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ELSEVIER Desal inat ion 116 (1998) 123-134

DESALINATION

Desalination in Tunisia: Past experience and future prospects

Fethi BenJemaa*, Imed Houcine, Mohamed Hachemi Chahbani

lnstitut National de Recherche Scientifique et Technique, B.P. 95, 2050 Hammam-Lif Tunisia, Tel.: +216-1-430044, -430053, Fax: +216-1-430934

Received 7 July 1998; accepted 16 July 1998

Abstract

The water desalination problem in Tunisia is reviewed. A detailed historical background is given and current as well as previous efforts in the domain of desalination research and development are presented. This paper covers a number of ambitious initiatives, which tackled the water shortage problem in Tunisia through the adoption of original programs and solutions adapted to the local environment. Such programs put forth effort to take advantage and harness the abundant and readily available renewable energy sources toward the production of fresh water. Presented are also the prospects of water desalination in Tunisia, which is currently pursued as a major component of an overall strategy to meet future water demand and enhance the quality of drinking water in various parts of the country. Current and projected water demand and water availability figures are given along with potential renewable energy sources. Strategic studies along with facts and statistics on the regional development show that brackish and seawater desalination is becoming a must for the growth and the sustainability of the socio-economic progress in the Mediterranean region.

Keywords: Tunisia; Renewable energies; Research and development

I. Introduction

Located on the southern border of the Mediterranean basin, Tunisia has very limited water resources, an important part of which is being brackish, thus, unsuitable for drinking or irrigation. Having an arid to a semi-arid climate, the country suffers from a lack of precipitation which is extremely variable not only in space but also in time. Some northern regions get an annual rainfall average of more than 1,300 mm per year (e.g., the

*Corresponding author.

northwestern region of Tabarka). In contrast, regions in the far south receive less than 100 mm of rain per year. Aside from regional va r i ab i l i t y , r a in fa l l q u a n t i t i e s vary dramatically from one year to the other with occasional drought periods extending for few consecutive years. Water transfer techniques are practiced locally (e.g., the aqueduct transferring water from the Medjerda river in the northwest to the citrus region of the Cap- Bon in the northeast). However , due to practical and economic constraints, water transfer to the southern regions proves to be impossible.

0011-9164/98/$ - see front matter © 1998 Elsevier Science B.V. All rights reserved PII S 0 0 1 1 - 9 1 6 4 ( 9 8 ) 0 0 1 8 9 - 1

124 F. BenJemaa et al. /Desalination 116 (1998) 123-134

In addition to fresh water scarcity, Tunisia is faced with an exponentially growing demand for water so as to fulfil the needs of its developing industrial, agricultural and demographic sectors. Aware of these challenges, the country was a pioneer in implement ing a comprehens ive water desalination research and development program focusing on the use of renewable energies for brackish and seawater desalination purposes.

2. Historical background

The need for adopting desalination to solve the fresh water availability problem in Tunisia, in particular, and in the southern border of the Mediterranean basin, in general, is becoming increasingly evident. Currently, water desalination is being practiced on a small scale in Tunisia, mainly in the southern part of the country where the water shortage is most acute and in the islands of Kerkenah. However, the situation is destined to change dramatically in the near future; and desalination will have to be undertaken on a larger scale. In fact, forecasts show an exponentially increasing water demand, whereas the country's finite water resources are being almost fully exploited at present.

Given its arid to semi-arid climate and the abundance of solar energy, Tunisia was a pioneer in exploring the feasibility of water desalination by means of solar energy. Research work and experiments in the domain were carried out as early as the 1920's. In 1927, an experimental apparatus for water desalination by evaporation using solar energy was designed in Tunis. Two solar evaporators were then installed in the south of Tunisia for supplying potable water to the French military troupes in the region. The first was installed in May of 1929 in the city of Ben Garden and the second was installed in March of 1930 in the city of Fort- Saint [1].

In 1962, a solar energy group was created within the Tunis ian Atomic Energy

Commiss ion (Commissar iat h l 'Energie Atomique, CEA) with the purpose of carrying out feasibility studies for the use of solar energy in social projects. In particular, were considered, water desalination projects for supply ing potable water to isolated communities, schools, hotels and resorts.

