Ionics Using Membrane Filtration

Ionics Technical Paper

Using Membrane Filtration as Pretreatment for Reverse Osmosis to Improve System Performance by Antonia J. M. von Gottberg and Janet M. Persechino, Ionics, Incorporated

Introduction Membrane filtration has been used for many years as pretreatment to reverse osmosis (RO) for industrial applications. This paper will discuss various configurations of membrane filtration systems, their advantages for RO pretreatment over more conventional pretreatment, and will use several case studies to illustrate the benefits of membrane filtration.

RO Pretreatment

Presented at the 2000 North American Biennial Conference of the American Desalting Association on August 7, 2000. Copyright © 2000, ADA. TP-388 E-US 0800-286

Printed on Recycled Paper in the USA

The need for appropriate pretreatment to ensure optimum performance of RO systems is well understood. RO system suppliers usually require the SDI15 (15 minute Silt Density Index) of the feed to the RO to be less than 5, and generally recommend that the SDI15 should be less than 3 to minimize problems caused by suspended solids blocking the brine spacers in an RO membrane module. When RO was first introduced to the marketplace, many applications of RO treated brackish groundwater.These well waters tend to be low in suspended solids, and so, generally, it is possible to achieve an SDI15 of less than 3 with simple multi-media filters followed by 5 micron cartridge filters. As RO has become widely accepted for both industrial and potable applications, the water sources that are being used to feed RO systems have become more and more challenging. For example, in the past,

many systems designed to produce boiler feed water from surface water sources used mixed-bed ion exchange directly to demineralize the surface water. Today, it is frequently more cost effective to use RO as a predemineralizer before either mixed-bed ion exchange or electrodeionization (EDI), to produce boiler feed water. Surface water sources usually have high levels of suspended solids and bacteria. In some cases, multi-media filters are sufficient to treat the water to feed an RO system, but in other cases a more complicated system of coagulation, flocculation, clarification and filtration is needed to achieve an SDI15 of 3. This conventional system consumes large quantities of chemicals, produces sludge, and requires operator attention to ensure that the appropriate chemical dosing is maintained as the surface water quality varies. Conventional RO pretreatment for surface water and other challenging water sources such as secondarytreated municipal effluent, recycled industrial waste, and some open intake seawaters, is expensive in capital cost and expensive to operate. When conventional systems are operated well, they can achieve an SDI15 of less than 3. There are many systems in operation using conventional pretreatment to an RO system that operate with few problems. However, upsets in the

performance of a conventional system can lead to solids causing excessive brine spacer plugging and increased pressure drop on the concentrate side of the membrane. Overdosing or use of the wrong chemicals in a conventional pretreatment system can increase transmembrane pressure, sometimes irreversibly. The result of upsets such as those described above will increase power consumption, increase chemical cleanings, reduce membrane life, and overall, increase the system’s operating and maintenance costs.

Membrane Filtration Problems such as those listed above have led to a growing number of people selecting membrane filtration for RO pretreatment. Membrane filtration is generally considered to include both microfiltration (MF) and ultrafiltration (UF). Microfiltration ranges in size from approximately 0.05 µm to 1.0 µm. Ultrafiltration is expressed in terms of molecular weight cut-off (MWCO), and ranges from 1,000 MWCO for a very tight UF membrane to approximately 500,000 MWCO for a very open UF membrane. There is some overlap between these two ranges, so a membrane with a pore size that might be considered to be a loose UF membrane might also be considered to be a tight MF membrane depending on the industry. For comparison, a 100,000 MWCO membrane has a pore size of about 0.01µm. Membrane sizes are usually nominal. A generally accepted definition is that a membrane with a given pore size or MWCO would be expected to remove 90% of material of this size. The largest pores in these membranes would 2

