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What is Reverse Osmosis?

What is Reverse Osmosis?-1

Reverse osmosis is a hot topic in the water treatment industry. With the lowest energy requirements, some of the highest recovery rates, and one of the best rejection rates on the market, it's no wonder people are interested in learning more about reverse osmosis systems. What is reverse osmosis, though? How does it work? Let's take a look into the heart of a Pure Aqua Reverse Osmosis System and break it down for you to understand.

 

Reverse osmosis, as anyone could guess, is the process of osmosis backwards. Osmosis is the passage of water through a protein membrane (like our skin, or the inside of a plant cell) to equalize the concentration of particles dissolved in the water. The protein membrane allows water to pass through, but molecules larger than water (things like minerals, salts, and bacteria) cannot. Water flows back and forth until the concentration is equal on both sides of the membrane, and an equilibrium is formed. 

Let's apply this knowledge to water purification. We want to drink water from a lake or stream, but it contains too high concentration of contaminants like salt, minerals, and bacteria, that make it undrinkable. By applying pressure to water as it passes through a membrane, the water can be forced to move away from the membrane rather than attempting to form an equilibrium like normal. This against-flow motion is where the "reverse" in "reverse osmosis" comes from. A pump works well for this process. Water is forced through the membrane, which like a super-fine particle filter, blocks an extreme majority of contaminants from coming through.

Reverse osmosis as a purification process has a number of advantages as well as disadvantages. In water treatment, reverse osmosis can usually remove between 96 and 99% of most contaminants, including salts and minerals, dyes, particles, bacteria, and hazardous metals. Because of the way reverse osmosis works, however, you can never truly remove all of a contaminant. You may purify down to a fraction of a fraction of a percent, but the contaminant can never be truly eliminated with reverse osmosis. Reverse osmosis also requires a high-grade pump, because the rejection rate is primarily dependant on the pressure applied to the membrane. That being said, smaller reverse osmosis units have smaller ratios of permeate (clean, purified product) to waste water. This makes media filtration or other conventional filtraiton more effective in smaller scales (like residential settings).

 

Now that we've answered the biggest question about Reverse Osmosis Systems by Pure Aqua, let's take a closer look at how we build our advanced water treatment systems and how they work.


How does a Reverse Osmosis System work?-1

Now that we know how Reverse Osmosis as a process works, let's take that and apply it to a real, working Reverse Osmosis System. If it only needed the membranes and a pump, it certainly wouldn't be so big, right?

A) Raw Water Storage Tank

 

Although some Reverse Osmosis Systems can draw water right out of a well or pipe feed, most Reverse Osmosis Systems start with a large tank that stores the contaminated water. Not having enough feed water can damage a pump, so having a large storage tank for your intake water is an easy way to make sure your pump lasts for as long as possible.

 

B) Feed Water Pump

 

A commercial or industrial pump provides the initial pressure for the Reverse Osmosis System. This motor usually provides enough water pressure to get through any pretreatment as well as the reverse osmosis membranes, but if it doesn't a booster pump may be necessary farther down the line.

 

C) Multi-Layer Filter

 

As much as we hate to admit it, there are some things that reverse osmosis can't purify. Nitrates, a common contaminant found in fertilizers and animal waste, are a good example of particles that dissolve too well in water for reverse osmosis to catch them. Things like foul odor and taste usually aren't prevented by reverse osmosis, either. A Multi-Layer filter can be filled with media that specifically targets the things your Reverse Osmosis System can't catch. If you need to eliminate these contaminants, a Multi-Layer Filter is a must.

 

D) Chemical Dosing System

 

Strong acids and bases can reduce the effectiveness of the membrane elements of a Reverse Osmosis System, or in some circumstances destroy them entirely. New chemicals may need to be introduced to your intake water to nullify these hazardous chemicals and preserve the life of your membrane elements. Chemical dosing is also an effective cleaning system for a Reverse Osmosis system, dramatically increasing the lifespan of membrane elements.

 

E) Reverse Osmosis System

 

We finally have our Reverse Osmosis System. If a booster pump is necessary, it will typically be just before this step. The Reverse Osmosis System can produce up to one million gallons of product water a day from a steady intake, as well as a sizable amount of waste. Usually the waste water can be dumped down the drain, but check with your local water authorities in case it needs to be handled with care.

