reducing energy consumption in seawater desalination

ELSEVIER Desalination 165 (2004) 299-312

DESALINATION

www.elsevier, corrdlocate/desal

Reducing energy consumption in seawater desalination

M. Busch*, W.E. Mickols Dow Deutschland GmbH & Co. OHG, POB 20, Industriestrasse 1, 77834 Rheinmiinster, Germany

Tel. +49 (7227) 91-3751; Fax +49 (7227) 91-3801; email: [email protected]

Received 11 March 2004; accepted 2 April 2004

Abstract

Water is an increasingly scarce resource and in many regions people are turning to new sources such as seawater and wastewater. Various membrane technology improvements in the last decade have led to a significant cost reduction in reverse osmosis (RO). This has triggered selection of seawater reverse osmosis (SWRO) technology for many large drinking water projects in the world. Since energy cost is the single largest factor in the cost of a seawater system (usually 20-30% of the total cost of water), it is an obvious target for cost reduction. Therefore new membrane developments have to focus on lower energy consumption. This requires the development of high productivity membrane elements and compatible design concepts that enable operation at lower pressure. This publication presents new products with higher flow (7500-9000 gpd at standard test conditions) and high rejection (99.70-99.75%) for the seawater desalination segment. The technical features of these products are explained and possible energy and other cost savings are calculated. It is shown that these products can be used across the entire seawater segment to reduce energy consumption down to levels of 2.0 kWh/m 3 or to reduce the capital cost of the membrane stage by up to 30%. Field data for these new products is presented at the end and confirms both, performance of the presented new products, and significant capital and operational cost savings.

Keywords: Seawater; Water cost; Low energy consuption; Capital and operation cost reduction; High productivity; NaCI rejection; Boron

1. Evolution of seawater desalination

Many areas of the world have limited fresh water sources and over the past 20 years have begun to look to the sea for fresh water. Reverse

*Corresponding author.

osmosis has become the method of choice for desalinating brackish waters. Over the past 15 years the pressure required to produce fresh water from brackish water has dropped by almost a factor o f 10. This tremendous cost reduction has spurred the rapid expansion of reverse osmosis in brackish water separation.

Presented at the EuroMed 2004 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Office National de l'Eau Potable, Marrakech, Morocco, 30 May-2 June, 2004.

0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved

doi; 10.1016/j.desal.2004.06.035

300 M. Busch, W.E. Mickols / Desalination 165 (2004) 299-312

Historically the cost of seawater separations has been a factor of four or five higher than that of brackish water separations. Over the past 10 years there has been a significant cost reduction in seawater desalination. In the early 1980's the desalination plant on the island of Malta operated at 30-33% recovery [1] and the cost of seawater desalination on the island was US$1.08/m 3. A governmental study in Spain estimated that a 5.3 million gpd plant in 1987 would produce water at US$ 0.9/m 3 [2]. Salinity plays a large role in the final cost also. In the late 1980s higher salinity feeds (42,000 rag/L) in the Arabian Gulf resulted in an estimated total cost of US$1.26/m 3 [3]. In the late 1990s' the cost of water had been estimated to drop to US$ 0.73-0.83/m 3 [4]. Recent cost proposals such as for the Ashkelon project have included numbers as low as US$ 0.47- 0.53/m 3 [5]. We can see that new financial models such as the BOOT (Build Own Operate Transfer) concept have triggered technology advances that have led to a reduction in the cost of water.

The inherent energy cost of desalinating seawater has been well known from basic thermo- dynamics. As an example, for 35,000 mg/L seawater at 40% recovery with a product water of 300 ppm, the minimum energy needed was

1.85 kJ/kg of permeate [6], which corresponds to 0.52 kWh/m 3. In the past 5 years we have seen more sophisticated thermodynamic analysis of system efficiency of reverse osmosis plants. These use second law considerations to estimate inefficiencies from concentration differences and con- verting one stream into two streams at different thermodynamic states. These same considerations were used to evaluate the inefficiencies of an RO plant. This exergy analysis of a California brackish water RO plant [7] showed the largest energy cost (73%) was the pressure loss across the membrane (feed to permeate section). The second largest cost was the inefficiency of the pumps and the use of throttling valves to control flow from pumps. The membrane manufacturers and system designers have the greatest impact on the pressure loss across the membrane and a key contribution to the reduced cost comes from membranes with higher flow rates.

In Fig. 1 the typical seawater spiral wound RO element initially (early 1990s) had a flow rate of 4000 gpd and a rejection of 99.4% (e.g. FILMTEC TM

SW30HR-8040) at standard seawater conditions (see footnote to Table 1). The typical plant in this period operated at a pressure of 70 bar and with typical recovery of about 3540%. Often two feed

10000 SW30-XLE-400 1

8000 SW30-8040 m 0.8

,ooo /_ / ooE-o,_, _ ,ooo s w 3 o , , . 8 • 5000 SW30HR-380 0.5

Q. 4000 ~ ~ 0.4 . .

F. 3000 ! N ~ ~ 0.3 t~ • Ik Standard element, flow rate (gpd) ~ i

2000 , imo- - " High flow element, flow rate (gpd) SW30HR LE-400 ~ 0.2 1000 Zx---- Standard element, salt passage (%)

- --o- - I-~gh flow element, salt passage (%) 0 T 1985 1990 1995 2000

Year

Fig. 1. Historical changes in 8" element flow rates and salt passages.

~ 0.1

10 2005

M. Busch, W.E. Mickols / Desalination 165 (2004) 299-312 301

stages were required, as well as a partial second pass was required to reach drinking water quality (500 mg/L without boron specification).

