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When viewing the lush vegetation on the Caribbean Island of Barbados, water seems to be plentiful. Although the island’s average annually rainfall is an abundant 1422 mm (55 inches), the dry season, from November to May, is often less than 25 mm. 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.
Challenge: Augment existing groundwater supplies in order to sustain the island’s potable water supply
As the drought conditions continued, plans to develop tourist attractions, such as golf courses were temporarily put on hold by the Barbados government. However, finding a solution for sustaining a potable water supply was of utmost importance to the island’s long-term economic growth. The Barbados Water Authority conducted extensive research in order 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, and the noise output from the plant could not exceed 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 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. GE enlisted its local partner, Williams Industries, Inc., to assist with the project. The agreement included design, engineering, manufacturing, financing, installation, start-up, commissioning, operations and maintenance for a 15-year period of the brackish water desalination plant in Spring Garden, St. Michael, located approximately two 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. The Barbados Water Authority was impressed with our comprehensive solutions, range of tools and our ability to look at the broader issues of infrastructure, energy and water.”
The Solution: A brackish water reverse osmosis desalination plant to produce potable water
On March 15, 1999, GE began development of the desalination plant with construction starting in June 1999. The plant began producing potable water on February 15, 2000, after only eleven 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, totaling 128. Each pressure vessel has six membrane elements that measure eight by forty inches.
The desalination process starts as brackish feedwater is pumped from ten separate 80-foot-deep wells located behind the Spring Garden plant. The water is then pretreated in order 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 utilizes 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 and ensuring 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 lower 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 Results: The largest brackish water RO desalination plant in the Western Hemisphere provides 25 percent of Barbados daily potable water demands
The Spring Garden, St. Michael 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 immediate 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.
GE is currently working on many water recovery projects in the Caribbean utilizing the Membrane Bioreactor Technology. The company has completed two wastewater treatment plants on the island of Tobago to assist in the preservation of environmentally sensitive marine environments. Three additional MBR wastewater treatment plants will be complete by the end of 2005 with two plants in Barbados and one in St. Lucia.