Reverse Osmosis

In 1959 Reid and Breton first demonstrated the ability of cellulose acetate membranes to separate dissolved salts from solution. Ever since, membrane separation technology has been expanding by leaps and bounds. Today, numerous commercial applications for membrane separations exist including high-purity water production, boiler feedwater, food and beverage processing, drinking water, waste removal and seawater desalination.

In this discussion on the fundamentals of reverse osmosis we will highlight the following topics:

  • Deionization process

  • Reverse osmosis – how does it work?

  • RO system design and operation

  • Pretreatment requirements

  • RO cleaning methods

  • Troubleshooting hints

THE DEIONIZATION PROCESS

As the universal solvent, water contains many impurities. These include substances that are dissolved or suspended, plus various dissolved gases.

Dissolved solids, as the name implies, are those impurities that are soluble in water. As water percolates through the earth’s crust, it dissolves the minerals from the local geology. These include calcium and magnesium (hardness), carbonate and bicarbonate alkalinity, and iron and manganese. Other impurities often found dissolved in water are sodium, potassium and barium salts of chloride and sulfate. Depending on the minerals found in the area, silica may also be present in significant quantity.

Suspended solids are those impurities that are not dissolved, but are carried in the water as filterable impurities. Some of these are visible to the eye, but others are very small particulate matter that can only be seen under magnification. These include sand and sediment, clays and colloidal material, oils and greases, microorganisms, and process contaminants.

Dissolved gases are also present in natural water supplies. These impurities are not visible, but are often detected by smell or taste. Common dissolved gases include carbon dioxide, oxygen and nitrogen. Carbon dioxide is common in well waters that are high in bicarbonate alkalinity. Depending on the surrounding conditions, other gases such as hydrogen sulfide, methane and ammonia may be present. Hydrogen sulfide is produced by bacteria. It imparts a rotten-egg smell even at low concentration. Methane may be present as the gas seeps into the aquifer from underground pockets of natural gas.

The goal of water treatment is to improve the quality of water by removing some or all of these dissolved and suspended impurities. Most of the suspended solids, for example, can be removed by filtration. Softening by ion exchange removes calcium and magnesium hardness along with most of the iron and manganese. Dealkalization decreases the carbonate and bicarbonate alkalinity. And deaeration is effective in reducing the concentration of dissolved gases.

Any water treatment process that removes essentially all of the dissolved and suspended solids is called deionization or demineralization. The goal of this process is to produce a water supply of exceptional purity. Two common methods for accomplishing this goal are ion exchange and reverse osmosis. In this discussion, we will review how reverse osmosis is used to demineralize or deionize water.

REVERSE OSMOSIS – HOW DOES IT WORK?

A membrane functions much like a filter, allowing water to pass through the membrane pores while preventing the passage of dissolved and suspended solids. Unlike traditional depth filtration where 100% of the flow is passed through the filter media to strain out impurities, membrane separation utilizes the principle of crossflow filtration. In this case, as feedwater flows over the membrane surface, a portion of the water permeates through the membrane with the remainder of the flow carried to waste. This separates the feedwater into two streams: a purified product water and a concentrated waste or brine. The water flow “sweeps” the membrane surface clean to prevent the accumulation of suspended and dissolved solids that would eventually block the flow of water through the membrane. The driving force for reverse osmosis is the pressure differential across the membrane. In this way, reverse osmosis can be thought of as the ultimate pressure filter as it removes chemical species as small as an ion.

If a concentrated salt solution is separated from a dilute salt solution by a semipermeable membrane (a membrane that allows water to pass through, but not dissolved salts), water will pass from the dilute solution through the membrane into the concentrated solution. This is a natural process called osmosis. As osmosis continues, the water flow will cause the level of the concentrated solution to rise. The height differential between the two columns represents the osmotic pressure. Flow will continue until the pressure exerted by the height of the concentrated solution equals the osmotic pressure. At this point the system is at equilibrium.

Osmosis can be reversed by applying pressure to the concentrated solution forcing water to flow from the concentrated solution through the membrane into the dilute solution; hence the name reverse osmosis. Since the salts in the concentrated solution can not pass through the membrane, this process results in the production of a purified water stream.

Membranes are classified based on the physical size or molecular weight of the substances that are filtered out by the membrane. Four basic membrane filtration systems exist — microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO).

