Reverse osmosis is a method used to separate a high percentage of pollutants from water by forcing it through a semi-permeable membrane. Reverse osmosis operates by utilizing advanced filtration technology which removes dissolved ions from water. Osmosis is an elemental force that draws fresh water to water with higher salt content. This is a process by which dissolved ions are removed from the water. This elemental force can be surmounted by applied pressure with the usage of pumps and semi-permeable membranes, which forces water through the membrane and filter out dissolved salt from the water with the lowest energy requirements, some of the highest recovery rates, and one of the best rejection rates.
the semi-permeable membrane allows water to pass through, but molecules larger than water cannot. Water flows back and forth until the concentration is equal on both sides of the membrane, and an equilibrium is formed. Reverse Osmosis works by filtering unwanted pollutants such as bacteria, viruses, and other microbiological organisms from water by pressuring the treated water which forces water substances through a semipermeable membrane. Throughout the process, the pollutants are separated from the water and flushed out, which produces ultra-pure water.
The reverse osmosis process works by utilizing high-pressure pumps to elevate the pressure on the concentrated side of the RO system and push the water through a membrane(s). It is a highly effective water treatment process that needs a higher degree of pressure depending on the total amount of contaminants existing in the feed water. Purified water is referred permeate or product water. The concentrated stream of contaminants left behind in the brine is referred to as reject water and is ultimately discharged.
The reverse osmosis membrane's sole responsibility is to ensure efficient filtration of all unwanted salts and minerals from the feed water as it passes through the system. Membranes from top-branded manufacturers are capable of eliminating up to 99% of dissolved solids from the water source. The rejected stream is not always thrown away, it can be recycled through the reverse osmosis system for water conservation.
The reverse osmosis process is more similar to a cross-flow filtration method than a conventional one. Cross-flow filtration is more effective since it consists of two outlets for the solution to go in order to handle higher concentrations of pollutants. The flow of the permeate and concentrated water in opposite directions allows the membrane surface to remain clean and greatly minimizes buildup. This filtration method ensures a longer life span of the system and components and reduced the need for membrane cleaning.
Here is the complete reverse osmosis process, including the pretreatment and post-treatment that is generally required for highly contaminated water:
Pre-Chlorination Dosing System
If the feed water contains traces of heavy metals or is contaminated, it is highly recommended to dose some chlorine to change the dissolved heavy metals to physical form, the media filter will be able to filter most of it.
Raw Water Storage Tank
Although some RO 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.
Feed Water Pump
A commercial or industrial pump provides the initial pressure for the Treatment System. This motor usually provides enough water pressure to get through any pretreatment as well as the RO membranes, but if it doesn't a booster pump may be necessary farther down the line.
Multi Media Filteration
As much as we hate to admit it, there are some things that membranes 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 RO system can't catch. If you need to eliminate these contaminants, a multi-layer filter is a must.
Activated Carbon Filter
Activated carbon filters are a good solution to reduce organic, bad taste, smell, and chlorine from the water.
Automatic Water Softener
Automatic water softeners are designed to remove water hardness, calcium, and magnesium ions, for small RO systems, we usually recommend water softeners instead of the antiscalant chemical dosing.
Antiscalant Chemical Dosing System
For larger RO systems, we use antiscalant dosing systems to dose antiscalant RO chemicals, which helps in preventing membrane fouling.
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 wastewater can be dumped down the drain, but check with your local water authorities in case it needs to be handled with care.
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 system would need to be running in order to have access to fresh water, which can be inconvenient. Sometimes, a RO 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.
Post-Chlorination Dosing System
If the permeate water is intended to be stored for more than one day, it is highly recommended to dose some chlorine to maintain clean and noncontaminated water.
Product Water Pump
This pump repressurizes the permeated water to the point of use ends. This is selected based on the overall traveling distance and required head. This pump must be selected in stainless steel to prevent any contamination of the permeated water.
