May 8, 2004

AL-Jubail is a small city, 80 kilometres north of Dammam in the Kingdom of Saudi Arabia. But, at the same time, it is the biggest industrial complex in the whole Middle East. It has hundreds of petrochemical industries providing four billion dollars of foreign exchange to the Kingdom.

To give this city a unique place not only in the Arabian Peninsula but the whole world, it has the world’s largest desalination complex producing 264 million gallons per day (MGD) of drinking water from its 46 multi-stage distillation plants and one seawater reverse osmosis (SWRO) plant adding 24MGD (91,000 cubic metres) per day. Three water transmission lines are supplying water to Riyadh, the capital of the Kingdom, 500km away.

The SWRO plant at Al-Jubail is also the largest operating reverse-osmosis plant on the Gulf. There is only one plant larger than that of Al-Jubail, which is at Yanbu and has a total production of 33MGD (148,000m3) per day. Another SWRO plant, with a mega size production is the Fujairah SWRO plant with a capacity of 37.5MGD (168,750m3) day. But that one is on the Gulf of Oman and still under commissioning stage.

The Al-Jubail SWRO desalination plant was constructed by a consortium of two companies — Preussag, a German desalination company and Yuksel Insaat, a Turkish company involved mostly in civil and electromechanical work. The construction progressed during February 1994 and September 1998.

The plant’s output was to be pumped to the Qassim Region through a 1500mm diameter pipeline. As this line was not ready, the plant was mothballed during September 1998 and September 2000. Commissioning recommenced in September 2000 and was completed in January 2001. It produces 20MGD (3,797 m3/hour; or 91,128m3/day) of potable water within specified purity levels (total dissolved solids less than 450mg/litre). The total contract value was around $166 million which gives an investment cost of $1,700 per metre3 per day of drinking water.

This cost, being extremely high because of stringent specifications laid down by the SWCC, should not be considered as guideline for the proposed desalination plants in Pakistan or for economic evaluation of RO technology.

The Al-Jubail desalination plant has 15 trains (or units), each train producing 253m3/hour (6,072m3 per day). A unique feature about this plant is that it uses both types of RO configurations —Hollow Fine Fibre (HFF) and Spiral Wound (SW). It is probably the only SWRO plant in the world with this feature. The plant has been constructed in a modular design such that any section or sections of the plant could be operated or shut down for maintenance without interrupting the operation of other sections.

The seawater, which has been pre-chlorinated to approximately 0.25-1.5ppm of chlorine, is supplied to the plant from the existing Al-Jubail Complex seawater header via two lines. Each pipeline is designed for a seawater flow rate of 6,500m3/hour to meet the RO plant requirements under worst condition.

For inline flocculation, ferric chloride is fed as a 40 per cent solution. The dosing rate of ferric chloride is maintained between 0.5-3.5ppm as iron to achieve optimum formation results.

Sulphuric acid required for the acidification has been designed such that approximately 75 per cent of the acid is fed upstream of the dual media filter unit. A lowering of the pH value upstream of the Dual Media Filter Unit has a positive effect on the inline flocculation and furthermore increases the efficiency of the chlorine.

The chemically treated raw water from the seawater intake is transported to an open distribution channel. The water is then distributed to the individual filters through culverts.

The DMF unit consists of two parallel trains with seven filter basins each. Each train is fed by one line from the seawater intake main, whereas a cross ink allows to feed both trains from one intake, too. The filter basins are made of concrete and provide a filter area of 110m2 each thus providing a total area of 1,540m2. The length and breadth of the beds are 22m and 5m, respectively. Sixteen, 5-micron cartridge filters are available for fine filtration and for the final protection of the RO membranes against suspended particles.

For the elimination of any residual chlorine, which would damage the membrane irreversibly, the injection of sodium bisulphate (SBS) is provided. For scale prevention and adjustment of optimum pH-value sulphuric acid is added. Upstream of cartridge filters, the pH-value of the water is set to approximately 6.7 in order to achieve the optimum pH value 6.9 for safe operation of the RO plant. The SBS is injected immediately downstream of the cartridge filters. The injection downstream of the cartridge filters has the advantage that the filters are fed with chlorinated water and therefore an infestation of microorganism can be prevented.