In the middle of the 1990s elements with flow rates of about 6000 gpd and rejections of 99.6% were introduced (FILMTEC SW30HR-380). The typical seawater element productivity of 6000 gpd became a standard among various membrane suppliers for the second half of the 1990s. It allowed a reduction of operating pressures or an increase of recovery. A second pass was no longer required in the majority of situations.

We can continue the above described trend of cost reductions by improving the design of the plants and by improving the efficiency of the reverse osmosis process. Recent technology advances show that elements with yet higher flow rates and higher rejections are possible and will lead to further cost reductions.

One example for such a product with reduced energy consumption (or increased productivity) is FILMTEC SW30HR LE-400 (previously SW30HR LE-380, refer to chapter 2) with 7500 gpd flow and 99.75% rejection which was introduced by Dow Chemical Company in 2003. Fol- lowing the trend of lowering capital and operation cost with higher productivity elements, an additional product with 9000 gpd and 99.70% rejection has been developed and is introduced as FILMTEC SW30XLE-400 this year. These two products are also included in Fig. 1 and we see 8" element productivities at unprecedented levels as well as a large reduction in salt passage.

2. The use of new low energy RO elements

The two new higher productivity elements rely on the following changes and developments: • The high rejection, low energy element

FILMTEC TM SW30HR LE-400 relies on various improvements to the SW30HR-380 membrane and element configuration. Various process improvements and optimizations in membrane production, as well as enhance-

ments in the composite structure of the element (membrane supports and materials of construction) over the last years have made it possible to enhance flow and rejection by this significant amount. New developments in element construction also allowed increasing the active area to 400 ft 2 without compromising the height of the 28 mil spacer. This allows increasing the productivity of a pressure vessel at constant membrane flux.

• The extra low energy element FILMTEC SW30XLE-400 relies on a modification of the FT30 membrane chemistry in a comparable configuration as SW30HR LE-400. The higher flow membrane is not produced by bleach or other oxidative treatment and subsequently the element displays the same stable rejection as seen in all our seawater membranes. The long term cleaning stability is unchanged.

• It will be noted here that the enhancements to FILMTEC SW30HR LE-400 enabled also offering an element which uses a wider spacer (34 mil in place of the standard 28 mil) for better cleaning in fouling situations and a lower pressure drop. This alternate e lement (SW30HR-320) now has 6000 gpd which is comparable to the standard SW30HR-380 from several years ago.

The two high productivity elements are available with the patented ILEC TM interlocking end caps design, as FILMTEC SW30HR LE-400i and SW30XLE-400i. This eliminates the incon- venience ofinterconnectors and the technical prin- ciples and benefits have previously been else- where [8,9]. The key characteristics of the above mentioned elements in the seawater standard test condition are briefly summarized in Table 1.

FILMTEC SW30HR-320 and FILMTEC SW30HR LE-400 have shifted the productivity by 20-25% while still keeping within the operating windows of most seawater plants. These productivity changes offer many sites the oppor- tunity to upgrade plant performance by a simple scheme of supplementing plant performance


Table 1 Characteristics of new SW desalination membrane elements

Product name Active area, Flow rate, NaCI rejection, Boron rejection, Maximum pressure, ft 2 (m ~) gpd (m3/d) 9'o % psi (bar)

FILMTEC 400 7500 Typical 99.75 91.0 1200 SW30HR LE-400 (35.3) (28.4) Minimum 99.60 (83)

FILMTEC 400 9000 Typical 99.70 88.0 1200 SW30XLE-400 (37.2) (34.1) Minimum 99.55 (83) FILMTEC 320 6000 Typical 99.75 91.0 1200 SW30HR-320 (29.7) (22.7) Minimum 99.60 (83)

Standard test condition: NaCI feed of 32,000 mg/L, recovery of 8%, 25°C, 55 bar, pH 8

during planned element replacement. In cases where pumps include variable frequency drives the cost savings can come directly as energy savings. In other cases higher recoveries or increased throughput could be the source of savings. The dual advantage of higher productivity and higher rejection allows for operating these elements at lower pressure while maintaining or improving the permeate quality.

The Dow Chemical Company is not only improving the productivity of its membranes but also the salt rejection. For example, the FILMTEC SW30XLE-400 with 9000 gpd and 99.70% rejection is shifting the productivity even more dras- tically, it has 50% higher productivity than our SW30HR-380 from 2002 and the same stabilized rejection. As little as two years ago the new rejection of 99.70% was only possible with the lower flow membranes from the Dow Chemical Company. Now it is possible to achieve previous high rejection with higher flow membranes.

The potential use of new higher flow products depends on the site conditions (feed water salinity, temperature), design and operation conditions (recovery, flux) and the permeate quality requirement. The rejection properties of seawater reverse osmosis elements will define the breadth of the source waters and designs that can be used with these elements. Based on the new rejection performance, the previously presented high productivity elements FILMTEC SW30HR LE-400

and FILMTEC SW30XLE-400 have a surprising range and should be useful through many new and old (retrofit) sites.

The highest flow seawater element FILMTEC SW30XLE-400 will be most useful in mid to lower salinity and temperature ranges as well as for relaxed permeate quality requirements. It will have limited use in highest feed salinity and highest temperature designs, while the new FILMTEC SW30HR LE-400 element can be used across the entire range. Perhaps the best way to demonstrate the wide utility of the SW30HRLE-400 and the SW30XLE-400, is to consider a standard plant with FILMTEC SW30XLE-400 and compare the permeate quality to the World Health Organization standard of 500 mg/L (Fig. 2).

This plant is assumed to use seven FILMTEC SW30XLE-400 elements per vessel and produces a flux of 15 L/h/m 2 at a recovery of 45%. It is very similar to the plant that will be considered later to consider possible cost savings in chapter 3, but we vary the feed salinity and temperature.

