Tasked with providing water for a population which more than quadruples with tourists throughout the year, the Caribbean island of Aruba is building a new 24,000 m3/day desalination facility to process seawater from beach wells. Paul Choules & Ron Sebek discuss technical details of the installation, set to replace older thermal desalination units.
The Island of Aruba's drinking water requirements have changed significantly since the early 1930s, when seawater desalination was first used to gain sufficient fresh water for its growing population of 17,000. Today, the island is populated by 100,000 people and attracts more than 700,000 international visitors annually. Desalination continues to provide the island's only source of water, and as Aruba continues to move forward, it is quickly adopting the latest seawater desalination technology to produce more affordable, high quality water.
The island is building a new 24,000 m3/day (cubic meters per day)/ 6.34-MGD (million gallons per day) desalination facility to treat seawater from beach wells.
This desalination project, which is being performed on a 22-month schedule, will use the latest reverse osmosis (RO) technology to replace the island's existing thermal desalination units. The contract was issued by Water en Energiebedrijf Aruba N.V. (W.E.B. Aruba N.V.), the island's electric and water utility.
Five prospective firms had been shortlisted for the project based on initial indicative pricing. Following the results of the official tender, two Veolia Water Solutions & Technologies (VWS) companies: N.A. Water Systems (N.A.WS) and OTV, were awarded the contract. The proposal included performance guarantees for water production and quality, minimal downtime, as well as electrical and chemical consumption. N.A. Water Systems is responsible for VWS design-build and desalination projects in the Americas, and OTV has been involved in conducting and executing large-scale desalination projects worldwide.
The new plant is being built next to the existing thermal desalination plant and the island's power plant, which are located in an industrial area. The thermal desalination units operate with low pressure steam from steam turbines powered by fuel fired boilers. The new facility will use cost-and-energy-efficient reverse osmosis (RO) technology, which will make it more economical to operate. The new plant will provide the island community with a more efficient treatment process and enable Aruba to phase out its older desalination units. The goal with the new desalination plant is to maintain the online availability throughout the life of the facility, while achieving an economical cost of water.
Three Main Processes
Seawater will go through three main processes at Aruba's new desalination plant: pre-treatment, RO, and post-treatment, prior to delivery to the island's potable water distribution system. Pre-treatment prior to membrane systems is the single most important factor in long-term, low-cost operation of RO. Seawater is pumped from 10 beach wells, where natural filtration occurs, and passes through strainers to remove any debris. It will then be pumped into a series of cartridge filters to remove suspended solids larger than five microns.
The plant will use a two-pass RO system to ensure that the treated water is always compliant with the island's stringent drinking water standards. The pre-treated water will be pumped at high pressure through semi-permeable RO membranes. No microorganisms or dissolved minerals pass through these membranes. The new system will employ both SWRO (Seawater RO) and BWRO (Brackish Water RO) stages. Four membrane trains will be employed to maximise plant availability and flexibility during sequenced maintenance activities or changes in production demand.
|Intake from SWRO area.|
Once the feed stream is pressurised and sent through the SWRO membranes, approximately 43% of this flow will cross the membranes as permeate water. The residual, under high pressure, will be sent to an energy recovery device where pressure from the reject side will be recovered. The energy recovery device uses the pressure of the reject flow to augment the pressure of the feed water, thereby lowering the energy required to operate the SWRO system.
SWRO permeate will be dosed with anti-scalant and caustic for pH adjustment and to enhance the boron rejection through the second-pass BWRO. BWRO, functioning primarily as a polisher, will ensure that the specified conductivity and boron limit is met in the final product water. Recovery levels of the BWRO are projected to be 85-90%.
|The plant will use a two-pass RO system to ensure that the treated water is always compliant with the island's stringent drinking water standards.|
Once passed through the membrane systems, the final permeate will receive post treatment for remineralisation, pH balancing and sterilisation before entering the distribution tank. This includes CO2 injection to lower pH to levels necessary for demineralisation to be effective; calcite filters to re-mineralise and buffer the product water, rendering it non-aggressive; and caustic soda dosing to achieve a positive Langelier Saturation Index (LSI).
Modularity is Key
Construction on the island presents logistical challenges in comparison to conventional design/build applications. Considering the unique requirements of the project, the two VWS companies are maximising the use of skidded equipment, piping spool pieces, modularity in assembly. In-house manufacturing will be used prior to expediting equipment to the construction site.
|Pre-treatment prior to membrane systems ensures long-term, low cost operation of RO.|
Employing this construction strategy reduces assembly time significantly. It also allows for enhanced equipment inspection because inspection can occur on complete equipment assemblies prior to shipment.
The components of the membrane system are being pre-assembled and mounted on carbon steel skids, designed to provide easy access for service, maintenance and system monitoring as well as maximum support and protection for the membrane system components.
In addition to the design-build work, the contract includes one year of on-site operations assistance.
"Pearl of the Caribbean"
Efficient water production and management to date has produced a market difference in the development and prosperity of the island.
W.E.B. Aruba N.V. has given its new desalination plant the moniker "Pearl of the Caribbean". When the facility is completed, Aruba will be celebrating its 80th year of seawater desalination since installing its very first desalination plant on the island in 1932. WWi
Author's note: Paul Choules is N.A. Water Systems vice president - Desalination & Reuse Americas, and Ron Sebek is project manager for N.A. Water Systems.