In the per iod of 1967-68, three desalination stations using solar energy were constructed in the cities of Chekmou, Chibou, and Mahdia [2]. These stations consisted of single basin solar stills covered with glass sheets. The station of Chekmou was made of a total of 60 solar stills arranged in two rows on a 1,800 m 2 platform which also served for collecting occasional rain water (approx. 150 mm/y) [3]. The station had 440 m 2 of glass- covered area and produced an average of 2 m3/d of distilled water from a source brackish groundwater of 6 g/1 salinity. Studies on the energy balance showed that due to poor insulation and high energy losses in the stills (such as ground and edge heat losses), only 40 to 50% of the total solar radiation received by the stills, served for the evaporation process.

Very similar to the one built in Chekmou, the desalination station of Chibou had a smaller capacity [4]. Constituted of a total of 10 solar stills with a glass covered area of 48 m 2, the station produced an average of 300 l/d of distillate. The source brackish groundwater had, however, a very high level of salinity (19 g/l).

Feasibility studies were carried out for several other stations, mainly the three desalination sub-stations of Sidi-Elhani [5]. These sub-stations were designed to include 150, 100 and 50 solar stills respectively with a total evaporation area (glass-covered) of 825 m 2, 550 m 2 and 275 m 2, respectively. The source brackish groundwater to be used for feeding the three sub-stations had salinity levels of t0 g/l, 5.5 g/1 and 9.2 g/l, respectively. The average distillate production was estimated to amount to 9900 1/d (i.e., 4950 l/d, 3300 l/d and 1650 l/d from each sub-station, respectively). Given the relatively high level of precipitation in the region (an

F. BenJemaa et al. /Desalination 116 (1998) 123-134 125

/~Aerogenerator 1.1 kW

Aerogenerator" . . . . . . . . . . . . . . . ~ 4.5 kW

Brackish - .. Brakish water . ,. water Wel l . . ~ ..

~ ~ (4 g/I) t anK ~.r d.d

• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . / '

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H m / .- 7' 7' ~ . . " - . - . - - / a n

. . . . ~ . . . " /z r~ . / ~ . . ~ | C o n t r o l I H I / [ .:/xr,#~Z~ ' ~.".~ i Room

Photovoltaic .::. !" J ~ r / ~ " p a n e l s ~ . / " ~ / / m ~ / " ~.."

i" i" ,..' / ; i ...................... " ........................................... ' / 0.25 m ' / h

v

Beachwell ~ 0.$8 mJha Seawater tank Brine tank

1.25 mVh at 1 g/I

i

/

/

water tank

1.25 mVh (7 g/l)

Sea

Fig. 1. Schematic site view of the experimental desalination station at the INRST.

average of 250 mm of rain per year), as compared to the southern part of the country which has a more arid and dry climate, the platforms of the three proposed stations, covering a total area of 4,937 m 2 (i.e., 2,480 m 2, 1,638 m 2 and 819 m 2, respectively), were est imated to contr ibute significantly in increasing the total fresh water that can be delivered by these stations. In all these solar desalination stations, underground tanks were planned to store the collected desalinated water as well as rainwater. The collected water flows to the tanks by simple gravity, therefore pumping would only be needed for distribution.

A typical solar still unit used in the stations described above consisted of a single basin solar still of dimensions varying from 6.14 m x 0.78 m (for the station of Chibou) to 9.42 m x 0.78 m (for the station of Chekmou). The stills were oriented facing south so as to

maximize the period of exposure to solar radiat ion. The cover ing is made of transparent regular glass sheets with a thickness of 2 mm and a slope of 10%. The stills had concrete blackened bases and the surrounding walls were built with hollow bricks.

In the feasibility studies of some proposed stations, additional pre-heating basins were introduced upstream so as to enhance the per formance of the station by further exposing the feed water to solar radiation prior to entering the stills. The basins were 5 m long, 2 m wide and 2 m deep and had a particular design in order to maximize the absorption of solar radiation. The southern facade, painted in black, had a slope of 45 ° so as the vertical cross-section of the basin would have a triangular shape. Consequently, the inclined and blackened facade would constitute a perfectly oriented heat collector,

126 F. BenJemaa et al. / Desalination 116 (1998) 123-134

which intercepts sunrays. The rest of the reservoir walls were made of two partitions between which is superimposed a layer of insulation filling composed of a maritime organic fiber in the form balls which can be collected locally on the beach.