be expected to be larger than the nominal pore size. The pore size variation depends on the membrane type and manufacturing process. Filtrate Water Quality Both UF and MF will remove suspended particles, algae and bacteria. UF will also remove viruses. Table 1 shows the expected filtrate water quality from MF and UF systems and compares with multi-media filtration. Microfiltration will guarantee an SDI15 less than 3 regardless of variation in the feed water quality. Ultrafiltration usually achieves an SDI15 below 2, frequently below 1. The lower SDIs guarantee the water quality to the RO system to ensure that the RO is easy to operate and maintain. The removal of suspended solids prevents fouling and blockage of the RO brine spacer. The elimination of large quantities of chemicals from the pretreatment avoids the risk of overdosing. The removal of bacteria by a membrane process reduces a potential source of bacteria into the RO system, which should help to reduce bio-fouling. Membrane Configurations Ultrafiltration and microfiltration membranes are available in a number of configurations. These include hollow fiber, spiral wound, flat sheet, tubular and ceramic. Tubular and ceramic membranes are generally not found to be economical when applied to large-

scale water treatment, but are used frequently in food and beverage applications as well as for the treatment of industrial waste. Flat sheet UF is applied to some niche applications, but generally is not competitive with spiral wound and hollow fiber technologies. Spiral Wound UF Spiral wound UF has been used for over 15 years as pretreatment to RO systems for industrial applications such as the production of ultrapure water from surface water.[1,2,3] Spiral wound UF modules look identical to spiral wound RO modules, and are housed in pressure vessels identical to those used for RO membranes. The membrane material is usually polysulfone. Feed water, pressurized to about 20-100 psi, is fed to one end of the spiral module. Feed water travels across the feed spacer, and is forced through the membrane, leaving behind suspended solids and particulates larger than the membrane MWCO. Filtered water then travels in a spiral to the filtrate core tube at the center of the membrane module, from where it can be transported out of the membrane module as UF product. Most spiral UF membranes cannot be physically backwashed, defined as applying filtered water to the opposite side of the membrane, because the membrane coating would come away from its backing cloth. Hence, the solids that are filtered out of the water by the

Table 1: Filtrate Water Quality Water Quality Turbidity SDI15

Multimedia

MF

UF

0.5 NTU

< 0.1 NTU

< 0.1 NTU

3-5

<3

<2

membrane are removed by means of a crossflow velocity that continuously scours the membrane surface. To maintain a crossflow velocity at the recommended levels, a portion of water that enters the system as feed leaves as concentrate. To achieve high water recovery, roughly half of this water is recycled to the feed stream, with a blowdown stream used to remove solids from the system. Over time, the spiral wound UF membrane system will foul. The feed pressures will increase from 20 psi to up to 100 psi over an operating cycle to maintain production. Once the feed pressure reaches its maximum design value, a chemical clean-in-place (CIP) is performed. A typical operating cycle ranges in length from a few weeks to several months. Hollow Fiber UF and MF Over the last five years, hollow fiber membrane systems for membrane filtration have gained wide acceptance for surface water treatment for potable water production, with over 200 mgd (million gallons per day) of installed capacity in operation by 1999.[4] For potable water applications, hollow fiber membrane systems can guarantee removal of bacteria such as giardia cysts and cryptosporidium oocysts because the integrity of the membrane system can be verified. Extensive application of hollow fiber UF and MF for potable water production has led to the costs of this technology coming down to the point where it is cost competitive with conventional water treatment and spiral wound UF systems for RO pretreatment. Hollow fiber membranes for water treatment may either be MF or UF membranes. The fibers are

typically 0.5 - 1 mm diameter. Several thousand hollow fibers are bundled into a membrane element. At either one or both ends of the membrane element, the fibers are potted in epoxy. Feed water can either be fed to the inside of the fibers, with filtrate passing to the outside of the fibers (inside-out), or else from the outside of the fibers, with filtrate passing from the inside of the fibers (outside-in). Membranes are manufactured from several different materials, depending on the membrane supplier. Typical membrane materials include polysulfone, PVDF, polypropylene, polyacrylonitrile, polyethylene and polyethersulfone. Many systems mount the hollow fiber modules vertically. A more compact design mounts the membrane modules in horizontal

membrane housings similar to RO vessels.[5] Hollow fiber systems are typically operated in a dead-end mode. Particulates are removed from the membrane surface by means of a physical backwash that forces the particulates out of the membrane pores and away from the surface of the membranes. The backwash may occur every 20 minutes to every few hours depending on the system and the feed water source. Since the system operates in a dead-end mode, operating pressures are generally low (usually around 10 psi), and there is no recirculation stream requiring extra pumping power. Over time, the physical backwash will not remove some membrane fouling. Most membrane systems allow the feed pressure to gradually increase over time to around 30 psi

