Crossflow Membrane Filtration High-Purity Water Systems

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INTRODUCTION

As the electronics industry continues to provide us with such products as high resolution television, notebook size portable computers and cars with automatic navigation systems, it becomes necessary to produce semiconductors with finer device geometries to allow for efficient placement of more circuits per a given area. This requires increased control of a semiconductor's material characteristics by eliminating impurities in the circuit material or on its surface.

Likewise, amazing drugs such as monoclonal antibodies and immunoglobulins used for the treatment of cancers, must meet the stringent pharmaceutical industry purity requirements. This need for increasingly precise and controlled manufacturing capabilities in a variety of industries has a common ingredient: high-quality purified water.

This paper discusses the acceptance of crossflow membrane filtration into these high-purity water applications. General system design considerations such as pretreatment and membrane selection are covered and the performance history of two operating systems is reviewed.

Several crossflow membrane device configurations exist: plate and frame, hollow fiber, tubular and spiral-wound. This paper references only spiral-wound membrane elements (sepralators) which are the most common configuration in the marketplace. Although all configurations have specific applications for which they are useful, sepralators provide the best combination of advantages for high-purity water applications. These specifically are: cleanability and fouling resistance, high packing density and low operating costs.

MEMBRANE TECHNOLOGY

Crossflow membrane filtration was first developed in the 1950's with initial focus being desalination. As the technology developed, many other uses were found: milk and juice concentration, metal and electro-deposition paint reclamation and waste water purification.

Membrane filtration involves separation of dissolved, colloidal and particulate constituents from a pressurized fluid using a microporous material (usually polymeric media). The size of the polymer's pores categorizes the membrane into one of the following groups: reverse osmosis, nanofiltration, ultrafiltration and microfiltration. Reverse osmosis (RO) membranes have the smallest pore size ranging from 5-15 angstroms, nanofiltration (NF) covers separations in the 15-30 angstrom size, ultrafiltration (UF) removes organics in the 0.002-0.2-micron range and microfiltration (MF) effects separation in the 0.02-2.0-micron range.

RO is both a mechanical and chemical filtration procedure in which the membrane's surface sieves organics and actually repels ions. The dielectric repulsion of ions from the membrane is influenced by the ion's charge density. Unlike RO membranes which have salt retentions of 80-99%, NF membranes reject 30-60% salts. UF and MF membranes have even larger pores and therefore pass most of the salts. Although a simplification, they can be considered a straightforward filtration mechanism which removes organics based on their size and shape.

In crossflow membrane filtration, two effluent streams are produced, the permeate and the concentrate. The permeate is the purified fluid that has passed through the semi-permeable membrane. The remaining fluid is the concentrate which has become enriched with organics and salts that could not permeate the membrane.

PHARMACEUTICAL CROSSFLOW MEMBRANE APPLICATIONS

The United States Pharmacopeia (USP) lists either RO or distillation as final treatment processes for both purified water (PW) and water for injection (WFI). However, difficulties in acceptance of crossflow filtration technology arise in the pharmaceutical industry due to validation concerns. Systems incorporating membrane filtration are relatively new, innovative designs. Understandably, pharmaceutical manufacturers have a conservative approach, tending to incorporate process designs which have already been approved and accepted. For them, this hopefully results in fewer difficulties with the Food and Drug Administration (FDA) validation procedure which can be a lengthy and costly process. However, the effectiveness and economics of using membrane filtration often outweighs these concerns. As more and more USP PW and WFI systems incorporate membrane filtration and become validated, the acceptance of crossflow membrane filtration continues to grow.

There are several areas in conventional USP systems where membrane filtration can replace a specific component or enhance an already existing treatment process. A properly designed and maintained RO system will consistently produce USP PW and WFI water from a variety of feedwater qualities. Two-pass RO configurations are frequently used to ensure these specifications are met with a wide margin of safety. Two-pass RO involves feeding permeate from a first staged RO machine into a second RO unit.

