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2.7 Non-renewable groundwater development in the Middle East

It is possible to distinguish two major types of aquifers in the Middle East. Along river valleys and beneath alluvial fans and plains, there are shallow alluvial aquifers. These are generally unconfined, small in area, and have water tables that respond rapidly to local precipitation conditions. The second type are deep rock aquifers of sedimentary origin, usually sandstone and limestone. These are often confined systems, sometimes of considerable areal extent, and store water that can in part be many thousands of years old.

The deep rock aquifers often extend over many thousands of square kilometres in area, with natural recharge occurring in upland and foothill zones where the rocks have surface outcrops. There is still considerable uncertainty as to the degree to which recharge is taking place in these large rock aquifers at the present day, partly because little is known about how much run-off is generated during the rare, but often intense, local storms.

The potential for conventional water resources such as river water and renewable groundwater is extremely limited in the Arabian peninsula and North Africa, excluding minor areas in the mountain ranges where annual rainfall exceeds 10 inches, or 250 mm. By overexploiting major rivers such as the Nile, Jordan, Tigris, and Euphrates, groundwater resources in deep sandstone aquifers, such as the Nubian sandstone aquifers and equivalent formations, could have been conceived as a major source of water for development in the Middle East and North Africa in the 1980s. Groundwater in the deep sandstone aquifers, however, is non-renewable or "fossil" water which may offer an opportunity for short-term and emergency uses. Large-scale deep sandstone aquifer development projects in Saudi Arabia, Libya, and Egypt are discussed in this section.

2.7.1 Groundwater resources in deep sandstone aquifers

NUBIAN SANDSTONES. Sandstones of Nubian facies underlie most of the Arabian peninsula and the Sahara, and represent one of the most extensive artesian groundwater basins of the world. Nubian sandstones derive from the Precambrian and from reworked sandy Palaeozoic deposits and have not been altered by metamorphic processes.

The sediments, as a rule, are deposited either in flood facies represented by poorly sorted, coarse- to medium-grained, commonly cross-bedded, brownish sandstones containing mud flakes and quartz pebbles, or in lacustrine facies consisting of clay banks and sandstone tongues and reaching a maximum thickness of 3,500 m. Rapid facies changes are typical and marine incursions common, particularly over the less stable parts of the platform.

The age of the Nubian sandstones is poorly defined. In Libya, Late Jurassic to Early Cretaceous age is indicated, while the formation extends into the Palaeocene in Egypt. It seems that the name "Nubian sandstones" is best regarded as a purely litho-stratigraphic unit that does not easily fit into a chrono-stratigraphic system.

GROUNDWATER BASINS IN THE SAHARA. The groundwater of the Sahara is to be found mainly in the following seven major basins: the Great Western Erg and Great Eastern Erg in the north, Fezzan and Tonezroft in the central region, the Western desert of Egypt in the east, and Chad and Niger in the south (fig. 2.33). A very large groundwater reservoir of fresh water is found in the Libyan part of the Sahara up to a depth of approximately 3,000 m. The water in the aquifers of Nubian sandstone correlates with the "continental intercalaire" in the western Sahara and is normally of good quality with the total dissolved solids content being usually less than 500 mg/L (Gishler 1979). The Nubian aquifer system of the north-eastern Sahara, which is one of the largest groundwater systems of the Sahara, covers an area of about 2 million km² and has two principal basins: the Kufra basin in Libya, northeastern Chad, and north-western Sudan, and the Dakhla basin of Egypt (fig. 2.34).

RECHARGE. Despite the hyper-arid climate, huge reserves of fresh groundwater are contained in Nubian sandstone in several thousand metres of saturated rock. The average rainfall is less than 5 mm per year, from which it is obvious that there has been no recent groundwater recharge in most of the system. For the occurrence of the groundwater, two flow mechanisms have been discussed, including steadystate and non-steady (Heinl and Brinkmann 1989). The steady state concept, which suggests renewable conditions, is based on observations of piezometric heads and postulates a large-scale flow from mountainous recharge areas in the south-west, such as the Tibesti mountains on the Chad/Libyan border and the Ennedi mountains on the border with Sudan, to a north-east discharge area along the Mediterranean Sea coast. Such artesian water generally moves very slowly over considerable distances from the recharge area.