 

F) Product Water Storage Tank

 

The permeate from the Reverse Osmosis System will usually go to a large tank, where it is held for use. If it didn't, the Reverse Osmosis System would need to be running in order to have access to fresh water, which can be inconvenient. Sometimes, a Reverse Osmosis System pumps water directly into a well or aquifer for recharging instead of being used in many of the normal industries or applications it is used in.


What are the basic components of a Reverse Osmosis System?-1

A Reverse Osmosis System is built of five basic parts:

 

1) Pressure Vessels & Membranes

 

Obviously, a Reverse Osmosis System wouldn't get very far without membrane elements. The proteins that make up membrane elements vary depending on the kind of intake water and ending clarity. There are membrane elements for brackish water, seawater, hospital-grade disinfection, and membranes designed to remove specific contaminants to name a few. If there is a water treatment need, you can be sure there is certainly a membrane element for the job. The size of the task (municipal, commercial, or industrial) will determine the size and number of membranes in a system. There can be anywhere from a single two and a half inch membrane (like in an under-sink Reverse Osmosis System) to hundreds of eight inch membranes all working togethor (a typical reverse osmosis plant).

 

2) Reverse Osmosis Skid

 

The best way make your Reverse Osmosis system as durable as possible is with a powder coated, carbon steel frame to mount all of your components on. It's resistant to the elements, designed for the heavy vibration of high-pressure pumps, and  mounts to the ground to ensure that it will last a lifetime.

 

3) Cartridge Filter

 

Most Reverse Osmosis Systems come with a cartridge filter to ensure that no particles large enough to damage the membranes come anywhere near them. This cartridge is usually a five micron spun polypropylene filter, but can vary upon request. The cartridge comes in a durable casing that can handle the pressure from the main feed or booster pumps.

 

4) Reverse Osmosis High Pressure Pump

 

Without a high-grade pump, the rejection rate for a Reverse Osmosis System isn't viable in most commercial or industrial settings. It is vital to the system to ensure that the pump is matched to the membrane quantity and size appropriately. Usually, the higher the horsepower on the intake pump, the better the rejection and recovery rates of permeate.

 

5) Control Panel

 

Lastly, the Reverse Osmosis System has to be controlled by a human operator. At Pure Aqua, we use advanced PLCs or a solid-state microprocessor depending on how advanced the controls need to be. The controls can also be used to manage multiple systems simultaneously, effectively making a one-man water production plant.

Reverse Osmosis Systems can have a number of other components built onto or into them as extra components as well. The entire skid can be built into a containerized system, for example, so your Reverse Osmosis System is always on the go. There are a number of supplemental skids that can be attached to a Reverse Osmosis System as well, for tasks like membrane cleaning, pretreatment, chemical dosing, and a number of other jobs as necessary.


If there is a necessity for water treatment, chances are a Reverse Osmosis System can do the job. There's a wide array of industries that benefit from having high-purity water, as well as a large number of applications where water treatment is required. Because of the extreme volumes of water required, a Reverse Osmosis System is often the ideal, economical solution, requiring less energy than most large-scale treatment methods. Because they consume less energy, a Reverse Osmosis System is often the  environmentally friendly solution as well. At Pure Aqua, we pride ourselves as being a go-to source for information and service helping you solve your municipal, commercial, or industrial water treatment needs.

Here's Just A Few Applications A Reverse Osmosis System is Good For:
  • Pretreatment of Boilers
  • Food and Beverage Services
  • Industrial wastewater purification
  • DI water pretreatment
  • Hotels and Resorts
  • Ice making
  • Car washes
  • Pure ethanol
  • Dairy industry
  • Maple syrup
  • Pharmaceuticals
  • Water Bottling
  • Hospitals
  • Agriculture
  • Humidification

Reverse Osmosis is an ideal water treatment solution in most types of water. Generally speaking, all major water sources from a treatment standpoint can be broken down into three major categories: tap water, also known as municipal sources, groundwater, which includes brackish water, and saltwater. The biggest distinction between these three types is the Total Dissolved Solids (TDS) content of each type. As a rule of thumb, the American Health Association requires that drinking water is under 1,000 PPM TDS.

