Q. How Reverse Osmosis Works
The phenomenon of osmosis occurs when pure water flows from a dilute saline solution through a membrane into a higher concentrated saline solution.
The phenomenon of osmosis is illustrated in Figure 1. A semi permeable membrane is placed between two compartments. “Semi permeable” means that the membrane is permeable to some species, and not permeable to others. Assume that this membrane is permeable to water, but not to salt. Then, place a salt solution in one compartment and pure water in the other compartment. The membrane will allow water to permeate through it to either side. But salt cannot pass through the membrane.
As a fundamental rule of nature, this system will try to reach equilibrium. That is, will try to reach the same concentration on both sides of the membrane. The only possible way to reach equilibrium is for water to pass from the pure water compartment to the salt-containing compartment, to dilute the salt solution.
Figure 1 also shows that osmosis can cause a rise in the height of the salt solution. This height will increase until the pressure of the column of water (salt solution) is so high that the force of this water column stops the water flow. The equilibrium point of this water column height in terms of water pressure against the membrane is called osmotic pressure. If a force is applied to this column of water, the direction of water flow through the membrane can be reversed. This is the basis of the term reverse osmosis. Note that this reversed flow produces pure water from the salt solution, since the membrane is not permeable to salt.
General Understanding of RO Membrane Maintenance and Cleaning :
Following gives you a clear understanding of Membrane working and effectiveness of maintenance and cleaning. If practice is adopted and seriousness is developed membrane can give you consistent flow of required specified quality water apart from longer life.
One RO design feature that is commonly over looked in reducing RO cleaning frequency is the uses of RO permeate water for flushing foul ants from the system. Soaking the RO elements during standby with permeate can help dissolve scale and loosen precipitates, reducing the frequency of chemical cleaning.
Eventually the day comes when your RO system will require cleaning. Cleaning is recommended when your RO shows evidence of fouling, just prior to a long-term shutdown, or as a matter of scheduled routine maintenance. Fouling characteristics that signal that you need to clean are- a 10-15% decrease in normalized permeate flow, a 10-15% decrease in normalized permeate quality, or a 10-15% increase in normalized pressure drop as measured between the feed and concentrate headers.
RO cleaning frequency due to fouling will vary by site. A rough rule of thumb as to an acceptable cleaning frequency is once in every 3 to 12 months. If you have to clean more than once a month, you should be able to justify further capital expenditures for improved RO pre-treatment or a re-design of the RO operation. If cleaning frequency is every one to three months, you may want to focus on improving the operation of your existing equipment but further capital expenditure may be harder to justify.
What you clean for can vary site depending on the foul ant. Complicating the situation frequently is that one more than one foul ant can be present. Typical foul ants are :
- Calcium carbonate scale
- Sulphate scale of calcium, barium or strontium
- Metal oxides of iron, manganese, aluminum, etc.
- Silica scale
- Colloidal deposits (inorganic or mixed inorganic/ organic)
- Organic material of natural origin or man-made origin
- Biological (bios lime, mold, or fungi)
There are a number of factors involved in the selection of a suitable cleaning chemical (or chemicals) and proper cleaning protocol. The first time you have to perform a cleaning, it is recommended to contact the manufacturer of the equipment, the RO element manufacturer, or a RO specialty chemical supplier. Once the suspected foulant(s) are identified, one or more cleaning chemicals will be recommended. These chemical(s) can be generic and available from a number of suppliers or can be private-labeled proprietary cleaning solutions. The proprietary solutions can be more expensive but may be easier to use and you cannot rule out the advantage of the intellectual knowledge supplied by these companies. An invaluable service offered some service companies pull that they will determine the proper cleaning chemicals and protocol by testing at their facility an element from your system.
It is not unusual to use a number of different cleaning chemicals in a specific sequence to achieve the optimum cleaning. There are times that a low pH cleaning is used first to remove foul ants like mineral scale, followed by a low pH cleaning. Some cleaning solutions have detergents added to aid in the removal of heavy biological and organic debris, while others have a chelating agent like EDTA added to aid in the removal of colloidal material, organic and biological material, and sulphate scale. An important thing to remember is that the improper selection of a cleaning chemical or the sequence of chemical introduction can make the foulant worse.
There are a number of precautions in cleaning chemical selection and usage for a composite polyamide membrane :
- Follow the manufacturer’s recommended chemical list, dosage, pH, temperature and contact time guidelines
- Use the least harsh chemical cleaning to get the job done. This will optimize the useful life
- Be prudent in the adjustment of pH at the low and high pH range to extend the useful life of the membrane. A “gentle” pH range is 4 to 10, while the harshest is 2 to 12
- Don’t mix acids with caustics. Thoroughly rinse the 1st cleaning solution from the system before introducing the next solution
- Flush out detergents with high pH permeates
- Verify that proper disposal requirements for the cleaning solution are followed
If your system has been fouled biologically, you may want to consider the extra step of introducing a sanitizing biocide chemical after a successful cleaning. Biocides can be introduced immediately after cleaning. Biocides can be introduced immediately after cleaning, periodically (e.g. once a week), or continuously during service. You must be sure however that the biocide is compatible with the membrane, does not create any healthy risks, is effective in controlling biological activity, and is not cost prohibitive.