For both PW and WFI, RO and UF are also used as pretreatment to final process equipment. If a still is used, pyrogens can carry over into the distillate because of their small molecular weight.5 Double distillation will eliminate this problem but requires very high energy consumption. The use of UF to remove pyrogens prior to distillation is an economical alternative to double distillation. RO will also remove the salts which result in scale build-up in a still. Another pretreatment application is to position RO prior to ion-exchange (ion-exchange is permissible as final treatment for PW). This reduces the load on ion-exchange resins and increases the economy of ion-exchange dramatically.

PHARMACEUTICAL HIGH-PURITY WATER SYSTEM DESIGN

For RO and UF, the selected membrane type is the main component around which the rest of the system is designed and built. To achieve USP bacterial and pyrogen standards, a durable membrane is required which, in addition to having good rejection capabilities, can easily and frequently be sanitized.

Two membrane polymers have proven to be the most effective for RO: cellulosic (CA) and polyamide (PA). Each has specific advantages and disadvantages which must be fully understood and designed for in the system's hardware as well as in its long-term operation and maintenance plans.

CA is chlorine tolerant and can be exposed on a continuous basis to up to 2.0 ppm of chlorine versus less than 0.1 ppm for PA. CA can also tolerate shock dosages of 20-25 ppm chlorine weekly for approximately 30-minute periods. Therefore, for applications with bacterial concerns such as USP water systems, it tends to be the most effective membrane type to work with. A disadvantage of CA is that it can require higher operating pressures than PA [typically 400-500 psig (27.6-34.5bar)] versus 200-300 psig (13.8-20.7 bar). PA also has higher salt rejection (98-99%) than CA (97-98%). However, CA sepralators can be 40-60% the price of PA which can result in a substantial savings.

Since CA membranes are chlorine tolerant, they can be exposed to the low levels of chlorine residual found in most city water supplies. These levels are usually higher than the recommended chlorine range for PA systems and therefore must be removed. Common methods of chlorine removal are granular activated carbon (GAC) and sodium metabisulfite injection. GAC filtration is a higher capital cost. However, they are more reliable since chlorine breakthrough is gradual in contrast to a chemical feed pump breaking down. Another drawback of GAC is its well known potential for becoming a micro-organism breeding site. This can increase the bacteria load to an RO and raise the possibility of downstream bacteria growth. If GAC is incorporated before an RO unit in a USP system, a sanitization step such as ultraviolet light (UV) should be used prior to the RO.

Pharmaceutical water treatment systems may also use GAC to remove organics. Since RO removes the majority of organics above 100-200 Molecular Weight, GAC filters are only necessary upstream of an RO unit if organic removal below the 200 Molecular Weight size is required.

High-temperature sanitization is a common procedure in USP systems. Many pharmaceutical manufacturers sanitize all components in their production loop at temperatures of 82°C (180°F) or higher. These high temperatures help ensure that all vegetative bacteria are killed. This procedure is not compatible with the crossflow membranes. Under normal operating pressures, the standard manageable temperatures for CA are 29°C (85°F) and with special design they can withstand 110° F maximum. When designed for high temperatures, PA membranes can be run up to 60°C (140°F) at low pressures which allows for a modified lower heat sanitization procedure to be used. This is believed to be effective against most bacteria found in high-purity water. However, membrane life may be shortened significantly. Therefore, to obtain maximum sepralator life, membrane manufacturers recommend special system design measures in conjunction with chemical sanitization over heat sanitization.

For USP PW and WFI crossflow systems, the permeate plumbing should be of sanitary design with special attention given to: sloped construction to permit complete system draining, sizing to ensure turbulent flow rates, connections which provide ease of disassembly for cleaning, the use of sanitary sample valves and elimination of dead legs, threads, crevices and leachable materials including joining and gasket compounds. Usually 304 or 316 stainless steel with 180 grit polish plumbing is used as most plastic polymer piping has been found unsuitable for meeting USP requirements.3,4 Stainless steel 'L' grade is recommended where welds are present. Non-permeate contact plumbing such as the feed and concentrate lines can be of PVC construction to reduce cost, unless high temperature sanitization is used. This is true even if the RO unit is run at low pressures during heat sanitization.

