REVERSE OSMOSIS (RO)
REVERSE OSMOSIS (RO)
Common membrane processes include ultrafiltration (UF), reverse osmosis (RO), electrodeionization (EDI). These processes (with the exception of UF) reduce most ions; RO and UF systems also provide efficient reduction of non-ionized organics and particulates. Because UF membrane porosity is too large for ion rejection, the UF process is used to reduce contaminants, and suspended solids.
Reverse Osmosis (RO) and Nanofiltration (NF) membrane technologies are widely recognized to offer the most effective and economical process options currently available. From small scale systems, through to very large scale desalination, RO and NF can handle most naturally occurring sources of brackish and seawaters. Permeate waters produced satisfy most currently applicable standards for the quality of drinking waters.
RO and NF can reduce regeneration costs and waste when used independently, in combination or with other processes, such as ion exchange. They can also produce very high quality water, or, when paired with thermal distillation processes, can improve asset utilization in power generation and water production against demand.
Below Figure gives an approximate representation of the salinity range to which the main desalination processes can be generally applied economically.
The most typical operating range of the four major desalination processes is shown in Figure Also shown is typical operating ranges for several generic FILMTEC membrane types.
The various filtration technologies which currently exist can be categorized on the basis of the size of particles removed from a feed stream. Conventional macrofiltration of suspended solids is accomplished by passing a feed solution through the filter media in a perpendicular direction. The entire solution passes through the media, creating only one exit stream. Examples of such filtration devices include cartridge filters, bag filters, sand filters, and multimedia filters. Macrofiltration separation capabilities are generally limited to undissolved particles greater than 1 micron.
MEMBRANE FILTRATION SPECTRUM
REVERSE OSMOSIS (RO)
Osmosis is the flow of solvent through a semi-permeable membrane, from a dilute solution to a concentrated solution. This flow results from the driving force created by the difference in pressure between the two solutions. Osmotic pressure is the pressure that must be added to the concentrated solution side in order to stop the solvent flow through the membrane. Reverse osmosis is the process of reversing the flow, forcing water through a membrane from a concentrated solution to a dilute solution to produce filtered water. Below figure illustrates the process of osmosis and reverse osmosis:
Figure: In the osmosis process, water flows through a membrane from the dilute solution side tohe more concentrated solution side. In reverse osmosis, applied pressure causes water to flow from the concentrated solution to the dilute solution.
Reverse osmosis is created when sufficient pressure is applied to the concentrated solution to overcome the osmotic pressure. This pressure is provided by feedwater pumps. Concentrated contaminants (brine) are reduced from the high-pressure side of the RO membrane, and filtered water (permeate) is reduced from the low-pressure side.
Reverse Osmosis System Removal
Below figure is a simplified schematic of an RO process:
Figure: A reverse osmosis system converts a feed stream into a purified stream (permeate) and a concentrated stream (brine).
Membrane modules may be staged in various design configurations, producing the highest-quality permeate with the least amount of waste. An example of a multistage RO configuration is shown in below figure:
Figure: A multistage reverse osmosis system configured to reduce the quantity of waste.
Typically, 95% of dissolved salts are reduced from the brine. All particulates are removed. However, due to their molecular porosity, RO membranes do not remove dissolved gases, such as Cl2, CO2, and O2.
RO Membranes. The two most common RO membranes used in industrial water treatment are cellulose acetate (CA) and polyamide (PA) composite. Currently, most membranes are spiral wound; however, hollow fiber configurations are available.
In the spiral wound configuration, a flat sheet membrane and spacers are wound around the permeate collection tube to produce flow channels for permeate and feedwater. This design maximizes flow while minimizing the membrane module size.
Hollow fiber systems are bundles of tiny, hair-like membrane tubes. Ions are rejected when the feedwater permeates the walls of these tubes, and permeate is collected through the hollow center of the fibers.
Concentrated brine is produced on the outside of the fibers contained by the module housing.
Below figure shows the construction and flow patterns in a spiral wound membrane configuration:
Figure: Spiral wound reverse osmosis modules are widely used. (Reprinted with permission from McGraw Hill, “Standard Handbook of Environmental Engineering.”)