Table 2: Hollow Fiber Versus Spiral Wound Membranes Hollow Fiber

Spiral Wound

Physical backwash

Possible

Not generally possible

Chemical cleanings

CIPs or CEBs possible

CIPs possible

Operating pressures

5 - 30 psi

20 - 100 psi

Membrane life

5 years

8 years

Prefilter requirements

100 - 500 micron strainer

5 micron cartridge filter

Operating mode

Dead-end

Crossflow

Valves

Requires several pneumatically operated valves for backwash sequence

Can be operated with manual valves

Control

Requires PLC and transmitters for monitoring to maintain performance

May be simple on/off with indicators for manual adjustment

Breaktank

Requires breaktank to supply backwash water and to continuously feed RO when offline for backwash

Operates continuously so no breaktank before RO system is required

Availability of membrane replacements

Each system is proprietary, with spare membranes only available from manufacturer

Spirals are a standard size, with replacements available from several vendors

Power consumption

0.05 - 0.2 kWh/kgal

0.2 - 0.8 kWh/kgal

Capital cost (based on 200 gpm system)

$0.50 - $0.90 /gpd

$0.40 - $0.60 /gpd

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and then perform a CIP. CIP frequency might vary from 10 days to several months. Another approach is to use a Chemically Enhanced Backwash (CEB), where, on a frequent basis (typically every 1 - 14 days), chemicals are injected with the backwash water to clean the membrane and maintain system performance at low pressure without going off-line for a CIP.[5] Hollow Fiber Versus Spiral Wound The selection of hollow fiber membrane filtration versus spiral wound membrane filtration depends on the application. Table 2 compares some of the advantages and disadvantages of the different configurations.

Membrane Filtration — RO Many comparisons of using UF or MF as RO pretreatment have previously been presented.[6,7,8] The most recent papers have shown that utilizing UF or MF for RO pretreatment is economical when one considers overall life-cycle costs. Benefits of using membrane filtration as pretreatment to RO fall into two areas. The first area is the benefit of membrane filtration in comparison with conventional pretreatment. The second area is the benefit of better feed water quality to the RO. Equipment Cost The equipment cost of membrane filtration systems is generally higher than the equipment cost of conventional pretreatment to RO. This obviously depends upon how extensive the conventional pretreatment needs to be in a particular situation. When compared with multi-media filters, membrane filtration systems are 4

obviously more capital intensive. However, when compared to clarifiers and filters, the cost difference between membrane and conventional systems is not so great. Spiral wound UF systems are generally lower in capital cost than hollow fiber UF systems since they have fewer automatic valves, controls, and monitoring requirements. However, spiral wound systems require additional prefiltration to ensure reasonable performance. For example, on many surface water sources, a media filter is required as spiral wound UF pretreatment where a backwashable strainer is sufficient for a hollow fiber system. Floorspace Requirements Membrane systems are generally more compact than conventional systems, taking up less than 50% of the area of a conventional pretreatment system. This means that building costs are lower. Especially in areas where space is limited, or areas where civil costs are high, a membrane system may be favored. Power Consumption Hollow fiber UF systems in particular use very low power. Since the RO systems do not foul or plug as much due to the better quality feed water, RO power consumption may also be reduced. Chemical Consumption Conventional pretreatment systems often use large quantities of chemicals. This may include lime softening to reduce turbidity, ferric or alum and polymers for coagulation. The RO system will also require more chemical cleaning with a conventional system.

Reduction in chemical cleaning of an RO system due to membrane pretreatment will reduce chemical costs, waste disposal costs, increase RO membrane life and reduce labor related to performing CIPs. Operator Attention Membrane systems operate in an automatic mode. The water quality from a UF or MF system will be consistent regardless of changes in feed water quality. When feed water quality does deteriorate, the most sophisticated membrane systems will compensate by adjusting operating conditions to minimize system fouling. Conventional systems require an operator to pay attention to the feed water quality and to adjust chemical dosing rates accordingly. In general, even a large UF or MF plant will only require an operator’s attention for a couple of hours per day to keep the plant running. A conventional plant needs an operator to check the water quality and make changes every few hours. Cartridge Filter Replacement Membrane filtration eliminates the need for cartridge filters in front of the RO system. This provides a small savings in capital cost, and a large saving in consumables over the life of the plant. Membrane Replacement Conventional systems do not have membranes that need to be replaced. Membrane systems obviously include membranes that need to be replaced. Depending on the type of membrane, membrane life is expected to be 5 - 10 years. Since the feed to the RO system is improved, and therefore fewer chemical cleanings would be expected, the RO membranes