The conditioned and filtered seawater is pumped through the membranes by high-pressure pumps. In the membranes, 35 per cent of the water is converted into drinking water and 65 per cent is returned to the sea as brine. The RO Plant consists of 15 trains each of which is equipped with a high-pressure pump. Each train consists of two racks known as “banks”.

High-pressure feed pumps are horizontal-type centrifugal pumps with multi-stage impeller design. The design capacity of the pump is 833.65m3/hour at a design pressure of 85.5 bar. However, the pumps are capable of meeting process flow requirements from 724.91 m3/hour to 900 m3/hour with corresponding discharge pressure.

The required process pressures at the membrane inlet are maintained by the control valves installed on the feed and brine discharge line. The energy recovery turbines are technically designed similar to the HP pumps (i e, multi-stage, horizontal, centrifugal) to run in reverse direction. The ERT’s use high-pressure brine as source of motive power, which imparts energy to the ERT shaft. The energy from the ERT shaft is transferred to the main motor, which is driving the high-pressure pump

The plant is controlled by the flow rates. The reference input is the produced permeate flow of each train. This ensures the guaranteed product water flow, independent from the raw water temperature. A constant permeate flow is continuously produced within the guaranteed range of 20-35oC.

If the feed pressure exceeds the maximum value for the actual feed temperature, which is an indication of a malfunction of one of downstream valves, the high-pressure pump is switched off. The shut-off pressure is a set value, which is calculated and set automatically according to the pressure- temperature profile.

Up to 0.5 mg/litre of chlorine is added to the product water for disinfection purposes. Although the resulting product water complies in all respect with WHO recommendations for the quality of drinking water and other relevant standards, a saturation index in the range of 0.1-0.3 is desired. For this purpose, additional CO2 and lime [saturated Ca(OH)2 solution, “lime water”] is dosed.

Plant water quality and quantity: After two years of operation, the plant is performing satisfactorily, complying with all obligations as regards to water quantity and quality in accordance with WHO Standards. During 2001 and 2002, the permeate conductivity was almost 400µS/cm for most of the period, seldom going above 600 µS/cm. Process optimization, carried out in the last two years, proved necessary and have led to a better plant performance. Further investigations were carried out on other possible improvements such as incorporation of variable frequency drives to high-pressure pumps.

Plant availability: The plant availability was 62 per cent during the first year of operation as from April 18, 2001, it was shut down for refurbishment by the contractor.

On November 2, the North side Dual Media Filters were emptied and the South Side was normalized. This was for the purpose of removing the failed coating on the walls and repainting by the contractor.

Consequently, after this date, a maximum of six trains were run at any time due to restriction in seawater flow and filtered seawater clear well level during the months of May, June and July, all the trains remained shut down due to contractor work Between the period July 01, 2001 to September 15, 2001, no product water was produced due to the modification of the lime silo unit where an air operated system for unloading lime from silo into the hopper was assembled along with a new cones.

During the year 2002, the Al-Jubail RO Plant was available for almost 90 per cent of the operating period. Only 10 per cent production loss was due to high SDI value of feed water. The average daily production was approximately 40,000m3/day that is 44 per cent of design.

Chemical cleaning: For Dupont trains, chemical cleaning has been carried out almost every fifteen days, followed by PT-B post treatment. For Toray trains, chemical cleaning was carried out almost every three months.

Energy consumption: As per the contract, the specified maximum energy consumption without energy recovery is 10kwh/m3 and guaranteed maximum is 9.48kwh/m3. With energy recovery, the actual average specific energy consumption is 7-8kwh/m3. It has been estimated that 30 per cent of energy is recovered.

Membrane replacement: There has been membrane replacement with every train but with different degree. However, overall membrane replacement was within membrane manufacturer’s guidelines.

During the two years operation, the performance of the plant was satisfactory with the exception of few incidents. Nevertheless, there is still room for improvement like installation of variable frequency drives, recycling of overflow back to filtered water Clear well, installation of Nanofiltration (NF) system as pretreatment. The aim of this article is to share the experience and knowledge gained through the operation of the plant in identifying the problems and their solutions.

The writer has been desalination engineer at Al-Jubail SWRO Plant

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