In the early 1980's, the desalination research and deve lopment program in Tunisia entered a new era. As a part of a national research program in the domain of solar energy, an experimental station for brackish and sea water desalination using renewable energies was built in cooperation with the French Commissariat for Atomic Energy (Commissariat ~t l'Energie Atomique (CEA), Cadarache, France) in Borj-Cedria in the southern suburbs of Tunis. This experimental desalination station, a part of the National Institute for Scientific Research and Technology (INRST), was completed in 1982 and included: - A reverse osmosis pilot plant for sea water

desalination with a capacity of 0.25 m3/h, - An electrodialysis pilot plant for brackish

water desalination with a capacity of 1.25 m3/h,

- Two aerogenerators of 1125 VA and 4500 VA used for pumping and desalination respectively, and

- Two photovoltaic generators of 0.384 kW and 3.5 kW used for pumping and desalination respectively.

Fig. 1 shows a site map of the experimental desalination station for brackish and seawater desalination using renewable energy sources at the INRST.

Since its establishment in 1982, the experimental desalination station of Borj- Cedria has served as the focal point for water desalination research in Tunisia and helped train a number of researchers in the domain. Numerous research projects have been carried out by the INRST team in collaboration with other research teams from nat ional and in te rna t iona l academic institutions and consulting firms.

In addition to the desalination pilot plants mentioned above, the desalination laboratory

at the INRST has built a mobile RO desalination plant with a capacity of 3 m3/h and a family size RO prototype with a capacity of 36 1/d. A considerable effort is currently needed to rehabilitate and upgrade the experimental desalination station of the INRST.

3. Current status of water desal ination in Tunisia

Currently a number of R&D projects are undertaken at the INRST Desalination Laboratory as well as in other key research institutions and universities in Tunisia [6, 7]. Numerous research projects on water desalination were launched by the Tunisian secretariat of State for Scientific Research and Technology since 1993, in addition to other projects in the f ramework of bilateral cooperation with a number of international institutions. These projects deal mainly with coupling of desalination processes with renewable energy sources (solar and wind). In fact, a lot of research work still needs to be done worldwide in order to optimize the use and coupling of renewable energies with desalination systems. Currently, desalination plants powered by renewable energy sources account for only 0.02% of the total production of desalinated water worldwide [8]. Among the renewable energy types that need to be considered are wind, geothermal, solar photovoltaic and active solar. These types of energy are very suited for use in small high reliability desalination systems for remote areas. Although the conversion of some types of these energies is still commercially unattractive, they are regarded as having a good long term potential given their geographical synergy with locations where desalination is most needed [9].

In 1983, an RO desalination station with a capacity of 4,000 m3/d was installed in the Kerkenah Islands to provide potable water to its inhabitants as well as to the huge influx of summer tourists. Summer population of the islands increases dramatically. For example,

F. BenJernaa et al. / Desalination 116 (1998) 123-134

Table 1 Design parameters for the two main RO plants for municipal water production

127

Parameters Kerkenah Gab~s

Technology RO Membranes Spiral wound cellulose acetate Total capacity, m3/d 4,000 Number of units 4-2 stages each Feed water salinity, g/l 3.6 Feed water temperature, °C 2 5 Feed pressure, bar 2 8 Recovery ratio, % 4 0 - 4 5

RO Spiral wound polyamide 22,500 (30,000 planned) 3-2 stages each (4th unit planned) 3.2 30 initially pumped at 70°C 16 6 5 - 7 4

vc (9~ 4220

Other (s%) 260o

ED (13%) 6O56

RO (70%) 32OOO

Fig. 2. Capacity (m3/d) distribution according to plant technology.

the water demand for the Kekenah Islands increased from 27,000 m3/d in January of 1995 to reach 86,000 m3/d in August of the same year.

