ELSEVIER Desalination 125 (1999) 17-23

DESALINATION

www.elsevier.com/locate/desal

Twenty-five years of desalination in the Canary Islands: an historical review of the application of reverse osmosis using case studies

and operational experience

Peter Moss a*, Jose Manrique de Lara y Gil b aKoch Membrane Systems (Fluid Systems), Unit 20, Berghem Mews, Blythe Road, London W14 OHN, UK

Tel. +44 (171) 371-4544; Fax +44 (171) 371-4450; email: FluidSystems UK@compuserve.com bKoch Membrane Systems (Fluid Systems), Fernando Calpena, 1-5 °A, Las Palmas de Gran Canaria, Spain

Abstract

The need for a good quality water in the Canary Islands was, and continues to be, the driving force for the use of reverse osmosis to desalinate brackish water and seawater for irrigation and potable use. A number of case histories are presented which show the development of the use of reverse osmosis, fxom the small plants built 25 years ago in the early 1970s to current-day use. The case histories show not only the increase in use of reverse osmosis but also plot the development of the performance of the available membranes. The case studies show the importance of bringing previous experience to the design and direct technical support to the customer by the membrane supplier in order to ensure satisfactory long-term stable system performance.

Keywords: Reverse osmosis; Brackish water; Seawater; Membrane performance history; Canary Islands

1. The early brackish water plants

The Centro Experimental de Los Moriscos of the Caja Insular de Ahorros in Las Palmas was part o f the pioneering work applying new technologies in the Canaries, in particular in

*Corresponding author.

agriculture. By the early 1970s, despite work on improving efficiency, the wells which had in the past supplied water for irrigation were insufficient to provide the required volume of water of a good enough quality and it was decided to use desalination to improve the quality and exploit aquifers of higher salinity.

After an initial abortive attempt using an HFF

Presented at the Conference on Desalination and the Environment, Las Palmas, Gran Canaria, November 9-12, 1999. European Desalination Society and the International Water Services Association.

0011-9164/99/$- See front matter © 1999 Elsevier Science B.V. All rights reserved PII: S0011-9164(99)00120-4

18 P. Moss, J.M. de Lara y Gil / Desalination 125 (1999) 17-23

Fig. 1. Photograph taken in 1979 of the Los Moriscos 200 m3/d brackish water RO plant.

reverse osmosis (RO) plant, contact was made with Fluid Systems in San Diego, and in 1978 a 200 m3/d brackish water RO plant was purchased and installed in the Centro Experimental de los Moriscos (Fig. 1). This plant used Fluid Systems' TFC ® 4600 4" elements with a polyetherurea thin-film composite membrane -- some of the early TFC ~ membranes produced. The plant consisted of seven tubes, each containing six elements, tubes in parallel operating at 50% recovery, producing a permeate with conductivity of 500-1,000 ~ts/cm from a feed of about 10,000/~S/cm at a pressure of 30kg/cm 2. The pretreatment was sodium hexametaphos- phate and acid and cartridge filters. The unit incorporated a cleaning system and degassing tower.

The feed water was pumped from an existing well on the site to a storage tank and from there transferred to the plant. The permeate was used to irrigate the crops in the Centro Experimental.

Studies were also carried out on the effect of irrigation water quality on the crops.

The plant operated very successfully, with the elements lasting between 5 and 7 years. Many farmers came to know about and see the plant and thus started the business of constructing RO plants on the island. The plant was finally closed and today stands partially dismantled.

Another of the early plants was the one at El Cardonal, Telde, built in 1985 with a capacity of 350 m3/d which was also used for crop irrigation. This plant used Fluid Systems' TFC ® 8021MP 8" elements with a polyetherurea thin-film composite membrane, which had improved performance over the earlier membrane. The plant consisted of four tubes each containing six elements, tubes in parallel operating at 50% recovery, producing a permeate with conductivity of 125/~S/cm at start-up from a feed conductivity of about 11,000 ~S/cm.