At 38,000 mg/L and 25°C, which can be considered an average condition for most seawater plants across the globe, the permeate TDS reached is 350 mg/L. In most conditions between 35,000 mg/L and 41,000 mg/L and 20 and 30°C the permeate TDS remains below 400 mg/L. This means the SW30XLE-400 will meet the permeate requirement in most conditions and hence can be considered very versatile.


450 . . . . . . . w

,~ 400 E

I-~ 350300 I

~ . , ~ . ~ - - ,--- 35,000 rng/L ¢l,,,

~ _ __ - - - - 41,000 mg/L 250 /

20 22.5 25 27.5 30 Feed temperature, *C

Fig. 2. Operation range ofFILMTEC SW30XLE-400.

For higher TDS feeds and temperature conditions the permeate may exceed normal design limits. As an example at 41,000+ mg/L and 30+°C, the permeate TDS will exceed 400 mg/L. If a lower permeate TDS is desired then we can either increase the permeate flux somewhat or use SW30HR LE-400. Another option to fine-tune a plant for the rejection requirement is to mix elements. As an example, FILMTEC SW30HR LE-400 could be used at the feed end of the vessel, in the first, second or third posit ion and SW30XLE-400 in the reject end of the vessel. This would not only allow reaching exactly the rejection required, but it also reduces flux on the front elements leading to a reduction in fouling. This is also attractive when it is intended to split permeate from the front and reject end.

While improved TDS rejection and productivity are the main driving forces in reverse osmosis, specific ion rejection is also very important. The chloride ion has traditionally been the most quoted specific ion in seawater separations but now boron removal is an important consideration [5]. A boron concentration of <1.0 mg/L can usually be achieved with a single pass or a small high pH second pass, preferably treating the brine end permeate, if FILMTEC SW30HR LE-400 is used. For boron removal to <0.5 mg/L usually a second

pass is required, which is large enough to treat at least 60% and in specific cases 80-100% of the first pass permeate. This is also applicable when we work against tighter limits for TDS (e.g. <50 mg/L), chloride (e.g. <25 mg/L) and other solutes. In that situation the first pass does not require the highest possible rejection, because the combined permeate has a very high quality and operation cost can be saved by using high productivity elements such as FILMTEC SW30XLE-400.

3. Reductions in operational and capital cost

New products with higher productivity enable a further cost reduction of both the capital expenses (CAPEX) and operational expenses (OPEX), which make up the desalination cost. Depending on the focus of the system designer these benefits may vary and it is the designers and/or operators choice to opt for either reducing capital or operation cost, or selecting a com- bination of both adapted to the individual economic frame conditions (energy cost, project lifetime, capital cost).

A capital cost oriented system designer will take advantage of the higher productivity at comparable conditions to increase capacity with the same number of pressure vessels and mem-

304 M. Busch, WE. Mickols / Desalination 165 (2004) 299-312

brane elements, or he will use fewer pressure vessels and membrane elements to achieve the same productivity.

Another approach to save system cost is to increase productivity and water recovery. This again results in savings in pressure vessels and membranes but it also reduces pre-treatment and pump capital and operation cost. A designer or

operator leaning more towards the reduction of operation cost will most likely reduce the operating pressure o f a membrane plant to reduce energy cost.

The possible options o f substituting conven- t ional 6000 gpd p roduc t s wi th h ighe r f l ow 7500 gpd and 9000 gpd elements are described in Table 2. This table lists the various possible

Table 2 Strategies for capital (CAPEX) and operational (OPEX) expense reduction with higher productivity membranes

Option Design strategy Resulting impact Practical implications

Option 1 Compared to design with lower Lower feed pressure results in a lower (1), (2) (OPEX reduction): productivity membrane elements, use the energy consumption of the feed pressure Reduce feed same number of pressure vessels and pump. Hence, savings in energy cost can pressure membrane elements, produce same flow be captured.

rate at same recovery (constant permeate flow)

Compared to a design with lower (3)-(7) productivity elements, use the same feed pressure and the same number of pressure vessels and elements

Option 2 (OPEX and CAPEX reduction): Increase plant output and recovery

Option 3 (CAPEX reduction): Higher flux operation at same recovery

Compared to a design with lower productivity membrane elements, use the same feed pressure and the same recovery, but increase average permeate flux

Increase water production and recovery. Higher water production means capital savings in pressure vessels and elements, higher recovery means less capital cost in pre-treatment and less operation cost for pumping and pre-treatment.

Option 3a: At same plant output, fewer pressure vessels and dements, savings in capital cost

Option 3b: At same number of pressure vessels and elements, capacity increase

(8), (9)

(3),(7)-(9)

(1)Possible with positive displacement pump and centrifugal pump (CP) with variable speed drive (VSD). For CP without VSD, impellers have to be trimmed or impeller sections of multi-stage pumps removed

(2)Operating higher productivity elements at lower pressure to match the design flow of lower productivity elements may result in an increase in permeate salt concentration, this should be checked.

(3)Verify that the brine control valves, product and brine tubing, storage and post-treatment can accommodate modified flow rates.

(4)Increasing average flux and recovery may result in minor permeate quality changes. (5)The impact of higher recovery on scaling in the brine should be checked and necessary precautions taken. (6)The higher expected brine concentration also should be compared to the discharge concentration limits and other

environmental regulations. (7)Higher permeate flow on the lead elements, higher average permeate flux and/or lower brine flow might result in a

modified fouling behavior. The possible impact should be assessed. (8)Increasing average flux at the same recovery leads to a reduction of salt passage, because salt flux remains constant.

