Project Case Study: Dry season relief

October 2006 » Feature Articles
A brackish-water desalination plant in Spring Garden, Barbados
Mario Mondo

The largest brackish-water reverse osmosis desalination plant in the Western Hemisphere meets 25 percent of Barbados' water demand.


Project
Brackish-water desalination plant, Spring Garden, Barbados

Joint venture build/own/operate
GE Water & Process Technologies, Watertown, Mass.
Williams Industries, Inc., Barbados

Product application
Reverse osmosis desalination plant provides secure source of potable water on a Caribbean island.


When viewing the lush vegetation on the Caribbean island of Barbados, water seems to be plentiful. Although the island's average annual rainfall is an abundant 55 inches, precipitation during the dry season, from November to May, is often less than 1 inch. The minimal rainfall, coupled with a high standard of living on the island, prompted the United Nations to identify Barbados as one of the most water-scarce nations in the world in the 1990s.

Barbados experienced a severe water shortage in 1994 and 1995 when the one-in-50-year drought caused more than 3,000 households to be without water for significant time periods. On July 31, 1995, virtually the entire town of Bridgetown, located on the southwestern corner of the island, was without water for a prolonged period. Water shortages in Bridgetown affected the Queen Elizabeth Hospital and many hotels filled with tourists. This drought prompted the Barbados government to take action to prevent the water shortage from endangering the safety of the island's residents and its economic development.

Augmenting groundwater supplies
As the drought conditions continued, plans to develop tourist attractions, such as golf courses, were temporarily put on hold by the Barbados government. Finding a sustainable potable water supply was of utmost importance to the island's long-term economic growth. The Barbados Water Authority conducted extensive research to find a solution to the island's potable water shortage.

Research results prompted the Barbados Water Authority to initiate an international request for proposal to bid on a project to design and construct a brackish-water desalination plant. The Barbados Water Authority chose the build-own-operate model, requiring the winning bidder to provide operations and maintenance at the plant for a period of 15 years. The RFP included other challenging specifications, such as the need to meet noise limitations of 50 decibels. Six bids were tendered, and before making a decision, a team of managers from the Barbados Water Authority toured four desalination plants across the world.

The Barbados Water Authority selected the joint venture of U.S.-based GE Water & Process Technologies, a unit of the General Electric Company, and Williams Industries of Barbados to help them eliminate the present and future water shortage problems. The agreement included design, engineering, manufacturing, financing, installation, start-up, commissioning, operations, and maintenance for a 15-year period for a brackish-water desalination plant in Spring Garden, St. Michael Parish, located approximately 2 miles from Bridgetown.

"GE was selected because of the many successful water scarcity projects it has developed," said David Staples, director of Ionics Freshwater, Ltd., a joint venture of GE Water & Process Technologies and Williams Industries, Inc. "We work closely with water scarcity solutions in developing countries, paying close attention to environmental impact and energy conservation."

Plant process
On March 15, 1999, GE began development of the desalination plant, with construction starting in June 1999. The plant began producing potable water on Feb. 15, 2000, after only 11 months of construction and commissioning.

In the Spring Garden plant, GE provided two, brackish-water, reverse osmosis (RO) trains, each a single array, with four banks of 16 pressure vessels (128 total). Each pressure vessel has six membrane elements that measure 8 inches by 40 inches.
The desalination process starts as brackish feedwater is pumped from 10, 80-foot-deep wells located behind the Spring Garden plant. The water is then pretreated to remove suspended solids and particulates, such as nitrates, minerals, and dirt. An antiscalant is also added to prevent mineral scale build-up.

Pretreated water is then sent through the RO system, a widely accepted membrane-based technology for separating the incoming feedwater into desalted and concentrated streams by the application of pressure to semi-permeable membranes. The Barbados RO plant uses Toray ultra-low-pressure brackish membrane elements that achieve 99.5 percent salt rejection at 110 pounds per square inch (psi). The desalinated water is further treated for public consumption in a lime dosing step, adding some mineral content to the water to ensure good taste. The water is then chlorinated by dosing it with a small amount of sodium hypochlorite.

Approximately 75 percent of the pressurized feedwater is converted into potable water, while the remaining concentrate stream is passed through an energy-recovery turbine to minimize energy consumption before it is returned to the sea via deep-well injection. Potable water is transferred to a storage tank where it is monitored to ensure consistent water quality and then delivered to the Barbados Water Authority for distribution.

Pump motor sets in the Spring Garden plant have provided for virtually noiseless operation at the plant, meeting the contract requirements of remaining quieter than 50 decibels. Building insulation also contributes to the low noise.