This plant also operated very successfully

P. Moss, J.M. de Lara y Gil / Desalination 125 (1999) 17-23 19

Table 1

Feed and permeate water analysis for the El Cardonal 350 m3/d brackish water RO plant from 1986 to 1990

Water analysis, mg/l

02/10/86 26/12/86 08/04/87 29/08/87 14/10/88 10/12/90

Feed

Ca 572 763 710 455 668 759

Mg 897 810 923 890 904 1,115

Na 768 776 800 670 780 850

K 52 52 57 43 55 43

HCO 3 777 762 711 682 622 679

SO4 269 470 648 440 868 330

NO3 64 51 97

CI 4,192 4,079 4,305 3,717 3,968 5,261

SiO2 49 50.5 23.8 45.5 25 43

pH 6.3 6.19 6.31 6.51 6.15 6.1

Conductivity 11,450 11,940 12,240 11,810 10,210 13,370

Sum ions 7,576 7,762.5 8,241.8 6,942.5 7,941 9,177

Permeate

Ca 3 3 3 2 10 21.6

Mg 1.5 1.5 4.3 2.4 11.2 22.6

Na 12.8 18.4 21.2 28.1 32.5 61

K 2 2 2 3.5 3.1 4.3

HCO 3 6 6.1 11.6 3.1 22.6 50.3

SO 4 0 14.4 0 3.4 3.4 2.5

NO 3 l 2

CI 47.8 43.31 52.5 62.8 85.2 163.6

SiO 2 4 4.1 5 6

pH 4.46 4.49 4.3 5.03 5.12 4.8

Conductivity 125 171 202 336 315 642

Sum ions 77.1 92.81 94.6 110.3 168 343.9

with the elements lasting 6 years due to the plant being serviced and maintained by the membrane supplier through their representative in the Islands. Good records were kept and a review of the water analysis (Table l) shows a feed water with increasing salinity during the latter period and a steadily increasing permeate salinity.

This increasing salt passage in the permeate (Fig. 2) was a feature of the polyetherurea membrane, which was not as robust as the modern polyamide membranes.

4 0 0 . . . .

300 ' i ~ 200

100

0 ' ~ - -1

0 500 1000 1500 2000

Elapsed Time (days)

Fig. 2. TDS vs. time for the E1 Cardonal 350m3/d

brackish water RO plant from 1986 to 1990.

20 P. Moss, J.M. de Lara y Gil / Desalination 125 (1999) 17-23

It was found that as a "rule of thumb" the salt passage doubled in 3 years which can be seen to hold true from a plot of the permeate quality. The permeate TDS started at about 70 mg/l and rose to 350 mg/l in the 6-year period.

In 1991 the TFC '~ 8021MP elements were replaced. Then in 1994 the elements were replaced and upgraded with TFCL 8821HR elements with a polyamide thin-film composite membrane.

The polyamide membrane was introduced in 1987; however, due to a lawsuit, Fluid Systems was prohibited from selling the polyamide membrane from 1990 to 1993. Upon conclusion of the litigation in favour of Fluid Systems in 1993, the polyamide was reintroduced.

The plant started up with increased capacity, operating at 55.5% recovery and producing a permeate with conductivity of 237#S/cm at a pressure of 22.5kg/cm 2. The plant was later extended by adding two tubes and an additional plant with four tubes was installed. Records show that the plant operated in a stable fashion to date, with permeate conductivity ranging from 190 to 240/~S/cm.

2. Current brackish water plants

A more recent plant is in Pedro Hidalgo, built in 1998 with a capacity of 780 m3/d. Again it is used for crop irrigation. This plant takes on board previous experience in terms of design of the layout, pipe-work design, control and flushing. The plant uses Fluid Systems' TFC® 8822HR 8" elements with polyamide thin-film composite membranes, which have the latest performance specification. The plant consists of six tubes of six elements in a 4:2 array, operating at 65% recovery, producing a permeate with conduct- ivity of 70/.zS/cm from a feed of 4380/~S/cm at a pressure of 22.5kg/m 2. It is interesting to note that, although this has a relatively low salinity, the feed water contains high levels of silica,

80-90 mg/l and some iron 0.13 mg/l. The silica is recorded at 230-260 mg/l in the concentrate. The silica is the limitation on recovery. With good design and operation, the plant has only been cleaned once since start-up to remove iron fouling.

In contrast the plant at Pozo Fabelo built in 1998 with a capacity of 1007m3/d, has a very high salinity feed, greater than 11,000 mg/l TDS, and high silica at 50 mg/l. This plant uses Fluid Systems TFC ® 8832HR Magnum ® elements with polyamide thin-film composite membranes. These elements are 60" long, as opposed to the regular 40", giving the benefit of needing only four elements per tube instead of the conven- tional six elements per tube. This eliminates some O-rings and interconnections, thus reducing by one-third the number of potential sites for leakage. The plant consists of eight tubes each containing four elements in a 5:3 array, operating at 70% recovery, producing a permeate with conductivity of 420/~S/cm at a pressure of 28 kg/m 2.