Tap water typically comes through a pre-existing infrastructure like city pipes or a damming system. Reverse osmosis is often used in a tap water environment to reduce hardness, or the debris deposited in water from traveling in metal pipes. Total dissolved solids is often a target of water purification in tap water systems. This type of Reverse Osmosis Systems are ideal in places like power plants, pharmaceuticals, laboratories, and hospitals, where an extreme purity of water is crucial to the industry. Tap water typically has a TDS of under 1,000 PPM.

Underground resevoirs of water are often brackish, meaning they contain large volumes of salt, but not enough to be considered salt water. Groundwater reverse osmosis is very common, and one of the best uses of a Reverse Osmosis System to date. Groundwater is most often purified for the agriculture industry, the mining industry, and for residential use. Groundwater is also a prized target of the bottling industry, because the unique mineral combinations often have an appealing taste. Brackish water usually has a TDS of 5,000 PPM or less, but can come in concentrations of up to 12,000 PPM.

Salt water reverse osmosis (sometimes referred to simply as desalinatoin) is the turning of saltwater into drinking water. Ocean water has up to 45,000 PPM TDS. Typically, for environmental reasons, a bore hole is dug in the ocean for this kind of reverse osmosis, but an open intake is more cost effective. The biggest uses of desalination come in providing water in areas that lack a regular supply of fresh water.


Is Pretreatment Necessary?-1

If you work with an RO, you understand that the feed water must be preconditioned to protect the membranes from fouling and premature failure. An RO membrane functions much like a cross flow filter. The membrane is constructed of a porous material that allows water to pass through the membrane, but rejects up to 99% of the dissolved solids at the membrane surface. The dissolved salts are concentrated in the Reverse Osmosis reject water, or brine stream, where they are discharge to waste.

As the RO System continues to operate, the dissolved and suspended solids in the feed water tend to accumulate along the membrane surface. If these solids are allowed to build up, they eventually restrict the passage of water through the membranes, resulting in a loss of throughput. (The throughput capacity of the membranes is commonly referred to as the flux rate, and is measured in gallons per square foot of membranes surface area per day.)

Early in the development of membranes systems, little was known about which impurities in the Reverse Osmosis feed water are likely to cause fouling and a corresponding reduction in flux. Today, many of these troublesome impurity treatments have been identified, and preventive treatments have been devised that greatly reduce membranes fouling, thus prolonging the life of the RO System.


Is Pretreatment Necessary?-2

Autopsies of failed membranes modules have revealed a build-up of foulants caused by mineral scales such as calcium carbonate; colloidal materials like clays and silica; dead and living microorganism; carbon particles; and chemical attach by oxidizing agents like chlorine, ozone, or permanganate. Likewise, dissolved metals like iron and aluminum, whether naturally occurring or added as a coagulant, can cause premature fouling and failure of the membrane.


Is it necessary to get a water analysis?-1

A detailed chemical analysis of the RO feed water is an absolute necessity for identifying potential foulants. This should include a measurement of the hardness (calcium and magnesium), barium, strontium, alkalinity, pH, and chlorine. The data from the chemical analysis can be used by the RO equipment designers to determine the optimum membrane array that will both minimize the tendency of scale and deposit formation and maximize the recovery and flux rate.

For example, the Langelier Stability Index (LSI), a measure of the calcium carbonate scaling tendency of the water, is computed from the water analysis to determine the maximum permissible concentration of dissolved minerals in the reject stream before scale deposition becomes a problem. Because of the number of variables that must be considered, these calculations are difficult to do with pencil and paper. Fortunately, the membranes manufacturers have developed computer programs that make these computations fast and easy to perform where the user can project the performance of membranes at actual feed conditions.

Although a water analysis is helpful in predicting the tendency of dissolved minerals to cause problems in the RO System, it does not always forecast the fouling tendency of colloids and other finely dispersed suspended solids. The Silt Density Index (SDI) is a useful tool for quantifying the fouling tendency of the feed water. This test is conducted by filtering a sample through a 0.45 micron (µm) filter and measuring the time required to collect a unit volume of filtrate. An index number is calculated from this data. Traditionally, a SDI value of less than 3.0 is desirable for RO feed waters. The SDI measurement has certain limitations in that it does not model the cross flow design of an RO membrane.