Fig. 2.33 Major groundwater basins in North Africa (Source: Gischler 1979)

Fig. 2.34 Deep sandstone aquifers in the Middle East

The non-steady concept, which suggests a non-renewable condition, is based on isotope dating of water samples, indicating ages of groundwater of 25,000-40,000 years. The apparent age of a groundwater sample, taken from a certain depth in an aquifer, is not influenced only by the flow time of the groundwater particle from the recharge area; to a large extent it is the result of diffusive and convective processes in the aquifer and of mixing within the well.

A recent model simulation study in 1989 (Heinl and Brinkmann 1989), which took into account palaeo-climatological factors in the Holocene period, showed that groundwater in Egypt and Libya was probably derived from precipitation during humid and semi-arid climatic periods and entered the aquifer in the unconfined parts of the aquifer.

AQUIFERS IN THE ARABIAN PENINSULA. Aquifers in the Arabian peninsula are found in arenaceous and/or carbonate formations, including the major formations Saq, Disi, Tabuk, and Wajid of Palaeozoic age; Minjur, Dhruma, Biyadh, and Wasia of Mesozoic age; and Umm er Radhuma and Damman of the Tertiary period (fig. 2.35).

The Saq and Disi sandstones, which are of Cambrian to Early Ordovician age, constitute the most extensive aquifer in the Arabian peninsula (fig. 2.34). The Saq formation in Saudi Arabia is equivalent to the Disi formation in Jordan. Its outcrops form the western and southern fringes of the Great Nafud basin of Saudi Arabia, which extends northwards into southern Jordan (fig. 2.36). It underlies at great depth the whole of Jordan and a large part of the Nafud and Sirhan basins in Saudi Arabia, and is composed of a complex sequence of cross-bedded quartz sandstone, shales, and siltstone more than 600900 m thick.

The mechanism of groundwater recharge in such a hyper-arid region is still under discussion among hydrogeologists, but isotope datings of water in the Disi and Saq sandstones indicate ages of up to 35,000 and 20,000 years respectively (NRAJ 1986). The current hypothesis is that the observed hydraulic gradients cannot be attributed to replenish ment and must be the result of dewatering of an ancient recharge area at outcrops. Groundwater reserves in the Disi/Saq aquifer are there fore most probably of fossil origin with very little, if any, additions from modern recharge.

Fig. 2.35 Geological map of the Arabian peninsula (Source: AOMR 1987)

Fig. 2.36 Statigraphic section of sandstone aquifer in Jordan and Saudi Arabia

DEVELOPMENT STRATEGY. The dominance and importance of nonrenewable groundwater reserves in national water planning is demonstrated in the 1985-1990 development plans of Saudi Arabia and Libya. These, and the New Valley project in Egypt, are described below.

2.7.2 Non-renewable groundwater development in Saudi Arabia

Non-renewable groundwater in the deep sandstone aquifers is concentrated in the northern, north-eastern, and central part of Saudi Arabia. The surface water and renewable groundwater is generally concentrated in the west and south-west, near the Hijaz and Assir mountains, while non-renewable groundwater with brackish quality in the Mesozoic to Neogene aquifers is found in extensive areas in the north-eastern part of the country as shown in fig. 2.37.

Fig. 2.37 Water resources of Saudi Arabia

Saudi Arabia today is one of the world's leaders in the production of wheat for self-sufficiency in food. The production of wheat, however, is dependent almost wholly on the mining of non-renewable groundwater resources.

According to the Fourth Development Plan (1985-1990) of the Ministry of Agriculture and Water, agricultural water demand in Saudi Arabia in 1985 amounted to 8 x 10⁹ m³ per year, while the demand for water for urban, rural, and industrial (M&I) use was 1.6 x 10⁹ m³ per year (MAWSA 1985). It was estimated that the total annual demand would increase to 16.5 x 10⁹ m³ by the year 2000, comprising an agricultural demand of 14 x 10⁹ m³ and an M&I demand of 2.5 x 10⁹ m³. This huge demand for water for agricultural use is based on the kingdom's policy of self-sufficiency in food. The wisdom of growing grain, which generally requires 2,000-3,000 tons of water per ton of grain, is constantly under discussion (Rogers 1986).