Therefore this is an option when we want to improve permeate quality. (9)It should be verified that capacity ofpretreatment and feed pump can accommodate the higher feed flow required in

this option.


high level options, the changes in the design strategy and the resulting impact. The practical implications of choosing one of the options are summarized quickly in the below footnotes to the table, but have been described in more detail in recent publications [10,11].

It can be seen that high productivity elements offer an energy cost reduction when the feed pressure is reduced with the same number of elements (option 1), but they can also offer capital cost reduction (pressure vessels and membrane elements), when the system footprint is reduced (option 3a). When the higher flux operation route suggested in option 3a is followed, existing pressure vessels in a plant can be used and the output increased (option 3b). Further below (Table 3) it will be shown, that conceptually both options, 3a and 3b, result in the same capital cost savings. Another strategy is to increase recovery and water production which enables both operational cost reductions (less pre-treatment costs, less pumping cost) and capital cost reductions (lower pre-treatment and membrane stage footprint). The three options represent basic strategic routes, which can of course be combined or varied at the discretion of the designer.

To illustrate the cost reduction of these various options, costs of a model plant using the new elements SW30HR LE-400 and SW30XLE-400 will be compared to a plant using FILMTEC SW30HR-380 elements ("reference case"). The plant is operated on a feed with 38,000 mg/L at 25°C (77°F). The design is 115 vessels with seven elements per pressure vessel, and a vessel produces 3.45 m3/h (15.2 gpm). The production of the plant is 9,500 mVd. The average flux of this design is 14.0 L/h/m 2 (8.2 gfd). Recovery of the plant is 45 %.

The cost of the membrane stage, which in- cludes the energy cost during 5 years of operation ("power cost", OPEX) and amortization of the investment cost for pressure vessels and membrane elements ("capital cost", CAPEX) will be evaluated. The pretreatment, cleaning and other

membrane plant costs will not be considered, since both the 7500 gpd and the 9000 gpd element use a very similar element construction and therefore can be cleaned as easily as a conventional 6000 gpd element. Due to higher recovery, there might be additional savings in the pretreatment cost, which will not be included in this analysis. The following are assumptions: • Operation time: 5 years ° Replacement rate: 20% ° Pump efficiency: 90% ° Power cost: US$O.O8/kWh

Two scenarios are included in this evaluation which is shown in Table 3: one with energy recovery devices (90% efficiency), and another one without energy recovery. In addition to the power and capital cost over 5 years, the value of con- verting the plant to the new designs with new elements will be determined. This will be reported as the value per element. The added value per element corresponds to the power and capital cost savings gained by replacing the conventional 6000 gpd element.

We will consider reducing the pressure as well as increasing output and recovery with FILMTEC SW30HR LE-400. In all these cases the permeate quality (253-258 mg/L) stays roughly the same as with a conventional 6000 gpd, 99.70% product (248 mg/L). The permeate quality is improved by + 13% to 216 mg/L when higher output is chosen at the original recovery of 45%. With FILMTEC SW30XLE-400, the permeate TDS increases to 359-368 mg/L in the case of reducing pressure or increasing output and recovery. In the case of operating at high output and the original recovery, the permeate concentration only increases slightly, from 248 to 261 mg/L. All of the scenarios meet a drinking water quality of 500 mg/L and offer a safety buffer of at least 27% for post-treatment and/or variation of operating conditions.

When operational cost savings are intended, then the operating pressure in this reference case can be reduced by 2.5 bar using the 7500 gpd pro-


Table 3 CAPEX and OPEX reductions with higher flow seawater elements

Parameter SW30HR-380 SW30HR LE-400 SW30XLE-400

reference case Option 1: Option 2: Option 3a Option 3b Option 1: Option 2: Option 3 Reduce Increase Higher Higher Reduce Increase Higher pressure output and flux flux pressure output and flux

recovery recovery

Permeate flow, m3/d 9,500 9,500 10 ,000 11,500 9,500 9,500 10,400 9,500 Recovery, % 45 45 47.3 45 45 45 49.2 45 Feed flow, m3/d 880 880 880 1,070 880 880 880 880 Feed TDS, mg/L 38,000 38,000 3 8 , 0 0 0 3 8 , 0 0 0 38 ,000 3 8 , 0 0 0 3 8 , 0 0 0 38,000 Permeate TDS 248 258 253 216 216 368 359 261 Pressure 58.3 55.8 58.3 58.3 58.3 53.8 58.3 58.3 Number of elements 805 805 805 805 665 805 805 567 Number of pressure 115 115 115 115 95 115 115 81 vessels

Pump efficiency 90%, energy recovery efficiency 90%

Energy use, kWh/m 3 2.27 2.17 2.34 2.27 2.27 2.09 2.19 2.27 Power cost, 2.39 2.29 2.47 2.89 2.39 2.21 2.31 2.41 mil US$ in 5 years Water cost, 19.0 18.5 18.5 18.1 18.1 18.0 18.3 17.6 US cent/m 3 Added NPV per 0 61 61 136 136 129 95 248 element, US$/element

Pump efficiency 90%, no energy recovery

Energy use, kWh/m 3 4.01 3.84 4.01 4.01 4.01 3.70 3.66 4.01 Power cost, 4.22 4.04 4.23 5.44 4.22 3.90 3.86 4.23 rail US$ in 5 years Water cost, US cent/m 3 29.6 28.6 28.2 28.7 28.7 27.7 27.2 28.0 Added NPV, 0 128 187 136 136 231 307 271 US$/element

duct, and by 4.5 bar using the 9000 gpd product. This corresponds to energy consumption reductions of 4 -8% and with high efficiency motors, pumps and energy recovery, 2.09 kWh/m 3 can be reached. It can be shown that a lower flux design would even enable energy consumptions of below 2.0 kWh/m 3. With FILMTEC SW30HR LE-400, the savings in water cost are US cent 0.5/m 3 with and 1.0 cent without energy recovery. They can be further enhanced with FILMTEC SW30XLE- 400 to 1.0 (with energy recovery) and US cent 1.9/m 3 (without). Under the financial conditions chosen in this case, the added value is between

61 (SW30HR LE-400 with energy recovery) and US$ 231/element (SW30XLE-400 without energy recovery). This means as long as the price delta is below these limits, the end user will still break even.