3. Historical trend in brackish water membrane performance

It is interesting to plot the development of the performance of the membranes with time. Fig. 3 shows the change of specific flux with time for the Fluid Systems' polyetherurea and polyamide brackish water membranes.

..... 0 .15 • ~ Premium ~ 1 1 p

o.1 Polyamide Membrane

0 .05 m Polyetherurea Membrane I,I.

o 77 79 81 83 85 87 89 91 93 95 97 99

Year

Fig. 3. Flux vs. year for Fluid Systems' TFC ® Brackish elements.

P. Moss, J.M. de Lara y Gil / Desalination 125 (1999) 17-23 21

100 Premium ~ o~,._.. 99.5 Polyamide Membrane - ~ - t l ~ [

.9 98.5

~ 98 ~ Polyetherurea Membrane

97.5 77 79 81 83 85 87 89 91 93 95 97 99

Yea r

Fig. 4. Rejection vs. year for Fluid Systems' TFC ® brackish elements.

The marked improvement from the polyetherurea to the polyamide can be seen clearly with a more than doubling of flux, the polyamide continuing to improve in the early 1990s.

The paper does not address the advance in the membrane technology during 1995 when the first ultra-low-pressure TFC '~' membranes were introduced, aimed at lower salinity and municipal applications. This type of membrane is not appropriate for the high salinity brackish waters of the Canary Islands.

Fig. 4 shows the change of standard rejection with time for the Fluid Systems polyetherurea and polyamide brackish water membranes.

Again, the marked improvement from the polyetherurea to the polyamide can clearly be seen with the salt passage reducing from 2% to 0.5% during the period. The advanced Premium polyamide membrane is shown with a salt passage of 0.3%.

TFC "' 2021SS 8" elements with polyetherurea thin-film membranes. The plant consisted of four trains, each with eight tubes each containing six elements; the tubes are in parallel operating at 40% recovery, producing a permeate with conductivity of 800 #S/cm from a feed of about 48,000#s/cm, at a pressure of 60kg/cm 2. The pretreatment was addition of scale inhibitor and cartridge filtration.

The plant was later extended, increasing the number of tubes to 10 on each train and increasing the capacity of the feed pumps.

The owner of the plant tried seawater elements from other manufacturers, partly due to the problems of supply due to the litigation mentioned earlier, and also partly to test and assess the performance of the various seawater elements available in the market in order to choose the best for the plant.

In 1995 one of the trains was replaced with Fluid Systems' TFC ® 2822SS 8" elements with a polyamide thin-film composite membrane. The output increased to 26 m3/h and the conductivity reduced to 580#S/cm. The train has now run in a stable fashion for 4 years. Due to the good operation, in May 1999, the elements in another of the trains were replaced with Fluid Systems' TFC ® 2822SS Premium elements. The Premium is one of the highest rejection membranes available with a minimum rejection of 99.8%. On start-up the output increased to 33m3/h and recovery to 45%. The permeate conductivity was reduced to 368 ~S/cm.

4. The early seawater plants

Many aquifers contain seawater, and in the late 1980s work began to exploit these as a source of water for irrigation. One of the earliest plants was built in Maspalomas in 1988 with a capacity of 2000 m3/d, the permeate being used for crop irrigation. The plant used Fluid Systems

5. Current seawater plants

A more recent plant is in Roquemar, built in 1998 with a capacity of 700 m3/d. Once again, it is used for crop irrigation. This plant takes on broad previous experience in terms of design of layout, pipework design, control and flushing. The plant uses Fluid Systems' TFC ® 2822SS Premium 8" elements with a polyamide thin-film

22 P. Moss, J.M. de Lara y Gil / Desalination 125 (1999) 17-23

composite membrane. The plant consists of 10 tubes each containing six elements, tubes in parallel, operating at 45% recovery, producing a permeate with conductivity of 335/~S/cm from a feed of 42,500#S/cm at a pressure of 56 kg/cm 2.