The successful cleaning of RO on-site requires a well-designed RO cleaning skid. Normally this skid is not hard piped to the RO skid and uses temporary hosing for connections. It is recommended to clean a multi-stage RO one stage at a time to optimize cross flow cleaning velocity. The source water for chemical solution and rinsing should be RO Permeate, DI Water or at least soft water. Components must be corrosion proof. Major cleaning system components are :
- RO Cleaning Tank : This tank needs to be sized properly to accommodate the displacement of water in the hose, piping, and pressure vessels. The tank should be designed to allow 100% drainage, easy access for chemical introduction and mixing, a recirculation line from the RO Cleaning Pump, proper venting, overflow, and a return line located near the bottom to minimize foam formation when using a surfactant.
- RO Cleaning Pump : This pump needs to be sized to develop the proper cross flow velocity to scrub the membrane clean. The cleaning rate for an 8-inch diameter vessel is 30 to 40 gpm and for a 4-inch diameter vessel is 8 to 10 gpm. The maximum recommended pressure is 60 psi to minimize the production of permeate during cleaning and reduce the convective re-deposition of foulant back on to the membrane surface.
- RO Cleaning Cartridge Filter : Normally 5 to 10-micron and is designed to remove foul ants that have been displaced from the cleaning process.
- RO Tank Heater or Cooler : The optimal temperature for cleaning is 35 to 45o C. One cannot forget that heat is generated and imparted by the RO Cleaning Pump during recirculation.
RO Tank Mixer: This is recommended to get optimal mixing of chemical, though some designers rely solely on the slow introduction of chemical while maintaining a recirculation through the RO Cleaning Pump back to the tank.
- Instrumentation : Cleaning system instrumentation should be included to monitor flow, temperature, pressure, and tank level.Sample Points : Sample valves should be located to allow pH and TDS measurements off the RO Cleaning Pump discharge and the concentrate side recirculation return line.
- Permeate Return Line : A small amount of the cleaning solution can permeate through the membranes and so, a permeate side return line is recommended.
RO Cleaning procedures may vary dependent on the situation. The time required to clean a stage can take from 4 to 8 hours. The basic steps of cleaning are :
- Perform a low pressure flush with feed or permeate water to remove service concentrate and foulants.
- Make the cleaning solution as per the manufacturer’s instructions.
- Introduce the cleaning solution to the first stage for 60 minutes. You may want to throttle the flow up slowly to minimize the plugging of the feed path with dislodged foulant. Send the displaced water and up to 20% of the fouled cleaning solution to drain before returning the cleaning solution back to the RO Cleaning Tank. Readjust the pH to the target when it changes more than 0.5 pH units.
- An optional soak and recirculation sequence can be used. The soak time can be from 1 hour to overnight depending on the manufacturer’s recommendations, but the cautions that the proper temperature and pH be maintained and that this does increase the chemical exposure time of the membrane.
- A low pressure Cleaning Rinse with permeate water is required to remove all traces of chemical from the Cleaning Skid and the RO Skid.
- Once all the stages of a train are cleaned, the RO can be placed back into service. It is not unusual for it to take from a few hours to a few days for the RO permeate quality to stabilize, especially after high pH cleanings.
It is exciting to have a successful cleaning and watch your pressures and permeate quality improve. On the flip side it is frustrating to have an unsuccessful cleaning. If the cleaning did not provide the results you were hoping for, you may want to consider talking to those suppliers who offer off-site services rather than proceed with a trial-and-error approach on site. Pull one or two elements from the front or back end and send them to a service company. A service company can determine the optimal cleaning procedure and also report how effective the cleaning was in restoring flow and salt rejection.
Understanding Reverse Osmosis
Semi permeable Membranes are the Heart of RO Systems
The process of reverse osmosis (RO) represents the finest level of liquid filtration available today. While ordinary liquid filters use a screen to separate particles from water streams, an RO system employs a semi permeable membrane that separates an extremely high percentage of unwanted molecules. For example, the membrane may be permeable to water molecules, but not to molecules of dissolved salt. If this membrane is placed between two compartments in a container as shown in Figure 1, and a salt solution is placed in one half of the container and pure water in the other, water passes through the membrane while the salt cannot.
