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 it. But what is it? How does it work? Let’s take a look into the heart of an industrial grade system and break it down for everyone to understand.
Reverse osmosis, 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 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 motion is where the “reverse” comes from. Water is forced through the membrane which blocks an extreme majority of contaminants from coming through.
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.
Although some systems can draw water right out of a well or pipe feed, most 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 strength pump provides the initial pressure for the RO system. This motor usually provides enough water pressure to get through any pretreatment as well as the membranes, but if it doesn’t, a booster pump may be necessary further down the line.C) Multi-Layer Filter
Things like foul odor and taste usually aren’t prevented by reverse osmosis. A Multi-Layer filter can be filled with media that specifically targets the things your 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 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 tool for an RO system, dramatically increasing the lifespan of membrane elements.E) Reverse Osmosis System
This is the membrane system itself or the heart of the purification process. It can produce up to one million gallons of product water a day from a steady intake.F) Product Water Storage Tank
The permeate pure water output will usually go to a large tank, where it is held for use. Sometimes, a Reverse Osmosis System pumps water directly into a well or aquifer for recharging instead of being used or reserved right away.
The systems can have a number of other components built for them as well. The entire skid can be built into a containerized system, for example, to prevent from weather elements or keep the entire system portable.
If there is a need for water treatment, chances are an RO 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, RO 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.
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.
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.
Underground reservoirs of water are often brackish, meaning they contain large volumes of salt, but not enough to be considered salt water. Groundwater is most often purified for the agriculture industry, the mining industry. Groundwater is also a prized target of the bottling industry, because the unique mineral combinations often have an appealing taste.
Salt water reverse osmosis (sometimes referred to as desalination) is the turning of saltwater into drinking water. Ocean water has up to 45,000 PPM TDS. The biggest uses of desalination come in providing water in areas that lack a regular supply of fresh water.
It is very important that feed water be preconditioned to protect the membranes from fouling causing premature failure.
The membrane is constructed of a porous material that allows water to pass through, but rejects up to 99% of the dissolved solids at the surface. The dissolved salts are concentrated reject water (brine stream), where they are discharged to waste. Removing things prior is key to letting the RO system do what it was meant to do.
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 feed water were 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 membrane fouling, thus prolonging the life of the RO System.
A detailed chemical analysis (LSI, SDI, or CFI) 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 engineers designing the system to determine the optimum membrane array that will both minimize the tendency of scale and deposit formation and maximize the recovery and flux rate.
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.
Ion exchange is a popular method for softening and 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 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, but the chlorine will simply destroy the protein membrane.
Just as acidic solutions 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.
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, organophosphates, and dispersants. 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.
Despite all efforts to protect the system from fouling and lose 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 for accomplishing a complete chemical cleaning of the RO modules.
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