In an effort to make up for the water shortage and to ameliorate the water quality for the southern part of the country, a new RO desalination station was built in 1995 in the city of Gab6s with a preliminary capacity of 22,500 m3/d. As the water demand of Gab~s City and its suburbs is expected to increase due to its growing population and to its expanding industrial compounds, the production capacity of the desalination plant will be increased to reach 30,000 m3/d at a later stage. The station is supplied with a

brackish groundwater transported over a 70 km distance from the Chott-El-Fejij region. The source feed water of 3.2 g/1 salinity is pumped out at a geothermal temperature of 70°C. Prior to being channeled via a pipeline to the desalination plant, the water has its temperature lowered down to 30°C by means of cooling towers in the form of aeration cascades. The main design parameters for the two RO desalination stations of Kekenah and Gabrs are shown in Table 1.

Currently, the total installed capacity of water desalination plants in Tunisia amounts to approximately 45,000 m3/d. From a production point of view, reverse osmosis is the predominant process with a total capacity of about 30,000 m3/d making almost 67% of the total production of desalinated water in Tunisia (Fig. 2). The RO process is followed by ED (14%) and VC (10%). Municipal capacity accounts for about 60% of desalted water produced. Concerning the source feed water, brackish water makes more than 90% of the total capacity; however, seawater desalination is also expected to increase especially at coastal and island resorts and towns.

Unlike the situation for the production of desalinated water for municipal use, where reverse osmosis is being the dominant process, thermal processes account for most of the industrial water production. In this context, we cite mainly the production of distilled water by the Tunisian Chemical

128 F. BenJemaa et al. / Desalination 116 (1998) 123-134

Table 2 Thermal desalination plants for industrial water production in Gab~s

Factory Process Units Vapor produced (ton/h) at 60 bar and 400°C

Year installed

Phosphoric acid MSF 1 (8 stages) 14 Phosphoric acid MED 1 (2 effects) 15 D.A.P MED 2 (2 effects) 2 × 20 Phosphoric acid MED 2 (2 effects) 2 × 13.5 Ammonitrate M I ~ 2 (2 effects) 2 × 10 Phosphoric acid M t ~ 1 (3 effects) 20 Phosphoric acid MED 1 (2 effects) 25

1972 1974 1979 1982 1983 1985 1998

Group (TCG) for the production of sulfuric, phosphoric and nitric acids. The choice of thermal processes stems from two main reasons: the need for ultra pure water and the readily available heat rejections from the acid production factories. Vapor produced from the seawater used for cooling in the acid production process is condensed to produce distilled water. Table 2 shows a listing of thermal desalination plants installed by the TCG in its chemical industry compound in Gab~s.

The TCG has also adopted the reverse osmosis process to produce industrial water in some of its factories, e.g., the Skhira factory in which an RO plant with a 9,000 m3/d is in operation since 1988. The membranes used are of cellulose acetate type operating under a pressure of 30 bars. The source brackish feed water has a salinity of 10 g/1. The Skhira RO plant for industrial water production has a recovery ration of 40-45%.

Other desalination processes such as ED, EDR and VC are used by a number of users throughout the country; we cite mainly the tourist resorts, the cement factories, the food industry and the petroleum exploration companies.

With its modest but diversified experience in the desalination field, Tunisia has gained a considerable expertise in operating and maintaining desalination units. A number of problems have been encountered and solved successfully. For the RO installations, the

encountered problems are mainly biofouling of the membranes, algae formation in the intake structures and the pretreatment stages, and corrosion due to acid dosages. For the case of the RO station of Gab~s, it has been noticed that a continuous chlorination during the first year of operation (1995) induced an aggravation of the biofouling problem causing a clogging of the membranes and a pressure drop in the RO modules. However, when continuous chlorination is stopped and carried out only periodically in the form of high dosage shocks, the problem was solved [10]. It has been found that the small number of bacteria which succeed to survive the chlorination stage find a favorable medium for proliferation downstream of the point of dechlorination by the sodium bisulphite. In fact, the chlorination oxidizes the organic matters present in the feed water breaking their long chains into smaller fragments that can be easily digested by the bacteria. Finding an abundant food source, the bacteria grow exponentially in numbers causing a biofoul ing problem in the membranes. Since the chlorination process was eliminated after the first year of operation (except for occasional shock dosages), and for the last two years, the RO station of Gab~s has operated with no biofoul ing problems. However , after discontinuing the chlorination, green algae proliferation was noticed in the oxidation basins and the sand filters. Nevertheless, the

F. BenJemaa et al. /Desalination 116 (1998) 123-134 1 29

problem of algae formation can easily be overcome by covering the filters to keep out sunrays and daylight necessary for the photosynthesis process needed for algae growth.

