of water with high salt concentration, separated by a semi-permeable membrane, the water with the lower salt concentration would start to move towards the container with the higher salt concentration.
A semi-permeable membrane allows some atoms or molecules to pass but not others. A simple example is a screen door which allows air molecules to pass through but not pests or anything larger than the screen holes. Another example is Gore-Tex clothing which has an extremely thin plastic film with billions of small pores just big enough to let water vapor through, but small enough to prevent liquid water from passing.
Reverse Osmosis
RO is the process of Osmosis in reverse. Osmosis occurs naturally without an external energy source, but reversing the osmosis process requires applying energy to the more saline solution to reverse the natural flow.
A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not most of the dissolved salts, organics, bacteria, and pyrogens. However, the water must be “pushed” through the RO membrane by applying pressure greater than the naturally occurring osmotic pressure.
The diagram on the right outlines the RO process. 
When pressure is applied to the concentrated solution, the water molecules are forced through the semi- permeable membrane while the contaminants are not allowed through.
How does Reverse Osmosis work?
RO works using a high-pressure pump to apply pressure on the salt side of the RO system and to force the water across the semi- permeable RO membrane, leaving almost all (95% to 99%) of dissolved salts behind in the reject stream.
The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure.
Permeate vs. Concentrate Water
In very simple terms, feed water is pumped into an RO system and two types of water come out: good water (permeate) and bad water (concentrate).
The “good” water has most contaminants removed and is called permeate. Another term for permeate is product water. Permeate is the water that was pushed through the RO membrane to remove nearly all contaminants. RO system sizes are based on permeate flow. For example, a 100 gallon per minute (gpm) RO system will produce 100 gpm of permeate water.
The “bad” water, called the concentrate, reject, or brine, is the leftover liquid will all the contaminants unable to pass through the RO membrane. All three terms are used interchangeably and mean the same thing. The simple schematic below shows how water flows through an RO system.
As the feed water enters the RO membrane under pressure (enough to overcome osmotic pressure) the water molecules pass through the semi-permeable membrane and the salts and other contaminants remain on the other side and are discharged from the system through the concentrate stream.
The concentrate either goes to a drain or, in some circumstances, is fed back into the feed water supply and recycled through the RO system to save water. The water that makes it through the RO membrane usually has approximately 95% to 99% of dissolved salts removed.
It is important to understand that RO systems employ cross filtration rather than standard dead-end filtration in which contaminants are collected within the filter media. With cross filtration, the solution passes through, or crosses, the filter with two outlets routing the filtered water one way while the contaminated water goes a different route. Cross flow filtration allows water to sweep away contaminant build up and allow enough turbulence to keep membrane surfaces clean.
What will Reverse Osmosis remove from water?
RO can remove 95-99% of dissolved salts (ions), particles, colloids, organics, bacteria, and pyrogens from feed water. An RO membrane rejects contaminants based on their size and charge. Any contaminant with a molecular weight greater than 200 will likely be rejected by a properly running RO system.
The greater the ionic charge of the contaminant, the more likely it will be unable to pass through the RO membrane. For example, a sodium ion has only one charge (monovalent) and is not rejected by the RO membrane as well as calcium, which has two charges.
RO systems cannot remove dissolved gases, such as carbon dioxide (CO2), very well because they are not highly ionized (charged) while in solution and have a very low molecular weight. Because RO systems do not remove gases, permeate water can have a slightly lower than normal pH level, depending on dissolved CO2 in the feed water since CO2 is converted to carbonic acid.
RO is very effective in treating brackish, surface and ground water for both large and small flow applications. Industries that use RO water include pharmaceutical, boiler feed water, food and beverage, metal finishing, and semiconductor manufacturing to name a few.