Below figure shows the construction and flow patterns in a hollow fiber membrane system:
Figure: Hollow fiber reverse osmosis modules
How to Use Reverse Osmosis and Nanofiltration in Practice
In practice, reverse osmosis and nanofiltration are applied as a crossflow filtration process. The simplified process is shown in below Figure.
Figure. Nanofiltration process
With a high pressure pump, feed water is continuously pumped at elevated pressure to the membrane system. Within the membrane system, the feed water will be split into a low-saline and/or purified product, called permeate, and a high saline or concentrated brine, called concentrate or reject. A flow regulating valve, called a concentrate valve, controls the percentage of feedwater that is going to the concentrate stream and the permeate which will be obtained from the feed.
The key terms used in the reverse osmosis / nanofiltration process are defined as follows.
Recovery – the percentage of membrane system feedwater that emerges from the system as product water or “permeate”. Membrane system design is based on expected feedwater quality and recovery is defined 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 solute concentration removed from system feedwater by the membrane. In reverse osmosis, a high rejection of total dissolved solids (TDS) is important, while in nanofiltration the solutes of interest are specific, e.g. low rejection for hardness and high rejection for organic matter.
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 feedwater introduced to the membrane element or membrane system, usually measured in gallons per minute (gpm) or cubic meters per hour (m3/h). Concentrate flow is the rate of flow of non-permeated feedwater that exits the membrane element or membrane system. This concentrate contains most of the dissolved constituents originally carried into the element or into the system from the feed source. It is usually measured in gallons per minute (gpm) or cubic meters per hour (m3/h).
Flux – the rate of permeate transported per unit of membrane area, usually measured in gallons per square foot per day (gfd) or liters per square meter and hour (l/m2h).
Factors Affecting Reverse Osmosis and Nanofiltration Performance
Permeate flux and salt rejection are the key performance parameters of a reverse osmosis or a nanofiltration process. Under specific reference conditions, flux and rejection are intrinsic properties of membrane performance. The flux and rejection of a membrane system are mainly influenced by variable parameters including:
– feed water salt concentration
Processes that rely on microporous membranes must be protected from fouling. Membrane foul-ing causes a loss of water production (flux), reduced permeate quality, and increased trans-membrane pressure drop.
Membrane fouling is typically caused by precipitation of inorganic salts, particulates of metal oxides, colloidal silt, and the accumulation or growth of microbiological organisms on the membrane surface. These fouling problems can lead to serious damage and necessitate more frequent replacement of membranes.
Water Chemistry and Pretreatment
To increase the efficiency and life of reverse osmosis (RO) systems, effective pretreatment of the feed water is required. Selection of the proper pretreatment will maximize efficiency and membrane life by minimizing:
– Membrane degradation
– Product flow
– Product quality (salt rejection)
– Product recovery
– Operating & maintenance costs
Fouling is the accumulation of foreign materials from feed water on the active membrane surface and/or on the feed spacer to the point of causing operational problems. The term fouling includes the accumulation of all kinds of layers on the membrane and feed spacer surface, including scaling. More specifically, colloidal fouling refers to the entrapment of particulate or colloidal matter such as iron flocs or silt, biological fouling (biofouling) is the growth of a biofilm, and organic fouling is the adsorption of specific organic compounds such as humid substances and oil on to the membrane surface. Scaling refers to the precipitation and deposition within the system of sparingly soluble salts including calcium carbonate, barium sulfate, calcium sulfate, strontium sulfate and calcium fluoride.
Pretreatment of feed water must involve a total system approach for continuous and reliable operation. Such inadequate pretreatment often necessitates frequent cleaning of the membrane elements to restore productivity and salt rejection. The cost of cleaning, downtime and lost system performance can be significant.
The proper treatment scheme for feed water depends on:
– Feed water source
– Feed water composition
Summary of Pretreatment Options
Summarizes the pretreatment options when specific risks for scaling and fouling are present. It is a quick reference for “possible” and “very effective” methods. A combination of “possible” methods may also be “very effective”.
