IntroductionWith continued population growth

and an overall expansion in tourismover the last few decades, a heavyburden has been placed upon thenatural resources of the Caribbean.Water, essential for life itself, is oneof the resources most drasticallyaffected, whether by over-pumping ofthe natural source that has sustainedisland nations for centuries, or theinadvertent contamination of sourcesthrough development. One of thechallenges for the people of theCaribbean and their governmentshas been to manage and sustain this natural resource. Most of theCaribbean, built over the millenniathrough the natural growth of coralreefs or volcanic eruption, does not typically have sufficient naturalreservoirs, aquifers, and rain rechargethat larger landmasses enjoy.

The Caribbean—and the rest of theworld—faces the unmistakable ironythat, although the world’s majorsurface component is made upprincipally of water, there is “water,water, everywhere and not a drop todrink.” The salinity of human blood is almost equal to that of the oceans,possibly hinting at our humblebeginnings, but we would perishmore quickly drinking seawaterthan drinking nothing at all.

So we turn back to the ocean, but this time we do so with a twist.

Advances in technology allow us thesmall miracle, that we mimic fromnature, of removing enough of the salt from the seawater to be able toproduce clean, safe, drinking water.The supply seemingly inexhaustibleat first glance, it might appear thatour problems are solved. But toaccomplish this remarkable alchemyrequires energy, and energy in theCaribbean, primarily imported,is a more dear and preciouscommodity than the water it isrequired to produce.

Seawater Desalination CostsThe next challenge comes in

applying the technology as efficientlyand as cost effectively as possible.The costs in seawater desalinationhave been reduced greatly overthe last twenty years, most notablythrough the advances in reverseosmosis (RO) as shown in Figure 1.In 1978, the cost to produce 1,000 U.S. gallons of potable water from

Ionics Technical Paper

Presented at CWWA 2001 Conference,Grand Cayman, October 2001. Reprintedwith permission from the CaribbeanWater and Wastewater Association.

Copyright © 2002, Caribbean Water and Wastewater Association.

TP-398 E-US 0802-658

Printed on Recycled Paper in the USA

New Sea Water Reverse Osmosis Plantsfor the Caribbean “Energy Recovery,Brine Recovery & Cost Reduction”

by Shawn Meyer-Steele and Antonia von Gottberg, Ionics, Incorporated, U.S.A.,Jose Luis Talavera, Ionics Iberica, Spain

$ p

er 1

,000

Gal

lon

s

25

20

15

10

5

01978 1988 1998

$3.00

Figure 1: Cost of Water Produced bySeawater RO 1978 - 1998

seawater in a large desalinationfacility was over U.S. $20. Today,the cost has decreased by a factorof six, to less than U.S. $3 per1,000 U.S. gallons today.

In the Caribbean, and inmost of the world, (except wherelarge amounts of waste heatfrom true co-generation facilitiesare available) reverse osmosishas proven to be, by far, the mostcost-effective technology for seawater desalination.1

RO is a pressure-driven processby which salt can be removed fromseawater. If a solution containingsalts is placed on one side of asemi-permeable membrane, with a more dilute solution on the otherside, water will be forced by naturalosmosis through the membranefrom the more dilute to the concen-trated side in an attempt to reachequilibrium. Reverse osmosisutilizes external pressure to over-come the natural osmotic pressure,and forces water through a semi-permeable membrane from themore concentrated to the lessconcentrated side.

RO is chosen for many desali-nation applications today becauseof its proven ability to produce high

quality water in an energy efficientmanner with the ability to with-stand, given appropriate pre-treatment, fluctuations in feedwater quality. A seawater reverseosmosis (SWRO) plant includesseveral major blocks, as shownin Figure 2: the site and building;feedwater supply; pretreatment;SWRO unit; post treatment;chemical systems; instrumentation;electrical and control system. Thesection below discusses each ofthese areas in more detail.

Site and BuildingThe site as a whole generally

includes:• the plant building• outdoor chemical storage area• production reservoir• wastewater collection and

discharge system• the site including civil works,

paving, fencing, and landscaping

Feed Water Supply

Feed water to the desalinationsystem is collected from a seawaterintake or beach wells located asnear the plant site as is possible.From the intake, a feed pumpwill convey feed water to thepretreatment system.

PretreatmentThe seawater must be treated

before it reaches the RO unit toremove suspended solids. Typically,multimedia filters are used toeffectively remove solids. The filtersare designed to operate at a loadingrate consistent with the overallplant design.

The suspended solids areremoved from the water into thefiltering media by a combinationof mechanisms including straining,interception, impaction, sedi-mentation, flocculation, andadsorption. A majority of thesuspended material in the rawseawater is removed during thisfirst filtration step.

Recently, there has been muchinterest in the application of back-washable, hollow-fiber ultrafiltration systems (UF) aspretreatment for SWRO systems.Today, the capital cost is higherthan with traditional media filters,but the space requirement issmaller and UF provides higherquality feedwater to the RO.Ongoing work in this area may soon provide a reduction in the cost of seawater desalination.Filtered water then flows to thereverse osmosis unit.

