Desalination System Helps Tampa Bay “Drought Proof” Water Supply
In the 1980s and early 1990s, Tampa Bay water officials began developing a master plan to meet the long-term water needs of the region.
By Dr. Harold Fravel
In the 1980s and early 1990s, Tampa Bay water officials began developing a master plan to meet the long-term water needs of the region. The west-central region of Florida was faced with periods of drought, dwindling groundwater resources and increasing demand from a growing population. To help “drought proof” the cities and surrounding counties, planners looked to the sea and desalination.
By 1995, the idea of converting salt water to drinking water started to become a reality. The wholesale water supplier that would become Tampa Bay Water approved a master plan that served as a blueprint for providing sustainable water from diverse sources, including water from the Bay. With technological advances that lowered the operating costs of desalination, the technology would become an important component to the long-term water supply plan.
Today, after a year of successful operation, the Tampa Bay Seawater Desalination Plant is North America’s largest such facility, built with a capacity to supply 25 million gallons per day of fresh water, enough to serve 10 percent of the area’s water needs. It has proven itself to be environmentally sustainable and, with built-in energy efficiencies and filtration technologies, produces water at a competitive cost.
Desalination is one component of Tampa Bay’s water supply network that also includes a surface water treatment plant and storage reservoir, groundwater treatment facilities and well fields. All combined, the system has worked to reduce groundwater pumping from a permitted average of 158 mgd to 90 mgd annual average by the end of 2008.
Road to Success
The success in integrating desalination into the water system didn’t come without setbacks and lessons learned. In 2001, construction began on a desalination plant that would begin operations in 2003 and then become fully operational by the end of 2007. The original plant, once up and running in 2003, failed to meet design standards due to a number of deficiencies, including equipment problems. Another shortcoming was the fouling of the original reverse osmosis (RO) membranes. The operational problems were so serious that in 2005 the plant was shut down for retrofitting and rebuilding.
The Tampa Bay Seawater Desalination Plant is the largest desalination plant in North America.
In a second effort to make desalination work, Tampa Bay Water hired American Water-Pridesa, a joint venture of Acciona Agua of Spain and American Water, to rebuild and operate the plant. There were many upgrades made to convert the facility into a successful operation. For the actual work of desalination, American Water chose to equip the facility with Filmtec™ SW30HR-380 reverse osmosis membranes from Dow Water Solutions to remove salt and other dissolved solids from the feedwater.
Another key component to the desalination plant’s redesign was a thorough pretreatment of the feedwater before it enters the RO process. Key to this strategy was the desalination plant’s location: adjacent to Tampa Electric’s (TECO) Big Bend Power Station in the south area of the Bay region. The power station withdraws and discharges up to 1.4 billion gallons of sea water every day from the Bay for cooling. The desalination plant’s raw water intake was set up beside the neighboring power plant’s four discharge tunnels, two of which are tapped to divert 44 mgd of water from the cooling outflow into the intake for pretreatment.
In the pretreatment phase, the intake water is treated with ferric chloride and then flows through a single-stage sand filter, where sediment and other particles that could clog the RO membranes are filtered out. The feedwater is further treated with diatomaceous earth filters which remove microscopic particles. The final stage of pretreatment is five micron cartridge filters before the water enters the RO system.
After pretreatment, the feedwater is ready for the reverse osmosis process. During this stage, high pressure pumps force the water through 9,408 semi-permeable RO membranes that make up seven independent trains that produce freshwater and leave behind a concentrated salt solution. The desalination plant’s use of TECO’s relatively warm discharge water increases water flux while the lower salinity properties of water from the Bay require less pressure. Both these factors help increase membrane efficiency and lower the cost of the reverse osmosis process. The plant’s contractors added another benefit to the facility - all of the plant’s highpressure RO feed pumps have energy recovery units that help reduce energy costs.
The Filmtec SW RO membrane elements themselves also reduce operational costs. Each membrane has 380 square feet of active area and is made of a spiral wound thin-film composite polyamide material. The spiral wound design has become the industry standard as it offers the highest active membrane surface area in a compact design.
