Desalination ofSeawater by

Reverse Osmosis

The MaltaExperience

IntroductionMalta, an archipelago of threeislands situated 50 miles south ofSicily in the middle of the clear blueMediterranean Sea, is blessed withan abundance of sunshine. Theseidyllic conditions are, however,marred by the sparse annual rainfallof only 250 to 450 mm. The Islands,whose dense population nownumbers about 350,000, have untilrecently been perpetually plaguedwith problems of obtaining freshwater. Over the years, special effortshave been required to cope with thisadverse situation.

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Stainless Steel forHigh Pressure Piping

in SWRO Plants

Are there anyOptions?

SummaryService experience from 27 seawaterreverse osmosis (SWRO) plantsconfirms that there is an evident riskof corrosion if wrong steel grade isused for the high pressure piping.Neither 316L nor 317L possessessufficient corrosion resistance. Noteven highly alloyed grades like 2205and 904L show reliable serviceperformance. However, no corrosionhas been reported for 254 SMO,which has been used in 16 fullstrength SWRO plants and severalplants using high salinity brackishwater.

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Pembroke R.O. Plant, Malta

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AVESTA SHEFFIELDCORROSION MANAGEMENTAND APPLICATIONENGINEERING

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Desalination of Seawater by Reverse Osmosis

The Malta Experienceby

Michael F. Lamendola and Arthur Tua, Polymetncs Seawater Systems, Inc., Malta

IntroductionMalta, an archipelago of threeislands situated 50 miles south ofSicily in the middle of the clear blueMediterranean Sea, is blessed withan abundance of sunshine. Theseidyllic conditions are, however,marred by the sparse annual rainfallof only 250 to 450 mm. The Islands,whose dense population nownumbers about 350,000, have untilrecently been perpetually plaguedwith problems of obtaining freshwater. Over the years, special effortshave been required to cope with thisadverse situation.

Efforts Over the YearsAncient temples predating Christ by afew thousand years were built withcisterns, so that to this day, rainwatercan be collected and stored.

To meet demads of the new city ofValletta in 1610, the Knights of St.John of Jerusalem started work on anaqueduct to transport water for over15 km by gravity from Rabat. Thewater was collected by subterraneanconduits from several natural springsand galleries cut into limited freshwater aquifer.

Since water was such a scarce anddifficult resource, the Knights made ita requirement by law, that eachhouse and building have its owncistern.

Towards the end of the nineteenthcentury, as the demand for freshwater increased, bore holes andsubterranean collection gallerieswere cut, in order to exploit the meansea level aquifer1). During the sameperiod sizable covered reservoirswere built in strategic locations. Thesewere further developed in the early

post World War II years, andcovered storage now exceeds450,000 m3.

Desalination Makes an EarlyAppearanceDesalination made an early appear-ance in Malta. In 1881 a thermaldistillation plant installed by thegovernment was desalting sea waterfor distribution to the local popula-tion. Further thermal desalination inthe form of multistage flash (MSF)evaporators followed more recentlybetween 1966 and 1969.

12 Years of Reverse OsmosisDesalinationAs the 1980s began, Malta's waterneeds became more critical. TheIsland evolved from a military baseunder British rule to an independent

country with a growing industry andtourist trade.

It was at this point that the MaltaGovernment made its first investmentin Reverse Osmosis (RO) Desalination.In 1983, after a construction period ofonly 11 months, the 20/000 m3/dayGhar Lapsi2) Seawater RO facilitybecame operational. This was soonfollowed by a second 4500 m3/dayhighly brackish RO facility in Marsa.(See Figure 1.)

The high reliability (i.e., on-linefactors >96%) and the relatively lowcost of water from these plants, ledthe Malta Government to expandthese plants and to purchase anotherthree seawater RO facilities. All of theMalta RO plants have been installed,operated and maintained byPolymetncs.

2) For many years, this facility was the largestSWRO plant in the world.

1) A fresh water lens or body lying over a layerof salt (sea) water.

Figure 1.History of RO desalination in Malta. Except for Marsa all plants desalt seawater.

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Plant DescriptionThe Malta SWRO systems utilizeboreholes as the raw water intakesystem. Acid (H2SO4l and micronfiltration pre-treat the seawater, priorto its arrival to the high pressurepumping system and membranes.The product is post-treated bydecarbonation, lime addition andchlorination. Figure 2 is a simplifiedflow diagram for a typical plant.

RO Plant FeaturesReliability through simplicity andautomation, low energy requirementsand a stable supply of seawaterprovided from wells, have been thesalient design features of all fivefacilities.

Key Design Features Reduced pre-treatment requirements

by taking advantage of the localgeology and drawing seawaterthrough boreholes. Pre-treatmentconsists only of acid injection andmicron filtration

Each plant is sub-divided into anumber of independent RO systems,referred to as trains

Single pass operation, utilizingPermasep B10, aramid, hollow fibermembranes

Energy recovery turbines reducepower consumption by 40%. Exceptfor the last phase of the Pembrokeplant, KSB Guinard integratedturbo-pumps are used

Simplified hydraulic controls, suchthat the only operating adjustmentsare one valve per train

An advanced control andmonitoring system that enables oneperson in a central control room tooperate the entire plant

Use of corrosion free plastic ma-terial for low pressure (

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Cirkewwa R.O. Plant

GeneralThe ambient environment in a SWROplant is relatively corrosive. On theLapsi plant structural steel works andother non plastic or non stainless steelcomponents deteriorated very rapid-ly, resulting in a shoddy appearanceand a need for regular maintenance.This led to an increased use of non-metallic components, the use of SSTLfor fasteners and the hot dip galvan-izing of all structural steel work.