Sea Water Reverse Osmosis UnitThe core desalination operation

is reverse osmosis. The process ele-ments of a typical desalination unitare cartridge filters; an RO feedpump, an energy recovery device, an RO membrane unit, and auxiliarysystems for cleaning and chemicaladdition.Figure 3 shows a photo-graph of a 3,000 m3/day (792,000 gpd)SWRO plant at Aqualectra, Curaçao,Netherland Antilles before expan-sion to its present 10,000 m3/day(2,690,000 gpd) size.

2

Feedwater Supply

Concentrate Discharge

ChemicalStorage RO Building

SWRO Unit

ProductReservoirPretreatment

Figure 2: Typical SWRO Plant

Cartridge FiltrationWater is conveyed to the cartridge

filters from the media filters. Thecartridge filter system is used toremove fine suspended matter fromwater, typically down to five micronsin size. A cartridge filter consists of a filter housing and filter elementsmounted to tube supports. Waterenters the housing and flowsthrough the filter elements. Thesuspended solids are trapped inthe fine fibers of the filter.

Reverse Osmosis In reverse osmosis, water under

pressure is forced across a mem-brane element with a portion of the feed permeating the membraneand the balance of the feed watersweeping along the membranesurface and exiting without passingthrough the membrane. In the caseof seawater, the membrane willfreely pass water but will rejectmost of the dissolved mineralsas well as any small particles. Anillustration of an RO membraneelement is shown in Figure 4.

SWRO plants typically employspiral-wound, thin-film compositepolyamide membrane elements

to separate dissolved salts fromthe seawater. In a typical one-stageSWRO system, the process canconvert, or “recover” approximately40% of the incoming seawater asdesalted product. The remainingapproximately 60% of the incomingwater is concentrated by the saltsrejected from the product and isreturned to the sea. Two-stageSWRO recovers approximately 60% of the incoming seawater as product with 40% returned to the sea.

The high pressure required forRO treatment is provided by a high-pressure pump. The RO systemincludes a single pass of treatmentin one stage.

Permeate from the RO systemflows to the post-treatment system.Concentrate (reject) from the ROsystem flows through the energyrecovery device and is thendischarged back to the sea.

Energy RecoveryThe pressure required for RO

treatment is provided by a high-pressure pump. Because of the rela-tively high energy requirements,most SWRO systems are equippedwith an energy recovery device thatrecovers energy from the pressur-ized RO concentrate leaving thesystem. The energy recovery systemtypically recaptures anywherefrom 20 - 50% of the initial pump-ing energy.

Concentrate DischargeConcentrate from the RO system

is discharged back to the seathrough a reject pipeline or througha deep well. This pipe is also usedfor disposal of wastewater such asfilter backwash.

Clean-In-Place SystemA membrane cleaning system

is normally provided to clean ROmembrane elements if suspendedsolids or precipitates foul them.Cleaning procedures are under-taken when operating pressuresor RO permeate production fallsoutside normal operating limits.The system used to effect thiscleaning (referred to as the Clean-In-Place, or CIP procedure) consistsof: a chemical tank; a pump torecirculate the cleaning chemicalsthrough the RO membranes; a cartridge filter to remove anysolid contaminants or scale whichis removed from the vesselsand/or piping.

3

Figure 3: SWRO Unit, Aqualectra, Curaçao, N.A.

Figure 4: RO Membrane Element

Post TreatmentPost-treatment of the RO

permeate is needed to create a potable water that is properly adjusted for storage and distribution. A calciumcarbonate filter and/or caustic soda addition systems are typically provided for pHadjustment and remineralization.Sodium hypochlorite and/or UV sterilization are used fordisinfection of the final product.After post-treatment, the product water is delivered to a production reservoir.

Chemical SystemsThe water treatment plant can

require the addition of chemicalsat certain points throughout thesystem, as indicated in the descrip-tions given in this section. Thechemicals used in the plant varydepending on the water source.

The chemical systems supplieddepend upon the nature of thechemical used but generally consist of a chemical storage tankof suitable capacity and materialof construction for the chemicalunder consideration, two chemicaldosing pumps (main and standby)and interconnecting piping. Inaddition, each system will be sup-plied with a mixer if necessary.

Instrumentation

Instrumentation characteristicallyincludes: pressure gauges, pressureswitches, conductivity indicatorsand transmitters, temperatureindicators and transmitters, levelindicators and switches, and flowindicators and transmitters. Thesystem is normally equipped with sample ports so that watersamples may be collected at various locations in the process.

Electrical and Control SystemThe power and control system

normally includes:• power distribution components• control panels, instrument panels• programmable control devices

The control system typicallyprovides for complete automaticoperation of the desalinationsystem. A central control systemwill control, monitor performance,and record operating data for all aspects of the desalting plant. It will provide plant-operatingstatus, alarm messages, datacollection, protective shutdowns,and automatic regulation of theplant equipment.