Pressure is applied to Reverse Natural Osmotic Flow
Now a fundamental scientific principle comes into play. That is, dissimilar liquid systems will try to reach the same concentration of materials on both sides of the membrane. The only way for this to happen in our example, is for pure water to pass through the membrane to the salt-water side in an attempt to dilute the salt solution. This attempt to reach equilibrium is called osmosis. But if the goal in our example water purification system is to remove the salt from water, it is necessary to reverse the natural osmotic flow by forcing the salt water through the membrane in the reverse direction. This can be accomplished by applying pressure to the salt water as it’s fed into the system, creating a condition know as “reverse osmosis.”( See Figure 1).
Q. How to Use Reverse Osmosis in Practice
The simplified reverse osmosis process is shown in Figure 2
With a high-pressure pump, pressurized saline feed water is continuously pumped to the RO system. In the system, consisting of a pressure vessel (housing) and a membrane element, the feed water will be split into a low saline product, called permeate and a high saline brine, called concentrate or reject. A flow-regulating valve, called concentrate valve, controls the percentage of feed water that is going to the concentrate stream and the permeated which will be obtained from the feed.
In the case of a spiral wound module consisting of a pressure vessel and several spiral wound elements, pressurized water flows into the vessel and through the channels between the spiral windings of the element. Up to seven elements are connected together within a pressure vessel. The feed water becomes more and more concentrated and will enter the next element, and at last exists from the last element to the concentrate valve where the applied pressure will be released. Permeate of each element will be collected in the common permeate tube installed in the center of each spiral wound element and flows to permeate collecting pipe outside of the pressure vessel.
Figure 2 : Reverse Osmosis Process
Pretreatment required for RO
Typical pretreatment consists of :
- Coarse filtration (~80 micron) to remove large materials
- Hypochloride addition to reduce biofouling potential
- Fine filtration using multimedia filters or clarification
- Bisulphide addition to reduce residual free chlorine
- Cartridge filter upstream of the feed pump and membranes
Additional pretreatment considerations: Waters with higher particle contents, measured by silt density index (SDI), require a higher degree of pretreatment to achieve acceptable quality. Systems using groundwater as the feed source frequently operate without hypochloride and bisulphide addition. Water with high hardness may require softening and / or acid addition. Activated carbon may be needed for water with high organic content. The in-line addition of antiscalants may be required for waters with high scaling potential.
Pore Size recommended for Filter Cartridge in an RO
Reverse Osmosis Module Designs
- Most commonly a filter with a micron rating of 5 micron absolute.
- With high levels of colloidal silica, lower micro-ratings are advisable.
Four basic types of RO module designs are in commercial use: tubular, plate-and-frame, spiral wound, and hollow fibre modules.
The tubular and the plate-and-frame devices date back to the early days of RO membrane technology. Both of these designs involve a high initial capital cost and a low membrane packing density (very low for the tubular design). However, these designs can operate on highly fouling feed waters. Thus, these designs find use in the food industry (examples: milk concentration for cheese manufacture, tomato juice concentration), and in concentration/treatment of wastewaters. They seldom compete with spirals and hollow fibre modules in desalination and water purification applications.
The design of spiral wound elements contains two layers of membrane glued back-to-back onto a permeate collector fabric (permeate channel spacer). This membrane envelope is wrapped around a perforated tube into which the permeate empties from the permeate channel spacer. Plastic netting is wound into the device, and maintains the feed-stream channel spacing. It also promotes mixing of the feed stream to minimize concentration polarization.
Comparisons of Reverse Osmosis System Types
- System Costs : Tubular, plate & frame >> Hollow Fiber, Spiral
- Flexibility in Design : Spiral >> Hollow Fiber > Plate & Frame > tubular
- Cleaning Behavior : Plate & Frame > Tubular > Spiral > Hollow Fiber
- System Space Requirements : Tubular > Plate & Frame > Spiral > Hollow Fiber
- Susceptibility to Fouling : Hollow Fiber >> Spiral > Plate & Frame > tubular
- Energy Requirement : Tubular > plate & frame > hollow fiber > spiral
The design of a hollow fiber permeator can package a tremendous amount of membrane area into a small volume. The difficulty in this approach, however, is that these fibers act almost like a string filter. This design requires a high level of feed water pretreatment to minimize the fouling potential of the feedwater. And when they are fouled, they are very difficult to regenerate by cleaning methods. Another aspect of hollow fiber permeators is that abrasion through fiber-fiber contact or via fiber contact with trapped particles appears to occur during RO operation. This results in gradual fall-off of salt rejection with time.
Above is a set of comparisons between the four basis module designs. Comparing their susceptibility to fouling for example, hollow fiber devices are much worse than spiral wound devices, which in turn are much worse than tubular devices and plate-and-frame devices.
