Membrane Technology
(RO)

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Mendeleev University Science Park

Reverse Osmosis Technology

      Technology. The membrane-based separation process, which began as a scientific curiosity in the 1960s, is now a commercial reality. During the last two decades significant advances have been made in the development and application of microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) processes. These processes have now become major players in the field of solid-liquid separation technology.
      This article provides the reader with a general understanding of principles involved in the purification of aqueous streams by a membrane separation process. Included herein are basic descriptions of membrane separation systems and overviews of commonly encountered RO system operating issues. The level of detail is intended to alert the water technologist to the methods of preventing and restoring the performance of membrane systems. This article also includes a discussion of RO applications of interest to water technologists.
      The characteristics of various membrane separation processes in removing various species from the feed water are shown in Table 1 and indicates that membranes act as barriers to mass movement but allow restricted or regulated passage of one or more species. Depending upon the feed water quality, any combination of the membrane separation processes can be cost effectively applied to achieve the desired product water quality. Among the membrane separation processes, RO is now widely applied in a number of industrial applications.
      Water purification by the RO process involves the separation of dissolved solids from the feed water by means of a semi-permeable membrane. Semi-permeable membranes allow water to pass through (permeate) readily, but are fairly impermeable to other constituents present in the feed stream. Figure 1 illustrates feed water passing over the membrane at a transmembrane pressure exceeding the osmotic pressure of the feed water with the result that the permeate (product stream) is selectively passed through the membrane. This product stream (on the low-pressure side of the membrane) is depleted of dissolved ions while the reject steam (brine or waste stream) is enriched in the dissolved material. In order to treat a feed water stream, a trans-membrane pressure must be applied that is higher than the osmotic pressure for permeation of the solvent. This osmotic pressure strongly depends on charge, number of ions, and molecular weight of the feed stream.

Reverse osmosis process

Figure 1. Fluid Streams in RO Operation

Table 1. Membrane Separation Process

Membrane TypeConstituents RemovedConstituents Passing
MicrofiltrationSuspended solidsWater, high molecular weight, low molecular weight, ionic species
UltrafiltrationSuspended solids and highmolecular weightWater, low molecular weight, ionic species
NanofiltrationSuspended solids, high molecular weight, and low molecular weightWater and ionic species
Reverse OsmosisSuspended solids, high molecular weight, low molecular weight, ionic species
Water

Applications
      RO systems are widely used for the desalination of sea and brackish waters for potable water production. The RO separation process plays a useful role in cleaning various industrial effluents including pulp and paper; recovery of metals from electroplating wastes; recovery of valuable products from acid mine drainage; municipal wastewater reclamation; and the production of ultrapure water for boiler, semiconductor, and pharmaceutical industries.
      Applications in the food processing industry are developing broadly and include the processing of milk, sugar, fruit and vegetable juices, and fats and meat by-products. RO technology is also used in the production of alcoholic beverages and carbonated soft drinks.
      RO applications are also found in a variety of purifying operations. In pharmaceutical applications, the large quantities of ultrapure water required for hemodialysis is of particular significance. Other uses include process water for the production of prescription medication and over-the-counter items such as contact lens cleaning solutions or eye drops. Medical laboratories also use a large quantity of ultrapure water in research and testing procedures.
      In semiconductor manufacturing, control of particles in rinse water is crucial. The presence of even the smallest of particles could result in device failure by lithographic blocking or chemical contamination. In a plating operation, high purity water is used to rinse off any excess plating solution before drying. Improper rinsing or rinsing with poor quality water may reduce the luster of the plate, cause spotting, or inhibit the plating process completely.

Terminology
      As a prerequisite to discussing RO technology and applications, this section presents definitions for several key terms.

Rejection.
The term rejection is used to quantify the removal of dissolved solute from the feed stream. Rejection means the amount of solute that does not pass through the membrane relative to the feed concentration and this is mathematically expressed as

    R = (Cf -Cp) / Cf or R = 1 - Cp/Cf         (1)

Where R, Cf, and Cp are rejection, concentration of solute in the feed and permeate, respectively.

Rejection (%). The % rejection of a particular ion is defined as

    % R = 100 x (1 - [permeate]i - [reject]i )    (2)

Rejection Rate. The degree to which dissolved solutes are repelled from an RO membrane under pressure.

Transmembrane Pressure. The transmembrane pressure (TMP) is defined as

    TMP = [(Pf - Pr )] / 2 - Pp              (3)     
       
Where Pf, Pp, and Pr are the pressures for the feed, permeate, and reject streams, respectively.

Pressure Drop. Pressure differential between feed and reject streams.

Flux. Flow through the membrane per unit time per membrane surface area.

