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RO system helps island meet drinking water needs
Water & Sewerage Corp. of the Bahamas (W&SC) contracted with Aqua Design Bahamas Ltd. for a new desalination facility on the island of Eleuthera under a build, own and operate business model. The new facility, located in Bogue, North Eleuthera, will provide 600,000 imperial gal per day of desalinated water and is required to meet the growing need for drinking water on the island. This new facility joins three other Aqua Design facilities on the island of Eleuthera. Aqua Design is wholly owned by GE Water & Process Technologies. The new facility will be operational in the first quarter of 2018.
The island of Eleuthera, is part of the Grand Bahama Bank located 50 miles east of Nassau. At 200 sq miles in area, the island is 110 miles long and in some places no more than 1 mile wide. The island extends from Lighthouse Beach in the south to Tay Bay Beach in the north and is home to more than 11,000 residents.
Eleuthera does not have a sufficient supply of fresh groundwater, making desalination necessary to support increasing population and tourism. W&SC is responsible for providing water to the island and contracted with Aqua Design for four desalination facilities on the island. The fourth plant recently was purchased and is now under construction in Bogue. W&SC retains responsibility for water distribution to the community.
Under build, own and operate contracts with W&SC, Aqua Design designed, supplied, installed, operates and maintains three existing facilities on the island of Eleuthera and is building the fourth facility. The locations of these plants are shown in Figure 1.
Groundwater production in North Eleuthera was not able to keep up with the rapidly growing demand for drinking water in the area. Following the success of the other three facilities on the island, W&SC decided to pursue construction of a fourth facility located in the north of Eleuthera to meet the growing demand.
Figure 1. Map of desalination facilities on Eleuthera
The system will be designed, built, installed, owned, operated and maintained under a commercial model called build, own and operate. Under this commercial model, Aqua Design guarantees the quality and quantity of product water and provides all capital necessary to install and operate the plant.
The system starts with raw seawater wells approximately 150 ft deep. Well water is pumped through two stages of filtration (cartridge and multi-media), which provide water with low suspended solids content. Filtered water is sent to a single-pass reverse osmosis (RO) plant, which removes 95% to 98% of the dissolved solids.
The RO filtration process prevents the passage of most dissolved solids. The water is divided into two streams —permeate containing water and a small fraction of dissolved solids and concentrate; and reject brine containing water and most of the dissolved solids present in the feed. Typically, approximately 50% of the inlet water is recovered as product water. Reject brine contains about two times the concentration of sea water.
The RO process uses high-pressure pumps to force water from the feed to the permeate side of the membranes. The high pressure is required to overcome the osmotic pressure of the membranes. Energy recovery devices are used to recover some of the energy in the recovered water pressure, boosting the incoming water pressure and lowering the system’s overall energy consumption.
Product water from the RO process is adjusted for pH, and calcium carbonate is added in a remineralization bed. This product water then is sent to storage for distribution to the community by W&SC. Backup power generation will be used when power supply is interrupted.
Dissolved solids removed by the process are injected into a brine disposal well, approximately 250 ft deep.
This process is commonly used throughout the Caribbean and is utilized at the other Aqua Design facilities on Eleuthera. W&SC has used the build, own and operate commercial approach on several islands of the Bahamas, as it allows W&SC to increase production while conserving capital and relying on the design, logistics and operating expertise of companies such as Aqua Design.