Particular examples of thesechanges include the exclusive useof plastic valves for low pressureapplications (as opposed to linedsteel valves) and the use of GRPcartridge filter vessels.

MembranesAll of the Malta Plants make exclusiveuse of DuPont "Permasep" B10permeators. Initially at the Ghar Lapsiseveral problems were experiencedparticularly with regard to SaltPassage. This was attributed toa) particular permeators which weredefective and replaced underwarranty and b) rapid loss of Pt-B4).The latter problem took some time toresolve and after considerable effortby both Polymetrics and Permasep asolution was found. This consisted inlowering the pH. It is interesting tonote that the pH required to maintaina stable or controllable Salt Passage4) Tannic Acid which is used to enhance

membrane rejection.

is site dependent and is normallydetermined experimentally. It is alsointeresting to note that no specificcause has been identified for thisrapid loss of Pt-B.

Analysis of the flow performanceof the Malta Plants shows that theguarantees by the membrane manu-facturer have always been met.

Pumps and EnergyRecovery TurbineAs part of the 27,600 m3/day exten-sion of the Pembroke RO plant, multi-stage split case pumps and energyrecovery turbines 5), manufactured byByron Jackson, were selected for highpressure service. For all RO units onMalta, commissioned prior to thePembroke extension, integrated turbo-pumps (TPMDX), manufactured byKSB Guinard were installed and havegiven exceptional service, particularlyon the Lapsi plant, where the totaldown time, due to pumps, was lessthan 2000 train hours 6) in over 12years of operation.

The change from integrated turbo-pumps to multi-stage split case pumpsand energy recovery turbines hasresulted in an energy reduction of0.28 kWh/m3 of product, and areduction in the noise level due tolower operating speed.5) Francis turbinea pump running in reverse.6) The Lapsi Plant has a total of twelve trains,

each with a nominal capacity of 2000m3/day.

OperationsOn-line FactorsPolymetrics is obliged by its Opera-tion and Maintenance contract withthe Client, The Water ServicesCorporation (WSC), to maintainminimum levels of production and tomaintain minimum plant on-line levels.Failure to meet these obligationswould result in large penalties. Theseobligations have already been met.Table 1 outlines the on-line per-formance for all the plants for thelast three years.

SDIOne of the most Important para-meters for meeting contractualobligations and obtaining highon-line factors is the SDI of thefeedwater. Through the use ofproperly constructed and maintainedSeawater boreholes, we have acontinuous supply of low (

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Table 1.On-line numbers for the last 3 years.

Down Time Down Time Down Time Overall PolymetricsCaused by Requested by due to Lack of On-line On-linePolymetrics WSC* Power Time Time**

1992Lapsi 1.87% 0.28% 1.73% 96.11% 98.09%Tigne 2.31% 0.49% 4.21% 95.79% 97.58%Cirkewwa 2.01% 2.68% 1.22% 94.09% 97.91%Pembroke 3.94% 0.85% 0.68% 94.54% 96.00%1993Lapsi 1.46% 0.56% 1.98% 96.65% 98.50%Tigne 1.23% 1.44% 3.32% 97.23% 98.71%Cirkewwa 1.39% 2.26% 2.17% 95.15% 98.55%Pembroke 1.60% 1.35% 0.91% 96.78% 98.36%1994Lapsi 1.25% 1.63% 2.99% 94.12% 98.69%Tigne 0.92% 2.16% 0.29% 96.63% 99.06%Cirkewwa 1.03% 2.39% 2.21% 94.37% 98.92%Pembroke 1.04% 6.86% 0.37% 91.74% 98.88%

* WSCWater Service Corporation.** Calculation based on system availability i.e. the denominator excludes down time by others.

ReferencesAndrews W.T. and R.A. Bergman: The MaltaSeawater RO Facilities, First Congress onDesalination and Water Re-use, Florence Italy,May 1983.

Andrews W.T: The Malta Seawater ROFacilitiesUpdate on the first two years ofOperations, Second Congress on Desalinationand Water Re-use, Bermuda, Nov 1985

Winning the War for Water, DuPont MagazineVol 82 No 6, Nov/Dec 1988

Schembri R.: Reverse Osmosis Water Costs inMalta, Workshop On Production Cost ofWater, May 1988

This paper was originally published inDESALINATION & WATER REUSE,Vol. 5/1, p. 18-22. See also 'A Notefrom the Editor' on page 6.

Ghar Lapsi R.O. Plant

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A Note from the Editor ofDESALINATION & WATER REUSEThe Malta desalting experience is anoutstanding example of currentdesalination technology. This articlenot only provides data on reverseosmosis unit design and experiencebut also provides a valuable insightas to the benefits of seawater supplyvia boreholes when the site geologypermits.

In keeping with D&WR editorialpolicy of seeking desalting water costdata, it seemed fitting to supplementthe information in this article withsome water cost data from one of thereferences, as presented at theBahrain Workshop on ProductionCost of Water, May 1988 by Mr R.Schembri of the Malta government.The cost data was later conformed tothe format endorsed by IDA, andpresented in a paper published in"Desalination 81" (1991) entitled"Total Water Costs on a StandardBasis for Three Large, OperatingSWRO Plants". The original cost data(1988) has been adjusted to reflectthe outstanding on-line performancefor this plant as shown in this article,97.23%, and an excerpt is shown tothe right:

Plant Capacity 15.000m3/day (4MGD)Total Direct & Indirect Costs $14,458,000Annual Fixed Charge Rate 0.1272

Annaul CostsOperating & Maintenance $979,000Fixed Charge 1,838,000Electric Energy @ $0.10/kWh 3,164,000Chemicals 40,000Membrane Replacement In Fixed ChargeTotal Annual Cost $6,021,000

Total Water Cost $/m3 $1.13 $/1000 US Gal $4.28