Guidelines for Acceptable RO Feed Water
Below Table summarizes the limits of quality parameters of the feed water. It is recommended to respect these limits to ensure successful operation of the membrane system. Otherwise, more frequent cleaning and/or sanitization may become necessary. The concentrations correspond to the entry to the membrane for a continuous feed stream, including any influences to the feed water from dosing chemicals or piping materials in the pretreatment line.
Comments & conditions Max. level Unit Component
5 1 SDI
0.5 NTU Turbidity
Target: <1 4 1 MFI0.45
0.1 mg/L Oil & grease
Synthetic organic compounds (SOC) have generally more adverse effects on RO/NF membranes compared with natural organic matters (NOM). 3 mg/L TOC
10 mg/L COD
Target: <5 10 μg/l Ac-C AOC
Target: <1 5 pg/cm2 ATP BFR
Under certain conditions, the presence of chlorine and other oxidizing agents will cause premature membrane failure. Since oxidation is not covered under warranty, FilmTec recommends removing residual free chlorine by pretreatment prior to membrane exposure. 0.1 mg/L Free chlorine
pH <6, oxygen <0.5 ppm 4 mg/L Ferrous iron
0.05 mg/L Ferric iron
0.05 mg/L Manganese
0.05 mg/L Aluminum
Higher concentrations may damage the element glue line In ug/L range mg/L VOC’s
Membrane feedwater should be relatively free from colloidal particulates. The most common particulates encountered in industrial membrane systems are silt, iron oxides, and manganese oxides.
Silt Density Index (SDI) testing should be used to confirm sufficient water quality for the specific membrane system employed. SDI evaluates the potential of feedwater to foul a 0.45 µm filter. Unacceptable SDI measurements can be produced even when water quality is relatively high by most industrial water treatment standards. Where pretreatment is inadequate or ineffective, chemical dispersants may be used to permit operation at higher-than-recommended SDI values. RO systems are highly susceptible to particulate fouling, EDI systems are more forgiving, and UF systems are designed to handle dirty waters.
The removal of suspended and colloidal particles by media filtration is based on their deposition on the surface of filter grains while the water flows through a bed of these grains (filter media). The quality of the filtrate depends on the size, surface charge, and geometry of both suspended solids and filter media, as well as on the water analysis and operational parameters. With a well-designed and operated filter, a SDI15 <5 can usually be achieved.
Some well waters, usually brackish waters, are in a reduced state. Typically, such waters contain divalent iron and manganese, sometimes hydrogen sulfide and ammonium, but no oxygen; therefore, they are also called anoxic. Often the oxygen has been used up (e.g., by microbiological processes) because the water is contaminated with biodegradable organic substances, or the water is from a very old aquifer.
The efficiency of media filtration to reduce the SDI value can be markedly improved if the colloids in the raw water are coagulated and/or flocculated prior to filtration. In-line filtration can be applied to raw waters with a SDI only slightly above 5. The optimization of the method, also named in-line coagulation or in-line coagulation-flocculation, is described in ASTM D 4188 /25/. A coagulant is injected into the raw water stream, effectively mixed, and the formed microflocs are immediately removed by media filtration.
For raw waters containing high concentrations of suspended matter resulting in a high SDI, the classic coagulation-flocculation process is preferred. The hydroxide flocs are allowed to grow and settle in specifically designed reaction chambers. The hydroxide sludge is removed, and the supernatant water is further treated by media filtration.
For the coagulation-flocculation process, either a solids-contact type clarifier or a compact coagulation-flocculation reactor may be used. For details, please refer to the general water treatment textbooks.
Microfiltration (MF) or ultrafiltration (UF) membrane removes virtually all suspended matter and, in the case of ultrafiltration, also dissolved organic compounds depending on their molecular mass and on the molecular mass cut-off of the membrane. Hence, an SDI <1 can be achieved with a well-designed and properly maintained MF or UF system.