It has been also noticed that the energy consumption of the Gab~s RO station depends on the feed water temperature [10]. As the temperature of the source water increases, the energy consumption of the desalination station decreases, hence its yield increases. Since the feed water of the station comes from a geothermal source (Chott-E1- Fejij) at 70°C, and has to be cooled down before entering the station, the coolers have been adjusted to provide an optimal feed water temperature for the RO plant while taking care not damage the membranes by ex cee d ing the m a x i m u m a l lowable temperature.

4. W a t e r r e s o u r c e s s i tua t ion

Potable water demand in Tunisia is expected to reach 877 million m3/y by the year 2025. Fig. 3 illustrates the projections for potable water demand for the five North African countr ies . Compared to the neighboring countries, Tunisia has the smallest increase rate in water demand over the 30-year period 1995-2025 this is due to the fact that Tunisia has the lowest population growth rate. Demographic studies predict that the growth rate of the Tunisian population will decrease from 1.9% in 1990 to reach 1.3% by the year 2015 and will remain constant thereafter. However, due to the increase in the living standards, the specific water consumption is expected to increase from 132 LCD in 1990 to 262 LCD in 2025. The notion of specific water consumption is a very important factor in estimating future demand and taking into account the population growth alone will give inaccurate water demand projections. As illustrated in Fig. 4, the specific water consumption per capita in the North African region will increase dramatically in the next few years.

12000-

i- o E

E

• -B--Algeria , -~- - Egyp4

--N-- Libya + M o r o c c o

1966 2000 2005 2010 2015 20~0

Year 20"25

Fig. 3. Projections for potable water demand in the North African region.

--e--Alge~a ,--Ik- Egypt

350 . -A . - Ubya -N- -Morocco

11~1 1 1 ~ 2(~0 20~ 2010 2015 ~ 20~5

Year

Fig. 4. Projected specific water demand in the North African region.

This increase, which is due to a progressive urbanization and a steady rise in the standards of living, will have a considerable impact on the increase rate in the demand for potable water.

Given the overall limited water supply in the region and the increase in the demand, it is estimated that the North African region will suffer a huge deficit in meeting its water needs in the near future (Fig. 5). By the year 2025, potable water deficit is estimated to

! 30 F. BenJemaa et al. / Desalination 116 (1998) 123-134

A

0

|

I -+-A~e.a -,--SQ~ 1 .~' • - i - - Ubys -N--Morocco / /

- - , y

Algeria

2000 2005 2010 2015 2020 2025 Year

Fig. 5. Projected water deficit in the North African region.

reach 500 million m3/y for Tunisia alone and approximately 15,000 million m3/y for the whole region. Potential water resources development in the future such as dam constructions, water transfers and water conservation projects can only cover the deficit partially. For remote and isolated locations (e.g., the south of Tunisia, the northwestern coast of Egypt, the southern region of Morocco and the western region of Algeria), brackish and seawater desalination projects are estimated to be the most economic and practical solution for solving the problem of fresh water scarcity.

5. Energy situation

Unlike the neighboring countries of Algeria and Libya, Tunisia has very modest fossil energy reserves. Aware of the challenges of energy availability, Tunisia was a pioneer in exploring the use of renewable energies for desalination. An abundance of solar, wind as well as geothermal energies are available and can be harnessed for use in different sectors mainly in the water desalination sector. Crude oil reserves in the

8000

~" 7000 0 Pc 6000 0

'~ 5000

4o00

3000

o 2000

~1000 J Libya Tunisia

Fig. 6. Crude oil reserves (million tons).

12

A e o- 10 c 0 ~. 8 ~2

> ,

~ 6 e -

~ 4

E "'- 2 o.