In addition to plumbing, sepralator design is another area where dead flow regions can be a concern for USP RO systems. The original standard sepralator designs have fiberglass (FRP) outer covers which enclose the membranes, and 'brine seals' which direct the incoming fluid into the element. Cleaning solutions, bacteria and other contaminants can become trapped downstream of the brine seal, between the outer FRP cover and the sepralator housing. The concentrate seal prevents this stagnant water from being flushed out or cleaned with chemicals. The membrane manufacturer's solution to this problem is the Full-Fit™ sepralator, designed with no outer FRP cover nor brine seal. When placed in a housing they expand to completely fill the vessel, eliminating the dead flow areas. Full-Fit sepralators were pioneered five to six years ago and today are the design of choice for applications that demand extreme purity and low microbial growth.

USP CASE HISTORY

A large southern-based cosmetic manufacturer has successfully used UF followed by RO to produce 30 gpm at 13°C (55°F) of USP PW water for their production process since 1987. This system replaced an ultraviolet sterilization unit which could not meet the required bacterial standards. It was assumed that the high silt and particulate levels in the feedwater source shielded the bacteria from the ultraviolet light. The customer wanted to avoid sand filtration and the UF served as the only pre-treatment to the RO machine. The city water feed source was surface water at 50 microsiemens.

Although the manufacturer had recommended CA membrane be used in the RO unit, the customer chose PA. This decision was based on the perception that the slightly better rejection PA offered over CA would be necessary to meet their water quality requirements. At this time PA was considered 'state-of-the-art,' being touted as a superior membrane to CA and viewed, in an over-simplified manner, as an improvement. The RO sepralators were originally supplied with the standard outer cover and brine seal design. Because the city water had residual chlorine, sodium metabisulfite was required prior to the RO. The UF contained polysulfone Full-Fit sepralators.

For their cosmetic production process, the customer required water containing less than 100 colony-forming units (CFU) per 100 mL sample (tighter than USP PW specification). In addition, they were required to have zero pathogens per 100 mL. After one month of running with PA sepralators and no continuous biocide, these requirements were not being met by the system.

Rigorous cleaning and sanitization efforts by the membrane manufacturer followed. Some by-pass around the concentrate seals was introduced, and several chemical cleaning procedures were conducted on both machines involving chlorine sanitization (in which the RO membranes were removed), citric acid/sodium bisulfite cleaning and hydrogen peroxide/peracetic acid sanitization. All of this required the system to be shut down for about five to six hours. After cleaning, bacterial readings were reduced to acceptable levels, but within 48 hours the system was again out of specification.

A day later another sanitization attempt was made after which bacterial readings were 0 CFU per 100 mL. However, within 24 hours of operation high levels of bacteria colonies were re-established. This demonstrated the necessity of a continuous biocide if low levels of bacteria are required.

A decision was made to replace the PA sepralators with Full-Fit CA sepralators and incorporate chlorine injection at a 0.2 ppm dosage level. Once these design changes were implemented, the RO successfully met all USP requirements and is still running satisfactorily, four years later, producing less than 1 microsiemen water through a circulating loop. Furthermore, the UF removes particulate matter prior to the RO unit so effectively (virtually 100%) that the manufacturer has not found it necessary to clean the RO more than every six months. Typical cleaning periods are every two to eight weeks.

ELECTRONICS CROSSFLOW MEMBRANE APPLICATIONS

In the electronics industry, crossflow membrane technology is now routinely incorporated into applications which produce semiconductor rinse water and those in which the chemicals for circuit chip manufacturing processes are made.

As in pharmaceutical water treatment systems, single and multiple-pass RO configurations are a means of primary dissolved solids removal for the electronics industry, both alone and prior to ion-exchange. This allows the desired water quality to be achieved more economically and without several combinations of twin-bed and mixed-bed deionization.

For electronics production, the required water quality required typically ranges from 5 to 18 megohm/cm with detail constituent specifications listed in Semiconductor Equipment and Materials Institute (SEMI) and ASTM Electronic Grade Water Guidelines. Two-pass RO systems are presently capable of producing 0.5 to 2.0 megohm/cm water. Therefore, a final polishing step is required to boost the RO water purity to the 5+ megohm/cm range. For feedwaters in the 50-150 ppm TDS range, the cost of operation is usually fairly equal between twin-bed ion exchange and RO/mixed-bed ion-exchange, depending on chemical and utility costs (see Figure 1). Above this level, operating costs for twin-bed ion-exchange increase greatly, making two-pass RO/ion-exchange the preferred system design.