A cartridge filter with an absolute pore size of less than 10 μm is the suggested minimum pretreatment required for every RO system. It is a safety device to protect the membranes and the high pressure pump from suspended particles. Usually it is the last step of a pretreatment sequence. A pore size of 5 μm absolute is recommended. The better the prefiltration the less RO membrane cleaning required. If there is a risk of fouling with colloidal silica or with metal silicates, cartridge filtration with 1 to 3 μm absolute pore size is recommended. The filter should be sized on a flow rate according to the manufacturer’s recommendation and replaced before the pressure drop has increased to the permitted limit, but at least every 3 months.
Scaling of RO/NF membranes may occur when spar ingly soluble salts are concentrated within the element beyond their solubility limit. For example, if a reverse osmosis plant is operated at 50% recovery, the concentration in the concentrate stream will be almost double the concentration in the feed stream. As the recovery of a plant is increased, so is the risk of scaling.
Due to water scarcity and environmental concern, adding a brine (RO concentrate) recovery system to increase recovery has become more popular. To minimize precipitation and scaling, it is important to establish well-designed scale control measures and avoid exceeding the solubility limits of sparingly soluble salts. In an RO/NF system, the most common sparingly soluble salts encountered are CaSO4, CaCO3, and silica. Other salts creating a potential scaling problem are CaF2, BaSO4, SrSO4, and Ca3(PO4)2. Solubility products of sparingly soluble inorganic compounds are listed in below Table.
The following design practices can be used to prevent scaling of a membrane.
Most natural surface and ground waters are almost saturated with CaCO3. The solubility of CaCO3 depends on the pH, as can be seen from the following equation:
Ca2+ + HCO3– ↔ H+ + CaCO3
By adding H+ as acid, the equilibrium can be shifted to the left side to keep calcium carbonate dissolved. Use food-grade quality acid.
Scale Inhibitor Addition
Scale inhibitors (antiscalants) can be used to control carbonate scaling, sulfate scaling, and calcium fluoride scaling. There are generally three different types of scale inhibitors: sodium hexametaphosphate (SHMP), organophosphonates and polyacrylates.
Softening with a Strong Acid Cation Exchange Resin
In the ion exchange softening process, the scale-forming cations, such as Ca2+, Ba2+ and Sr2+, are removed and replaced by sodium cations. The resin is regenerated with NaCl at hardness breakthrough. The pH of the feed water is not changed by this treatment and, therefore, no degasifier is needed. Only a little CO2 from the raw water is present that can pass into the permeate, creating a conductivity increase there. The permeate conductivity can be lowered by adding some NaOH to the softened feed water (up to pH 8.2) to convert residual carbon dioxide into bicarbonate, which is then rejected by the membrane. The rejection performance of the FT30 membrane is optimal at the neutral pH range.
Dealkalization with a Weak Acid Cation Exchange Resin
Dealkalization with a weak acid cation exchange resin is used mainly in large brackish water plants for partial softening to minimize the consumption of regeneration chemicals.
Lime softening can be used to remove carbonate hardness by adding hydrated lime:
Ca(HCO3)2 + Ca(OH)2 → 2 CaCO3 + 2 H2O
Mg(HCO3)2 + 2 Ca(OH)2 → Mg(OH)2 + 2 CaCO3 + 2H2O
The noncarbonate calcium hardness can be further reduced by adding sodium carbonate (soda ash):
CaCl2 + Na2CO3 → 2 NaCl + CaCO3
The lime-soda ash process can also be used to reduce the silica concentration. When sodium aluminate and ferric chloride are added, the precipitate will include calcium carbonate and a complex with silicic acid, aluminum oxide, and iron.
With the hot lime silicic acid removal process at 60–70°C, silica can be reduced to 1 mg/L by adding a mixture of lime and porous magnesium oxide.
In some applications, scaling is controlled by preventive membrane cleaning. This allows the system to run without softening or dosage of chemicals. Typically, these systems operate at low recovery in the range of 25%, and the membrane elements are replaced after 1–2 years. Accordingly, these systems are mainly small single-element plants for potable water from tap water or seawater. The simplest way of cleaning is a forward flush at low pressure by opening the concentrate valve. Short cleaning intervals are more effective than long cleaning times (e.g., 30 seconds every 30 minutes).