The RO membrane consists of three layers that ensure durability and long-term performance: an ultra-thin polyamide barrier layer that allows high water flux, a microporous polysulfone interlayer and a polyester support web. The microporous polysulfone interlayer provides a substrate for the salt barrier layer while the polyester support web provides the major structural support to the membrane and helps it withstand high pressures.
Finding membrane elements that could handle severe fouling was a key concern for the plant’s remediation contractors. Since Filmtec RO membrane elements are constructed by a fully automated system that allows more leaves in the element, the increased active area means more water can pass through the membrane. This results in a higher flow rate and greater salt rejection. The membranes at use in Tampa Bay have a flow rate of 6,000 gpd and can remove up to 99.7% of salt and other dissolved solids and inorganic molecules. An additional benefit to the automated construction is that there are uniform rolling of the elements and minimum gluelines to cover active area. Between the membrane leaves are thick feed spacers that make the elements easier to clean.
Split Permeate Design
In a typical desalination system the reverse osmosis membrane elements are encased in pressure vessels with permeate being drawn from one end. In an operating system, the water produced in the lead element is of the best quality because it sees the highest pressure and produces permeate with the lowest Total Dissolved Solids (TDS). In the Tampa Bay facility, the plant’s designers sought to increase the efficiency of the RO process even more. They implemented a unique split permeate design in which water could be drawn from both ends of the pressure vessel, specifically the front three and the back five elements.
Eight inch RO membranes help produce 25 mgd of water at the Tampa Bay Seawater Desalination Plant.
Because the permeate quality demanded is difficult to meet 100% of the time with a one pass system many plants operate with a two-pass system. The water passes through one RO pass to produce permeate of a fairly good quality, and then passes through a final, even lower pressure RO pass for a high quality permeate of at most 500 ppm TDS.
Though Tampa Bay has a generally low level of salinity, in the hottest months of the summer or during periods of drought, the salinity level of the water rises and the desalination process can require more pressure. During those times, the high-quality water from the front three elements can go straight to the distribution tank, while only the permeate from the final five RO elements needs to go through the second pass. This makes the second pass usage a rarer occurrence and keeps the plant running at optimum capacity.
Once the water leaves the RO units, it is still not quite ready for distribution and use. Chemicals – including sodium hypochlorite to chlorinate the water and calcium hydroxide to introduce hardness – help stabilize the water, which is then blended with treated drinking water from other sources.
In addition to 25 mgd of freshwater, the RO desalination process produces 19 mgd of a concentrated salt solution. While this was an initial environmental concern in the early stages, the facility’s location next to the power plant provided yet another advantage. The salt solution from the desalination facility is returned to the neighboring power plant’s cooling water where it is diluted with up to 1.4 billion gallons of regular seawater before being returned to the Bay. The cooling water dilutes the concentrated salt solution 70 – 1. This new mixture then passes through a discharge channel to be blended with more seawater. Once the blended water reaches the Bay, its salinity level is roughly the same as the seawater. This process ensures that the desalination plant does not discharge water with a significant increase to the salinity of the water in the bay, thereby minimizing any detrimental effect on marine life. Continuous environmental monitoring is confirming these environmental benefits.
At present, the Tampa Bay Seawater Desalination Plant is providing safe drinking water to 2.5 million residents and is a key component of the region’s drinking water system. Though the plant currently produces 25 mgd, officials say the plant capacity could be expanded to 35 mgd in the future. By integrating key energy efficient features – warmer, lower salinity water input and energy efficient RO membrane elements – the system has become a water solutions model that has attracted visitors from around the world. It’s only a matter of time before the facility is replicated in other parts of the nation dealing with strains on their public water supplies. For the Tampa Bay area, the benefits of desalination are here today as residents enjoy water from an economically viable and sustainable process to harvest water from the sea.
About the Author:
Based in Jupiter, FL, Dr. Harold Fravel is a senior industry manager in membrane technology at Dow Water Solutions, a business unit of The Dow Chemical Company, with headquarters in Edina, MN.