Advances in SWROThe developments that account

for most of the advances seen inRO cost reduction over the last20 years in Figure 1 came abouton five different fronts:

1. through the use of energyrecovery devices

2. through new membranechemistries that allowed forthe same salt rejection withlower feed pressure, whichrequires less energy and hence lower operating costs

3. through new membrane ele-ments and pressure vesselsthat allow operation at higherpressures, and hence highersalinities at similar saltrejection rates

4. through larger basic RO units(trains) that provide some costreduction through economiesof scale for key componentssuch as pumps, piping, andpressure vessels

5. through the establishment ofrelationships with municipal-ities, whereby each side lendsits own expertise and ability tolower the capital and operatingcosts to the project

While the fourth front is valid,there have been industry-wide“growing pains” resulting frommaking the basic trains too large.The fifth front is very projectspecific. In this paper, we will limitour focus to the first three areas.

When designing a SWRO systemtoday, the engineer must decidewhat kind of energy recoverydevice to use, and whether or notto use a brine conversion system.By optimizing the system for localconditions, one can minimize theoverall life-cycle costs of producingdesalted water.

Energy Recovery DevicesThere are a number of devices

available commercially that arecapable of reducing the unit powerconsumption of reverse osmosisunits. This is primarily accom-plished by reducing the powerconsumption of the high-pressurepump by capturing and returningthe energy in the concentratestream (which was waste energybefore the development of energyrecovery devices). For typicalsingle-pass, single-stage seawaterdesalination, the concentratepressure can be from 55 to 65 bar(800-950 psi). Three of the mostsuccessful are the turbocharger, the Pelton wheel, and the work or pressure exchanger.

TurbochargerThe turbocharger has been

successfully used for SWRO energy recovery since 1989.

4

The turbocharger essentially acts like a reverse running pump,where the RO concentrate is used to turn a turbine that iscoupled to a pump section with its impeller on the turbine shaft.The energy transfer from the RO concentrate to the RO feed through the turbo-chargerincreases the pressure of the RO feed and thus reduces theexternal energy requirement for the RO feed. Figure 5 illustratesthe process flow diagram (PFD) of a typical RO system using aturbocharger for energy recovery.

Since the turbocharger is a high-speed rotary machine it requireslittle maintenance. It is compactin size and low weight. The wastestream is pressurized so it can bedisposed of without re-pumpingbeing required. The nominalefficiency of the device is quite low,however, due to high viscous lossesin the device. Also, flow deviationswith respect to the design point,such as temperature fluctuationor change in water recovery, hasa potentially large impact onperformance. The turbocharger is generally used on smaller units.

Pelton WheelPelton wheel technology was

first evaluated for energy recoveryalmost 20 years ago.2 The firstprototype machines were basedon standard hydroelectric turbinehydraulics. Development of thistechnology over the past twodecades has led to the widespreaduse of Pelton wheels in SWROsystems, accounting for about 80%of energy recovery devices fitted toSWRO plants over 1 mgd capacity.

A nozzle valve is used to direct ajet of high-pressure RO concentrateonto the bucket type blades of the Pelton wheel.3 This causes thewheel to turn. The kinetic energyof the jet is converted into rotatingmechanical energy. By coupling theshaft of the Pelton wheel to a motoror pump, this energy can be used toreduce the electrical energy that isneeded to pump the RO feedwater.Figure 6 illustrates the PFD of atypical RO system using a Peltonwheel for energy recovery.

Pelton wheels are reliable andeasy to maintain. Typical deviceefficiency ranges between 84 and90%. Since the discharge from aPelton wheel is at atmosphericpressure, either the waste must be able to drain by gravity, or else it has to be re-pumped.

Work ExchangersThe original work exchanger was

built for the U.S. Government in1980. There are a number of similarproducts on the market. Theauthors’ company markets theirversion of the work exchangerunder the name “Dyprex”. The workexchanger uses a system of pistonsand valves to transfer the pressureof the RO concentrate to part ofthe RO feed. A high-pressurebooster pump then pumps this

5

FilteredWater

HP FeedPump

HP Feed

HPConcentrate

LPConcentrate

Drain

Reverse Osmosis(RO)

DesaltedProduct

Figure 5: Process Flow Diagram—Turbocharger

FilteredWater

HP FeedPump

HP Feed

Motor

Drain

Reverse Osmosis (RO)Membrane Array

DesaltedProduct

Concentrate

Turbine NozzleControl Valve

PeltonWheel ERT

Figure 6: Process Flow Diagram—Pelton Wheel

pre-pressurized feed to the requiredRO feed pressure. The remainingRO feed is pumped by a high-pressure pump. Figure 7 illustratesthe PFD of a typical RO systemusing a work exchanger for energy

recovery. Since the work exchangerdirectly transfers energy from theconcentrate to the feed rather thanthrough rotating machinery, it hashigher efficiency in comparison tothe Pelton wheel and turbocharger.

However, the work exchanger islimited in size, and, although add-ing units in parallel can increasecapacity, the capital cost is high forlarge plants. Work exchangers alsohave a large number of movingparts that can be subject to wear.Figure 8 shows a photograph of awork exchanger at a 26,400 m3/day(360,000 gpd) SWRO plant atHandsome Bay, BVI.

PEI has introduced a smallerversion of a work exchanger (thePressure exchanger) built on aprinciple similar to the Dyprex withfewer moving parts. It is marketed ashaving the same type of efficienciesbut has not been adequately provenin long-term operation and onlyused to-date on smaller plants.