Characteristic

Microfiltration

Ultrafiltration

Nanofiltration

Revese Osmosis

Particle size, nm

100 to 1000

1 to 100

1

<1

Removes

Suspended solids and large colloids

Proteins and large organics

Organics and dissolved solids

Dissolved salts and organics

Molecular weight cut off

> 100,000

1,000 to 100,000

200 to 400

100

Operating pressure

10 psig

10 to 100 psig

50 to 225 psig

200 to 800 psig

RO MEMBRANE DESIGN

A typical reverse osmosis membrane consists of a dense surface skin and a porous substructure. Salt rejection occurs at the surface skin layer with the permeate passing into the porous sublayer.

Two basic types of membranes are in commercial use – asymmetric and thin-film composit. Asymmetric membranes are formed by using the same polymer for the dense surface skin and the porous sublayer. Cellulose acetate, cellulose triacetate, and polyamide are common polymers used in asymmetric membrane manufacture.

In thin-film composite membranes the surface skin and microporous sublayer are formed from two different polymers. Commonly, aromatic polyamide is used for the surface skin with a graded polysulfone resin used in the sublayer.

Each membrane type offers certain advantages and disadvantages for the end user. Cellulose acetate membranes are lower in cost and resistant to chlorine attack. However, the membrane tends to chemically degrade (hydrolyze) outside a pH range of 5 to 8. They are also susceptible to biological degradation and, therefore, require chlorine addition to the feedwater to control bacteria growth.

Aromatic polyamide membranes offer hydrolytic stability, better salt and organic rejection and are not biodegradable. But they are higher in cost than cellulose acetate membranes and have no tolerance for chlorine.

Other membrane polymers are available, each with their own set of advantages and disadvantages. In general, thin-film polyamide composite membranes (TFC) offer certain performance advantages over cellulose acetate membranes (CA). TFC membranes have half the salt passage, higher flux rates and require half the net driving pressure of CA membranes, but they come at a higher cost.

TFC versus CA Membranes

Cellulose Acetate

Thin Film Composite

410 to 600 psi

150 to 500 psi

0 to 30 oC

0 to 45 oC

pH 4 to 6.5

pH 2 to 11

Flux 5 to 18 gfd

Flux 10 to 205 gfd

70 to 95% rejection

97 to 99.5% rejection

Chlorine stable

Chlorine intolerant

Lower cost

Higher cost

There are seven US manufacturers of reverse osmosis membranes and one manufacturer of electrodialysis membranes. These companies have engineered the various membrane materials into unique reverse osmosis element designs. The four basic types of RO element design are (1) tubular, (2) plate-and-frame, (3) spiral wound, and (4) hollow fiber. Tubular and plate-and-frame designs represent higher initial cost and lower membrane surface area per unit. As a consequence, spiral wound and hollow fiber elements dominate the water treatment marketplace.

In a spiral wound membrane design, two layers of membrane material are glued to a permeate collector fabric. Plastic mesh is used to form a feedwater channel between the membrane layers. These layers of membrane, permeate collector and feedwater spacer are rolled around a hollow, perforated center tube that collects the product water.

The spiral wound membrane module is inserted into a pressure vessel housing. Several membrane elements can be linked together inside a single pressure vessel. High pressure feedwater is directed into the end of the element. The permeate is collected in the permeate channel and flows toward the center tube. The concentrated brine exits at the other end of the element.

The hollow fiber membrane module (also called a permeator) incorporates a bundle of hollow fiber membranes in a single element. Feedwater enter the inside diameter of the hollow fiber with the permeate collected in the perforated center tube. Alternatively, hollow fiber modules are designed with ouside-in permeate flow. Feedwater that is not recovered as produce exits as concentrate.

Hollow fiber elements offer an advantage over the spiral wound design in that they pack a large membrane surface area in a single element. The spiral wound membrane type is used more often than the hollow fiber design because of lower operating pressures and energy requirements. Because of this, hollow fiber membranes have been used sparingly since the 1980’s except for seawater desalination projects.

RO MEMBRANE OPERATING CHARACTERISTICS

The operating characteristics of the various membrane materials are best defined by the percent rejection of salts at the membrane surface, the flux rate, percent recovery, differential pressure, net driving pressure, and normalized permeate flow. These parameters are also useful for monitoring the performance of RO systems in service and for scheduling routine cleaning.

Percent rejection defines the ability of the membrane to remove dissolved salt from solution. An ideal membrane would allow only water to pass through it, rejecting 100% of the dissolved salts and organic molecules at the membrane surface. In actual practice, RO membranes are not perfect and allow some passage of salts and low molecular weight molecules through the membrane. Typically, 1 to 2% of the salts in the feedwater will pass through. This is equivalent to 98 to 99% salt rejection. A 1.5% rejection rate is considered very good. The passage of low molecular weight organics can be significantly higher, up to 75% in some cases. On average, expect a 95 to 99.5% rejection rate for most inorganic salts.