For an extreme OPEX view, the financial examples of our standard scenario can be modified to consider a longer project lifetime than in the example (e.g. 10 instead o f 5 years), a lower replacement rate (10 instead of 20%) and use of energy recovery. The water cost then drops from US cent 13.6/m3 to US cent 12.7/m 3 and the added value per element is US$ 227.

In case of a focus on lower capital cost, the


number of pressure vessels and elements can be reduced by 17% with FILMTEC SW30HR LE- 400 and a 30% reduction is possible with SW30XLE-400. This corresponds to a reduction in water cost of US cent 0.9/m 3 (added value US$136/element) with the 7500 glad product and US cent 1.6/m 3 (US$ 260/element) with the 9000 gpd product.

There are lots of other options available to the plant designer and operator, and they can be optimized according to the financial frame conditions (e.g. project duration, replacement rate, energy cost, interest rate) and the design approach (e.g. elements per vessel, average flux, recovery).

4. Field experience with low energy elements

In the field testing period prior to commer- cialization, elements have been used in various configurations in a variety of plants. The field experience will be presented in this final section of this paper. For each of the 2 new high flow products, the list of sites where they have been shipped is shown and then operation data for some of the sites is presented.

The stabilized product performance will be established by comparing the operation data at each of the sites to the reverse osmosis system analysis (ROSA) design program predictions. The following approach is used to compare the specified performance under standard test conditions (see footnote to Table 1) with the actual performance of the plant and reports a "normalized performance" (performance of the elements in the plant transformed to reflect performance of an element in the standard test condition): ° We determine the fouling factor (FF) for a

certain operation condition (feed TDS and temperature, permeate flow, recovery) to accurately predict the actual pressure by using ROSA. The normalized flow is then the specified flow (on the spec sheet, can be calculated using ROSA under standard test conditions) multiplied with the fouling factor [Eq. (1)].

Calculating the expected permeate concentration (TDS, chloride or boron) in the operating condition and establishing the salt passage factor (SPF), the ratio of the observed concentration to the predicted concentration, which can be used for transforming the specified rejection to the normalized rejection in a condition [Eq. (2)].

Q~o= = FF. Q~p~¢ (1)

R..~ = 1-SPF. (1- R ~ ) (2)

4.1. FILMTEC SW3OHR LE-400

Table 4 shows 14 field test sites, delivered projects and systems that use the new product FILMTEC SW30HR LE-400 and its predecessor version SW30HR LE-380. The smaller numbers of elements usually refer to piloting sites; whereas, the larger numbers are industrial scale installa- tions. 2670 elements in 12 projects were delivered and installed between 2002 and 2004 and there are orders for 5190 more elements in 2 projects. Detailed operation data is only available for some of these sites, and is presented further below.

The Dhekhelia plant treats a 40,700 mg/L (40500-41,000), 28°C (24-31), pH 6.7 (6.4-7.0) open intake feed with SDI of 2.7 (2.1-2.9) and boron of 5.7 mg/L (5.1-6.2), and also uses Dupont hollow fine fiber membranes. A test vessel in the plant is equipped with FILMTEC SW30HR LE- 380 elements. With a feed pressure of about 72.5 bar (72-73), permeate back pressure of 2.5 bar (1.5-3.5) and the feed flow is 8.0 m3/h (7.5-8.5), the resulting performance is as follows: at a recovery of 52% (49-55%), the vessel of six elements produces 4.0 m3/h (3.4-4.6 mVh) permeate with a salinity of 230 mg/L (180-280) and the permeate contains boron at 1.1 mg/L (0.75-1.35), which corresponds to a normalized performance of 7500 gpd, 99.69% NaC1 rejection and 90% boron rejection at start. During operation, a flux reduc-


Table 4 Reference sites for FILMTEC SW30HR LE-400

Location Cotmtry End user/OEM Application No. of Element Feed water Year elements installed

Temeuzen Netherlands Dow Chemical Process 528 SW30HR LE-380 Sea, open 2002 Belize Bel ize Consolidated Potable 48 SW30HR LE-380 Sea, well 2002

Water Galdar Spain Agragua Irrigation 360 SW30HR LE-400 Sea, well 2003 Unknown G e r m a n y K a e r c h e r Unknown 48 SW30HR LE-380 Sea 2003 Lanzarote Spain INALSA Potable 18 SW30HR LE-380 Sea, well 2003 Dhekhelia Cyprus Caramondani Potable 6 SW30HR LE-380 Sea, open 2003 Unknown Italy Epuro Unknown 12 SW30HR LE-400 Sea 2003 Undisclosed Undisclosed Undisclosed Potable 48 SW30HR LE-380 Sea, open 2003 Lanzarote Spain Inalsa Potable 560 SW30HR LE-400 Sea, well 2004 Ummluj Saudi Arabia SWCC Potable 6 SW30HR LE-400 Sea, open 2004 Undisclosed Undisclosed Undisclosed Potable 800 SW30HR LE-400 Sea, open 2004 Boudjour Morocco Proagua Potable 238 SW30HR LE-400 Sea, well 2004 Fuerteventura Spain La Oliva Potable 154 SW30HR LE-400 Sea, well 2004 Rambla Morales Spain Tecnicas Potable 5040 SW30HR LE-400 Sea, well 2004

tion has been observed while NaC1 and boron rejection improved.