RO System Performance and Design Calculations
There are a handful of calculations that are used to judge the performance of an RO system and for design considerations. An RO system has instrumentation that displays quality, flow, pressure and sometimes other data like temperature or hours of operation. To accurately measure the performance of an RO system you need the following operation parameters at a minimum:
- Feed pressure
- Permeate pressure
- Concentrate pressure
- Feed conductivity
- Permeate conductivity
- Concentrate flow
- Permeate flow
- Temperature
Salt Rejection %
This equation tells you how effectively the RO membranes are removing contaminants. It does not tell you how each individual membrane is performing. However, it will tell you how the system overall is performing on average.
A well-designed RO system with properly functioning RO membranes will reject 95% to 99% of most feed water contaminants (of a certain size and charge).
The following equation can be used to determine how effective the RO membranes are at removing contaminants:
Salt Rejection % = ((Feed water conductivity – Permeate water conductivity) / Feed water conductivity) x 100
Salt Passage %
This is simply the inverse of salt rejection described in the previous equation. This is the amount of salts, expressed as a percentage, passing through the RO system.
The lower the salt passage, the better the system is performing. A high salt passage can mean the membranes require cleaning or replacement.
Salt Passage % = (1 – Salt Rejection %) x 100
Recovery %
Recovery is the amount of feed water emerging from the system as good, permeate water.
Another way to think about recovery is as the amount of water collected as permeate or product water instead of being sent to drain as concentrate. Higher recovery percents mean you are sending less water to drain as concentrate and saving more permeate water. However, if recovery percents are too high for the RO design, it can lead to larger problems from membrane scaling and fouling.
Design software establishes RO system recovery rates by considering numerous factors such as feed water chemistry and pre-treatment before the RO system. Therefore, proper RO system recovery depends on the design. Calculating the recovery facilitates rapid determination that the system is operating outside of the intended design.
The calculation below expresses the recovery rate as a percentage.
% Recovery= (Permeate Flow Rate gpm / (Permeate Flow Rate gpm + Concentrate Flow Rate gpm)) x 100
For example, if the recovery rate is 80% then for every 100 gallons of feed water entering the RO system, you are recovering 80 gallons as usable permeate water while 20 gallons go to the drain as concentrate. Industrial RO systems typically run between 50% to 85% recovery depending on feed water characteristics and other design considerations.
Concentration Factor
The concentration factor is related to RO system recovery and is important for RO system design. The more water you recover as permeate (higher recovery %), the more concentrated salts and contaminants you collect in the concentrate stream. When the concentration factor is too high for the system design and feed water composition, the system may experience increased scaling on RO membrane surfaces.
Concentration Factor = (1 / (1 – Recovery %)
The concept is no different than that of a boiler or cooling tower with purified water exiting the system as steam leaving a concentrated solution behind. As the concentration increases, solubility limits may be exceeded and precipitate on equipment surfaces as scale.
For example, if your feed flow is 100 gpm and the permeate flow is 80 gpm, then the recovery is (80/100) x 100 = 80%.
To find the concentration factor, the formula would be 1 ÷ (1 – 80%) = 5.
A concentration factor of 5 means the water going to the concentrate stream will be 5 times more concentrated than the feed water. If the feed water in this example was 500 ppm, then the concentrate stream would be 500 x 5 = 2,500 ppm.
Flux
Flux expresses the amount of water passing (permeating) through a reverse osmosis membrane during a given time, measured as gallons per square foot per day (GFD) or liters per square meter per hour (l/m²/hr).
A higher flux means more water is permeating through the RO membrane.
Designers establish RO systems to operate within a specific flux range to ensure that the water flowing through the RO membrane is neither too fast nor too slow.
Flux Gfd = (Permeate flow gpm * 1,440) / (# of RO elements in system * square footage of each RO element)
For example, you have the following:
· The RO system is producing 80 gpm of permeate.
· The system has 3 RO vessels each holding 6 RO membranes for a total of 3 x 6 = 18 membranes.
· The RO system membranes are Toray TMG20D-400 which has 400 square feet of surface area per membrane.
To find the flux (Gfd):
Gfd= (75 gpm X 1,440) / (18 x 400)
= 16 Gfd
This means 16 gallons of water passed through each square foot of each RO membrane per day.