Since RO also removes organics, resin fouling due to negatively charged particles such as humic or tannic acids is reduced when a unit is positioned upstream to ion-exchange. This type of organic fouling hinders full regeneration, reduces resin life and is a major operational problem and cost for ion-exchange equipment.

UF can be used as a final filtration step, replacing expensive sub-micron filters previously used to prevent resin fines from passing into the manufacturing process.2 The UF also removes colloidal material, pyrogens and large organic solutes from the final polishing loop.

ELECTRONICS HIGH-PURITY WATER SYSTEM DESIGN

Silica is often a critical concern in electronics manufacturing applications, not only because it can cause loading on ion-exchange equipment but even more seriously, it can cause defects in semiconductor chips. Both CA and PA reject 100% of the colloidal silica with PA having somewhat higher ionic silica rejection capabilities than CA (98-99% versus 90-95%). Therefore, if silica removal is a primary objective, there is yet another factor to consider when selecting a membrane type.

As discussed for USP systems, sanitization concerns exist for PA due to bacterial membrane fouling which results in flow loss and system contamination. One solution is to use a CA/PA two-pass RO system. This approach allows chlorine feed to the first-pass. Sodium metabisulfite can then be added between stages to eliminate residual chlorine in the first-pass permeate and protect the PA second-pass. The first-pass RO water is of such good quality that bacterial fouling of the PA membrane is no longer a concern.

ELECTRONICS CASE HISTORY

In California, a magnetic recording wafer manufacturer utilizes a 35 gpm two-pass RO in their electronics-grade ultrapure water system (see Figure 2). The system consists of chlorine addition, manganese greensand dual-media filtration, CA membrane first-pass RO, forced-draft degasification, sodium metabisulfite injection, PA membrane second-pass RO and mixed-bed deionization. Both RO stages are mounted on a single 134'L x 72'D x 72'H skid which also holds a prefilter cartridge housing (five-micron), pumps, motor starters and instrument panel.

Chlorine injection is not only done to control microbiological growth but also to oxidize trace dissolved iron or manganese prior to the dual-media filter. Iron and manganese can cause serious RO membrane fouling. Feed levels to an RO are required to contain iron and manganese levels of less than 0.2 ppm and 0.05 ppm respectively.

Once oxidized, the metal particulates precipitate and are filtered by the dual-media filter prior to the RO machine. The manganese oxide coating on the greensand grains also has the capability to oxidize iron and manganese. This provides the system with a temporary back-up should the chemical feed pump malfunction or run out of chlorine. Used continuously, however, the greensand would require regeneration.

As in most crossflow membrane applications, acid injection is used in this system to prevent precipitation of calcium bicarbonate scale on the membranes. This converts alkalinity to carbon dioxide and pH levels are usually reduced to 5.0-6.0. Since alkalinity is effectively rejected by the RO whereas carbon dioxide passes, acid injection can result in increased loading to downstream ion-exchange beds. Forced-draft degasification is used downstream of the first-pass RO to remove carbon dioxide and lengthen ion exchange runs between regenerations. A Hepa-type filter is positioned on the degasifier air intake to prevent bacterial contamination.

The two-pass RO machine was designed to provide complete isolation of both passes. This is a necessary requirement for CA/PA systems since cleaning chemicals that are compatible with one membrane type can be harmful to another.

The CA/PA two-pass RO system produces 3.5 microsiemen permeate from a 1066 microsiemen feed source. Following mixed-bed deionization, the required 16 to 18 megohm/cm quality water is sent to various wafer electroplating, etching and rinse sites. Ion-exchange regeneration is necessary only every two months versus every two to three days for the old system.

SUMMARY

Industries which require high-purity water can reduce costs and improve water quality by incorporating crossflow membrane technology into their processes. Proper system design will ensure both the removal of inorganic contaminants as well as the maintenance of very low microbial levels. Crossflow membrane technology is now standard for electronics manufacturing applications and is quickly becoming accepted as a key component in water treatment for the pharmaceutical industry, providing technical and economic synergy.

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