The Ambergris Caye plant in Belize is operated with a 37,500 mg/L, 25-28°C feed from an anoxic well, hence containing humic acids and high iron. The pretreatment consists only of cartridge filtration (5 lxm). Any further pretreatment is avoided in order to maintain the anoxic condition of the water and to avoid iron precipitation. The SW30HR LE-380 element was mixed with old elements to enhance plant performance. Due to the mix of different elements, normalized performance data for SW30HR LE-380 can not be reported. Nevertheless, it can be said that installa- tion of the new low energy elements led to a reduction in operation pressure from 67.9 bar to 57.9 bar under the same operating conditions (recovery of 42%, production of 70 m3/h) while the permeate TDS stayed at the same level of 320 mg/L.

The RO plant in Lanzarote is operated with a sand beach well having an SDI around 1, and a salinity of 38,500 mg/L with a temperature range between 20-22°C. The tests were carried out with six elements in a single pressure vessel. With SW30HR LE-380, the flow per pressure vessel

under these conditions was 4.7 m3/h, which is equivalent to a flux of 22 L/h/m2; the recovery was 34%. Feed pressure was 63.5 bar and the permeate pressure was 1.5 bar and this corresponds to a normalized flow rate of 7200 gpd. The permeate TDS is 100 mg/L vs. projected 120 mg/L, which corresponds to a normalized rejection of 99.78%. With a feed boron of 5.6 mg/L and a pH of 7, the permeate boron concentration is 0.60 rag/L, which corresponds to a normalized boron rejection of 92.0%.

An undisclosed plant uses a feed of 40,700 mg/L (40,500--41,000), at 18-21 °C, with the pretreatment of the open intake feed consisting of flocculation-multi-media filtration which results in the RO feed having an SDI continuously below 3. Six pressure vessels with 7000 gpd prototype elements were installed in the normal plant. Operating at 70-71 bar (back pressure 0.8 bar) and a recovery of 49-50%, a typical 8 element pressure vessel produces 4.6--4.8 m3/h with a permeate TDS of 220 mg/L. This corresponds to a normalized performance of 7200 gpd and 99.73% NaC1 rejection. At a pH of 7.0, and a feed boron content of 5.5 mg/L, the permeate con-


tains 0.85-1.01 mg/L boron which corresponds to a normalized boron rejection of 90.7%.

The RO-plant in Terneuzen is a 2 pass plant producing process water, operated with estuary North Sea open intake water, pretreated with microfiltration membranes. The feed salinity is between 15,000 and 20,000 mg/L, the temperature variation due to seasonal variations is between 12 to 17°C. The configuration of the plant is two stages with 44 pressure vessels and six elements per vessel (264 elements). The recovery is 55% at a flux of 22 L/h/m 2. The pressure is 36.9 bar, corresponding to an average standard element performance of 7860 gpd. The TDS in the permeate was 47 mg/L corresponding to 99.79% rejection. With a feed boron concentration of 2.55 mg/L and a pH of 7.3 the permeate concentration was 0.21 mg/L corresponding to a normalized boron rejection of 91%. Five to ten days after start-up the flow decreased to 90-95% of original value (7100-7500 gpd) and is perform- ing at this level. The salt passage came down to about 90% as well, corresponding to an average standard element rejection of 99.81%.

The Galdar plant in Gran Canaria produces water for agricultural purposes and was originally designed for DuPont hollow fine fiber RO elements. One train was retrofitted for spiral wound RO elements using 60 vessels each with six FILMTEC SW30HR LE-400 elements. These have allowed the operators to reduce pressure and increase water production at reduced pressure. With a 39,000 mg/L feed at 21 °C and a recovery of 45%, the plant now produces 207 mS/h of

170 mg/L permeate at a feed pressure of 63.2 bar and a permeate pressure of 3.2 bar. This corresponds to a normalized flow rate of 7200 gpd and a normalized rejection of 99.79%.

4.2. FILMTEC SW30XLE-400

The first prototypes of SW30XLE-400 were sent out in 2003 to various sites with the main purpose of testing them under different conditions (Table 5).

The feed water of one plant (undisclosed location) is 41,000 mg/L TDS, 5.4 mg/L and is at 21 °C. The eight element vessels are operated with a feed pressure of 70.7 bar and a recovery of 50%. It produces 5.3 m3/h of water with a permeate TDS of 310 mg/L and a boron content o f 1.18- 1.36 mg/L. This corresponds to a normalized flow performance of 8000 gpd, a normalized TDS rejection of 99.67% and a normalized boron rejection of 88.0%.

The Bodrum plant operates on a beach well feed of 38,700 mg/L and 21°C. To produce a permeate flow of 21 m3/h with six element pressure vessels with a recovery of 40%, the required pressure was 55 bar. This corresponds to a fouling factor of 0.9 or a nominal flow rate of 8100 gpd. The batch of elements delivered to Bodrum was designed for a lower flow rate in order to determine the most suitable production conditions, which explains the somewhat lower than specified productivity. The plant uses latest developments in axial piston pumps and pressure exchangers [ 12] and the energy use is only 2.04 kWh/m 3 [ 13]

Table 5 Reference sites for FILMTEC SW30XLE-400

Location Country End user/OEM Application No. of Element elements

Feed water Year installed

Undisclosed Undisclosed Bodrum Turkey Lanzarote Spain Undisclosed Undisclosed

Undisclosed Potable 8 SW30XLE-400 OWV Potable 36 SW30XLE-400 INALSA Potable 16 SW30XLE-.400 Undisclosed Potable 8 SW30XLE-400

Sea, open 2003 Sea, well 2003 Sea, well 2003 Sea, open 2004

310 M. Busch, WE. Mielmls / Desalination 165 (2004) 299-312

which confirms the estimations of energy use of 2.09 kWh/m 3 presented and discussed in context with Table 3. The permeate TDS was 295 mg/L, which was somewhat higher than the expected 255 mg/L and it corresponds to a normalized element rejection of 99.65%. The plant has been operating for a year and performance is stable.