Referring to system costs, spiral wound and hollow fiber systems are relatively equal on well water sources. For surface water sources, pretreatment costs tend to be higher for hollow fiber systems because of their fouling potential. Tubular and plate-and-frame systems are far more expensive than hollow fiber and spiral wound devices, and are relatively cost competitive to each other. As for system space requirements, tubular modules require the most space, hollow fiber and spiral modules require the least space.
One specific advantage of spiral wound units is that they can be linked together into series of two to seven elements within a single pressure vessel. Thus, up to seven times the flow of product water can be handled with only a single set of plumbing connections for feed, concentrate and permeate to a pressure vessel. In the case of hollow fiber modules, each hollow fiber unit requires installation of one feedwater inlet, one concentrate outlet, and one permeate outlet. For large modular systems for field application, a significant percentage of the system cost will be in the plumbing connections.
Factors Affecting RO Membrane Performance
Reverse osmosis (RO) technology can be a complicated subject, particularly without an understanding of the specific terminology that describes various aspects of RO system operation and the relationships between these operating variables.
This bulletin defines some of these key terms and provides a brief overview of the factors that affect the performance of RO membranes, including pressure, temperature, feed water salt concentration, permeate recovery, and system pH.
Effect of Pressure
- Recovery : The percentage of membrane system feedwater that emerges from the system as product water or “permeate”. Membrane system design is based on expected feed water quality and recovery is fixed through initial adjustment of valves on the concentrate stream. Recovery is often fixed at the highest level that maximizes permeate flow while preventing precipitation of super-saturated salts within the membrane system.
- Rejection : The percentage of solids concentration removed from system feedwater by the membrane.
- Passage : The opposite of “rejection”, passage is the percentage of dissolved constituents (contaminants) in the feedwater allowed to pass through the membrane.
- Permeate : The purified product water produced by a membrane system.
- Flow : Feed flow is the rate of feed water introduced to the membrane element, usually measured in gallons per minute (gpm). Concentrate flow is the rate of flow of non-permeated feedwater that exists in the membrane element. This concentrate contains most of the dissolved constituents originally carried into the element from the feed source. It is usually measures in gallons per minute (gpm).
- Flux : The rate of permeate transported per unit of membrane area, usually measured in gallons per square foot per day (gfd).
- Dilute Solution : Purified water solution, RO system product water.
- Concentrated Solution : Brackish water solution such as RO system feedwater.
Feed water pressure affects both the water flux and salt rejection of RO membranes. Osmosis is the flow of water across a membrane from the dilute side toward the concentrated solution side. Reverse osmosis technology involves application of pressure to the feed water stream to overcome the natural osmotic pressure. Pressure in excess of the osmotic pressure is applied to the concentrated solution and the flow of water is reversed. A portion of the feed water (concentrated solution) is forced through the membrane to emerge as purified product water of the dilute solution side.
As shown in Figure 2, water flux across the membrane increases in direct relationship to increases in feedwater pressure. Increased feedwater pressure also results in increased salt rejection but, as Figure 2 demonstrates, the relationship is less direct than for water flux.
Because RO membranes are imperfect barriers to dissolved salts in feedwater, there is always some salt passage through the membrane. As feedwater pressure is increased, this salt passage is increasingly overcome as water is pushed through the membrane at a faster rate than salt can be transported.
However, there is an upped limit to the amount of salt that can be excluded via increasing feedwater pressure. As the plateau in the salt rejective curve (Figure 2) indicates, above a certain pressure level, salt rejection no longer increases and some salt flow remains coupled with water flowing through the membrane.
Effect of temperature As figure 3 demonstrates, membrane productivity is very sensitive to changes in feedwater temperature. As water temperature increases, water flux increases almost linearly, due primarily to the higher diffusion rate of water through the membrane.
Increased feedwater temperature also results in lower salt rejection or higher salt passage. This is due to a higher diffusion rate for salt through the membrane.
The ability of a membrane to tolerate elevated temperatures increases operating latitude and is also important during cleaning operations because it permits use of stronger, faster cleaning processes.
Effect of salt concentration Osmotic pressure is a function of the type and concentration of salts or organics contained in feedwater. As salt concentration increases, so does osmotic pressure. The amount of feedwater driving pressure necessary to reverse the natural direction of osmotic flow is, therefore, largely determined by the level of salts in the feedwater.
Figure 5 demonstrates that, if feed pressure remains constant, higher salt concentration results in lower membrane water flux. The increasing osmotic pressure offsets the feedwater driving pressure. Also illustrated in Figure 5 is the increase in salt passage through the membrane (decrease in rejection) as the water flux declines.