Comparison of EnergyRecovery DevicesWhen selecting the most

appropriate energy recovery

device for a given application,

one needs to consider several

factors including the cost of

power, expected variation in plant

operating conditions, maintenance

6

Figure 8: Work Exchanger, Handsome Bay, B.V.I.

Motor

HP Feed

Motor

Drain

RO MembraneArray

DesaltedProduct

FilteredSea Water

Motor

HP FeedPump

HP

Co

nce

ntr

ateFree Floating Piston

Pressure Tubes

LP C

on

cen

trat

e

HP

Feed

LP Booster Pump

HP Booster Pump

VFD

SuctionAccumulator

LP Feed

Figure 7: Process Flow Diagram—Work Exchanger

requirements and capital cost.

Table 1 compares the features of

the three energy recovery devices

discussed above. Note that the

capital cost refers to only the

capital cost of the energy recovery

device; when selecting an energy

recovery device in a particular

application, one needs to look

at the overall cost of pumps, energy

recovery devices and VFDs to

determine which scheme is

optimum in that case. Recent

innovative approaches include

combining the turbocharger

and Pelton wheel to minimize

energy consumption and to

recover energy as efficiently

as possible over a wide range

of operating conditions.4

Toray Seawater Second-StageBrine Recovery

Conventional wisdom has held

for years that to maximize efficien-

cy of the systems, the optimum

recovery and configuration was

35-40% recovery and a single

stage system. It has always been apparent that

the low recovery of historical SWROmeant that a lot of water had to bepretreated, then pumped to highpressure, and then 60-65% of thiswater was just dumped back to thesea. Limitation in the membranemodule design, however, preventedSWRO systems from operating athigher water recoveries.

As the water recovery increases,the concentration of salt in thebrine stream also increases. Hence,the pressure that must be appliedto overcome the osmotic pressureof the brine stream increases. Mostspiral wound RO membranes canoperate up to 82.7 bar (1,200 psi) attemperatures below 29°C. If waterrecovery is increased, the pressurelimitation of the membranebecomes a limit on recovery beforeany limits on water chemistry arereached. If water recovery were tobe increased to the water chemistrylimit rather than the membranepressure limit, then the RO mem-brane had to be capable of oper-ating at pressures up to 98 bar(1,420 psi).

Toray Industries, Inc., for someyears now, has been manufacturingwith great success a spiral woundRO membrane that can operate athigh pressure and can achieve over99.7% rejection working on theconcentrate reject from the firststage unit.5 This brine second-stagesystem, called a Brine ConversionSystem (BCS), is capable of recov-ering an additional 50% of the con-centrate for recoveries of up to 60%with no appreciable increases inproduct salinity or energy per unitof product produced.

7

Table 1: Comparison of Energy Recovery Devices

Device Turbocharger Pelton Wheel Work Exchanger

Capital Cost Low Low-Medium High

Efficiency Low (55-60%) Medium (84-90%) High (> 95%)

Efficiency Curve Slopes downwards Varies Flatat lower flows

Crossleak from Minor via center Connected via motor Low (< 3%)reject to feed journal bearing so not an issue

Capacity Range < 2.5 mgd Up to multi mgd < 2.5 mgd

Reliability High-speed rotary High-speed rotary Multiple valves andmachine—easy machine—easy other parts subjectto overhaul to overhaul to wear

Footprint Compact Compact, but Largerequires civil work for atmospheric drain

Discharge Pressure Pressurized Atmospheric Pressurized

Effect of deviation Wide operating range Wide operating range Moderate impactfrom design point on performance

Table 2: Comparison of One- and Two-Stage SWRO Plants

Parameter Performance 1 Stage Performance 2 Stage

Net Production 7,500 m3/day 11,300 m3/day(2 mgd) (3 mgd)

Product Salinity 200-350 mg/l TDS 220-375 mg/l TDS

Salt Removal Rate 99.9% + 99.9% +

Product Water Recovery 40% 60%

Operating Temperature 24-28°C 24-28°C(75-83°F) (75-83°F)

Operating Pressure 55-62 bar 76-83 bar(800-900 psi) (1,100-1,200 psi)

The authors’ company hasformed a joint-venture companywith Toray to manufacture and sellthese membranes in the Americasand the Caribbean.

Single-Stage versus Two-Stage SystemTable 2 compares the perfor-

mance of a conventional one-stageSWRO system with that of a two-stage system employing the Toraybrine recovery membrane. Theperformance of the desalinationsystem is based on typicalCaribbean seawater compositionand a process temperature of 28°C(83°F). As feed water conditionsvary, system performance willchange. Table 3 compares therelative cost of water production

for a traditional single-stage systemand a two-stage system.6

The two stage system saves agood deal of capital costs andfootprint area because the intake,outfall, the pretreatment, and theamount of seawater taken in by theintake pumps is only 67% of that fora conventional first-stage system.The energy of this second stage canbe minimized by using an energyrecovery device such as a turbo-charger to boost the pressure of thefirst stage concentrate using thesecond-stage brine. Hence, theelectricity consumption of the two-stage system can be lower than thatof the single stage system. Thesesavings in water production costcan reduce the cost of desalinatedseawater by 16%.