The feed water of the Lanzarote plant is 38,500 mg/L with a temperature of 22°C. Lanza- rote III, train 4, where the SW30XLE-400s are installed consists of 2 feed stages in series, one low recovery and one high recovery stage using 60 and 48 pressure vessels (six elements per vessel). One vessel of SW30XLE-400 elements was installed in the low recovery stage and one vessel in the high recovery stage, where the feed TDS increases to 57,000 mg/L. In a third vessel in the low recovery stage, a vessel was installed containing two FILMTEC SW30HR-320 elements (6000 glad flow, 99.75% NaCI rejection) and four SW30XLE-400 (9000 gpd, 99.70%). The average of this mix of elements should perform like an element with 8000 gpd flow and 99.72% NaCI rejection. Table 6 shows the operational results in these conditions.

We can see that in the high recovery stage we observe a lower than expected productivity but

much higher than expected rejection. This can be due to the low operating and high osmotic pressure resulting in very low net driving pressure (about 10 bar) and predictions from ROSA are more uncertain, because slight measurement inac- curacies may have a large effect. It might also be due to a slightly different than expected behavior of this membrane at high TDS, which would however be desirable, because it further boosts the rejection and despite low water production and high feed TDS, the vessel can produce <500 mg/L quality. Due to the somewhat higher uncertainty with this data point it will not be included in the final conclusions.

The vessels in the low recovery stage (150 and 170 mg/L) produce permeate with excellent permeate quality. They exactly mimic the predictions with the rejection specification of 99.70%. The vessel flow rates of 5.3 and 5.8 m3/h require fouling factors >1.0 to simulate the observed pressure (FF = 1.03-1.04). The normalized SW30XLE-400 element performances then are 9300 and 9400 gpd, which is excellent perform- ante. The average permeate fluxes are remarkably high, 26.5 and 27.5 L/h/m 2 and are roughly double of the original design flux of the Lanzarote plant.

Table 6 Performance data of Lanzarote plant

Stage Product Feed TDS, Water temperature production,

recovery

Feed, brine Normalized Normalized and permeate performance, performance, pressures flow rejection

Low SW30XLE-400 38,500 5.8 rn3/h, recovery mg/L, 21°C 42%

Low 2 SW30HR-320 38,500 5.3 rn3/h, recovery and mg/L, 2 I°C 40%

4 SW30XLE- 400

High 57,000 rng/L 57,000 2.4 m3/h, recovery mg/L, 2 I°C 20%

65.5, 64.4 P(f) predicted and 2.7 bar 65.9 bar, FF adjusted

1.04 or 9400 glad

65.2, 64.1 P(f) predicted, and 2.3 bar 65.9 bar, FF adjusted

1.03 which is 8200 for elements mix and 9300 glad for SW30XLE-400

63.8, 62.5 p(f) predicted and 1.4 bar 61.7 bar, FF adjusted

0.80 or 7200 gpd

TDS predicted 170 mg/L, TDS observed 150-170 mg/L, R = 99.72%

TDS predicted 160, TDS observed 150 mg/L, R = 99.74% and 99.72% for SW30XLE-400

TDS predicted 470 mg/L, observed 340 mg/L, R = 99.81%

M. Busch, WE. Mickols ~Desalination 165 (2004) 299-312 311

The concept of mixing lower flow elements in the lead positions and higher flow elements in the tail positions is used to reduce the flux and fouling tendency on the front elements and is more likely to prevent fouling and sustain the high flux operation over the long-term. Even though such high fluxes should not be used to design a major plant, the initial results confirm the estimations in Table 3 of significant plant productivity increase and water cost reduction with the new elements.

5. Summary and conclusions

Due to an increased demand for fresh drinking water and a trend to use desalinated seawater as a source, there have been significant advancements in the development of seawater desalination technology. These have led to cost reductions in the past, and further significant cost reductions are possible.

Some of the technology enhancements rely on RO elements with up to 50% higher productivity and significantly increased salt rejection. The potential use of these products will depend on the site conditions (feed water salinity, temperature), design and operation conditions (recovery, flux) and the permeate quality requirement. Projections with the highest productivity element FILMTEC SW30XLE-400 show we reach a permeate TDS of below 400 mg/L up to feed salinities of 41,000 mg/L and temperatures of 30°C, which is confirmed by field data in three plants where a permeate TDS of 150-310 mg/L is achieved at a feed TDS of 38,500--41,000 mg/L, recoveries of 40-50% and temperatures around 21°C. For higher salinities and temperatures or somewhat tighter permeate quality requirements (e.g. includ- ing boron removal to 1.0 mg/L), we have to mix with FILMTEC SW30HR LE-400 or use it exclusively. For the most stringent permeate quality requirements (e.g. boron of 0.5 mg/L or very low TDS and chloride requirements), a second pass is needed. In this case energy savings are possible with the highest flow product.

Field data from 5 field sites with FILMTEC SW30HR LE-400 indicates flows in the range of 7200-7800 gpd with an average of 7400 gpd and a standard deviation of 270 gpd. NaCI rejection is 99.76% on average with a range of 99.69- 99.79% and a standard deviation of 0.04%. Boron rejection is 90-92% and 91% on average (standard deviation of 0.8%). The 4 FILMTEC SW30XLE- 400 field observations selected for final evaluation indicate flow performance between 8000 and 9400 gpd with an average of 8700 gpd and a standard deviation of 750 gpd. NaC1 rejection is 99.69% (99.65-99.72) with a standard deviation of 0.04% and boron rejection 88.0%. This means that flow and rejection specifications indeed match very well with the field performance observations.

