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)

ERA	PERIOD	FORMATION	MEMBRE	APPROXIMATE THICKNESS (m)	LITHOLOGY	HYDROGEOLOGICAL SIGNIFICANCE
QUANTERNARY	Recent	Superficial		5	Aeolian sand,bioclastic limestone, beach deposits	Unsaturated.
	Pleistocene	Superficial		10	Sand, sabkha deposits	Unsaturated.
TERTIARY	Oligocene-Miocene	Jabal Cap		33	Dolomitic bioclasic limestone, algal coral breccia	Forms cap to Jabal Dukhan.
		Neogene		10-66	Marl with subordinate sandy limestone	Confines Dammam aquifers. Basal limestone forms part of the 'A' aquifer.
	Eocene	Damman	Alat Limestine	15-25	Fossilifeous dolomitised limestone	Main 'A' aquifer. Formerly sustained small artesian flows. Low productivity. Used in NE and W coast.
			Orange Marl	19-15	Orange-brown dolomitic marl	Confines Aquifer B when present
			Khobar Dolomite	30-39	Dolomitic limestone	Main 'B' aquifer, usually confined. Highly permeable in top 5-10m. Main source of freshwater.
			Khobar Marl	Discontinuous	Marl and shale	Forms part of the 'B' aquitard.
			Alveolina Limestone	c. 10	Friable brown dolarenite
			Sharks Tooth Shale	8-20	Shale with silty dolomitic limestone	Aquitard
		Rus		60-150	Chalky dolomitic limestone, shale, gypsum and anhydrite	Part of 'C' aquifer. Aquitard if evaporites present.
	Paleocene	Umm Er Radhuma		115-350	Dolomitic limestone and calcarenite, often argillaceous and bituminous	'C' aquifer in upper UER and Rus. Salinity stratified. Lower UER saline with low permeability.
MESOZOIC	Cretaceous	Aruma		c. 400	Mainly shale in the upper part, limestone predominat below	Aruma shales form hydraulic base to Umm Er Radhuma.

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).