QUALITY AND WATER USE. The quality of groundwater in the deep sandstone aquifers is generally fresh, with a low salinity, in the range between 300 and 1,000 mg of TDS per litre. This water is used mainly for growing wheat, with a total yield of 741,000 tons per year. The unit water requirement is calculated to be 10.8 m³ per kilogram of wheat (Akkad 1990). The most commonly used method of irrigation in Saudi Arabia is the central-pivot sprinkler system, which loses a significant amount of water through evaporation.

PROBLEMS IN SUSTAINABLE DEVELOPMENT. Salt accumulations in surfacial soil layers and/or underlying aquifers, which is a typical and difficult problem for groundwater irrigation in the arid region, cannot be neglected in any long-term development project. In Saudi Arabia this has already caused a substantial depletion of non-renewable groundwater resources.

Water demand in various sectors is increasing at an alarming rate. Measures to control demand have become increasingly important to water-resource planners and decision makers in balancing the needs of agricultural development against the depletion of non-renewable groundwater resources, and strategic parameters for self-sufficiency in food.

According to the Fifth Development Plan (1990-1995), total water use in Saudi Arabia will be reduced by 8%, from 16.2 x 10⁹ m³ per year in 1990 to 14.9 x 10⁹ m³ per year in 1995, compared with a total increase of 89% during the Fourth Plan period. The reduction in water consumption will be the result of a projected decline in annual agricultural consumption from 14.6 x 10⁹ m³ at the beginning of the period to 12.7 x 10⁹ m³ at the end. This change in the consumption rate is expected to take place through changing crop patterns, the intensification of water-saving techniques, and other appropriate measures, all of which will not affect the desirable growth rate of agricultural production or its value added. This 8% of reduction in the national water supply may be the world's first initiative to conserve non-renewable groundwater resources (MAWSA 1990). Many countries in the Middle

East must consider such a conservation policy for sustainable development, including the reduction of national water supply.

2. 7.3 The Great Man-Made River project in Libya

SAHARA/LIBYAN DESERT. Libya is located in the northern part of the Sahara desert in Africa, and extends from 19° to 33° north latitude and from 9° to 25° east longitude, with a land area of 1,759,540 km² (see fig. 2.34). Except for the Mediterranean coastal belt, the country consists of barren rock deserts, undulating sand seas, salt-marsh depressions, and mountains that rise to 1,200 m in the south-west and 1,800 m in the south-east. Climatically Libya is influenced by both the Mediterranean and the Sahara. The coastal region has a Mediterranean climate: winters are mild with 250-400 mm of rain, and summers are hot and dry. Conditions in the desert interior are extremely hot and arid, with an annual rainfall of 0-120 mm.

Hydro-meteorologically Libya is a desert in which the surface hydrology is of no direct practical importance, while huge amounts of fossil groundwater are stored in the Nubian sandstones that underlie wide areas of the Libyan desert. Groundwater development and/or mining of the Nubian sandstones in the inland desert depressions, named the "Great Man-Made River project," is the key to the nation's development strategy.

GREAT MAN-MADE RIVER PROJECT. A vast aquifer estimated to hold an amount of fresh water equivalent to the total flow of the Nile River over a 200-year period was discovered accidentally by an American geologist during crude-oil exploration in the Sahara desert in the early 1960s. The Libyan government saw an opportunity to pump the water, at a rate of 5.7 million m³ per day (66 m³/sec), then convey it over 600 km north to farms on the Libyan coast. The total length of the water pipeline is estimated at 4,000 km, which will be the world's largest water pipeline system (fig. 2.38).