Case Study 1:Maspalomas II SWRO PlantThe authors’ company owns and

operates this 20,400 m3/day SWROplant as well as a 20,000 m3/dayelectrodialysis reversal (EDR) plantfor brackish water desalting. Thefacility is located on Gran Canaria,Spain. The original SWRO systemwas installed in 1987 and has sincebeen expanded.

Description of ConventionalSWRO PlantThe raw seawater is delivered

via an offshore, open, submergedintake. The seawater is filteredthough two sets of vertical mediafilters containing anthracite andsand. The filtered seawater thenpasses though two sets of cartridgefilters sized at 10 and 5 microns.The conventional SWRO plant atMaspalomas II consisted of fivetrains. The seawater intake capacityis 41,000 m3/day. The SWRO systemrecovered 40% of the seawater asproduct water, with 60% of thewater being rejected to the seathrough a brine water outfallsystem. The seawater feed contains35,000 mg/l TDS. The originalSWRO plant used Francis Turbinesfor energy recovery.

Pilot Test of Toray BrineConversion SystemIn the late 1990s, the system

needed to expand again. A pilot testof the Toray BCS was undertakenat the site.7 Table 4 compares theactual data from the pilot teststo the targets.

Full-scale BrineConversion SystemBased on successful pilot testing

of the Brine Conversion System atthe SWRO plant, the decision wasmade to expand the facility using

8

Table 4: Pilot Test Data from Maspalomas

Feed 1st Stage Permeate BCS Permeate Product

Actual

Water Quantity (m3/day) 350 140 70 210

Water Quality (mg/l) 35,438 165 173 168

Water Recovery — 40% 33% 60%

Target

Water Quantity (m3/day) 350 140 70 210

Water Quality (mg/l) — < 350 < 350 < 350

Water Recovery — 40% 33% 60%

Table 3: Comparison of Water Production Cost

% 1 Stage 2 Stage

Capital Cost 46% 37%

Electricity 36% 30%

Membrane Replacement 5% 6%

Chemicals 4% 2.5%

Other 9% 8.5%(Labor, maintenance, etc.)

Savings — 16%

a second-stage SWRO system torecover reject from one train of theexisting single-stage SWRO facility.The advantage of this approachwas that the seawater intake andpretreatment systems did notrequire expansion. This was thefirst full-scale plant in the worldto use the new BCS, and it hasbeen in operation since 1999. Inthis system, the brine from theconventional SWRO system ispressurized up to 90 bar withbooster pumps. The pressurizedbrine then flows into the brineconcentrator membranes, whichrecover 33% of the water as productwater. A Pelton wheel recovers theresidual energy in the reject water.At the time of writing, a BCS hasbeen installed on three of the fiveSWRO trains. Trains BCS1 and BCS3 consists of 28 vessels of fivemembrane elements per vessel.Train BCS4 has 56 vessels of fivemembrane elements per vessel.Table 5 compares the product flow rate, the water recovery, andthe product quality of the three BCS units. Figure 9 shows theprocess flow diagram for one of the trains and Figure 10 illustratesthe module rack.

Performance Table 6 shows the water quality

of the feed, first-stage and BCSpermeates and first-stage BCSrejects. The BCS is producing

Table 5: BCS Trains atMaspalomas II

BCS1 BCS3 BCS4

Product flow (m3/h) 40 44 110

Recovery (%) 26 28 29

Product Quality 960 920 560(µS/cm)

9

PeltonTurbine

Permeate60 m3/h

BoosterPump

28 Vessels Parallel (+2 spare)

Permeate120 m3/h

Existing SWRO Train1st Stage

180 m3/h23 bar

50 bar

900 m3/h

feed

180 m3/h 65 bar Brine Stage 1

Figure 9: Process Flow Diagram for One BCS Unit at Maspalomas II

Front SideFeed 180 m3/h

Check Valve

Booster Pump

Back Side

Spare Tubes

Permeate

Figure 10: BCS Module Rack at Maspalomas II

Table 6: Water Quality Data

RO 1st Stage BCS 1st Stage BCS Feed Permeate Permeate Reject Reject

Sodium (mg/l) 11,900 134 106 19,700 31,000

Calcium (mg/l) 432 1.6 0.8 780 1,080

Magnesium (mg/l) 1,407 3.4 2.9 2,549 3,635

Potassium (mg/l) 430 5.0 4.0 650 1,075

Chloride (mg/l) 21,800 222 170 37,000 55,200

Bicarbonate (mg/l) 115.9 43.7 3.7 190.3 345.3

Sulfate (mg/l) 3,300 9.0 9.0 5,400 7,200

TDS (mg/l) 39,391 379 298 66,282 99,547

pH 6.98 6.14 6.18 7.15 7.37

permeate of higher quality thanthe first stage, even though the concentration of feedwater to theBCS is higher than to the first stage.The membranes in the BCS wereinstalled later than the membranesin the first stage, and this is the

reason for the better quality fromthe second stage.

Figure 11 plots the feed pressureto the first stage and the BCS versustime. The feed pressure to bothstages has been constant during

the operation of the plant, at about68 bar for the first stage and 90 barfor the BCS. Figure 12 plots the firststage and BCS product quality aswell as the percent water recoveryversus time.