should last longer with membrane pretreatment. For example, if membranes were expected to last three years with conventional pretreatment, they should last five years with membrane pretreatment. Waste Disposal Single stage membrane systems generally have a water recovery of 85 - 95%, whereas a conventional filtration system would be expected to have 95 - 97% water recovery. When water recovery is important, a secondary membrane system can be used to treat the backwash water from the first system to increase water recovery to over 99%. The product water from the secondary system is the same quality as that from the primary system, and so it reduces the size of the primary system. For large plants, a secondary system adds a relatively small amount to the system cost since multiple trains would be installed. For small plants, the increase in complexity translates into significant cost increase that may not be justifiable. Membrane filtration systems use few chemicals, and hence there is no sludge to dispose of. Where a waste handling system has to be installed, or where waste must be transported, a membrane system may show considerable cost benefits over a conventional system. Higher Design Flux Rates RO membrane manufacturers typically allow RO systems to be designed at higher flux rates with UF feed than with conventional pretreatment. For example, Hydranautics suggests a maximum lead element flux of 28 gfd with UF/MF permeate, versus 20 gfd for a low fouling surface water.[9] Design guidelines for FILMTEC

elements recommend a maximum permeate flux per element of 27 gfd and 18 gfd for a surface water supply.[10] Higher design flux rates reduce the capital cost and size of the RO system.

Case Studies Carbery Milk Products Carbery Milk Products is located in Ballineen, County Cork, Ireland. The dairy requires water for steam generation and potable quality process water. Due to the expansion of the cheese whey manufacturing business, the existing ion-exchange system at the dairy was reaching the limits of its capacity, as was the pretreatment system. The feed water source is the adjacent fishing river, the River Bandon. The existing water treatment system included clarification and filtration, followed by an ion-exchange plant for demineralization. The clarifier was operating at its maximum capacity, and the ion-exchange system was life expired. In 1998, a new water treatment system was commissioned. The existing clarifier was retained to supply only the plant process water. A new water treatment system utilizes a UF plant to feed an RO system. The RO system reduces the feed water total dissolved solids (TDS) from 140 mg/l to less than 10 mg/l. RO product directly supplies the required demineralized water quality for boiler feed. The system production rate is 180 gpm. The combination of UF/RO was selected to save on chemical usage, both from the existing pretreatment system and from the ion-exchange plant, and to reduce the effluent load on the nearby river.

The UF system utilizes hollow fiber UF membranes made by X-Flow, which is part of NORIT Membrane Technology, BV. The membranes are 35 m2 and are installed in horizontal membrane housings. This membrane was selected because of its compact design and low operating and maintenance costs. The river water contains organic acids, and is faintly yellow. Once the system started up, the organics were found to cause two problems. The first was that the organics fouled the UF membrane, causing a reduction in membrane permeability and an increase in chemical cleaning costs. The second was that the organics, which are smaller than the MWCO of the UF membrane, went through the UF membrane and started to foul the RO membranes. A pilot testing program was undertaken to determine how to handle the fouling problems. It was found that addition of 5 mg/l alum could completely control the fouling. The alum coagulates TOC so that it can be removed effectively by the UF system without fouling the UF. This ensures a clean feed to the reverse osmosis system. Salt Union Salt Union Ltd. is a subsidiary of Harris Associates located at Weston Point in the United Kingdom. Salt Union makes salt crystals from brine made by solution mining some 40 km away. Next to the site is a chemical works that supplies superheated steam to the plant. The chemical works will buy high quality waste condensate from the multiple effect evaporators that Salt Union utilizes to manufacture salt crystals. Unfortunately, the evaporators only provided a low5

