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.

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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.

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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.

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.

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.

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.

Increased microorganism growth on RO membranes tends to easily foul membranes since there is no biocide present to prevent growth.

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.

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.

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.

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.

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).

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.

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.

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.

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.

Reverse osmosis

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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.

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.

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.

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.

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).

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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.

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.

AMOT has been a trusted manufacturer of 3 Way TMVs since 1948 and pioneered the wax element technology used today. These rugged, reliable valves provide a “Fit and Forget” solution where a mixing configuration is needed. Our factory pre-set elements are accurate, and our service kits contain all the components necessary to make easy work out of valve maintenance. Simple to install, operate and maintain, AMOT TMVs provide years of trouble-free, reliable temperature control without the need for external power sources.

For example, in a non-condensing boiler system, if the water returned to the boiler drops below 140oF (60°C), condensation of the flue gas occurs. This, in turn, causes corrosion on the copper fins of the boiler. Conversely, if the temperature of the returned water is too hot, it can lead to thermal shock, a condition where a sudden change in temperature causes rapid and uneven expansion and contraction of the boiler’s structure. Typical boiler systems that benefit from a TMV include in-floor radiant heating, hydronic heating, heat pumps, soil heating and various manufacturing operations.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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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.

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.

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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).

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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.

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.

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.

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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.

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.

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.

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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.

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.

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.

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.

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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.

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.

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.

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).

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.

Reverse Osmosis system

With the correct system design, maintenance program, and experienced service support, your RO system should provide high purity water for many years.

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.

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.

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.

Maintaining control of a tempered system’s outlet water temperature can be difficult. Outlet temperature is affected by the temperature of the water provided to the system, which can fluctuate depending on its source and the location’s climate. Installing a TMV, rather than a static valve, improves the safety and comfort of the user.

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.

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.

Tempered water systems use TMVs to protect against scalding in municipal water systems, emergency showers, eye wash stations, and care facilities like hospitals and nursing homes. Most facilities require tempered water systems that meet ANSI international safety standards. ANSI Z358.1 states that the temperature of water delivered by tempered systems should be 60-100°F (15-38°C). Temperatures over 60°F (15°C) allow for continuous flushing without causing hypothermia or shock. The upper limit of 100°F (38°C) protects against scalding and bacteria growth, a concern when water is stored at high temperatures.

The lower the salt passage, the better the system is performing. A high salt passage can mean the membranes require cleaning or replacement.

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.

AMOT has been in the business of developing the best temperature control technologies since 1948. Our earlier blog, “What is a Temperature Control Valve,” explained how these devices work, the different types of valves available, and how the valves are used in different applications. In this article, we’re going to dive a bit deeper into a specific type of temperature control valve: thermostatic mixing valves (TMVs).

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.

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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:

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.

With so many sizes, body materials, and flange options, AMOT is sure to have the right valve for your application. It’s easy to select the options and request a quote right from our website. Give the filters a try on our thermostatic control valve page or contact us if you need assistance.

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.

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A temperature control valve that is actuated by internally sensing and controlling fluid temperatures is called a thermostatic valve. This type of valve is self-contained without any external power source.  AMOT pioneered this technology in 1948 when we introduced into our valve design a special wax that remains in a semi-solid form and is highly sensitive to temperature changes. As the temperature of the fluid changes, it causes the wax to expand or contract which slides the valve up or down, opening or closing ports.

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.

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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.

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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.

ROwater System

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.

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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.

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.

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.

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.

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.

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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.

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.

Thermostatic mixing valves often are used in industrial, residential, or commercial applications that need to ensure a constant outlet temperature to prevent damage or injury.

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.

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.

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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.

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.

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.