Recovery (%). The ratio of the RO product water to the feed water is called recovery (Y)                     

    Y= (product flow rate)/(feed flow rate) x 100   (4)        

Concentration Factor. Assuming that the amount of any ion passing through the membrane is close to zero, then the concentration factor (CF) for any ion is given by:

    CF = 1 / (1 - Y)                 (5)

Membrane configurations and materials
       RO membranes typically remove greater than 99% of the dissolved salts, microorganisms, and colloids and in some cases more than 90% of the soluble silica and TOC (total organic carbon) from the feed stream. Hollow fiber and flat sheet are the most commonly used RO membrane configurations. Hollow fiber membrane is extruded like fishing line with a hole in the center to create a tiny (100 to 200 micron) hollow fiber strand. Flat sheet membrane is manufactured by applying the semi-permeable material to a woven or non-woven cloth. It is manufactured as a continuous sheet and rolled up like a large paper towel roll. Flat sheet membrane is used in "spiral wound" (SW) and "plate and frame RO elements. Hollow fiber membrane is used in hollow fine fiber" (HFF) and "hollow fiber" (HF) RO elements. Table 2 summarizes advantages and disadvantages of HF and SW membrane configurations.
      Over 100 different materials are used to make RO membranes. However, the two most commonly used membranes are made from cellulose acetate (CA) and thin film composite (TFC). The characteristics and performance of these membranes differ significantly. Table 3 summarizes the comparative data for CA and TFC membranes.

Table 2. Advantages and Disadvantages of Hollow Fiber and Spiral Wound Membranes

Membrane

Advantages

Disadvantages

Hollow Fiber

· High membrane surface area to volume ratio
· High recovery in individual permeator
· Easy to troubleshoot
· Easy to change bundles in the field

· Sensitive to fouling by colloidal materials
· Limited number of membrane materials and manufacturers

Spiral Wound

· Good resistance to fouling
· Easy to clean
· Variety of membrane materials and manufacturers

· Moderate membrane surface area
· Difficult to achieve high recovery

 

Table 3. Comparison of Cellulose Acetate and Thin Film Composite Membranes

Parameter

Cellulose Acetate (CA)

Thin Film Composite (TFC)

Operating pressure (psi)

410 to 600

200 to500

Operating temperature (°C)

0 to 30

0 to 45

Operating pH

4 to 6.5

2 to 11

Membrane degradation potential

Hydrolyzes at low & high pHs

Stable over broad pH range

Permeate flux (gfd)

5 to18

10 to 205

Salt rejection (%)

70 to 95

97 to 99

Stability to free chlorine

Stable to low (< 1 ppm) levels

Attacked by low levels (>0.1ppm)

Resistance to biofouling

Relatively high resistance

Low resistance

Manufacturer

Several

Several

Cost

Lower

50 to 100% more

Operating challenges and solutions
       Fouling resulting from the foulant accumulation on the membrane surface is the major cause of RO system failure. RO membrane fouling is a complex phenomenon involving the deposition of several different but related types of foulants on the membrane surface. RO system fouling problems are becoming more prevalent as the use of low quality feed water increases. In addition, surface water treated with cationic organic flocculants poses very serious and challenging fouling problems. Operating costs increase when performance problems arise. These costs are associated with membrane cleanup, replacement, and system downtime. The success of an RO system depends largely on three factors: system design, pretreatment (e.g., chemical conditioning), and system maintenance.
      RO system designs typically include a number of unit operations placed in a series. Figure 2 illustrates a typical RO system consisting of several unit operations: pretreatment, membrane unit, and post treatment. The pretreatment system adjusts the water quality of the feed water chemistry to optimize the post treatment. The primary considerations are the quality and quantity of both water entering the system (feed) and the finished water leaving the treatment process (product).

Membrane system

Feed water and pretreatment
       The performance of an RO system is largely controlled by the composition of the feed water. Feed water quality will determine the amount and the type of pretreatment necessary to make RO an economical process. This balance is the primary limiting factor of most RO systems in operation today. The close relationship between water chemistry and membrane performance is why membrane manufacturers require periodic water analysis in order to maintain membrane warranties. Water sources vary widely around the world, across the country, and even within local areas. All natural waters contain organic and inorganic, dissolved and suspended contaminants. The water composition dictates the pretreatment process(es) that are used.
Designers and operators of the system benefit greatly from having current, accurate water analyses for all aspects of an RO system, from actual design of arrays and membrane area to the tracking of the performance and identification of trends. Periodic analyses can alert operators to changes in the feed water composition and its possible impact on the RO system and facilitate pretreatment adjustments. In addition to feed water characterization, analyses such as cartridge filter, SDI (silt density index) pad digestions, and membrane autopsies provide valuable information in troubleshooting existing systems.
      Pretreatment is an essential design consideration and a key to the successful long-term performance of an RO system. Membrane surfaces are prone to fouling by particulate matter, inorganic scales (i.e., carbonate and sulfate salts of alkaline earth metals), oxides and hydroxides of aluminum and iron, organic material (i.e., humic, tannic, fulvic acids, etc.), and biological materials (e.g., bacteria, fungi). Pretreatment techniques used to alleviate these problems are nearly as varied as the problems themselves. It is often necessary to pilot test pretreatment unit operations to verify the efficacy of the process for a particular application.