[ -~-Availsble 1 -e - - Demand ]

0 i , i J i , i

1979 1994 2001 2010 Year

Fig. 7. Energy demand and availability in Tunisia.

1000

J ~ \ I'-"-F "b. 700 / # f , , , . , , ~ t...,_~r ' 600 f t J / _ ~\~\~ 1 '''-M"y

I l l / f ~ \~ t I + Jun. j ~= soo JlJT/ ~ \ ~\~ I -+'Jul I

4oo #tIT ~ ~ \ ~ I-~ Aug. I

300 # t# /F ~ \ ~ \ ~ j--.-o=, j 200 ~' , f## " ~ I " - Nov. I

100

0 3 5 7 9 11 13 15 17 19 21 23

Hours

Fig. 8. Hourly variations of solar intensity in Tunis City.

F. BenJemaa et al. /Desalination 116 (1998) 123-134 13 1

35 251000

G 30

® 201000 25

~'20 ~ 151000-

g15 ¢> ~ 101000" m 10 "

t • 1- 5 51000-

0

1 3 5 7 9 11 13 15 17 19 21 23 Hours

Fig. 9. Hourly variations of ambient temperature in Tunis City.

, tAt tAA Jan Feb ~ Apr May Jun Jul Aug Sept Oct Nov Dec

Months

Fig. 12. Total monthly solar radiation in Tunis City.

9OO

800

~7oo

• ~ 400

~ 30o 2OO

0

"'J=~ r -4 . -F .b I f ~ k . \

- ' - " I R ' / F r ~ \ ~ % - ' - J " I 1 1 1 [ . , , . ' ~ k ~

. - . - Auo I I [ I f y,,,o..%~ " ~ X - - sop # # t l v ~ ~91

j--.-Nov t #I14/[ X~X~X

3 5 7 9 11 13 15 17 19 21 23 Hours

Fig. 10. Hourly variations of solar intensity in Gab~s City.

40

~30

E i

=

5 t /

i 0 } , , , , . . . . . . . . . . . . . . . . . . .

1 3 5 7 9 11 13 15 17 19 21 23

Hours

Fig. 11. Hourly variations of ambient temperature in Gab~s City.

251000

201000- E

._~ 151000-

~ 101000-

k-

Jan Feb Mar Apt May Jun Jul Aug Sep Oct Nov Dec

Months

Fig. 13. Total monthly solar radiation in Gab~s City.

country are well below those available in other North African countries (Fig. 6).

Similar to the water demand, energy demand in Tunis ia keeps increas ing exponentially and is expected to reach 10.2 million tep/y by the year 2010. However, available primary energy (crude oil and natural gas) is progressively declining due to reserve depletion. In fact, from the late- 1990's, Tunisia started to have an energy deficit and the production no longer covers its demand (Fig. 7).

Unlike the fossil energy situation, Tunisia has an abundance of renewable energy

13 2 F. BenJemaa et al. /Desalination 116 (1998) 123-134

sources [11]. Located within the solar belt of the world, Tunisia has ample solar energy throughout its territories. The average solar radiation intensity varies from 350 cal/cm2/d in the north to 450 cal/cm2/d in the south with a total sunshine period of 3,500 hours per year. Figs. 8 and 9 show the monthly averages of hourly variations of solar in tens i ty and ambien t t e m p e r a t u r e throughout the year in Tunis City. Figs. 10 and 11 show the monthly averages of hourly variations of solar intensity and ambient temperature for the city of Gab~s located fur ther south where usual ly h igher temperatures are registered. Figs. 12 and 13 shows the monthly sums of solar radiation received in Tunis and Gab~s, respectively.

6. Future prospects

Statistics on water resources availability and water demand projections show that the global i m p l e m e n t a t i o n capac i ty of desalination is expected to double within the next 20 years to reach 40-50 million m3/d worldwide [12]. By no coincidence, the leading user markets for desalination by the year 2015 will be the countries of the Mediterranean Basin along with the Arabian Peninsula, the Canary and the Caribbean Islands. In Tunisia, fresh water deficit is already a major problem in the southern part of the country and in the Kerkenah and Djerba Islands. To face the problem, new desalination installations are currently being constructed and future projects are planned.