Table 7 compares the single-stageSWRO system, the combined SWROand BCS system, and the projectedsystem with a BCS added to alltrains. Since both systems use the same feed flowrate, the intakesystem did not have to be expandedto achieve more production. Also,the pretreatment system did nothave to be expanded, and theamount of chemicals used in thepretreatment system per m3 ofproduct is reduced. The projectedmaximum water recovery with allSWRO brine feeding a BCS systemis 60% rather than 40%. The costsof operation of the intake andpretreatment system would bereduced by 33% per m3 of product.This expansion was possible withno capital investment in seawaterintake, pretreatment system orbrine outfall system. For a newfacility designed with a BCS,there would be capital cost savingsof 33% per m3/day of installedcapacity for the pretreatmentand discharge systems.

Energy BalanceFor the conventional SWRO train,

the production rate is 118 m3/h.The total power consumed by thehigh pressure pump, minus thepower recovered by the Francisturbine, is 445 kW. The totalelectrical energy consumptionof this train is 3.77 kWh/m3.

For the trains with BCS unitsinstalled, the SWRO product flowis 118 m3/h, and the BCS productflow is 41 m3/h, so the total flowis 159 m3/h. The total power con-

10

Table 7: Maspalomas II Flowrate

SWRO System SWRO + BCS System Projected System

Feed Intake (m3/day) 41,000 41,000 41,000

SWRO Product 16,400 16,400 16,400

SWRO Waste 24,600 12,479 —

BCS Feed — 12,121 24,600

BCS Product — 4,000 8,000

Total Product 16,400 20,400 24,600

Total Waste 24,600 20,600 16,400

Water Recovery 40% 49.75% 60%

95.00

90.00

85.00

80.00

75.00

70.00

65.00

60.00

55.00

50.00

Kg

/cm

2

days

BCS Pressure

1° Stage Pressure

00 10 20 30

Figure 11: Feed Pressure versus Time

850.00

800.00

750.00

700.00

650.00

600.00

550.00

500.00

450.00

400.00

µS/c

m

days

60.0

58.0

56.0

54.0

52.0

50.0

BCS Product Quality

Total Recovery

1° Stage Product Quality

%

00 10 20 30

Figure 12: Product Quality and Water Recovery versus Time

sumed by the high pressure pumpand the BCS booster pump, minusthe power recovered by the Peltonwheel, is 533 kW. Hence, the totalelectrical energy consumption ofthis train is 3.35 kWh/m3.

The energy consumption per unit of water produced by theSWRO train with BCS is lower thanthe energy consumption per unit of water produced by the conven-tional SWRO train.

Concentrate DisposalThe average concentration of

the reject from Maspalomas II isover 90,000 µS/cm. A study wasperformed to evaluate the effect onflora and fauna in the area near thedischarge.8 This study showed thatthe discharge from Maspalomas IIdid not have any effect on flora andfauna near the outfall.

Case Study 2:Aqualectra, CuraçaoAqualectra is the municipal

supplier of potable water andelectricity for the Caribbean Islandof Curaçao, the largest of the fiveislands of the Netherlands Antilles.Faced with increasing demand for potable water and an aging

distillation plant, Aqualectraawarded a contract to the authors’company to build, own and operatea SWRO facility. The original facilitybecame operational in 1996. Theoriginal capacity was 3,000 m3/day.The plant was expanded in 1999and 2000, and now produces10,200 m3/day.

Description of System This SWRO system consists of a

first stage RO system using conven-tional SWRO membranes and asecond stage BCS to improve waterrecovery. Pelton wheels are usedto recover energy from the SWROreject. The SWRO permeate is fedto a BWRO system so that theproduct of the reverse osmosisfacility matches the product qualityof the thermal desalination unitsat about 20 mg/l TDS (TotalDissolved Solids).

Water QualityTable 8 shows the water quality

of the feed, first stage and BCSpermeates and first stage and BCSrejects. One might expect that thepermeate from the first stage wouldbe lower salinity than the BCSpermeate. However, it can be seen

that the BCS permeate is actuallyslightly better than the first stagepermeate. This is due to differencesin age of the membranes, andshows that the use of the BCSmakes no detrimental differenceto the product water quality.

Aqualectra Plant Summary: 10,200 CMD (2,692,800 US GPD)

Configuration: Open seas intake,Feedwater Pumps, MMF’s, CF, PositiveDisplacement HP Pumps, Calder PeltonWheel Turbines, Conventional 1st pass, BCSPass, 3 stage second pass, UV disinfection,and product pumping

Recovery: 40% first pass, 58% overall

Product Quality: < 40 mg/l

Power: 2.6 kWhr/m3 1st pass / 4.2 kWhr/m3

overall (includes 1st pass, BCS stage, 3 stage2nd pass, UV disinfection, and productpumping)

Performance: Some start-up problems,currently over 95% on-line

Case Study 3:Anguilla Plant ExpansionThe Crocus Bay desalination

Plant in Anguilla, West Indies isthe municipal supplier of potablewater and electricity for the EasternCaribbean Island of Anguilla. Facedwith increasing demand for potablewater and brackish water wells thatwere becoming increasingly saline,The Anguillian Governmentawarded a contract to the authors’company to build, own and operatea SWRO facility. The original facilitybecame operational in 1999. Theoriginal capacity was 60,000 U.S.gallons per day. The plant wasexpanded in 2000 to produce 90,000U.S. gallons per day by installing aBCS system on each of the fourindependently operating SWROsingle-stage trains.