Effect of recovery as shown in Figure 1, reverse osmosis occurs when the natural osmotic flow between a dilute solution and a concentrated solution is reversed through application of feedwater pressure. If percentage recovery is increased (and feedwater pressure remains constant), the salts in the residual feed become more concentrated and the natural osmotic pressure will increase until it is as high as the applied feed pressure. This can negate the driving effect of feed pressure, slowing or halting the reverse osmosis process and causing permeate flux and salt rejection to decrease and even stop (please see Figure 6).
The maximum percent recovery possible in any RO system usually depends not on a limiting osmotic pressure, but on the concentration of salts present in the feedwater and their tendency to precipitate on the membrane surface as mineral scale. The most common sparingly soluble salts are calcium carbonate (limestone), calcium sulphate (gypsum), and silica. Chemical treatment of feedwater can be used to inhibit mineral scaling.
Effect of pH
The pH tolerance of various types of RO membranes can vary widely. Thin-film composite membranes are typically stable over a broader pH range than cellulose acetate (CA) membranes and, therefore, offer greater operating latitude (please see Figure 4).
Membrane salt rejection performance depends on pH range. Water flux may also be affected. Figure 7 shows that water flux and salt rejection of a good membrane are essentially stable over a broad pH range.
As illustrated in Figure 4, the stability of membrane over a broad pH range permits stronger, faster, and more effective cleaning procedures to be used compared to CA membranes.
Factors Influencing Reverse Osmosis Performance
Permeate Flux and salt rejections are the key performance parameters of a reverse osmosis process. They are mainly influenced by variable parameters, which are as follows:
- Feedwater salt concentration
The following graphs show the impact of each of those parameters when the other three parameters are kept constant. In practice, there is normally an overlap of two or more effects.
Not to be neglected are several main factors which cannot be seen directly in membrane performance. These are maintenance and operation of the plant as well as proper pretreatment design. Consideration of these three ‘parameters’, which have very strong impact on the performance of a reverse osmosis system, is a must for each OEM (original equipment manufacturer) and end user of such a system.
With increasing effective feed pressure, the permeate TDS will decrease while the permeate flux will increase as shown in Figure 3.
If the temperature increases and all other parameters and kept constant, the permeate flux and the salt passage will increase (see Figure 4).
The recovery is the ratio of permeates flow to feed flow. In the case of increasing recovery, the permeate flux will decrease and stop if the salt concentration reaches a value where the osmotic pressure of the concentrate is as high as the applied feed pressure. The salt rejection will drop with increasing recover (see Figure 5).
Feedwater Salt Concentration :
Figure 6 shows the impact of the feedwater salt concentration on the permeate flux and the salt rejection. Table 1 shows a summary of the impacts influencing reverse osmosis plant performance.
Basics of RO and NF: Element Construction
Thin film composite membranes packed in a spiral wound configuration. Spiral wound designs offer many advantages compared to other module designs, such as tubular, plate and frame and hollow fiber module design for most of the reverse osmosis applications in water treatment. Typically, a spiral wound configuration offers significantly lower replacement costs, simpler plumbing systems, easier maintenance and greater design freedom than other configurations, making it the industry’s standard for reverse osmosis and Nano Filtration (NF) membranes in water treatment.
The construction of a spiral wound membrane element as well as its installation in a pressure vessel is schematically shown in Figure 1.13. A element contains from one, to more than 30 membrane leafs, depending on the element diameter and element type. Each leaf is made of two membrane sheets glued together back-to-back with a permeate spacer in-between them. The consistent glue lines about 1.5 in (4 cm) wide that seal the inner (permeate) side of the leaf against the outer (feed/concentrate) side. There is a side glue line at the feed end and at the concentrate end of the element, and a closing glue line at the outer diameter of the element. The open side of the leaf is connected to and sealed against the perforated central part of the product water tube, which collects the permeate from all leaves. The leaves are rolled up with a sheet of feed spacer between each of them, which provides the channel for the feed and concentrate flow. In operation, the feed water enters the face of the element through the feed spacer channels and exists on the opposite end as concentrate. A part of the feed water – typically 10-20% - permeates through the membrane into the leaves and exists the permeate water tube.
When elements are used for high permeate production rates, the pressure drop of the permeate flow inside the leaves reduces the efficiency of the element.
In membrane systems the elements are placed in series inside of a pressure vessel. The concentrate of the first element becomes the feed to the second element and so on. The permeate tubes are connected with interconnectors (also called couplers), and the combined total permeate exits the pressure vessel at one side (sometimes at both sides) of the vessel.
Degasifier use for RO
Many people ask questions regarding degasification. Typically degasification is not necessary. However, for specific needs, the use of a degasifier depends upon quality requirement needed and the composition of the feed water. Reasons to need a degasifier may be that a reduction of the CO2 or the H2S content is necessary. The use of a vacuum degasifer is preferable when H2S is present in the water or when airborne contaminations are present in the environment.