Media Filters
      The most common (and oldest) means of removing solids from the feed stream is media filtration and includes: slow sand filtration; rapid downflow or upflow sand filtration; single media anthracite, garnet, or green sand filtration; or more recently multimedia filtration. Each type of media has distinct advantages and disadvantages that are important system design considerations, a summary of which appears in Table 4
      Media filters alone may not provide sufficient RO system pretreatment because colloidal suspended matter is often too small to be removed efficiently and particles may be charged such that they are repelled by the media itself. In these instances, a flocculant/coagulant may be added as a filter aid or coagulant that functions by adsorbing onto the surface of a colloid and to neutralizing the surface charge and allowing small particles to agglomerate or coagulate. Popular coagulants and flocculants include alum, other aluminum and iron salts, and synthetic cationic polymers (e.g., diallyldimethyl ammonium chloride).

Table 4. Commonly Used Filtration Media

Filter Media

Advantages

Disadvantages

Sand

Inexpensive

May not remove small colloids

Anthracite

May remove oxidants

Provides site for biological growth

Garnet

May adsorb organic material

Heavy or difficult to use

Green sand

Oxidizing media

Expensive

Cartridge Filters
Nearly every RO system is equipped with cartridge filters before the high-pressure pumps to prevent suspended matter from entering the system. Cartridge filters are available in a variety of sizes, configurations, and materials of construction. Most membrane manufacturers suggest 5-micron or smaller filters to provide adequate protection. In some cases it is beneficial to use cascade filtration, i.e., the use of larger filters followed by small ones to reduce individual filter loading by better depth filtration.

Microfilters and Ultrafilters
Microfiltration (MF) and ultrafiltration (UF) membranes have been introduced in recent years. MF and UF membranes are not as tolerant as media filters to suspended solids, they are also more expensive, and require additional equipment for their operation. However, MF and UF membranes provide consistent, good quality low SDI water which in many cases may be fed directly to an RO system with little or no additional pretreatment. Additionally, they are fairly rugged (compared to RO). It is often beneficial if the fouling problem can be transferred from the RO to MF or UF membranes. These membranes can also usually tolerate a wide range of harsher cleaning chemicals. Some MF and UF membranes can be back flushed with air or permeate water.

Membrane fouling
       The success of an RO system depends upon membrane life and performance, the repeatability and the reproducibility of the process that the membranes are designed to perform, and periodic cleaning of the membranes to restore capacity. Membranes lose performance and are replaced due to the deposition of unwanted materials on the surface. In addition, a decrease in membrane performance may be due to other factors, i.e., degradation by chemical (oxidation, hydrolysis, etc.) and/or mechanical (compaction, telescoping) processes. Extending membrane life and thereby minimizing replacement costs is a key success factor for RO systems.
      The types of foulants most commonly encountered in RO systems include:
• Inorganic fouling (scaling)
• Colloidal fouling
• Biological fouling
• Organic fouling

      Scaling of RO membrane surfaces is caused by the precipitation of sparingly soluble salts from the concentrated brine. The presence of suspended solids in the water, such as mud and silt, tends to cause gross plugging of the device rather than fouling of the membrane surface.
      Biofouling is a special case of particulate fouling that involves living organisms and can be a serious problem. Biological material growing on membrane surfaces not only causes loss of flux but may physically degrade certain types of membranes. Hydrocarbon oils (naturally occurring or as a result of pollution) have also been known to cause performance deterioration. Synthetic cationic polymers have been known to carry over to the membrane system due to clarifier upset or media filters channeling. Cationic polymers are known to be incompatible with many of the acrylic acid-based antiscalant in use today and may influence membrane performance. In addition, it has also been reported that, in high hardness water, polyacrylate based antiscalant can form an insoluble salt with calcium, thus leading to membrane fouling. Detailed descriptions of fouling phenomena and their effects on membrane performance has been described elsewhere.

Scaling-foulant control alternatives
      Several methods as discussed below are used for reducing or preventing membrane fouling caused by the deposition of mineral scales and include softening, system recovery, acid, and antiscalant / dispersant.

Softening
      Hot and cold process lime softening and sodium cycle cation exchange are commonly applied methods to remove hardness ions from feed water. Sodium salts are rarely scale forming and, therefore, can be tolerated.