Product Water UV Sterilizer
The UV sterilizer is placed after the storage tank and as a final disinfection device. Most of the time, we either use post chlorination as a disinfectant agent or ultraviolet sterilization.
Basic Components of RO System
Pressure Vessels & Membranes
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 reverse osmosis membranes in a system. There can be anywhere from a single, two, and half-inch membrane (like in an under-sink reverse osmosis water system) to hundreds of Eight-inch membranes all working together (a typical reverse osmosis plant).
Reverse Osmosis Skid
The best way to 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.
Most reverse osmosis water 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.
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.
RO Control Panel
Lastly, the reverse osmosis system has to be controlled by a human operator. and 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.
A reverse osmosis system can have a number of other components built onto or into it as extra components as well. The entire skid can be built into a containerized system, for example, so your Desalination by RO System is always on the go. There are a number of supplemental skids that can be attached to an RO System as well, for tasks like membrane cleaning, pretreatment, chemical dosing, and a number of other jobs as necessary.
TYPES OF COMMERCIAL/INDUSTRIAL APPLICATIONS OF RO WATER TREATMENT USE
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 highly pure reverse osmosis 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 Water Lanka, we pride ourselves on being a go-to source for information and service helping you solve your municipal, commercial, or industrial water treatment needs.
- Boiler water pretreatment
- Food and beverage industry water treatment
- Industrial wastewater purification
- Deionized (DI) water treatment
- Hotels, resorts, and apartments portable water treatment
- Dry Ice making
- Spotless vehicle service stations
- Pure ethanol manufacturing
- Dairy industry water treatment
- Pharmaceutical manufacturing water treatment
- Water bottling plants water treatment
- Hospitals water treatment
- Agriculture water treatment
Types of water sources of RO system can treat
Reverse Osmosis is an ideal water treatment solution for 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.
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 are often a target of water purification in tap water systems. RO water is ideal in applications 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 reservoirs of water are often brackish or highly brackish, meaning they contain large volumes of salt, but not enough to be considered saltwater. 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 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.
Reverse osmosis saltwater (sometimes referred to simply as desalination) is the turning of saltwater into drinking water. Ocean water has up to 45,000 ppm TDS. Typically, for environmental reasons, a borehole is dug in the ocean for this kind of reverse osmosis water, 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 freshwater.
If you work with a reverse osmosis system, you understand that the feedwater 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 discharged 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 the RO water through the membrane, 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 membrane surface area per day.)
Early in the development of membrane 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 membrane fouling, thus prolonging the life of the RO plant.
Autopsies of failed membranes modules have revealed a build-up of foulents caused by mineral scales such as calcium carbonate, colloidal materials like clays and silica, dead and living microorganisms, carbon particles, and chemicals attached 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 reverse osmosis membrane.
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 under actual feed conditions.
Although a water analysis helps predict 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, an SDI value of less than 3.0 is desirable for reverse osmosis feed waters. The SDI measurement has certain limitations in that it does not model the cross-flow design of an RO membrane.
Analysis of 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 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 water systems need an antiscalant injector or water softener to prevent damage to the RO 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 due to this process 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 RO 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.
Just as acidic solutions aren't good for membranes, caustic solutions are equally damaging to reverse osmosis membrane elements. The acid injection may be incorporated into the RO pretreatment system to control pH and minimize the scale-forming tendency of the feed water. The 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 are effective in extending the intervals between chemical cleanings of the RO membranes. These products are generally formulated to include inorganic phosphates, organophosphates, and dispersants. Use antiscalant products that have been approved by the membrane manufacturer, and follow all directions in applying and controlling the product dosage. Some antiscalants contain negatively charged polymers and dispersants that can react with cationic polymers that might be dosed upstream prior to the media filters. The antiscalant must be compatible with these polymers, otherwise, the reaction product will foul the membranes.
RO Chemical Cleaning
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 connections and fittings required to accomplish 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 conditions, 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.
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 reverse osmosis 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 membrane cleaning frequency, and prolong the useful life of the RO equipment.