Understanding and reacting to the performance of a reverse osmosis (RO) system is necessary for continued successful operation. It is this interaction that allows us to quickly and correctly identify and correct issues that may arise. The following discussion is intended to explain the importance of RO maintenance.
First, we must understand why maintenance is needed. The following question will answer some of these needs.
- Have you ever had a problem with a RO unit?
- Have you ever had the same problem occur more than once?
- Has anyone ever asked about the performance before the problem occurred?
- Did anyone have documentation illustrating past performance?
Understanding and performing routine RO maintenance can prevent most problems before they occur. RO maintenance is more than repairing and replacing parts. It means taking steps to reduce or replacing parts. It means taking steps to reduce or prevent problems from occurring and being aware that a problem may be coming before it happens. Ensuring the RO is properly applied to the project and that feed water pretreatment (and the feed water itself) is checked on a regular basis also are instrumental. You also must check that normal scheduled maintenance occurs. If the system is large enough, daily log sheets are to be filled out. Maintenance is a combination of all these.
An RO unit is only as good as the application allows. The first step in preventative care is to ensure the feed water is of satisfactory condition. Customers just don’t want to pay for that all-important feed water analysis, yet it cannot be stressed strongly enough. Larger the system is, the greater the importance. Be aware of your feed water source. Surface water can produce needs that groundwater does not and vice versa. If your community mixes the two, it can be a “double whammy.” The point is to understand your feed water and install the proper pretreatment.
Understand both the amount of water and how the water will be used. Try to avoid traps such as knowing it needs to be 3,000 gallons per day and not knowing the day is eight hours. Make sure the unit is properly applied to the application and that any post treatment will allow proper flow and pressure.
Know the correct operating flows of the unit. Be sure to stay within manufacturer guidelines of pressure, product flow and recovery. Changes in feedwater temperature and total dissolved solids will change with the seasons. Expect these changes to cause minor adjustments to the unit.
On initial installation, main plumbing lines are flushed and any pretreatment is properly working before you run water into the RO. Double check for the lack of carbon fines and water hardness. Carbon fines from improperly rinsed carbon beds or cartridges will cause premature failure of the 5-micron pre filter used for particle protection just prior to the RO. Hardness leakage may lead to fouling of the membranes. Once installed be certain to set the unit flows to those recommended by the RO manufacturer and put these settings on a start-up sheet. Instruct the operating personnel on the importance of proper operation. Ensure it is understood to keep a watch on any pretreatment and check it on a regular basis. For instance, if carbon filters are used, periodically test for chlorine.
Water softeners should be tested for hardness and/or iron leakage. Don’t test just the treated water; also test the incoming water. Remember the best time to test a water softener is just before it regenerates. Testing a freshly regenerated water softener (when regenerating properly) is of little use. Cities that have multiple sources of water will have varying amounts of constituents. The pretreatment must be capable of performing in the worst conditions. When checking pretreatment, be sure to check any regenerates such as salt in the brine tank. Do not let the brine tank run low or become empty. Salt on a skid next to a brine tank can cause problems.
Regardless of system size certain tasks need to be performed on a regular basis. The most frequent maintenance is changing cartridge pre filters. These usually are nominally rated as 5 micron and are used to protect the RO membrane from particle fouling. Run length or time before changing is based on pressure drop. As these filters trap particles from the water supply, a reduction in pressure to the RO will occur. Most RO units include a low-pressure switch that prevents the RO from running if feed pressure drops too low. Check with your filter supplier to determine the allowable pressure drop across the cartridge and compare this to the incoming feed pressure. Applications with low feed pressure may not allow full use of these filters, requiring more frequent changes.
Carbon filters are commonly used for chlorine removal. Small systems may use carbon cartridges, while larger units may have backwashing carbon as well as other filter units. The time for carbon filter replacement is dependent on each application. Carbon cartridges should be replaced at least (if not before) every three months and backwashing filters should be changed annually if not before. Regardless of which type may exist, the change frequency is dependent on the application, size and type of cartridge and carbon as well as feed water make-up. Make certain the cartridges and any backwashing filters are well rinsed before sending any water to the RO. Backwashing filters should have an overnight presoak prior to use. It also is important to note that the use of some carbons is well-rinsed prior to placement online.
Check any RO pretreatment for correct operation. Check RO product water quality, system flows and pressure. Pressures include pre and post filters, RO pump discharge and waste and product water pressure. Keep a log sheet on this information and compare old data with new. Check the unit carefully for any leaks. Listen for any unusual noises. Pumps will exhibit a problem usually associated with a noise or leak prior to failure. Depending on size, the RO pump may be coupled to the motor and include an oil bowl reservoir. Be sure to check its level. Check to ensure that pressure switches and / or level controls are properly functioning.