Some agricultural development has already begun around the desert oasis of Kufra, using the self-flowing artesian wells in the depression. Acres of wheat, barley, and alfalfa grow where there were only desert and gravel plains before. According to an article in the British journal New Scientist, the amount of sustained yield of groundwater resources is in some doubt; Professor Ahmad, a hydrogeologist at the University of Ohio, says that water is moving into the two aquifers that are to be tapped at a rate of 80 m³/sec, whereas Dr. E. Wright of the British Geological Survey says that the figure is closer to 5 m³/sec. The life of the Nubian sandstone aquifer is estimated to be between 20 and 200 years, owing to the lack of data for estimating groundwater recharge through the wadi beds and/or the depressions during occasional and temporary flash floods. The total pipeline system is therefore designed on the assumption of an aquifer life of 50 years.

Fig. 2.38 Great Man-Made River project

In 1984 the Libyan government began the first phase of construction for the Great Man-Made River project. This comprises a 2 million m³ per day twin pipeline in eastern Libya, leading from wellfields in the Tazerbo and Sarir regions, deep in the desert, to the small coastal town of Agedabia. The trunk main is 667 km long. The line splits into two spurs, one a 150-km link to Benghazi, the other going south-west to Marsa el-Brege. From there the line will extend west to Sirte. By the end of 1992 Libya had spent more than US$5 x 10⁹ of the initial US$14 x 10⁹ allocated to the project, and the first section had been completed (Bulloch and Darwish 1993).

The second phase, consisting of a 600-km-long prestressed concrete pipeline to convey 2 million m³ per day from beneath the western deserts to the Tripoli area on the coast, began in 1986.

The constant increase in the price of the total scheme will have to be taken into account when figures are worked out for the cost of growing the wheat to be irrigated. The whole idea of using this valuable resource for agriculture is very much open to question, in which the groundwater irrigation accumulates substantial salts in the irrigated land. The total cost was estimated in 1990 at US$27 x 10⁹, but that figure is likely to rise further: in 1985 the total cost was expected to be US$20 x 10⁹ and in 1980, US$14 x 10⁹ (Bulloch and Darwish 1993).

2.7.4 New Valley project in Egypt

THE SAHARA DESERT IN EGYPT. Egypt is located in north-eastern Africa, extending from 22° to 31.5° north latitude and from 25°-36° east longitude, with a land area of 1,002,000 km² (fig. 2.34). About 96% of Egypt is desert. The area west of the Nile is an arid plateau some 200 m high, crossed by belts of sand dunes in the centre and west. The Nile is Egypt's most important feature. It divides 25 km north of Cairo into the Rashid and Dumyat, the two main channels of the 22,000-km² delta. Rainfall is minimal: Cairo receives only 60 mm annually, while the desert often has no rain at all. A narrow stretch of the Mediterranean coast is milder and wetter, with 250 mm of rain a year.

Hydro-meteorologically Egypt is a desert, however, in which the surface hydrology of the Nile River is of direct practical importance. The Nile is the basic source of water and, with the aid of dams and barrages, supplies an extensive network of distributary canals. West of the Nile, Nubian sandstones that store a huge amount of fossil to semifossil water underlie the desert. Groundwater development in the depressions, where the saturated Nubian sandstone aquifer underlies, is the worthy complement of the green revolution in western Egypt.

Fig. 2.39 Groundwater development in the New Valley project in Egypt (Source: Shahin 1987)

THE KHARGA AND DAKHLA OASES IN THE WESTERN DESERT. In 1 950 about 24 km² out of a total of 4,000 km² in the Kharga oasis was cultivated. Abstraction of groundwater from shallow wells amounted to 38.7 million m³ per year in the Kharga oasis and 92.7 million m³ per year in the Dakhla oasis (fig. 2.39). Seven deep boreholes drilled between 1938 and 1952, with depths varying between 342.5 and 509.3 m, encountered artesian flow. The yield of these deep wells was 20.6 million m³ per year in total; however, the yields decreased after a few years of operation by not less than 40% of their initial values.

