Desalination is the process of removing salt from water making it suitable for drinking. Distillation, electro dialysis, freezing and reverse osmosis are all methods of desalination. (“Water, desalination of, 2006) In the distillation method, saltwater is heated and the water evaporates leaving the salt behind. Due to fuel cost of turning water into vapor other distillation methods were developed. The multi-stage flash distillation method is conducted by either applying a vacuum to reduce the boiling temperature or exposing a thin film of water to high heat causing flash evaporation. Plants using this method account for 85% of all desalinated water in the world. (“Multi-stage flash distillation”, 2010) In electro dialysis salt is dissolved in water splitting up the salt into ions. The ions are then filtered by membranes and allow desalinated water to be tapped. This process uses a great amount of energy and is impractical. The most promising approach is the reverse osmosis process, which applies pressure to saltwater to force it through a special membrane that only allows pure water to pass.
The reverse osmosis process is being put to use in North America’s largest desalination plant. The Tampa Bay Seawater Desalination Plant began construction in 2001 to help meet the long-term water needs of the west-central region of Florida. This region has often suffered from long periods of drought and dwindling groundwater resources. (Fravel,) The region is also trying to meet the increase need due to a growing population in the region. The desalination process in place at the Tampa Bay Desalination Plant includes a pretreatment of the water to be desalinized, the actual reverse osmosis process of desalinization and a post-treatment stage.
In the pretreatment stage, water from the cooling tanks at the neighboring Big Bend power station is diverted into the intake. It is then treated with ferric chloride and sent through a single-stage sand filter to filter out particles and sediment that could clog the reverse osmosis membranes. It is then sent through diatomaceous earth filters and five-micron cartridge filters.
After the pretreatment stage the water is now ready for the reverse osmosis stage. In the reverse osmosis stage high-pressure pumps force the water through 9,408 semi permeable reverse osmosis membranes. The membranes make up seven independent trains that leave behind a concentrated salt solution. The membranes have a spiral design allowing a large surface area in a compact design. Each membrane has 380 square feet of active area and is made up of 3 layers. The ultra thin polyamide barrier allows a high water flux. The microporous polysulfone interlayer is the substrate for the salt barrier layer. (Fravel, 2008) The polyester support web provides the structural support to help the membrane withstand high pressure. These membranes have a flow rate of 6,000 GPD (gallons per day) and remove 99.7% of salt and other particles from the water. The trains are set up on both ends of the pressure vessel; three on the front and five on the back, known as the split permeate design.
In the post-treatment stage chemicals, sodium hypochlorite and calcium hydroxide are added to the water. Sodium hypochlorite is added to chlorinate the water and calcium hydroxide is added to harden the water. Both chemicals aid in stabilizing the water. The concentrated salt solution that was extracted from the water is added to the 1.4 billion gallons of cooling water at the neighboring power plant.
The Tampa Bay Desalination Plant has had to overcome several issues to remain in production. The high-energy cost of desalinating water is one that prevents desalination from being a leading provider of drinking water. The Tampa Bay Plant was designed to minimize energy use. The plant was purposely built next to Tampa Electrics Big Bend Power Station. This strategic location alleviates the cost of pumping water from the ocean and the relatively warm water increases water flux. The power station gets the water from the bay, which has lower salinity properties that require less pressure to desalinate. Another way the plant minimizes energy costs was by equipping the reverse osmosis feed pumps with energy recover units. The use of the split-permeated design maximizes the pressure and results in water having to be sent through the membranes a second time a rare occurrence.
Other issues that the plant has had to overcome include: equipment failure or inefficiencies leading to economic issues, environmental issues and political issues. In April 2003, just after the opening of the plant, a ruptured pipe forced the plant to shut down and in March of 2009 a leaking intake pipe caused the plant to shut down for 24 hours and lowered the rate of production to less than 19 mgd for a week. A test to determine the amount of water to be produced led to the discovery that cartridge filters seemed to be going bad quickly and at $10 per unit could quickly become costly, resulting in the plant changing membranes to Filmtec SW30HR-380 membranes. The disposal of 19 million gallons of concentrated salt solution a day posed an environmental issue for the plant but due to the strategic placement the solution was easily mixed with the 1.4 billion gallons of water in the cooling tanks of the power plant, minimizing the environmental effects. Political issues slowed the opening of the plant. Before the opening of the plant it’s developer, Covanta Tampa Construction, filed for bankruptcy protection. Covanta filed for bankruptcy protection due to Tampa Bay Water indicating their desire to have an outside firm take over resolution of problems that Covanta had failed to resolve in the agreed upon time frame. Tampa Bay Water says that Covanta’s motion for bankruptcy protection was a, “betrayal of the public trust that simply delays fixing and operating the plant.” (Landers, 2004)
Despite the set backs of the Tampa Bay Desalination Plant, the plant currently produces 25 million gallons of drinkable water per day, approximately 10% of the Tampa Bay regions drinking water. The plant has produced more than 18 billion gallons of water in two years. General manager of Tampa Bay Water said that, “the facility provides an important, drought-proof component to the region’s water supply system and is a true example of a successful public-private partnership.” (“Tampa Bay Milestones, 2010) The systems energy efficient features have attracted visitors from around the world and will likely be replicated in other parts of the nation very soon. (Fravel, 2008)
References
Fravel, H. (2008). Desalination System Helps Tampa Bay “Drought Proof” Water Supply. WaterWorld, 24(12), 8. Retrieved from MasterFILE Premier database.
Landers, J. (2004). Tampa Bay Desalination Dispute Goes to Court. Civil Engineering (08857024), 74(2), 26. Retrieved from MasterFILE Premier database.
Multi-stage flash distillation. (2010, September 27). In Wikipedia, The Free Encyclopedia. Retrieved 00:19, October 27, 2010, from http://en.wikipedia.org/w/index.php?title=Multi-stage_flash_distillation&oldid=387341213
Pittman, C. (2009, March 17). More problems for tampa bay water desalination plant. St. Petersburg Times. Retrieved October 20, 2010, from http://www.tampabay.com/news/environment/water/article984409.ece.
(2010). Tampa Bay Desalination Plant Achieves Performance Milestones. Underground Construction, 65(4), 7. Retrieved from MasterFILE Premier database.
Water, desalination of. (2006). In The Columbia Electronic Encyclopedia. Retrieved from