Sieves & Screens
Converting seawater
Powder-handling equipment from Flexicon improves filtration at an American desalination plant
The 2800 m2
Tampa Bay Seawater Desalination Plant produces up to 95 million litres of drinking water per day, making it the largest seawater desalination plant in North America. Since March 2007, the plant has desalinated over 11 billion litres of drinking water from the Tampa Bay. In 2005, however, the plant was shut down, as it could not meet the expected operational sustainability. Tampa Bay Water, the government agency responsible for the plant, assigned remediation work to American Water and Acciona Agua, through their operating partnership American Water-Pridesa, a Group that has designed and built more than 50 desalination plants worldwide.
Among the firm's many improvements is the addition of Precoat Filtration using a bulk handling system for diatomaceous earth. This has proven to be instrumental in re- establishing the plant as a major source of drinking water for the Tampa Bay region.
Reverse osmosis converts seawater
Desalination plants rely on reverse osmosis (RO), which uses high pressure to force water through semi-permeable membranes that remove salt from seawater. To ensure efficient RO, seawater must be pretreated to remove particulates. During remediation at the Tampa plant, American Water Acciona Agua
ABOVE: At the flexible screw conveyor’s discharge end, DE flows through a transition adapter into the 1136 L tank where the DE is put in suspension with water
ABOVE: One of two bulk bag unloaders unloads and transfer 1814-2722 kg per day of diatomaceous earth (DE) for Tampa Bay desalination plant’s DE filtration system. The crane deposits a lifting frame holding the bulk bag onto the unloader frame
22 Solids & Bulk Handling • May 2010
improved pretreatment by adding coagulation and flocculation, improving the operation of the existing sand filters and installing a diatomaceous earth (DE) filtration system to eliminate microscopic materials from the water prior to RO. DE is a silica powder (hydrated silicon dioxide) comprised of the cell walls of phytoplankton called diatoms. Applied to the pressure side of filter elements, DE traps micron-size particles that would otherwise pass through ordinary filter media. DE powder is added to seawater upstream of the filter, forming a cloud of DE particles that coats the filter medium and, in turn, traps solid contaminants as water passes through the DE coating. When contaminants build up, indicated by pressure increases, the filter is backwashed, after which another dose of DE is added to the water to re-coat the filter medium. The Tampa Bay plant consumes 1800 to 2725 kg per day of DE, which arrives in 400 kg bulk bags that are stored in a temperature- and humidity-controlled area to prevent compaction of the material. When the RO process calls for DE, a crane moves a bag from storage to either of two bulk bag weigh batching systems that feed the DE to a 1140 litre tank where it is put into suspension with water to a 5% concentration. The suspension is then metered into the saltwater upstream of the filter by peristaltic pumps.
www.solidsandbulk.co.uk
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