Do not rush in and out. It is important to check the complete system both pre and post treatment. Make note of any deficiencies and take corrective action. Cleaning of the membranes usually is needed when the product flow rate falls 10 to 15 percent. When checking, it is necessary to ensure the loss of production is not caused by low feed water pressure, dirty pre filters or low feed water pressure. As a general rule, hardness scaling causes both a loss of water quality and flow rate. Biological fouling causes a loss of flow rate. If you are uncertain but believe a problem exists check with your RO supplier.
Data log sheets are used to represent the past and present performance of an RO system. These sheets provide a window into expected future performance. Not all RO units need or have data logs. Some units are small enough that these sheets just do not make sense. You’ll know if your RO is large enough to need one of these. Just check the unit manual. If it needs one, you will find it there. Typically, log sheets include all available operating data such as date, time, run time in hours, pre and post filter pressures, feed, concentrate and permeate pressures, feed water quality, permeate water quality, SDI, feed water hardness (ppm), chlorine, pH and others. The size and options selected will affect the data required for logging.
Use of these sheets allows the operator to spot trouble ahead of time. Through filling out these sheets, the operator will see patterns developing indicating normal or abnormal operation. Abnormal operation will indicate the type of problem that is occurring, allowing corrective action to take place. For example, if pressure drop rises, product water decreases and quality is falling, the need for cleaning is indicated. In this example, the need for an acid cleaning is indicated. In this example, the need for an acid cleaning to remove hardness scale is indicated. This is merely one example. A hardness fouling condition is described above for two reasons. The first one is that it indicates a problem is coming and how to correct it. The other is to illustrate that you now can go one step further. Hardness fouling indicates a possible failure in pretreatment or a change in raw water hardness level. This allows a correction of the problem and correction of the cause.
Q. When to Clean an RO Plant ?
Generally it is recommended to clean an RO plant when a 10% decrease in normalized flux is observed. For orientation, cleaning frequency can be in the range of 4 year with an SDI of less than 3. With an SSDI of 5, the cleaning frequency could double. However, cleaning frequency will depend on the specific situation.
Reversing Problems in Reverse Osmosis
The useful life of reverse osmosis (RO) membrane elements is reduced by scaling, fouling and chemical attack. By preventing these processes from occurring, you can maximize membrane life.
Scaling results from the precipitation of certain feed water dissolved substances within an RO unit due to the concentration of feed water within the unit. The two most common scalants are calcium carbonate (limestone) and calcium sulphate (gypsum).
Scaling reduces permeate flow and increases permeate conductivity. Sharply pointed scale crystals may come into contact and cut the membrane, causing irreversible damage. Scalants other than polymerized silica generally will effectively be removed by chemically cleaning the affected elements with hydrochloric acid (muriatic acid) or citric acid. The pH of the cleaning solution should be no lower than the minimum pH allowed by the membrane manufacturer.
Polymerized silica scales are generally removed with a high pH cleaning solution. Caustic (sodium hydroxide) at the maximum pH allowed by the membrane manufacturer will remove polymerized silica scales. However, it will take many hours to remove a silica scale.
Most scaling can be eliminated by installing an upstream sodium-cycle ion exchange unit, commonly called sodium zeolite softeners. Silica scaling is less common, but more difficult to treat cost-effectively. An upstream dolomitic lime softener or the injection of a silica scale inhibitor will be required.
Fouling results when feedwater suspended particles are deposited within an RO unit. The most common fouling particles are bacteria, followed by aluminium, iron and silica.
Fouling reduces permeate flow rate. Bacterial fouling usually does not cause the permeate conductivity to increase until the system is extremely plugged. Other particles may cause the permeate conductivity to gradually increase. Sharp particles may cut the membrane and cause irreversible damage.
Most biological and particulate silica foulants can be removed y a high-pH detergent solution. The maximum pH is established by the membrane manufacturer. Appropriate and inappropriate detergents are specified by the membrane manufacturers. Aluminum and iron foulants are generally removed by scale-removing cleaning solutions.
Many vendors sell cleaning chemicals that are effective in cleaning RO membrane elements that can be used in place of the cleaning solutions described above or those recommended by the membrane manufacturers.
If biofouling is a problem, pretreatment may have little impact on RO unit fouling. For example, installing a UV (ultraviolet) unit upstream may have little effect on RO unit fouling. While the UV unit will kill or deactivate 90 to 100 percent of the feedwater bacteria, the same number of fouling particles (living and dead) enters the RO unit. The living bacteria can use the decomposing dead bacteria as food to promote swift re growth.
The action of chlorine, ozone, or other oxidizing compounds breaks down essential bacterial molecules, killing the bacteria or making it unable to reproduce. Additionally, these oxidizing compounds break down relatively large feedwater organic molecules that bacteria can’t eat, turning them into smaller, easily digested food molecules. The regrowth after chlorination/ dechlorination or after ozonation/deozonation may be very high.