Adjustment of Operating Variables
When other scale-control methods do not work, the operating variables of the plant have to be adjusted in such a way that scaling will not occur. The precipitation of dissolved salts can be avoided by keeping their concentration below the solubility limit. This is accomplished by reducing the system recovery until the concentrate concentration is low enough.
Silica Scale Prevention
Dissolved silica (SiO2) is naturally present in most feed waters in the range of 1–100 mg/L. The prevailing forms of silica are meta silicic acids as (H2SiO3)n with low n numbers. Since silicic acid is a weak acid, it is mostly in the undissociated form at or below a neutral pH. Supersaturated silicic acid can further polymerize to form insoluble colloidal silica or silica gel, which can cause membrane scaling. The maximum allowable SiO2 concentration in the concentrate stream is based on the solubility of SiO2.
The scaling potential for the concentrate stream will be quite different from that of the feed solution because of the increase in the concentration of SiO2 and the change in pH. It can be calculated from the feed water analysis and the RO operating parameters.
As the pH exceeds neutral, silicic acid dissociates into the silicate anion (SiO32-)n. This can react with calcium, magnesium, iron, manganese or aluminum to form insoluble silicates.
Colloidal and Particulate Fouling Prevention
Assessment of the Colloidal Fouling Potential
Colloidal fouling of RO elements can seriously impair performance by lowering productivity and sometimes salt rejection. An early sign of colloidal fouling is often an increased pressure differential across the system.
The source of silt or colloids in reverse osmosis feed waters is varied and often includes bacteria, clay, colloidal silica, and iron corrosion products. Pretreatment chemicals used in a clarifier such as aluminum sulfate, ferric chloride, or cationic polyelectrolytes are materials that can be used to combine these fine particle size colloids resulting in an agglomeration or large particles that then can be removed more easily by either media or cartridge filtration. Such agglomeration, consequently, can reduce the performance criteria of media filtration or the pore size of cartridge filtration where these colloids are present in the feed water. It is important, however, that these pretreatment chemicals become incorporated into the agglomerates themselves since they could also become a source of fouling if not removed. In addition, cationic polymers may coprecipitate with negatively charged antiscalants and foul the membrane.
Several methods or indices have been proposed to predict a colloidal fouling potential of feed waters, including turbidity, Silt Density Index (SDI) and Modified Fouling Index (MFI). (see Table) The SDI is the most commonly used fouling index.
Measuring these indices is an important practice and should be carried out prior to designing an RO/NF pretreatment system and on a regular basis during RO/NF operation (three times a day is a recommended frequency for surface waters).
Figure 2.13 shows the equipment needed to measure SDI, including
• 47 mm diameter membrane filter holder
• 47 mm diameter membrane filters (0.45 μm pore size)
• 10–70 psi (1–5 bar) pressure gauge
• needle valve for pressure adjustment
1. Assemble the apparatus as shown in Figure 2.13 and set the pressure regulator at 207 kPa (30 psi or 2.1 bar).
2. Place the membrane filter carefully on its support.
3. Make sure the O-ring is in good condition and properly placed. Replace the top half of the filter holder and close loosely.
4. Bleed out trapped air, close the valve and tighten the filter holder.
5. Open the valve. Simultaneously, using a stopwatch, begin measuring the time required for the flow of 500 ml. Record the time ti. Leave the valve open for continued flow.
6. Measure and record the times to collect additional 500 mL volumes of sample, starting the collection at 5, 10, and 15 minutes of total elapsed flow time. Measure the water temperature and check the pressure as each sample is collected.
7. After completion of the test, the membrane filter may be retained for future reference. Alternatively, the filter may be left in operation after the test until clogged in order to collect suspended matter for analysis with analytical methods.
T = total elapsed flow time, min (usually 15 min, see Note)
ti = initial time required to collect 500 mL of sample, sec
tf = time required to collect 500 mL of sample after test time T, sec (usually 15 min)
Note: For this test method, 1-(ti/tf) should not exceed 0.75. If 1-(ti/tf) exceeds this value, use a shorter time for T; (i.e., 5 or 10 minute measurements in Step 6).