grade condensate that suffered from ‘carry over’ of fine clay and salt, and from corrosion products (iron). If these could be removed, then they would have a saleable by-product. A 240 gpm integrated membrane system was installed in 1997 to achieve Salt Union’s objective of economically producing a saleable by-product. Table 3 shows the feed and product water quality from this plant. To deal with the silt and iron problem, an ultrafiltration plant is used. This is a hollow fiber system using X-Flow membranes. The hollow fibers are 0.8 mm internal diameter. A clean water backwash flushes the accumulated dirt off the membranes every 15 minutes. A periodic chemically enhanced backwash maintains the system performance. This was primarily selected over a continuous crossflow spiral wound UF system due to the low energy requirement. The clean UF filtrate water is then processed through a reverse osmosis plant to give demineralized water. The RO reject wastewater is utilized on site for washing and other utility purposes. Table 3: Feed Water Quality Feed Quality Conductivity (µS/cm) 400 - 2,000 SDI15

5

Iron

0.1 mg/l

Temperature (°F)

95 - 104

Product Quality 4 - 25 0.1

Northern California Power Agency In 1994, Northern California Power Agency (NCPA) installed a new 49-MW power generation facility in Lodi, California. NCPA

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signed a 10-year own-and-operate agreement with Ionics Ultrapure Water Corporation. Ionics Ultrapure built a water treatment plant to treat clarified secondary-treated municipal effluent from the Lodi White Slough Water Pollution Control Facility. The recycled water is first chlorinated and filtered for general use at the power generation facility. 200 gpm of filtered water is used as cooling tower make-up. An integrated membrane system consisting of UF, UV, RO and mixedbed polishing ion-exchange, treats 600 gpm of filtered water for use as boiler feed. The UF system selected was a spiral-wound UF system. UF was selected as pretreatment to the RO to minimize operating costs of the system. In 1994, hollow fiber membrane filtration had only been recently introduced to the marketplace and was not considered as an option. The UF system used Ionics’ Barrier™ spiral-wound UF elements. UF reduces organics and silt in the filtered water. UV further reduces organics to ensure that the RO is not susceptible to organic fouling. This system has been operating reliably with few membrane cleanings.

Summary The significant benefits of membrane filtration are leading to more and more industrial applications of MF and UF for RO pretreatment. This trend is expected to continue as industries look to tougher water sources to ensure availability of water at a reasonable price.

References

6.

Leslie, G. L., W. R. Mills, W. R. Dunvin, M. P. Wehner, R. G. Sudak, “Performance and Economic Evaluation of Membrane Processes for Reuse Applications”, Proceedings, American Water Works Association Water Reuse Conference, Lake Buena Vista, FL, February 1998.

7.

Schexnailder, Sandy J., “Choosing Membrane-Based Water Treatment for Advanced Boiler Makeup in the Power Industry”, presented at PowerGen Americas ’93, Dallas, TX, November 1993.

Chellam, Shankararaman, Christophe A. Serra, Mark R. Wiesner, “Estimating costs for integrated membrane systems”, Journal AWWA, Volume 90, Issue 11.

8.

American Water Works Association, “Current Issues in Membrane Applications and Research”, AWWA Membrane Technology Conference Preconference Workshop, February 1999.

Rosberg, Rick, “Ultrafiltration (new technology), a viable costsaving pretreatment for reverse osmosis and nanofiltration— A new approach to costs”, Desalination 110 (1997), 107 - 114.

9.

Bates, W., “Hydranautics’ Industrial RO Design Guidelines”, roguide.xls, Rev. 1 (1/20/98)

1.

Katz, William E., and Frederick G. Clay, “Demineralization— Triple Membrane Demineralizers”, Ultrapure Water, September/October 1986.

2.

Valcour, Jr., Henry C., “Triple Membrane Makeup Water Treatment at Four Nuclear Power Plants,” 52nd Annual Meeting International Water Conference, Pittsburgh, PA, October 1991.

3.

4.

5.

van Hoof, Stephan, “Semi dead-end ultrafiltration in potable water production”, Filtration + Separation, January/February 2000.

10. Dow, “FILMTEC Membranes; Membrane System Design Guidelines”, Product Information, April 1998.

Worldwide Headquarters Ionics, Incorporated 65 Grove Street Watertown, Massachusetts 02472 - 2882 USA Telephone: (617) 926-2500 1 (800) 4-IONICS (USA & CAN) Fax: (617) 926-4304 Web: www.ionics.com E-mail: sales @ ionics.com