This number could be good or bad depending on the feed water chemistry and system design. Below is a general rule of thumb for flux ranges for different source waters. This can be better determined with the help of RO design software.
Feed Water Source Gfd Flux
RO Permeate Water 20-30
Brackish Well Water 14-18
Brackish Surface Water 10-14
Sea Water 8-12
Sewage Effluent 5-10
Understanding passes and stages in a Reverse Osmosis (RO) system
The term ‘stage’ and ‘pass’ are often mistaken for the same thing in an RO system, and the terminology can be confusing for an RO operator. It is important to understand the difference between a one- and two-stage RO and a one- and two-pass RO.
Difference between a One and Two Stage RO System
In a one-stage RO system, the feed water enters as one stream and exits the RO as either concentrate or permeate water.
In a two-stage system, the concentrate (or reject) from the first stage then becomes feed water for the second stage. The permeate water collected from the first stage is combined with permeate water from the second stage. Additional stages increase the RO system’s recovery.
Array
In a reverse osmosis system, an array describes the physical arrangement of the pressure vessels in a two-stage system. Pressure vessels contain RO membranes (usually from 1 to 6 RO membranes are in a pressure vessel), and each stage can have a certain amount of pressure vessels with RO membranes.
The reject of each stage then becomes the feed stream for the next successive stage. The two-stage RO system above is a 2:1 array, which means the concentrate (or reject) of the first two RO vessels is fed to the next single vessel.
RO System with Concentrate Recycle
If an RO system cannot be properly staged and the feed water chemistry permits, you can use a concentrate recycle setup where a portion of the concentrate stream is fed back into the feed water of the first stage to enhance system recovery.
Single Pass RO vs Double Pass RO
Think of a ‘pass’ as a standalone RO system. The difference between a single-pass RO system and a double- pass RO system is how many RO systems the water passes through.
In a double-pass RO, the permeate from the first RO (the first pass) becomes the feed water to the second pass (or second RO). A double-pass RO system produces a much higher quality permeate because it has essentially gone through two RO systems.
In addition to producing a much higher quality permeate, a double-pass system also provides the opportunity to remove carbon dioxide gas from the permeate by injecting caustic between the first and second pass. C02 is undesirable when using mixed bed ion exchange resin beds after the RO system.
Adding caustic after the first pass raises the pH of the first pass permeate water and converts CO2 to bicarbonate (HCO3) and carbonate (CO3-2), which RO membranes in the second pass reject more effectively.
This process is not feasible with a single pass RO system because injecting caustic and forming carbonate (CO3-2) in the presence of cations like calcium leads to scaling of RO membranes.
RO Pretreatment
Proper pretreatment using both mechanical and chemical treatments is critical for an RO system to
prevent fouling, scaling and costly premature RO membrane failure and frequent cleaning requirements. Below is a summary of common problems an RO system experiences due to lack of proper pretreatment.
Fouling
Fouling occurs when contaminants accumulate on the membrane surface effectively plugging the membrane. Many contaminants in municipal feed water are naked to the human eye and harmless for human consumption. However, they are large enough to quickly foul (or plug) an RO system.
Fouling typically occurs in the front end of an RO system and results in a pressure drop across the RO system and a lower permeate flow. This translates into higher operating costs and eventually the need to clean or replace the RO membranes.
Fouling will take place eventually due to an RO membrane’s extremely fine pore size no matter how effective the pretreatment protocols or cleaning schedule. However, proper pretreatment will minimize the need to address fouling related problems.
The following can cause fouling:
- Particulate or colloidal matter (dirt, silt, clay, )
- Organics (humic/fulvic acids, )
- Microorganisms (bacteria, )
- Breakthrough filter media upstream of the RO unit
Bacteria are one of the most common fouling problems. This is because RO membranes cannot tolerate disinfectants such as chlorine and microorganisms are often able to thrive and multiply on the membrane surface. Microorganisms may produce biofilms that cover the membrane surface and result in heavy fouling.