The guideline is to maintain SDI15 at ≤5. To minimize the fouling, however, SDI15 at <3 is recommended. A number of pretreatment technologies have proven effective in SDI reduction, including media filtration (such as sand/anthracite), ultrafiltration and cross-flow microfiltration. Polyelectrolyte addition ahead of filtration sometimes improves SDI reduction.
Methods to prevent colloidal fouling are outlined in the following.
Cleaning and Sanitization
The surface of a reverse osmosis (RO) membrane is subject to fouling by foreign materials that may be present in the feed water, such as hydrates of metal oxides, calcium precipitates, organics and biological matter. The term “fouling” includes the build-up of all kinds of layers on the membrane surface, including scaling.
Pretreatment of the feed water prior to the RO process is basically designed to reduce contamination of the membrane surfaces as much as possible. This is accomplished by installing an adequate pretreatment system and selecting optimum operating conditions, such as permeate flow rate, pressure and permeate water recovery ratio.
Occasionally, fouling of the membrane surfaces is caused by:
• Inadequate pretreatment system
• Pretreatment upset conditions
• Improper materials selection (pumps, piping, etc.)
• Failure of chemical dosing systems
• Inadequate flushing following shutdown
• Improper operational control
• Slow build-up of precipitates over extended periods (barium, silica)
• Change in feed water composition
• Biological contamination of feed water
The fouling of membrane surfaces manifests itself in performance decline, lower permeate flow rate and/or higher solute passage. Increased pressure drop between the feed and concentrate side can be a side effect of fouling.
Cleaning can be accomplished very effectively because of the combination of pH stability and temperature resistance of the membrane and the element components. However, if cleaning is delayed too long, it could be difficult to remove the foulants completely from the membrane surface. Cleaning will be more effective the better it is tailored to the specific fouling problem. Sometimes a wrong choice of cleaning chemicals can make a situation worse. Therefore, the type of foulants on the membrane surface should be determined prior to cleaning. There are different ways to accomplish this:
• Analyze plant performance data.
• Analyze feed water. A potential fouling problem may already be visible there.
• Check results of previous cleanings.
• Analyze foulants collected with a membrane filter pad used for SDI value determination .
• Analyze the deposits on the cartridge filter.
• Inspect the inner surface of the feed line tubing and the feed end scroll of the element. If it is reddish-brown, fouling by iron materials may be present. Biological fouling or organic material is often slimy or gelatinous.
Clean In Place (CIP) Washing System Mobile
In normal operation, the membrane in reverse osmosis elements can become fouled by mineral scale, biological matter, colloidal particles and insoluble organic constituents. Deposits build up on the membrane surfaces during operation until they cause loss in normalized permeate flow, loss of normalized salt rejection, or both.
Elements should be cleaned when one or more of the below mentioned parameters are applicable:
• The normalized permeate flow drops 10%
• The normalized salt passage increases 5 – 10%
• The normalized pressure drop (feed pressure minus concentrate pressure) increases 10 – 15%
If you wait too long, cleaning may not restore the membrane element performance successfully. In addition, the time between cleanings becomes shorter as the membrane elements will foul or scale more rapidly.
Below Table lists suitable cleaning chemicals. Acid cleaners and alkaline cleaners are the standard cleaning chemicals. Acid cleaners are used to remove inorganic precipitates (including iron), while alkaline cleaners are used to remove organic fouling (including biological matter). Sulfuric acid should not be used for cleaning because of the risk of calcium sulfate precipitation. Specialty cleaning chemicals may be used in cases of severe fouling or unique cleaning requirements. Preferably, RO/NF permeate should be used for the preparation cleaning solutions, however, prefiltered raw water may be used. The feed water can be highly buffered, so more acid or hydroxide may be needed with feed water to reach the desired pH level, which is about 2 for acid cleaning and about 12 for alkaline cleaning.
Table .Simple cleaning solutions