Flux rate refers to the production capability of the membrane; that is, the amount of product water produced per square foot of membrane surface area per day (gfd). The membrane flux rate increases with increasing driving pressure and feedwater temperature. Great advances have been made in increasing the flux rate. In 1970, a typical production rate from a 4-inch membrane module was 375 gallons per day (gpd) at 600 psig with a 97% rejection rate. Twenty years later, production rates have increased to 1800 gpd at 225 psig with a 97% rejection rate. Typical flux rates for cellulose acetate membranes are 5 to 18 gfd and 10 to 205 gfd for thin film composite membranes.

Membrane flux rates are measured at a standard temperature of

77 oF. Lower feed water temperatures reduce the flux rate. Warmer water increases the flux rate. For every 1 oF change in temperature, the flux rate increases or decreases by 1.5% of rated capacity.

Percent recovery indicates the amount of feedwater that is recovered as treated product water(permeate) With cross-flow filtration some of the feedwater is lost as concentrated brine. Typically, 75 to 80% of the feedwater is recovered as permeate with 20 to 25% being rejected to waste as brine.

Differential pressure measures the pressure drop between the feedwater and the concentrate (brine). Fouling or scale formation in the feedwater flow channels will cause an increase in differential pressure across the RO system. Routine monitoring of the pressure differential helps determine the degree of buildup.

Net driving pressure indicates the pressure drop between the feedwater pressure and the permeate pressure. This is the pressure differential across the membrane. An increase in net driving pressure indicates trans-membrane fouling.

Normalized permeate flow corrects the measured permeate flow back to standard operating conditions at 77 oF. Warmer feedwater temperatures produce a higher flux rate, and colder feedwater reduces the flux rate. Normalized permeate flow calculations compensate for these differences in feedwater temperature by applying a temperature correction factor (supplied by the membrane manufacturer) and changes in net driving pressure to the measured permeate flow. This permits an apples-to-apples comparison between permeate flow rates under varying conditions.

NPF = (NDPnew/NDPnow) x TCF77 X Fp

NPF = normalized permeate flow

NDPnew = net driving pressure when new

NDPnow = net driving pressure when tested

TCF77 = temperature correction factor

Fp = permeate flow in gpm

A reduction in normalized permeate flow indicates fouling of the membrane surfaces.

PRETREATMENT REQUIREMENTS

Both spiral wound and hollow fiber membranes are susceptible to fouling. Feedwater impurities and over concentration of salts in the brine stream promote the accumulation of scale and foulants on the membrane surface. This causes reduced flux and higher operating pressures. For these reasons, pretreatment of the RO feedwater is required to maintain the operating efficiency of the system and prolong the useful life of the membranes.

A water analysis, Langelier Saturation Index (LSI), and Silt Density Index (SDI) are used to determine the precise pretreatment requirements for a particular RO system. Since water supplies vary considerably from one location to another, each pretreatment requirement will be different. The following pretreatment methods are commonly used in RO systems.

Carbon filtration is used to remove chlorine and organic molecules from the RO feedwater. RO systems that are made with cellulose acetate membranes are susceptible to attack by bacteria that feed on the cellulose acetate material. To minimize this problem, CA membranes require 0.3 to 1.0 ppm chlorine residual in the feedwater for bacteria control. Thin-film composite membranes, however, are intolerant of chlorine. Systems that use TFC membranes are designed with pretreatment provisions that remove chlorine and other oxidizing agents like ozone or permanganate. Carbon filtration accomplishes this task easily.

Carbon filters have an infinite capacity for chlorine and produce a final effluent of less than 5 ppb total chlorine. Activated carbon is also capable of removing total organic carbon (TOC) and silt, which cause fouling problems on the membrane surface.

Several grades of activated carbon are available. The carbon media should be made from higher rank coals like sub bituminous or bituminous instead of lignite-based coal. Lignite coals do not offer the same abrasion resistance as the bituminous coals and are more likely to produce carbon particles in the filter effluent. These carbon fines, if they get into the RO module, will cause fouling and a loss of flux. When specifying activated carbon, ask for acid washed carbon media made from bituminous coal having an Iodine Number of at least 800.

Multimedia filtration is used to reduce the suspended solids and colloidal material in the feedwater. These filters contain at least four types of media. The light coarse material is on top of the filter bed followed by progressively less coarse material in the lower layers. This design permits the unit to function as a depth filter by removing the filterable solids throughout the depth of the bed rather than just on the surface of the media. A properly designed multimedia filter will have a flow rate of not more than 7 gpm per square foot of surface area and will have a particle size cutoff of 10 microns.