GROUNDWATER DEVELOPMENT IN THE NEW VALLEY PROJECT. Extensive deep production wells were drilled in the mid-1950s, to correspond with the New Valley project which aims to expand the cultivated area in the Kharga and Dakhla oases. At first, much of the water was self flowing under artesian conditions. The pressure quickly fell, however, and an increasing use of water required ever greater amounts of pumping. Saline water began to contaminate some wells, limiting the crops which could be grown. In 1963 the combined discharge of shallow wells and deep production wells in el-Kharga amounted to about 117 million m³ per year, but this had dropped to a level of 80 million m³ by the end of 1967. The construction of deep production wells in the Dakhla oasis, completed by 1966, increased the combined yield of the shallow and deep systems up to 190 million m³ per year, but this had decreased to a level of 159 million m³ by the end of 1969. The response of the head of water to the growing abstractions from the deep production wells in Kharga and Dakhla oases from 1956 to 1975 is shown in fig. 2.40. The Egyptian authorities are planning to augment extraction until it reaches 2.4 x 10⁹ m³ per year by the year 2000 (Shahin 1987). Extraction of the target volume will lead to a further decline in the piezometric head to cease the artesian flow. Another problem of the development project is the human problem that many of the managerial staff do not like living in such isolated areas. Overall, the project cannot be considered a success (Beaumont et al. 1988).

Fig. 2.40 Water head in relation to abstractions from deep wells in the Dakhla and Kharga oases

2.8 Brackish-groundwater reverse-osmosis desalination in Bahrain

Groundwater in the Damman aquifer on Bahrain island has been seriously contaminated by seawater intrusion or upward leakage from the underlying saline aquifer of Umm er-Radhuma since the 1960s, owing to intensive pumping which exceeded the safe yield. The world's largest reverse-osmosis (RO) plant for the treatment of saline groundwater, which is located at Ras Abu-Jarjur, 25 km south of Manama, the capital of Bahrain, was commissioned in 1984. The plant has an installed capacity of 45,500 m³ (10 million imperial gallons [mig]) per day, whose source of raw water is the highly saline brackish groundwater in the Umm er-Radhuma formation. The RO plant was designed to meet the domestic water demand of Manama city, taking into account its several advantages over a seawater distillation (MSF) plant: (1) short construction time, (2) lower energy cost, and (3) ease of operation and maintenance (Akkad 1990). The use of reverseosmosis desalination for saline groundwater in Bahrain island began in 19841986. The data from its monitoring, examined here, provide one of the key sources of experience in the development of marginal water resources in the Middle East.

2.8.1 Background

The state of Bahrain consists of 33 islands, islets, and coral reefs in the Arabian Gulf between Saudi Arabia and Qatar, between the latitudes 25°45' and 26°27' north and longitudes 50°25' and 50°54' east. The country has a land area of 662 km², of which the island of Bahrain itself, with its capital Manama, occupies 85% (fig. 2.41). The population was estimated at 427,271, with a growth rate of 4.2% per year in 1985 (Beaumont 1988).

The climate is arid to extremely arid. The mean monthly temperature varies from 17°C in January to 34°C in July and August. Owing to the surrounding Arabian Gulf, the humidity is generally high. Rainfall is confined to the period between November and April, with an annual average of 76 mm, which occurs essentially in a form of ephemeral thunder showers. There are no rivers, streams, or lakes.

The country is occupied by Tertiary sediments, which are rather gently folded on a regional scale into elongate domes or periclines of near north-south trend. Bahrain island is dominated by one such dome, developed principally in carbonate sediments of Cretaceous-Tertiary age, which dip gently outwards. The Bahrain dome is elongate (about 30 km x 30 km) and with slight asymmetry, as seen in fig. 2.41.

The sequence is composed of three formations: Damman, Rus, and Umm er-Radhuma, as seen in the schematic geological profile in fig. 2.42. The Damman formation, which consists of fossiliferous dolomitized limestone, dolomitic marl, and dolomitic limestone, has two forms, known as Alat limestone and Khobar dolomite, from the Middle Eocene. The Rus formation of the Lower Eocene consists of chalky dolomitic limestone, shale, gypsum, and anhydrite. The Umm er-Radhuma formation of the Palaeocene is composed of dolomitic limestone and calcarenite with some argillaceous and bituminous facies, which is underlain by shales, marls, and argillaceous limestone of the upper Arma formation of the Cretaceous. The geological sequence and aquifer characteristics are shown in fig. 2.43.