Adjusting System Recovery
       In RO systems, membrane fouling by mineral scale can be controlled by operating the system under conditions where solubility of scale forming salts is not exceeded, operating the RO system at lower recovery. This technique is not always effective due to concentration gradients within the membrane not controlled by the bulk flow.

Acid Feed
      Acids are among the oldest treatments used to control calcium carbonate scale formation. Acid is injected into feed water to reduce alkalinity to prevent calcium carbonate precipitation. Normally, sulfuric acid is used and is relatively inexpensive. The use of sulfuric acid for alkalinity reduction increases the potential for sulfate scale (e.g., calcium sulfate, barium sulfate) formation. Though calcium sulfate is relatively soluble, strontium sulfate is becoming a problem in certain areas of the world and barium sulfate is extremely difficult to remove once it is formed. When acid is used to control pH, the product water is often degassed to remove the resultant carbon dioxide. Gases are not rejected by RO membranes and will pass directly into the permeate stream which decreases permeate quality.

Antiscalant/Dispersant Addition
      Nearly every RO water treatment program used today can benefit from the use of suitable pretreatment chemicals (e.g., antiscalant, dispersant, etc.). Depending on the system and treatment program, the pretreatment chemicals can be hexametaphosphate, homopolymers based, or copolymers. In some cases, proprietary blends of polymers and other scale control agents may be used to provide well-balanced treatment technology. These formulated products typically have both membrane manufacturer compatibility and National Sanitation Foundation International (NSF) potable water approvals. The mechanisms by which these antiscalant/dispersant function and associated performance data were discussed in earlier publications. Specially formulated products can provide excellent performance in controlling scaling (calcium carbonate, calcium sulfate), stabilizing metal ions (i.e., Fe, Mn, Zn, etc.), and dispersing particulate matter.
      Silica (SiO2) commonly found in ground water deserves a special comment. Silica usually exists in the weakly ionized soluble form. As soluble silica is concentrated in the RO process, it polymerizes to form an insoluble colloidal silica or silica gel that will foul membranes. The easiest method for preventing silica fouling is to reduce the conversion rate. The solubility of silica increases with increasing temperature and at high pH values. Operating at warmer temperatures may lessen the chance of silica fouling. Silica can be removed from the feed by lime softening, but it is very expensive and usually not practical unless other pretreatment requirements dictate lime softening.
      Certain polymers have been shown to be capable of dispersing fine particles of amorphous silica once they have formed. Although these polymeric dispersant may minimize the impact of the fouling, they do not address the root problem of silica polymerization. However, a recently introduced antifoulant can effectively inhibit the silica polymerization and also disperse particulate matter. Development of this new antifoulant is a major technological breakthrough as it can facilitate the operation of RO systems with concentrate (reject) streams containing greater than 500 mg/L soluble silica.

Antiscalant selection based on water chemistry
The prediction of reject (brine) and permeate chemistry based on feed water is integral to the design and optimization of RO technology. The dissolved salts that concentrate in the brine develop a scaling potential dependent on make-up water chemistry, pH, and recovery. Several predictive tools have been developed to predict the scaling potential of water. Recent developments in system simulation can facilitate prediction of scale potentials for a number of scale forming salts in RO systems. A scale inhibitor dosage model can be correlated to predict the required product (i.e., antiscalant, dispersant, etc.) level to reduce the potential of scaling using predictive modeling computer technology. The following example demonstrates the use of a predictive model for selecting a product to achieve desired performance from an RO system.
      Table 5 shows analyses of an RO system raw water, feed water (pH adjusted with sulfuric acid to depress alkalinity), product water, and brine 75% recovery. The use of an antiscalant is important in this case in order to control the precipitation of calcite and gypsum.
      If an RO system feed water contains trivalent cations such as iron or aluminum in concentrations above 0,05 ppm, it is likely that a high performance multifunctional antiscalant will be required, especially for higher LSI conditions.

Table 5. Analyses of Raw, Feed, Product, and Brine Water Streams

Parameter

Raw

Feed

Product

Brine

Calcium

152.0

152.0

10.13

577.60

Magnesium

65

65

4.33

247.0

Barium

0.40

.040

0.03

1.52

Strontium

2.20

2.20

0.15

8.36

Sodium

381.0

381.0

25.40

1447.8

Iron

0.05

0.05

0.00

0.19

Chloride

675.0

675.0

45.0

2565.0

Sulfate

401.0

444.0

29.60

1687.20

Bicarbonate

96.9

86.4

5.8

353.80

Carbonate

0.9

0.30

0.0

35.7

Dissolved CO2

5.9

17.1

1.1

4.3

pH

7.50

7.00