There are basically two types of commercial membranes used in RO applications today: cellulose acetate (CA) and thin film (TF). The former is considered an integral membrane, the latter a composite membrane.
The following is basic criteria to compare one type to the other:
1.- CA membranes
CA initially achieved acceptable results with brackish water, but never with seawater because of the compressibility of the membrane itself under the necessarily high pressures. Today, its pH, temperature and performance limitations have reduced its overall popularity. However, CA's superior chlorine and fouling resistance still make it the membrane of choice for many applications.
2.- TF membranes
Invented in the 1980s, TF membranes were a breakthrough in achieving flows and rejections suitable for seawater desalination. In general, these composite membranes consist of a polysulfone membrane as support for the very thin polyamide layer.
Most desalination applications use a two-layer configuration, while process applications often select three-layer configurations for extra durability and performance. TF membranes have good temperature and pH resistance, but typically are not tolerant of oxidizing environments, especially chlorine.
The following are the range of TF membrane elements used for desalination, each offering slightly different features and benefits:
Brackish water - low-pressure
Brackish water - high rejection
Brackish water - super high rejection
Sea water - high rejection
Sea water - high flow
Membrane element construction
Numerous construction options are available that can maximize element performance:
The center tube (CT) - Also called the permeate collection tube, this component is the center of the element around which membrane leaves, permeate carrier and spacer material is wound. It is perforated to allow the permeate to flow spirally through the permeate carrier to the center of the element.
The center tube provides structural strength to the element, as well as integrity toward thermal and chemical impact from the working environment. It is extremely important to choose the right material for the center tube. Materials typically available include:
Both Noryl and ABS materials typically are used in low pressure, ambient temperature environments with few chemical compatibility problems.
PVC typically is selected when an inexpensive material is needed for high-pressure, seawater applications.
This material is used under conditions of wider temperature and pH ranges, and is resistant to many chemicals.
Aluminum is used in extremely high-pressure environments.
6. Stainless steel
Stainless steel is also used in extremely high-pressure environments when chemical resistance is also required.
Anti-telescoping device (ATD) - Normally attached to the ends of the element, ATDs fill the space between elements in a pressure vessel, center the element within the housing and facilitate the flow of feedwater from one element to the next. Facilitating this flow helps prevent pressure buildup and subsequent telescoping and unrolling of the element.
The device can be bonded to the center tube or loosely attached to the center tube to form an integrated part of the inter-connector. The material chosen is usually the same as for the center tube.
The inter-connector (IC) - The IC connects the center tubes of neighboring elements and directs the flow of the permeate to one or both ends of a pressure vessel.
The material of the inter-connector is normally the same as that of the center tube and anti-telescoping device, but can also be another material with similar physical and chemical resistance.
The inter-connector seals into the center tube by 0-rings or other methods of sealing. The materials of the seals need to be pliable and are chosen to conform with the operating environment.
Buna is typically chosen for routine pure water applications, EPDM is selected for more chemical-intensive process applications, and Viton** typically is designated for highly acidic applications.
The product end adapter (EA) and dead-end plug (DEP) - Either an open-end adapter or a closed dead-end plug is used, depending on whether permeate will flow from both ends or one end of the element.
If the permeate volume is high, then it may be preferable to discharge from both ends, requiring two end adapters.
The more common configuration is to discharge permeate from only one end of the housing, with one end adapter and one dead-end plug per housing.
The materials used for the end adapter and dead-end plugs are chosen with the same requirements as for the inter-connector, and the same holds true for the sealing material.
The permeate carrier - This is a sheet of material inserted between the backsides of the membranes, forming a membrane envelope to promote the flow of permeate towards the center tube for discharge at the ends of the pressure vessel.
The permeate carrier material must be able to withstand the pressure of operation without collapsing and blocking the flow, and the surface of the permeate carrier must be smooth to prevent intrusion of the membrane backing material into the permeate carrier.
For most normal operating pressures - up to 600 pounds per square inch (psi) - a polyester knit tricot stiffened with polymeric materials is used. At pressures up to 1,500 psi, combinations of tricot and other polymeric material are used.
For extreme conditions of pressure, temperature or aggressive environments, various patterns of a metallic web or netting can be used.
The feed spacer - This is the material inserted between neighboring membrane surfaces to create the best possible flow conditions over the membrane.
Some of these conditions are high viscosity, suspended solids, high temperature, presence of fouling species, and precipitation or crystal formation. Feed spacers available include:
The diamond pattern feed spacer - This spacer is the most popular, promoting a high degree of turbulence over the membrane surface under fairly clean water conditions.
The parallel pattern feed spacer - This configuration helps prevent fouling from suspended solids without sacrificing turbulence. It is somewhat of a hybrid between the diamond and corrugated feed spacers.
Corrugated feed spacer - This spacer forms discrete flow channels over the membrane surface along the length of the element, providing the spiral wound element with some of the characteristics of tubular and plate-and-frame designs. It is used when high levels of solids are causing element plugging.
All of these feed spacers are normally made of polymeric material, but when the environment demands it, feed spacers of a metallic web or netting can be selected.
The outer wrap - This is an important selection depending on the use of the element. The tape-wrapped option is used for light duty cases, such as low pressure water purification. A fiberglass wrap will support the structural integrity of the element under higher pressure applications. A polypropylene mesh outer wrap can be used for sanitary conditions.
Adhesive sealant - Standard types of adhesive sealant typically are used to form membrane envelopes of high mechanical integrity. Only under extreme operating conditions will alternative adhesives be specified.
The choice of membrane types and element construction options today are numerous, allowing dealers and designers wide flexibility in addressing various application challenges. Selecting the right configuration will not only optimize performance today, but will avoid problems tomorrow.
Inge Bisconer is marketing communications manager for Osmonics Desal, Vista, CA.
Bjarne Nicolaisen, Element Construction, Desal Membrane Product, October 1996.
Jorgen Wagner, Membrane Filtration Handbook - Practical Tips and Hints, Wagner Publishing, August 1997.
Munir Cheryan, Ultrafiltration Handbook, Technomic Publishing Co. Inc., 1986.
Pure Water Handbook, Osmonics, 1997.
Reprinted with permission from Water Technology. For more information, contact Editor, Water Technology, 13 Century Hill Drive, Latham. NY 12110; (518) 783-1281.
*Noryl is a trademark of General Electric Company.
**Viton is a trademark of E.I. DuPont deNemours and Company, Inc.