In contrast, the plant for Consorcio de Fuerteventura, built in 1997 with a capacity of 3600m3/d, uses a unique design which is a combination of Fluid Systems' TFC ® 2832SS Magnum elements and TFC ® 2822SS Premium elements, both with polyamide thin-film composite membranes [1]. The Magnum ele- ments are 60" long, as opposed to the regular 40", giving the benefit of needing only four elements per tube instead of the conventional six elements. This eliminates some O-rings and interconnections, thus reducing by one-third the number of potential sites for leakage. The Premium elements have a minimum rejection of 99.8%. The Consorcio de Fuerteventura plant is unique in that it is a 26:14 array; the 26 tubes in the first bank contain four TFC ® 2832SS Magnum ® elements and the 14 tubes in the second bank contain TFC ® 2822SS Premium elements. This combination is to give the optimum quality of permeate under the given operating conditions. The plant operates at 45% recovery producing a permeate with conductivity of 600/~S/cm from a feed of 50,400k~S/cm at a pressure of 62 kg/cm 2 [2].

The daily operating data are normalized using the Fluid Systems' NORMPRO program. The program normalizes the operating data to specific conditions and calculates the mass transfer coefficients for water and salt and the pressure drop coefficient; from these a normalized flow and rejection can be obtained. The normalized flow (Fig. 5) shows a steady trend.

The scatter is due to the plant flow being varied on demand. The normalized rejection (Fig. 6) again shows a steady trend.

The figures show that the performance has been excellent, the combination of membranes is consistently producing a permeate with conductivity less than 600 #S/cm.

6. Historical trend in seawater membrane performance

As before, it is interesting to plot the development of the performance of the mem- branes with time [3]. Fig. 7 shows the change of specific flux with time for the Fluid Systems' polyetherurea and polyamide seawater membranes.

The marked improvement from the polyetherurea to the polyamide can be seen clearly, with a significant increase of flux. Fig. 8 shows the change of standard rejection with time

1.4

121 • 14. -=

M

Z Q

=.

0.8

0.6

0.4

v v v v

J • 6 0 50 100 150 200 250 300 350 400 450

Time (days)

Fig. 5. Normalized flow vs. time for the Consorcio de Fuerteventura 3600 m3/d s ea wa ter RO plant.

500

P. Moss, J.M. de Lara y Gil / Desalination 125 (1999) 17-23 23

A 1

"~ .9

0 50 100 150 200 250 300 350 400 450 500

Time (days)

"=~ 100 Premium I~

(~ 99.5 I / ~ ' - -

u 99 Potyamide Membrane Polyetherurea Membrane

"~" 98.5 n, 82 84 86 88 90 92 94 96 98

Year

Fig. 6. Normalized rejection vs. time for the Consorcio de Fuerteventura 3600 m3/d seawater RO plant.

A 0.06 Premium

0.04 • , \ • ~ / Polyamide Membrane X O. 0 2 Polyetherurea Membrane ,-i ,7 0

82 84 86 88 90 92 94 96 98

Year

Fig. 7. Flux vs. ycar for Fluid Systems' TFC sea

elements.

for the Fluid Systems polyetherurea and polyamide seawater membranes.

Again the marked improvement from the polyetherurea to the polyamide can clearly be seen with the salt passage reducing from 1.1% to 0.4% during the period. The advanced Premium polyamide membrane with a salt passage of 0.2% is shown.

7. Conclusions

The case histories presented show the development of both the brackish water and seawater membranes and illustrate the many improvements made in membrane performance

Fig. 8. Rejection vs. year for Fluid Systems' TFC ® sea

elements.

in terms of increased flow and enhanced rejection. They demonstrate the successful use of this technology over a 25-year period. The Magnum ® and Premium elements are superior products and, with the innovative designs, promise significant future benefits. The good operation confirms the importance of bringing field experience to the design and also the advantage of technical support direct to the customer from the membrane supplier in order to ensure satisfactory, long-term, stable system performance.

References

[1] H. Beets and R.L. Truby, Seawater desalting using membranes -- past, present and future, KAE NV Conference, Curacao, 1998.

[2] J.A. Medina, C. Castaneira and A. Rodriguez, Enlargement and Refurbishing of Seawater Desalination Facilities in Fuerteventura, IDA Conference, San Diego, 1999.

[3] J.H. Sleigh and R.L. Truby, Development of commercial reverse osmosis spiral wound seawater desalting systems, IDEA Conference, Puerto Rico, 1975.