The new 7500 gpd and 9000 gpd membrane elements can either enable pressure reductions by 2.5-4.5 bar in average conditions for optimum OPEX savings, or reduce the number of elements and pressure vessels by 17-30% for substantial CAPEX savings. Both OPEX and CAPEX saving estimations have been validated in the field trials. It was shown that when the extra-low energy element SW30XLE-400 is used together with high efficiency motors, pumps and energy recovery, estimated energy consumptions as low as 2 kWh/m 3 are reached. The successful operation at high and very high fluxes, as shown with remarkable >25 L/h/m 2 in Lanzarote, confirms also the possibility of significant capital cost reductions. Most designers will likely tailor the capital and operating expense reductions to obtain the greatest economic value for the life cycle cost for the project.

With FILMTEC SW30HR LE-400, the water cost reductions in the range considered are between US cent 0.5/m 3 and US cent 1.4/m 3, with an average of US cent 0.9/m 3. With FILMTEC SW30XLE-400, the possible reductions are between US cent 0.7/m 3 and US cent 2.4/m 3, on average US cent 1.5/m 3. The average cost reductions correspond to an added value of US$123 per element for the replacement of


F I L M T E C S W 3 0 H R - 3 8 0 wi th F I L M T E C SW30HR LE-400 and US$214 per element for r e p l a c i n g S W 3 0 H R - 3 8 0 w i t h F I L M T E C SW30XLE-400.

In conclusion, demonstrated significant enhancements in the rejection performance and the productivity o f seawater reverse osmosis offer the potential o f large reductions in water cost.

A c k n o w l e d g e m e n t s

We would like to express our sincere gratitude to the designers and operators o f the mentioned field sites for their thorough evaluation work. In particular we would like to thank • Antonio Sanchez, Juan Manual Bethencourt,

Femando Castro and Julio Gonzalez (INALSA) for Lanzarote data

• Jeroen B o o m (Rossmark), Marc Slagt and Peter Vaal (Delta) for Temeuzen data

• O l g a S a l l a n g o s ( C a r a m o n d a n i ) fo r the Dhekhelia data

• Juan Vargas, Maria del Carmen Velazquez (Cadagua) and Toni Casanas (DOW) for Galdar data

• Peter Mace (OWV) and Peter Sehn (DOW) for the Bodrum project data

• John Kavanaugh (Consolidated Water) and Craig Broden (DOW) for Belize data.

R e f e r e n c e s

[1] W. Andrews and R. Bergrnarm, The Malta seawater RO facility, Paper resented at First World Congress on Desalination and Water Re-Use, Florence, Italy, 23- 27 May, 1983.

[2] M. Farinas, Reverse Osmosis in Seawater Desalination Plants. Cost per m 3 of desalinated water, PRIDESA, 1996, based on The Eighth Commission of the Spanish Water Supply and Sanitation Association, 1987.

[3] G. Leitner, Costs of seawater desalination in real terms, 1979 through 1989, and projections for 1990, Desali- nation, 76 (1989) 201-213.

[4] M. Will and K. Klinko, Optimization of seawater RO system design, Desalination, 138 (2001) 299-306.

[5] J. Redondo, M. Busch and J.P. De Witte, Boron removal

from seawater using FILMTEC high rejection SWRO membranes, Desalination, 156 (2003) 229-238.

[6] Y. Cerci, Y. Cengel and B. Wood, The minimum separation work for desalination processes, Proc. ASME Advanced Energy Systems Division AES, 39 (1999) 545-552.

[7] Y. Cerci, Exergy analysis ofareverse osmosis desalination plant in California, Desalination, 142 (2002) 257- 266.

[8] J. Jonson, M. Hallan, M. Peery and L. Johnson, Say good-bye to the weakest link - - introducing a new method for coupling membrane elements, IDA World Congree on Desalination and Water Reuse, Paradise Island, Bahamas, 28 Sept.-3 Oct. 2003.

[9] A. Gorenflo, J. Redondo, M. Busch, A. Casanas and J. Jonson, Innovaciones tecnologicas en la osmosis inversa de agua de mar, IV Congreso national AEDyR - - Desalacion y Reutilizaci6n Mirando al Futuro, Las Palmas de Gran Canada, 19-21 Nov. 2003.

[10] Dow Chemical Company Literature Archive, How FILMTEC's new high rejection low energy seawater dement can reduce your desalination costs, Form No. 609-00437-803, Midland (2003), http://www.dow.com/ liquidseps/prod/sw.htm

[11] M. Busch, High-flow, high-rejection RO membranes reduce production costs by 20%, Water & Wastewater International, 18(8) (2003).

[12] F.Ludvigsen and A. Valbjoem, Newest pump technology uses only 2.7 kWh/m 3 for SWRO, Desalination and Water Reuse, 13(1) (2003).

[13] P. Mace, Publication of operation results for the seawater desalination plant in Turkey, Press release by Saloon Austria GmbH, 10 July 2003.

D i s c l a i m e r

No freedom for any patent owned by Dow Deutschland GmbH & Co. OHG/FilmTec Corporation or others is to be inferred. Because use conditions and applicable laws may differ from one location to another and may change with time, Customer is responsible for determining whether products and the information in this document are appropriate for Customer's use and for ensuring that Customer's workplace and disposal practices are in compliance with applicable laws and other governmental enactments. Dow Deutschland GmbH & Co. OHG/FilmTec Corporation assume no obligation or liability for the information in this document. No warranties are given; all implied warranties of merchantability or fitness for a particular purpose are expressly excluded.

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