Biofouling is usually more effectively handled by periodically cleaning and sanitizing the RO units and upstream equipment and piping that is not continuously chlorinated. Fouling by non-living particles is usually handled by installing sediment filters or cartridge filters with a nominal filtration rating of five micron or less.
When to Clean
Residential / commercial RO units typically are too small to justify the costs of installing instrumentation to determine when to chemically clean them. For a larger, industrial RO unit the following instruments are the minimum required to adequately monitor performance:
- Feedwater : Temperature, Conductivity, Pressure
- Interstage : Pressure
- Permeate : Conductivity, Flow, Pressure
- Concentrate : Conductivity, Flow, Pressure
Based upon the daily readings, graphs are generated that track normalized permeate flow (NPF), differential pressure (DP), and percent salt rejection (%SR). NPF tracks the pressures, conductivities, and temperature and determines if the amount of permeate produced is appropriate given the current conditions or whether too much pressure is required, which indicates whether scaling and /or fouling is present.
DPs are a measurement of the resistance to flow rate. A higher DP across one or more RO elements indicates the presence of a scalant and / or foulant.
Percent salt rejection measures the membrane’s ability to reject dissolved substances. Scaling and membrane damage cause the %SR to drop.
By monitoring the performance of an RO unit, the need for chemical cleaning becomes apparent. When the NPF drops by 10 to 15 percent, of if the DP across any stage increases by 10 to 25 percent, it is time to clean.
Cleaning at an early stage can remove most scalants and foulants. By waiting too long, however, the elements can become so plugged that channeling occurs. Small channels of high flow will develop in the elements. Cleaning solution will then go through the same channel as during normal service, but the plugged areas will remain plugged.
Again, for small residential / commercial units, instruments generally aren’t required to monitor performance. If you wait until the customer complains about taste, odor, and /or low – flow problems, the elements may be plugged significantly and a chemical cleaning may be ineffective or may take many hours to get the elements back to its almost – original performance. Periodic cleanings on a preventive maintenance contract may be worthwhile for certain customers.
Chlorine is the most common agent to chemically attack and destroy frequently used polyamide thin-film membranes. Activated carbon blocks are generally used upstream on smaller RO units to remove the chlorine. For larger units, dechlorination is accomplished either by an upstream granular AC (activated carbon) bed or by injecting a sulfite generating (SO3-2) compound.
Active chlorine-consuming sites on the activated carbon material are depleted over time. Eventually dechlorination will diminish and finally quit, damaging downstream chlorine – sensitive membranes.
Activated Carbon blocks, or Activated Carbon beds, must be replaced as frequently as Activated Carbon manufacturers specify. However, if you periodically sanitize the Activated Carbon units with a chlorine solution or hydrogen peroxide (or other oxidant) solution, you’ll exhaust the Activated Carbon unit quicker than the design. Also, if the chlorine compound concentration in the feedwater increases, this will reduce the useful life of the Activated Carbon units. For example, it is common today for municipal drinking water treatment systems to periodically super-chlorinate the distribution system.
During most of the year, both gaseous chlorine and ammonia are commonly injected into municipal drinking water in order to provide microbiological control without creating excessive amounts of trihalomethane compounds. Since chloramines (chlorine plus ammonia) compounds are not as biocidal as free chlorine compounds, periodic super-chlorination with free chlorine residual is required to maintain microbiological control. If the expected useful life of an AC unit is based upon the usually low chloramines levels and doesn’t take into account a month or two per year of higher free chlorine residual, the AC unit may exhaust prematurely and cause downstream membrane damage.
Changing Reverse Osmosis Membranes
To open a PVC pressure vessel, first remove any pins holding one end plug in place. (Note: many fiberglass pressure vessels use a snap ring to hold the plug in place. Properly – sized snap ring pliers should be used to safely remove the snap ring). Next, remove the fittings from the plug.
Thread a nipple and tee into the feed or concentrate port long enough to extend past the end of the pressure vessel. Apply a coating of glycerin to the inside and rime of the pressure vessel from the plug to the end so the plug will slide out more easily. Pry off the plug using a pry bar. We either use a ball joint separator, also called a “pickle fork,” which is an automotive tool, or a slide hammer. Remove the fittings used to remove the plug.
Next, remove the membrane from the pressure vessel, noting which side the brine seal is no. Insert the new membrane with the brine seal on the feed side of the pressure vessel. Replace any damaged O-rings. Lubricate the plug with glycerin. Tap the plug securely into place using a piece of wood and a rubber mallet. Replace any pins to hold the plug in place. Reattach the fittings to the pin.
With a new membrane, flush all permeate and concentrate to drain for 30 minutes at low pressure to flush out the preservative.