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Table 8: Water Quality Data

RO 1st Stage BCS 1st Stage BCS 2nd PassFeed Permeate Permeate Reject Reject Permeate

Sodium (mg/l) 11,741 200 167 18,263 23,074 10

Calcium (mg/l) 466 3.6 1.6 696 937 0.03

Magnesium (mg/l) 1,406 10.4 4.8 2,179 2,800 0.07

Potassium (mg/l) 460 8.1 7.5 714 936 0.42

Chloride (mg/l) 20,695 330 272 32,553 41,922 15.2

Bicarbonate (mg/l) 142 4.9 4.9 221 288 3.66

Sulfate (mg/l) 2,952 19.3 8.4 4,596 6,045 0.13

TDS (mg/l) 37,862 576 466 59,222 107,208 29.48

PH 8.1 6.8 6.6 8.0 7.9 6.5

Description of System This SWRO system consists of a

first-stage RO system using conven-tional SWRO membranes and asecond-stage BCS to improve waterrecovery. Pelton wheels are usedto recover energy from the SWROreject. The SWRO permeate is fedto a BWRO system so that theproduct of the reverse osmosisfacility matches the product qualityof the thermal desalination unitsat about 20 mg/l TDS.

Crocus Bay, Anguilla Plant Summary: 3,409 CMD (900,000 U.S. GPD)

Configuration: Open sea intake, FeedwaterPumps, MMF’s, CF, Positive Displacement HPPumps, Calder Pelton Wheel Turbines,Conventional 1st pass, BCS pass, substantialproduct pumping

Recovery: 58%

Product Quality: < 800 µS/cm

Power: 2.85 kWhr/m3 1st pass, 4.0 kWhr/m3

overall

Performance: Over 95% on-line after initialshakeout of plant

Case Study 4:WEB, BonaireWEB is the municipal supplier of

potable water and electricity for theCaribbean Island of Bonaire, partof the Netherlands Antilles. Facedwith increasing demand for potablewater and an aging distillationplant, WEB Bonaire awarded acontract to the authors’ companyto build, own and operate a SWROfacility. The original facility becameoperational in 1998. The presentcapacity is 1,633 m3/day.

Description of System This SWRO system consists

of a first-stage RO system usingconventional SWRO membranesand a second-stage BCS to improve

water recovery. A Dyprex workexchanger is used to recover energyfrom the SWRO reject. A portionof the SWRO permeate is fed to aBWRO system so that the productof the reverse osmosis facilitymeets the stringent product qualitystandards of the Bonairian govern-ment at about 40 µS/cm.

WEB, Bonaire Plant Summary: 1633 CMD (431,112 U.S. GPD)

Configuration: Open sea intake, FeedwaterPumps, MMF’s, CF, Positive Displacement HPPumps, DYPREX Work Exchanger,Conventional 1st pass, partial 2nd pass,product pumping

Recovery: 58% overall

Quality: < 40 µS/cm

Power: 2.85 kWhr/m3 overall

Performance: Over 95% on-line sincecommissioning

Case Study 5:Handsome Bay, BVIThe Ionics plant at Handsome

Bay is one of several municipalplants, strategically located tosupply potable water for the manyislands of the British Virgin Islands.The BVI government awarded acontract to the authors’ company to build, own and operate a SWROfacility in Handsome Bay in 1993.The present capacity is 568 m3/day(150,000 U.S. GPD).

Description of System This SWRO system consists of a

first-stage RO system using conven-tional SWRO membranes. A Dyprexwork exchanger is used to recoverenergy from the SWRO reject.

Handsome Bay, BVI Plant Summary: 568 CMD (150,000 U.S. GPD)

Configuration: Open sea intake, FeedwaterPumps, MMF’s, CF, Positive Displacement HPPumps, DYPREX Work Exchanger,Conventional 1st pass

Recovery: 40% overall

Quality: < 400 mg/l TDS

Power: 3.00 kWhr/m3 overall

Performance: Over 95% available on-linesince commissioning

Environmental AdvantagesWhen considering seawater

desalination processes, an impor-tant factor is the potential effect onthe environment. Using the Toraybrine recovery system does increasethe concentration of the brinedischarge to the ocean, and at firstglance, one might consider this tobe a disadvantage of this process.However, when one considers theoverall advantages of the process,the environmental benefits of thetwo-stage process are significant.

The size of the seawater intakesystem is 33% smaller than a one-stage system, so this system hasless of an effect on the environ-ment. Since the pretreatmentsystem is smaller, the chemicalconsumption and related wasteis reduced by 33%.

While the total salt concentrationin the brine is higher, the volumeof brine is reduced by 50% over aone-stage plant. Since the volumeis lower, the area around thebrine discharge point that seesan increased salinity over normalseawater concentrations is muchsmaller than with a singlestage plant.

Also, since the electricity con-sumption is reduced, the amountof CO2 gas exhausted in the

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electricity generation process isreduced by 10-15%.

