Reverse osmosis system meets water quality needs in the Bahamas
Abundant ocean waters may surround Eleuthera in the Bahamas, but freshwater supplies are scarce and uncertain on this arid, sand-and-coral island about 240 miles southeast of Miami.
The only available sources of freshwater on Eleuthera are pools of collected rainwater, known as “lenses” because of their shape. With a lower relative density, these freshwater lenses float on top of brackish and saline waters. Although their number, extent and thickness are controlled by the island’s geography, geology and rainfall, they tend to occur at shallow depths and therefore are vul- nerable to environmental risks.
Moreover, over-extraction of water from these lenses in many areas has caused fresh and brackish waters to mix, resulting in a steady and unacceptable rise in the salinity of the water supplies upon which the population depends.
Since June 2011, however, residents and tourists in the Tarpum Bay and Rock Sound areas of southern Eleuthera have enjoyed clean water produced by a seawater reverse osmosis (SWRO) desalination plant from GE that was pre-engineered and factory-built for high performance, fast installation, space savings and economy.
Controlling Water Quality
Reverse osmosis (RO) is the process by which dissolved impurities in water are removed using a semi-permeable membrane. RO involves the rever- sal of flow from a high salinity or concentrated solution to the high-quality stream on the other side of the membrane. Pressure is used as the driving force for the separation. The applied pressure must be in excess of the osmotic pressure of the dissolved contaminants to allow flow across the membrane.
The standardized, containerized plant has a capacity of 200,000 imperial gal of desalinated water per day. It was manufactured at GE’s facilities in Guelph, Ontario, Canada, and shipped to the island for installation on a prepared site—all in less than six months. It withstood Hurricane Irene, which passed directly overhead in late August 2011, and suffered no damage. In the days after the hurricane, it was the only water treatment system producing clean water on the island.
GE owns and operates the plant and will pro- vide the government-owned Bahamas Water and Sewerage Corp. (WSC) with a guaranteed minimum 150,000 imperial gal of desalinated water per day for 15 years. WSC incurred no capital costs beyond an investment into two 250,000-gal storage tanks, so the new plant represents an economical way for WSC to acquire reliable, secure supplies of clean water. WSC also has the right to take more water, up to the plant’s capacity.
The new SWRO plant is the latest step WSC has taken in its mission to develop the country’s water resources, control water quality, and allocate and distribute water effectively.
According to WSC’s Philip Beneby, nearly 20 years ago the organization began to plan for the eventual desalination of the water supply throughout the Bahamas. Even now in the most populous island of New Providence, about 90% of the water supplied is produced by RO desalination.
Even though WSC carried out major projects in the mid-1990s and early 2000s to address water infrastructure needs in Eleuthera—for example, a distribution system in Tarpum Bay and Rock Sound—groundwater continued to be the source of supply in those areas, and poor water quality was an ongoing fact of life. The new plant solves that problem.
The new GE SeaTECH-84 SWRO desalination plant on Eleuthera is the island’s third GE SWRO plant. It brings the number of GE water supply and treatment plants in use in the Bahamas to 11 and adds to the growing number of GE water and wastewater plants used elsewhere in the Caribbean region.
The new Eleuthera plant is a pre-engineered solution for small-to-mid-sized applications. It is housed in a standard cargo shipping container, delivered to the site on a flatbed truck, and comes equipped with all of the equipment needed for independent operation, such as piping, instruments and controls, electrical components and connections. Water pretreatment precautions are site-specific, however.
A local contractor cleared the Tarpum Bay site and poured the concrete pad upon which the SWRO plant sits. Seawater is drawn from a deep well and pumped through a series of filters that screen out anything larger than 5 μ. The system recovers 35% to 45% of the water treated and the remaining concentrated brine is disposed of in an injection well. Bahamian law forbids discharge into the ocean.
The plant is operated by four technicians from Eleuthera who are trained and employed by GE, and who are on site or on call 24 hours a day, seven days a week. Two are solely dedicated to the plant, while two also serve another GE SWRO plant in central Eleuthera.
The site is large enough to add at least one more SWRO plant as needed and thus will be able to keep pace with any increase in economic activity the new water supply may help generate. A new well would need to be drilled for additional SWRO plants, however.
The site also houses WSC’s two 250,000-gal storage tanks. From there, the water enters WSC’s distribution system to supply the Rock Sound and Tarpum Bay settlements.
Beneby said that the system had to be shut down when Hurricane Irene hit the Bahamas, but once the storm passed, GE was able to determine quickly there was no damage, and the plant ran on a standby generator until electrical power could be restored. The major impact of the hurricane was to postpone the grand opening ceremony and dedication until later in the year, by which time the plant had demonstrated flawless operation, as it has continued to do.