Filter media upstream of the RO unit breakthrough may involve GAC carbon beds and softener beds developing an under-drain leak. Without adequate post filtration, the media can foul the RO system.
Analytical tests determine if the feed water to your RO has a high potential for fouling. mechanical filtration helps prevent RO system fouling. The most popular methods to prevent fouling are the use of multi-media filters (MMF) or microfiltration (MF). In some cases, cartridge filtration will suffice.
Scaling
As certain dissolved (inorganic) compounds become more concentrated (remember discussion on concentration factor) scaling can occur. If these compounds exceed their solubility limits, they can precipitate on the membrane surface as scale. Scaling causes higher pressure drops across the system, higher salt passage (less salt rejection), and low permeate flow.
Common scale that tends to form on RO membranes is calcium carbonate (CaCO3).
Chemical Attack
Modern thin film composite membranes are not tolerant to chlorine or chloramines. Oxidizers, such as chlorine, will ‘burn’ holes in the membrane pores and can cause irreparable damage. The result of chemical attack on an RO membrane is higher permeate flow and higher salt passage (less salt rejection).
Increased microorganism growth on RO membranes tends to easily foul membranes since there is no biocide present to prevent growth.
Mechanical Damage
Part of the pretreatment scheme should involve pre and post RO system plumbing and controls. If ‘hard starts’ occur, the system may experience mechanical damage to the membranes. Likewise, too much backpressure on the RO system can cause mechanical damage to the RO membranes.
These can be addressed by using variable frequency drive motors to start high pressure pumps for RO systems along with installing check valve(s) and/or pressure relief valves to prevent excessive back pressure on the RO unit that can cause permanent membrane damage.
Pretreatment Solutions
The pretreatment solutions for RO systems listed below can help minimize fouling, scaling and chemical attack.
Multi-Media Filter (MMF)
Multi-Media Filters help prevent RO systems from fouling. An MMF typically contains three layers of media consisting of anthracite coal, sand, and garnet, with a supporting gravel layer at the bottom. These are the medias of choice because of the differences in size and density. The larger (but lighter) anthracite coal will be on top, and the heavier (but smaller) garnet will remain on the bottom.
The filter media arrangement removes the largest dirt particles near the top of the media bed and retains smaller dirt particles deeper within the media. The entire bed acts as a filter allowing much longer filter run times between backwashes and more efficient particulate removal.
A well-operated MMF can remove particulates down to 15 – 20 microns. An MMF with an incorporated coagulant removes particulates down to 5 – 10 microns by inducing tiny particles to join and form larger particles that can be filtered. To put this in perspective, the width of a human hair is around 50 microns.
When the feed water Silt Density Index (SDI) value exceeds 3 or when turbidity exceeds 0.2 NTU, experts recommend using a multi-media filter. While there’s no exact rule, following these guidelines helps prevent premature fouling of RO membranes.
It is important to have a 5-micron cartridge filter placed directly after the MMF unit to prevent the MMF media from damaging downstream pumps and fouling the RO sustem if the MMF under drains fail.
Microfiltration (MF)
Microfiltration (MF) is effective in removing colloidal and bacteria matter with a 0.1-10µm pore size and is helpful in reducing RO unit fouling potential. Membrane configuration can vary between manufacturers, but the “hollow fiber” type is the most common.
Typically, pumps draw water from the outside of the fibers, while clean water collects inside the fibers. Microfiltration membranes used in potable water applications usually operate in “dead-end” flow. Specifically, all the water fed to the membrane is filtered through the membrane. Periodically backwash the installed filter cake to remove it from the membrane surface.
Recovery rates are normally greater than 90 percent on feed water sources with high quality and low turbidity feeds.
Antiscalants/Scale Inhibitors
Antiscalants and scale inhibitors, as their name suggests, are chemicals added to feed water before an RO unit to help reduce the scaling potential. Antiscalants and scale inhibitors increase the solubility limits of troublesome inorganic compounds.