Optimizing Energy Recoveryand Brine Recovery In a single-stage SWRO plant, as

water recovery increases, energyconsumption decreases since lesswater has to be pressurized toproduce the required amount ofproduct water.5 The relationshipthat higher water recovery willreduce energy is one of the reasonsfor the development of the BCS.Obviously, since the salinity ishigher in the second stage, thepressure required for the BCS ismuch higher than for the first stage,and so as water recovery increases,the benefit of energy savingsdecreases. Also, the efficiency ofpumps and energy recovery devicesworking at different operatingconditions changes the amount ofelectrical energy consumed, andthe amount of energy that can berecovered. There is also powerconsumed by the intake, pretreat-ment and outfall systems, and thispower is lower with a higher waterrecovery system.

In situations analyzed by theauthors’ company, the comparisonof power consumption between alower recovery single-stage SWROdesign and a higher recovery two-stage design varies depending onthe feedwater salinity and the typeof pumps and energy recoverydevices selected. In some cases,the single-stage design uses theleast energy. In other cases, asdemonstrated at Maspalomas II,less energy is consumed with a two-stage than with a single-stage design.

There are several ways to com-bine energy recovery devices with

a BCS to minimize the electricalenergy required per unit volumeof water produced. The right choicefor a particular plant will dependon several factors including thesize of the plant, the cost of power, the capital cost of variousenergy recovery devices and themaintenance requirements ofthe customer.

At the Maspalomas II plant,energy is recovered from the BCSbrine reject by a Pelton wheelattached to the first-stage high-pressure pump. A booster pumpis used to increase the pressureof the first-stage reject to the BCS feed pressure.

An alternative way to minimizethe overall energy consumption per unit of water produced wouldbe to use a combination of a BCS with a turbocharger. A high-pressure pump is used to feed thefirst stage SWRO system. The rejectfrom the first stage can be boostedto the BCS feed pressure using aturbocharger.6 The turbochargerrecovers the energy it uses to boost the first stage reject from the BCS reject.

ConclusionsOver the last 20 years, innovation

in the RO field such as in the areas of RO membrane design andapplication, and energy recoverydevices has significantly reduced the cost of producing desalinatedseawater with RO. In the 21st

century, developments in theseareas and others will continue toimprove upon seawater desalinationtechnology, delivering more optionsand more affordable water for the Caribbean.

The Brine Conversion System is a proven technology for recovering

up to 60% of seawater as productwater. The BCS produces water of approximately the same quality as a conventional SWRO plant.Higher water recovery allowsexisting plants to expand withoutrequiring additional investment in intake and discharge structures,and pretreatment.

The electrical energy consumedper unit volume of water producedis approximately the same for asystem using a BCS as for a single-stage SWRO system, and in somecases is lower for the high recoverysystem. The combination of BCSand the appropriate energy recoverydevice can minimize the electricalenergy consumption of a plant. Asthe BCS is applied more widely tofull-scale plants, it is expected thatenergy recovery devices will be usedin creative ways to reduce powerconsumption further.

Energy recovery devices needto be evaluated on a case-by-casebasis. Turbochargers, Pelton wheelsand work exchangers each havedifferent merits and each produceenergy savings with the greatestsavings inversely proportional totheir relative capital costs.

Work exchangers have proven,reliable performance histories andhave demonstrated significantlylower energy consumption thanother available products. For long-term, larger applications whereenergy costs are high, althoughthey have higher capital costs, work exchangers can be the least expensive overall solution(considering operating andcapital costs).

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References1. Morris, A. B., “Curaçao, K. A. E.

World Leader in SeawaterDesalination Since 1928Adds Seawater ReverseOsmosis Plant in 1997,”Int’l Desalination & WaterReuse Quarterly.

2. Gruendisch, A., “Re-Engineering of the PeltonTurbine for Seawater &Brackish Water EnergyRecovery,” Int’l Desalination& Water Reuse Quarterly,Vol. 9/No. 3, November/December 1999.

3. Calder Bulletin, “Pelton WheelEnergy Recovery Turbines,”PUB. 9008, 1997.

4. Uchiyama, T.; Oklejas, M. andMoch, Jr., I., “Using a HydraulicTurbo Charger and PeltonWheel for Energy Recovery inthe Same Seawater RO Plant,”Proceedings, IDA WorldCongress on Desalination andWater Reuse, San Diego, CA,September 1999.

5. Moch, Jr., I., “The Case forand Feasibility of Very HighRecovery Sea Water ReverseOsmosis Plants”, Proceedings,ADA North American BiennialConference & Exposition, SouthLake Tahoe, NV, August 2000.

6. Toray, “Brine Conversion2-stages Seawater DesalinationSystem”, commercial literature.

7. Kurihara, M.; Yamamura, H.and Nakanishi, T., “HighRecovery/High PressureMembranes for BrineConversion SWRO ProcessDevelopment andits Performance Data”,Desalination 125 (1999),pp. 9-15.

8. Talavera, J. and Ruiz, J.Quesada, “Identification of the Mixing Processes in BrineDischarges Carried Out inBarranco del Toro Beach, Southof Gran Canaria”, Desalination139 (2001), pp. 277-286.

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