Reverse Osmosis is used to remove the fresh water from the salt water.
Tampa Bay Seawater Desalination uses a process called reverse osmosis (RO) to produce drinking water from seawater. RO has been successfully used in nearly 200 water and wastewater treatment plants throughout Florida and produces some of the highest quality drinking water in the world.
Tampa Electric’s Big Bend Power Station already withdraws and discharges up to 1.4 billion gallons a day of seawater from Tampa Bay, using it as cooling water for the power plant. The Tampa Bay Seawater Desalination plant “catches” approximately 44 million gallons (mgd) of that warm seawater a day, separates it into drinking water and concentrated seawater.
The unused concentrated seawater is returned to TECO's cooling water where it is diluted with up to 1.4 billion gallons of water before it is discharged to the bay.
There are basically three main treatment elements in the desalination process: pretreatment, reverse osmosis, and post-treatment.
Pretreatment
Pretreatment must be rigorous to remove sediment, organic matter and other microscopic particles to ensure efficient reverse osmosis operations.
Seawater entering the plant is first treated with chemicals to allow eventual settling of particles. It then goes through traveling screens that filter out shells and other larger debris. The screened water then goes through settling chambers. Similar to a traditional surface water treatment process, particles in the conditioned water clump together and settle out.
The next step in pretreatment is sand filtration, where smaller particles are filtered from the water. Next, diatomaceous earth filters eliminate microscopic materials before the water passes through cartridge filters, the last barrier before the RO process.
Reverse Osmosis
Reverse osmosis is what distinguishes a desalination plant from a traditional surface water treatment plant. During RO, high pressure forces the pretreated water through semi-permeable membranes, separating saltwater from freshwater and leaving salt and other minerals behind in a salty solution.
The size of each RO membrane pore is about .001 microns, which is about 1/100,000th the diameter of a human hair.
Post-treatment
The Tampa Bay Seawater Desalination Plant produces up to 25 mgd of desalinated drinking water. Before that water is delivered to Tampa Bay Water, chemicals are added to stabilize the water. The water is then pumped to the regional facilities site, where the desalinated seawater is blended with treated drinking water from other supply sources before being delivered to Tampa Bay Water’s member governments.
Concentrate Return
At full capacity, the RO process will leave about 19 mgd of twice-as-salty seawater behind which will be returned to Big Bend’s cooling water stream and blended with approximately 1.4 billion gallons of cooling water, which will dilute it 70-to-1. At this point, its salinity will be only 1.0 to 1.5 percent higher, on average, than water from Tampa Bay. Environmental scientists say this slight increase falls within Tampa Bay’s normal, seasonal fluctuations in salinity.
This cooling water mixture then moves through a discharge canal, blending with more seawater, diluting the discharge even further. By the time the discharged water reaches Tampa Bay, its salinity is nearly the same as the bay’s. And, the large volume of water that naturally flows in and out of Tampa Bay near Big Bend will dilute it even further, preventing any long-term build-up of salinity in the bay.
Monitoring during the plant’s first year of operations showed no measurable changes in salinity, even when the plant was operating at maximum capacity.
There are many advantages to locating Tampa Bay Seawater Desalination beside the Big Bend power plant in addition to large volumes of cooling water. Tampa Bay’s relatively low salinity and the warm temperature of the power plant’s cooling water help optimize the RO process, keeping costs down. Tampa Bay’s frequent flushing also helps prevent the build-up of salinity.