Fig. 2.41 Bahrain island (Source: Birch and Al-Arrayedh 1985)

Fig. 2.42 Schematic geological profile of Bahrain and the Arabian peninsula (Source Birch and Al-Arrayedh 1985)

2.8.2 Water resources

Historically, Bahrain has utilized groundwater for both agriculture and municipal requirements. Natural fresh-water springs used to flow freely in the northern part of Bahrain, but, with increased demand, spring flow has decreased and pumped boreholes became the normal means of obtaining water. Before 1925, the water supply depended on free flowing springs and some hand-dug wells, whose discharge was estimated to be 93 million m³ per year in total. With increased water demand after the exploration of offshore reservoirs of crude oil and gas in 1946, spring flow decreased and pumped boreholes became the normal means of procuring water. Groundwater use in Bahrain at that time was estimated to be 153 million m³ per year in total, which included 138 million m³ of tube-well abstraction, 8.1 million m³ of land springs, and 6.6 million m³ of marine springs (Mussayab 1988). During the 1980s, most of the springs ceased flowing, and further increase in water demand has caused deterioration in water quality, including the intrusion of seawater into the aquifer system.

Faced with rising demand and the contamination of the aquifers by seawater intrusion, Bahrain turned to desalination of seawater to provide for the increasing demand for M&I water supply. On the basis of a 1983 groundwater model study (Birch and Arrayedh 1985), which included the recommendation to reduce groundwater abstraction from the Damman aquifer to the level of 90 million m³ per year, the Ministry of Works, Power, and Water instigated a crash programme to increase Bahrain's desalinated water capacity from 22,730 m³ (5 mig) to 204,570 m³ (45 mig) per day. Production of water for M&I water supply was estimated to be 101 million m³ per year in 1987, including 53 million m³ of groundwater and 48 million m³ of desalinated water (Birch and Arrayedh 1985).

2.8.3 Hydrogeology and seawater intrusion

The principal aquifers are pervious limestone units in Palaeocene to Eocene sedimentary rocks. Damman and Umm er-Radhuma are the important aquifers in Bahrain.

Fig. 2.43 Geological sequences of Bahrain (Source: Birch and Al-Arrayedh 1985)

The Alat limestone in the upper Damman formation used to sustain small artesian flows or springs in the northern island. The Khobar dolomite in the lower Damman formation, a highly pervious unit, was the main productive aquifer to produce fresh groundwater, with a typical salinity of 2,500 mg of TDS per litre. Due to excessive abstraction, however, piezometric levels in the Khobar aquifer declined continuously with substantial increase in water salinity (figs. 2.44 and 2.45). This aquifer has become saline in the Ali-Buri area, due to upward leakage of brackish water, and on Sitra, due to seawater intrusion (fig. 2.45). Significant upward leakage of brackish water from the underlying aquifer of Umm er-Radhuma occurs only in eastern and central Bahrain, where the evaporite layers in the Rus formation have been removed by solution.

Fig. 2.44 Piezemetric-level changes in the Khobar aquifer in Bahrain (Source: Birch and Al-Arrayedh 1985)

Fig. 2.45 Total dissolved solids (TlDS) in the Khobar aquifer (Source: Birch and Al-Arrayedh)

The deeper aquifer of Umm er-Radhuma, composed of dolomitic limestone and calcarenite, is a salinity stratified aquifer with a total thickness of about 200 m. A further highly saline groundwater contains hydrogen sulphide and hydrocarbons from bitumens as specific contaminants.

2.8.4 Desalination

Since it has become the policy to curb the abstraction of groundwater resources in the Damman aquifer and to improve its quality, such as the salinity of domestic water supply, further development of water resources will undoubtedly be by means of desalination, either by a thermal process or reverse osmosis. The choice will depend on the sitespecific conditions and economy or cost.