What determines the precise pretreatments for a particular RO?-1

In one word: analysis. Every source of water is different, and you never know what's in your water until you have it analyzed. The water analysis, LSI, SDI, or CFI values are used to determine the precise pretreatment requirements for a particular  RO System. Since water supplies vary considerably from one location to another, each pretreatment requirement will be different. On average, most Reverse Osmosis Systems need an antiscalant injector or water softener to prevent damage to the membrane.


Ion exchange is a popular method for reducing the potential for mineral scale formation on the membrane surface. Ion exchange softening uses sodium to replace scale-forming ions such as calcium, magnesium, barium, strontium, iron, and aluminum to prevent damage to the membrane elements.. The sodium forms very soluble salts, which are readily rejected by the Reverse Osmosis System and do not readily form mineral scales on the membrane surface. A sodium-cycle softener is regenerated with sodium chloride brine. The spent regenerant, along with the softener rinse water, must be discharged to waste. It is because of this that ion exchange is recommended for applications that have high metal contents in the treated water.


Generally speaking, chlorination is a double-edged sword when it comes to Reverse Osmosis Systems. As a method of disinfection, chlorination is not only efficient and practical, but it is cost-effective as well. The only problem is that chlorine is too caustic for membrane elements, and can cause serious damage. Dechlorination is a kind of chemical injection that adds a chemical that forms salts with chlorine, making it readily rejected by the membrane elements. In this circumstance, dechlorination is a must as far as water treatment is concerned. Without dechlorination, reverse osmosis membranes are not only ineffective when it comes to chlorinated water, but the chlorine will simply destroy the protein membrane.


Is acid injection necessary?-1

Just as acidic solutiosn aren't good for membranes, caustic solutions are equally damaging to membrane elements. Acid injection may be incorporated into the RO pretreatment system to control pH and minimize the scale-forming tendency of the feed water. Acid injection is indicated if the scale-forming tendency of the brine stream is above +0.3 as measured by the LSI. Either sulfuric or hydrochloric acid can be used for this purpose. However, sulfuric acid is less costly, and is more commonly used.


Antiscalants have been shown to be effective in extending the intervals between chemical cleanings of the RO membranes. These products are generally formulated to include inorganic phosphates, organophosphonates, and dispersants. Use Antiscalant products that have been approved by the membrane manufacturer, and follow all direction in applying and controlling the product dosage. Some Antiscalant contain negatively charged polymers and dispersants that can react with cationic polymers that might be dosed up stream prior to the media filters. The Antiscalant must be compatible with these polymers; otherwise, the reaction product will foul the membranes.


Do all systems require Chemical Cleaning?-1

Despite all efforts to protect the RO System from fouling and loss of flux, eventually the membranes will require chemical cleaning. A well-designed RO System will include provisions for a cleaning skid to facilitate the cleaning process. The skid should include a chemical tank, solution heater, recirculating pump, drains, hoses, and all other connection and fittings required accomplishing a complete chemical cleaning of the RO modules.

Various chemical cleaning agents are available for maintaining RO membranes. The type and amount of foulant will dictate the most effective cleaning agent. Acid cleaners’ best remove mineral scale deposits. Hydrogen peroxide is commonly used to clean and sanitize membranes to correct or prevent biofouling problems. In some cases, a mild solvent such as methanol is used. Because of the number of variables involved in the selection and application of these cleaning agents, contact the membrane manufacturer, equipment supplier, or a qualified chemical consultant for specific advice and recommendations on how to accomplish an effective cleaning.

The operation of the RO System should be carefully monitored to predict when the membranes would require cleaning. As a rule of thumb, cleaning is indicated when the normalized flux rate decreases by 10%. Under ideal condition, assuming that the RO pretreatment system is properly designed and operated, the frequency between membrane cleanings should be 3 months or more. Cleaning every 1 to 3 months is considered a fair performance, and suggests that some improvements in the pretreatment system should be considered. Cleaning frequencies every month or more indicate a change in raw water quality, a problem with the pretreatment system, or a problem with the operation of the RO unit.


Summary-1

Reverse osmosis is a reliable method for producing high-purity water. However, most water supplies require some form of RO pretreatment such as softening, media filtration, activated carbon, or chemical injection to protect the membranes from premature fouling or failure. The pretreatment requirements will vary from location to location, but the overall objective remains the same: to maintain the design flux rates, minimize the membranes cleaning frequency, and prolong the useful life of the RO equipment.