At times it is necessary to inject a polymer coagulant upstream of the multimedia filter to enhance the removal of the colloidal material. These polymers are high molecular weight materials, normally cationic (positively charged), which coagulate the fine colloids and suspended solids into larger particles that are readily removed in the filter.

Softening the RO feedwater by ion exchange is a popular method for reducing mineral scale formation on the membrane surface. Sodium softening exchanges sodium for scale-forming ions such as calcium, magnesium, barium, strontium, iron and aluminum. Sodium forms very soluble salts that do not form mineral scales on the membrane surface or feedwater flow channels.

A sodium softener is regenerated with sodium chloride brine. The spent regenerant along with the softener rinse water is discharged to waste. This, along with the concentrated brine stream from the RO, can add significantly to the total waste flow from the RO system. For this reason, ion exchange softening should be considered only on very high hardness feedwater or those waters containing appreciable amounts of barium or strontium.

Cartridge filters are installed ahead of the RO high pressure pumps to remove any last traces of suspended solids or biomass that pass through the multimedia or carbon filters. A 5 micron absolute-type filter consisting of a spun filament depth filter with a pore size gradient is generally recommended for this type of application.

Dechlorination, using a chemical reducing agent, is sometimes required to remove the last traces of chlorine or other oxidizing agents prior to the RO membranes. Sodium bisulfite or sodium metabisulfite is used for this purpose. Sodium bisulfite reacts with chlorine to produce sodium sulfate, which is rejected by the RO membrane into the concentrated waste stream.

Acid injection may be required to control the pH and minimize the scale-forming tendency of the feedwater. Acid injection is indicated if the Langelier Saturation Index (LSI) of the brine stream is above +3.0. Either sulfuric or hydrochloric acid is used for this purpose. Sulfuric acid is less costly, however, and therefore more commonly used.

Antiscalants are sometimes effective in extending the intervals between chemical cleanings of the RO membranes. These products are formulated to include inorganic phosphates, organophosphonates, and dispersants. Use antiscalant products that are approved by the membrane manufacturer and follow all directions in applying and controlling the product dosage.

Some antiscalants contain negatively-charged polymers and dispersants that react with cationic polymers that might be injected upstream prior to the multimedia filters. The antiscalant must be compatible with these polymers to avoid any adverse reactions that would foul the membrane.

MEMBRANE CLEANING METHODS

As a membrane system continues to operate, the dissolved and suspended solids in the feedwater accumulate along the membrane surface or in the feedwater flow channels. As these solids build up, they restrict the passage of water through the membrane. This causes an increase in driving pressure or a loss of flux.

In the early days of RO operation, little was known about the impurities that cause membrane fouling. Today, many of these impurities have been identified. Autopsies of failed membrane modules have revealed a build up of mineral scales like calcium carbonate, colloidal materials like clays and silica, dead and living microorganisms, carbon particles, and chemical attack by oxidizing agents such as chlorine, ozone or permanganate. Likewise, dissolved metals like iron and aluminum, whether naturally occurring or added as a coagulant, will cause premature fouling and failure of the membrane.

Despite all efforts to protect the RO system from fouling and loss of flux, eventually the membranes will require chemical cleaning. Continuous monitoring of the RO operating parameters is required to pinpoint when clean is required. The following guidelines are useful in determining the best time to clean

  • When the pressure differential increases by 15%

  • When normalized permeate flow decreases by 15%

  • When the salt rejection rate decreases by 15%

Under ideal conditions, assuming the RO pretreatment system is properly designed and operated, the frequency between membrane cleanings should be 6 months or longer. Cleaning every 1 to 3 months is considered a fair performance and suggests that some improvements in the pretreatment systems are required. Cleaning frequencies every month or more indicate a change in raw water quality, a problem with the pretreatment system or in the operation of the RO unit.

A well-designed RO system will include provisions for a cleaning skid. The skid should include a chemical tank, solution heater, recirculating pumps, drains, hoses and all other connections and fittings required to accomplish a complete chemical cleaning. Specifications and drawings for a cleaning skid are available from the equipment manufacturer or membrane supplier.

Fouling impurities fall into one of two categories: inorganic deposits and organic foulants. Inorganic deposits include calcium salts, metal oxides, colloids and silica. Organic foulants include biofilms and organic molecules. The chemical nature of the foulant determines the best cleaning chemicals for the job. Acid cleaners are used to remove inorganic deposits and alkaline cleaners are required for organic deposits.