Two RO stations similar to the one built in Gab~s are currently under construction and expected to be in operation by the year 1999. One plant at the island of Djerba and the other in the coastal city of Zarzis, each with a production capacity of 12,000 m3/d. After completion of these RO plants, the share of RO desalinated water in the country will increase to make more than 75% of the total production of desalinated water.

Planned is also a cogeneration desalination project to be implemented in the Gab~s

region at the city of Bou-Chemma [13]. This project consists of coupling an MSF plant for desalting seawater, with a production capacity of 2,500 m3/d, to the already existing 25 MWe gas turbine electricity generator. The innovative feature of this cogenerat ion project is the use of a heat storage pond for storing the waste heat recovered from the gas turbine generator. As the electricity power plant operates only for a period of four hours daily, the waste heat is stored in a 7,800 m 2 storage pond by mean of heat exchangers. Consequently, the MSF desalination plant will have ample thermal energy available throughout the day allowing it to operate continuously. The storage pond will have a depth of 6 meters and a total volume of 17,000 m 3 of water. Similar to solar ponds, the heat storage pond will have a salt gradient where the concentration increases with depth allowing the retaining of heat energy within the lower and most concentrated layer of water. As the demand for electricity increases in the future, the gas turbine will have to be operated for longer periods. As a result, the desalination plant capacity can be extended without having to increase the storage pond area. The MSF desalination plant will have 12 to 15 stages operating at temperatures ranging from 95°C to 75°C.

In addi t ion to the industr ia l size desalination installations, laboratory and pilot size plants are considered by research institutions throughout the country. Current research projects are focusing on critical topics in the field of desalination. Among the main topics considered are energy issues, cogeneration, operation and maintenance issues [6].

7. Summary and conclusions

Given its energy situation, with modest fossil energy reserves in one hand and an abundance of renewable energies (especially solar energy) in the other hand, desalination research and development efforts in Tunisia have emphasized on the use of renewable

F. BenJemaa et al. /Desalination 116 (1998) 123-134 133

energies , wi thout h o w e v e r neg lec t ing other c o n v e n t i o n a l a l t e r n a t i v e s . T h e T u n i s i a n e x p e r i e n c e in the f ie ld o f desa l ina t ion is worth being closely studied. In fact, Tunis ia makes a typica l case s tudy for desal inat ion needs a s s e s s m e n t in the M e d i t e r r a n e a n region.

Desp i t e the e c o n o m i c a l cons t ra in t s that ac ted in f a v o r o f o ther a l t e rna t ives than desal inat ion in the last 30 years, desal inat ion - e s p e c i a l l y p o w e r e d b y c o n v e n t i o n a l energies - have ga ined grounds notably in the last d e c a d e . L i k e w i s e , and due to technological advances in the domain and the a g g r a v a t i o n o f the w a t e r and e n e r g y p r o b l e m s , d e s a l i n a t i o n by m e a n s o f renewable energy sources is soon expected to b e c o m e a compe t i t i ve opt ion in m a n y parts o f the world.

Given the presented statistics on the needs and availabil i ty of water and energy, coherent regional s t ra tegies have to be e labora ted to f a c e f u t u r e w a t e r s h o r t a g e s in the Medi te r ranean region with desal inat ion being a m a j o r c o m p o n e n t . It is h igh t ime to s e r i o u s l y c o n s i d e r the d a n g e r o f an impend ing wate r shor tage with its d ramat ic social and economic consequences .

References

Ill Tunisian Atomic Energy Commission, R61e de l'6nergie solaire dans le dessalement de l'eau, Rapport CEA No. 14, 1965.

[2l Tunisian Atomic Energy Commission, Quelques exp6riences sur le dessalement solaire de l'eau saum~.tre par r6nergie solaire, Rapport CEA No. 30, 1968.

[3] Tunisian Atomic Energy Commission, Etude technico 6conomique d'une installation de dessalement solaire ~t Chekmou, Rapport CEA No. 27, 1968.

[4] Tunisian Atomic Energy Commission, Etude technico 6conomique d'une installation de dessalement solaire ~ Chibou, Rapport CEA No. 28, 1968.

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