"This desalination plant has a very minimal impact on the environment," said Staples. "The brine resulting from the RO process is used by the Spring Garden plant's downstream neighbors—a rum distillery and Barbados Light and Power—for cooling water." Any water not used by these plants is returned to deep wells where it percolates through limestone before eventually making its way to the sea.

The Spring Garden plant is the largest, brackish-water, RO desalination facility in the Western Hemisphere. It has a capacity of 7.5 million gallons per day, and is able to meet 25 percent of the island's daily water demands. The plant is also the most secure source of potable water on the island, strategically positioned to supply water to a very large section of the island's 264,000 people through a series of lift stations and intermediate reservoirs.

"The Spring Garden plant has had zero days that it has been inoperable as a result of mechanical issues," said Staples. "The plant has an extremely dedicated staff and has surpassed all efficiency and availability expectations set by the Barbados Water Authority."

With the output of potable water from the Spring Garden facility, Barbados is now able to withstand the continuing patterns of water shortage.

Mario Mondo is operations manager for GE Water & Process Technologies. He can be contacted at mario.mondo@ge.com .


Storing water: A precious island resource
Hawaii is surrounded by an ocean. Nevertheless, water is one of the state's most precious natural resources, and potable water tanks are essential lifelines in every community. The Honolulu Board of Water Supply (HBWS) recently constructed a 6 million-gallon, prestressed concrete tank in Ewa, Oahu, Hawaii, to help meet increased water demands by increasing potable water storage capacity and providing safe and dependable drinking water for the community.

The HBWS is a semi-autonomous agency of the city and county of Honolulu, and the primary purveyor of water to residential and commercial users on the island of Oahu. It is one of the largest water utilities in the United States, and its mission is to ensure Water For Life—Ka Wai Ola. In addition to ensuring that there is sufficient capacity to meet Oahu's present water needs and maintaining the high quality of Oahu's potable water supply, the HBWS also considers the long-term viability, protection, and sustainability of the island's water resources.

The Ewa district on the island of Oahu has undergone significant development in recent years. Fertile agricultural land has given way to numerous residential, commercial, and industrial developments, which have impacted the region's available water resources because of increased potable water demands and accelerated withdrawal of water from potable water aquifers.

Recognizing these demands, the HBWS analyzed several factors to determine the most economical, efficient, and maintenance-free configuration for its water-storage needs. When reviewing the various tank systems available for its project, HBWS considered numerous factors, including construction time and costs, reliability, aesthetics, seismic performance, security, and future maintenance expenses. Another challenge it faced was that the tank would be subjected to the harsh, corrosive environment prevalent on the island. After careful consideration of the long-term benefits, the HBWS determined that a prestressed concrete tank would offer the highest-quality, longest-lasting, and lowest-maintenance water storage structure for the application.

Tank design and construction
Marc M. Siah and Associates, Honolulu, consulting engineer; Parsons Engineering, Ewa, Hawaii, general contractor; and DYK Inc., El Cajon, Calif., tank manufacturer comprised the project team. The floor, footings, columns, wall, and roof of the 6 million-gallon tank were all designed and constructed with concrete. It has an inside diameter of 226 feet, a wall height of 21 feet, and a service water depth of 20 feet. The walls were poured full height through openings in the sides of the wall forms to avoid horizontal joints and to prevent subsequent leakage. The floor, wall footing, and roof slab were poured in alternate quadrants and allowed to cure for seven days before pouring adjacent sections. Eighty-eight, 18-inch-diameter concrete columns were poured to support the 9-inch-thick, cast-in-place, two-way flat slab concrete roof.

The 12-inch-thick, poured-in-place concrete tank corewall was prestressed both vertically and circumferentially. Vertical prestressing used 180, 1-3/8-inch-diameter, high-strength Dywidag threadbars, which were subsequently grouted with epoxy. The tank walls were circumferentially post-tensioned by a strandwrapping machine that applied force to the 0.3750-inch-diameter galvanized strand. This machine continuously and electronically monitored the applied stressing force on the strand as it was applied. The strand was encapsulated with several coats of machine-applied shotcrete. Post-tensioning the concrete corewall both vertically and horizontally yielded an efficient structure to contain the large loads produced by the stored water and the subsequent backfill and design live loads.

Seismic considerations
Hawaii can experience high seismic activity, therefore, seismic design of the tank was of particular importance. Because of the high probability that this tank would be subjected to seismic ground motions, specialized, flexible seismic connections were incorporated into the wall-base and wall-top details. To account for the high seismic forces anticipated during the service life of the tank, freed wall-to-wall footing and freed wall-to-roof connections were used. These seismic connections were designed to allow for maximum ductility under a seismic event to ensure the structure would continue to serve the HBWS should the tank undergo horizontal and vertical ground accelerations.

Source: DYK, Inc.


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