The cleaning method is best determined by an assessment of the chemical nature of the deposits. An autopsy of a sacrificial membrane is required the first time around to verify the foulant. The most effective cleaning chemicals are selected based on the results of the autopsy analysis.

Membrane Cleaning Chemicals

Cleaning Chemical

Effectiveness

0.1% (W) NaOH

0.1% (W) Na-EDTA

pH 12

30 oC max

Best for biofilms

OK for silica and organics

0.1% (W) NaOH

0.1% (W) Na-DSS

pH 12

30 oC max

Good on biofilms, organics and inorganic colloids

1.0% (W) STP

1.0% TSP

1.0% (W) NaEDTA

Good for biofilms and organics

0.5% (V) HCl

Best for inorganic salts

0.5% (V) H3PO4

Good for metal oxides

OK for inorganic salts

2.0% (W) Citric acid

OK for inorganic salts

0.2% (W) NH2SO3H

OKk for inorganic salts and inorganic colloids

2 to 4% (W) Na2S2O4

Good for metal oxides

2.4% (W) Citric acid

2.4% (W) NH4F-HF

pH 12

30 oC max

Best for silica

OK for inorganic salts and colloids

(W) indicates percent by weight (V) indicates percent by volume

NaOH sodium hydroxide

NaEDTA sodium ethylene diamine tetraacetic acid

NaDSS sodium dodecylsulfate

STP sodium triphosphate

TSP trisodium phosphate (Na3PO4– 12H2O)

HCl hydrochloric acid

H3PO4 phosphoric acid

C3H3(OH)(CO2H)3 citric acid

NH2SO3H sulfamic acid

Na2S2O4 sodium hydrosulfite

Each cleaning procedure is unique depending on the nature of the foulant and the system operation. In general, it is best to remove organic foulants first with an alkalinity cleaning (high pH) followed by an acid cleaning (low pH) for inorganic scale and metals removal.

Begin the alkaline cleaning by filling the CIP (clean in place) tank with permeate. Circulate the permeate through the RO reject line and back to the tank at low pressure (40 to 60 psig). Heat the water to approximately 80 to 90 oF and slowly mix the cleaner to the desired strength. Alternate circulating and “soak” cycles at 5 to 30 minute intervals for up to 4 hours, if necessary. Repeat with a fresh solution of cleaner if the membranes are severely fouled.

After the alkaline cleaning is done, flush the system with permeate and neutralize any residual alkaline cleaner with mild acid, if necessary. The pH should be neutral (pH 6 to 8). Refill the CIP tank with fresh permeate and heat to temperature. Mix the acid cleaner to the required concentration and alternate circulating and “soak” cycles at 5 to 30 minutes intervals as described for the alkaline cleaner. Monitor the cleaner pH and add more acid to “sweeten” the solution if the pH drops by 0.5 pH unit. Once the acid cleaning step is complete, flush the system with fresh water and neutralize to pH 6 to 8.

Start the RO and operate normally with the permeate discharged to drain until the conductivity of the permeate returns to normal operating levels. Place the system back into service after the proper water quality is achieved.

DISPOSAL OPTIONS FOR CONCENTRATED BRINE

Reverse osmosis produces a high purity water stream and a concentrated brine or reject stream. Approximately 75 to 80% of the RO feedwater is recovered as useful permeate, but 20 to 25% of the feedwater ends up as waste. For a 75 gpm system operating 24 hours per day, this is equivalent to 250,000 gallons of brine per week.

Finding a suitable method for disposal of the brine stream is one of the challenges of RO design. The salt content of the brine stream is approximately 4 times that of the feedwater. For most installations, however, the reject is still of sufficient quality to permit discharge directly to the environment by one of the following options.

  • Surface water discharge

  • Deep well injection

  • Spray irrigation

  • Municipal waste water treatment plant

  • Evaporation

  • Drain field and bore holes

In many cases, the water is of sufficient purity that it can be recycled and reused prior to discharge. Typical designs for recovering RO reject include capturing the brine stream in a storage tank for use in washing and rinsing floors and equipment or as backwash water for filters. Reuse is the best option as it conserves water resources and improves the overall efficiency of RO operation.

SUMMARY

Reverse osmosis is a useful technology for producing high purity water for many industrial, commercial and residential applications. Proper design, operation and maintenance of the RO system is required to insure minimal problems with membrane fouling, shortened membrane life, and increased operating and maintenance costs. Proper pretreatment is required to prolong the useful life of the membranes. Eventually, however, cleaning is necessary to restore the design flow rates and pressures. If this is done properly, a reverse osmosis system will provide a continuous source of high purity water for 5 to 7 years or more between membrane replacements.

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