The first multi-stage flash (MSF) distillation plant was introduced in Bahrain in 1976. The total installed capacity of this plant was 22,730 m³ (5 mig) per day in 1981, which was 15% of the total demand of 154,000 m³ (34 mig) per day. The present installed capacity of desalination plants in Bahrain is 205,000 m³ (45 mig) per day, including 160,000 m³ (35 mig) of seawater distillation by MSF and 45,000 m³ (10 mig) of desalination of brackish groundwater by RO. A further 45,000 m³ per day of seawater desalination capacity by RO is under construction (Mussayab 1988).

2.8.5 Brackish-groundwater reverse-osmosis desalination

The RO desalination plant at Ras Abu Jarjur, 25 km south of Manama, with an installed capacity of 45,000 m³ per day, the world's largest RO plant with seawater membranes in the 1980s, was commissioned in 1984 (Al-Arrayedh 1985). The raw water source is a highly saline groundwater (13,000 mg of TDS per litre) in the Umm er-Radhuma formation, containing hydrogen sulphide and hydrocarbons from oil as specific contaminants. It is predicted that the water quality will deteriorate with time, implying significant increases in the hydrocarbon concentration from a trace to 2 mg/l, the hydrogen sulphide concentration from about 2 mg/l initially to about 13 mg/l, and the total dissolved solids (TDS) from about 13,000 mg/l up to about 30,000 mg/l after 20 years' operation. The design TDS for the plant is 19,000 mall; it is predicted that this concentration will be reached after 10 years' operation. The predicted range in feed-water salinity is shown in fig. 2.46. The permeate is being produced from highly brackish well water at a conversion rate averaging 65%, of which the salinity averages as low as 210 mg of TDS per litre, well below the design criterion of 500 mg/l. The plant contains five basic systems: a well-water supply, pre-treatment, RO desalination, post-treatment, and product-water transfer systems, as shown in the process flow diagram in fig. 2.47.

Fig. 2.46 Predicted range of feed-water salinity for RO desalination plant in (Source: Birch and Al-Arrayedh 1985)

WELL WATER SUPPLY SYSTEM. Raw water is pumped from 15 boreholes, which include 13 duty wells and 2 standby wells. Submersible pumps are designed to abstract an average of 3,200 m³ of brackish groundwater per hour from a group of boreholes. Four anti-surge tanks at the high and low points of the wellfield are installed to protect the collection pipes from sudden pressure surges. The anti-surge tanks are pressurized with nitrogen gas to prevent oxidation of hydrogen sulphide in the well water.

PRE-TREATMENT SYSTEM. TO protect the RO system, well water entering the plant is filtered and chemically treated to remove silt, oil, and other hydrocarbons. The raw water passes through a series of dual media filters and carbon filters. Sodium hexametaphosphate and sulphuric acid are then injected downstream of the carbon filters to prevent scaling of the system.

Fig. 2.47 Flow scheme of RO system m Bahrain (Source: Birch and Al-Arrayedh 1985)

RO SYSTEM. Before entering the heart of the RO system, the water passes through eight micro-guard filters (10-micrometre) with polypropylene cartridge elements. Seven horizontal multi-stage diffusertype high-pressure pumps are installed to feed water with an average pressure of 60 bar (maximum pressure 69 bar). Each pump is equipped with Pelton wheel impulse-type energyrecovery turbines. The RO membrane unit comprises a total of 2,100 permeators. The permeators are hollow fibre-type, such as DuPont B-10.

POST-TREATMENT. Since the well water contains a high level of hydrogen sulphide, the RO product water must pass through a series of stripping towers to remove the gas. Adjustment of the pH of the permeate with sulphuric acid is also needed before stripping for maximum removal of the hydrogen sulphide. In-line mixers are installed in the pipeline for post-treatment with chlorine, lime, and carbon dioxide.

2.8.6 Development strategy for R0 desalination

As stated earlier, officials of the Bahrain Water Supply Directorate chose reverse osmosis desalination over multi-stage flash distillation because of the short construction time, lower energy cost, and ease of operation and maintenance. The parameter that most readily demonstrates the performance of the system is the energy consumption per unit of product. The specific electric power consumption per product water is estimated to be as low as 5.3 kWh/m³, the mean value over two years' operation (1984-1986) (AlArrayedh 1987).

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