By increasing the solubility limits, you can concentrate the salts further than otherwise would be possible, achieving a higher recovery rate and operating at a higher concentration factor.
Antiscalants and scale inhibitors work by interfering with scale formation and crystal growth. The choice of antiscalant or scale inhibitor and correct dosage depends on feed water chemistry and RO system design.
Water Softening
A water softener can be used to help prevent RO system scaling. Water softeners exchange scale forming ions with non- scale forming ions. As with MMF units, it is important to have a 5-micron cartridge filter placed directly after the water softener if the under drains fail.
Sodium Bisulfite
Sodium bisulfite (SBS or SMBS), a reducer, added to the water stream before an RO at the proper dose can remove residual chlorine and chloramines.
Granular Activated Carbon (GAC)
GAC removes both organic constituents and residual disinfectants (such as chlorine and chloramines) from water. Manufacturers make GAC media from coal, nutshells, or wood. Activated carbon removes residual chlorine and chloramines by a chemical reaction. It involves a transfer of electrons from the surface of the GAC to the residual chlorine or chloramines. The chlorine or chloramines ends up as a chloride ion that is no longer an oxidizer.
The disadvantage of using a GAC before the RO unit is that the GAC will remove chlorine quickly at the very top of the GAC bed. This will leave the remainder of the GAC bed without any biocide to kill microorganisms. A GAC bed will absorb organics throughout the bed, which is potential food for bacteria. Eventually, the GAC bed can become a breeding ground which can pass easily to the RO membranes.
Also, a GAC bed can produce very small carbon fines under some circumstances that have the potential to foul an RO. Place a cartridge filter after GAC and before RO to protect membranes from carbon fines.
RO Data Trending and Normalization
RO membranes are the heart of the RO system. It is important to collect certain data points to determine its health. These data points include system pressures, flows, quality, and temperature.
Water temperature is directly proportional to pressure. As the water temperature decreases it becomes more viscous. Thus, the RO permeate flow will drop as more pressure is required to push the water through the membrane. Likewise, when the water temperature increases, the RO permeate flow will increase. Normalize RO system performance data to prevent mistaking flow variations for abnormalities when no actual problem exists.
Calculate, graph, and compare normalized flows, pressures, and salt rejection to baseline data. Obtain baseline data when commissioning the RO or after cleaning or replacing the membranes. This helps you troubleshoot problems and decide when to clean or inspect the membranes for damage.
Data normalization helps show the RO membranes’ true performance. As a general rule, investigate the cause and clean membranes when there is a normalized change of +/- 15% from baseline data. Otherwise, RO membrane cleanings may not be effective at bringing the membranes back to near new performance.
RO Cleaning
RO membranes will inevitably require periodic cleaning – usually between 1 to 4 times a year depending on feed water quality. Generally, if the normalized pressure drops or the normalized salt passage has increased by 15%, it is time to clean the RO membranes. Or if the normalized permeate flow has decreased by 15% then it is also time to clean the RO membranes.
The RO membranes can be cleaned in place (if equipped) or removed from the RO system and cleaned off site by a specialized service company. Offsite membrane cleaning delivers more effective cleaning than onsite cleaning skids.
RO membrane cleaning involves low and high pH cleaners to remove contaminants from the membrane. We address scaling with low pH cleaners and treat organics, colloidal matter, and biofouling with high pH cleaners.
Cleaning RO membranes is not only about using the appropriate chemicals. Many other factors, such as flows, water temperature, water quality, and properly designed and sized cleaning skids, require the involvement of an experienced service provider to clean RO membranes properly and prevent any damage that would necessitate replacement.
Summary
Reverse Osmosis is an effective and proven technology to reduce water contaminants.
Further post treatment after the RO system, such as mixed bed deionization, can increase RO permeate quality and make it suitable for the most demanding applications. Proper pretreatment and RO system monitoring are crucial to preventing costly repairs and unscheduled maintenance.
With the correct system design, maintenance program, and experienced service support, your RO system should provide high purity water for many years.
