Seawater filters through 18 diatomaceous earth (DE) vessels before being forced at pressures of up to 1,000 psi through semipermeable membranes that trap salt before the purified water moves to the final treatment stage. Photo: American Water

Though it's been approved to supply local utilities since 2008, Tampa Bay Water's 25-mgd seawater desalination facility had to pass four performance tests before receiving $31.25 million that the Southwest Florida Water Management District had pledged to remediate the facility. All four tests have been successfully completed.

In 1998, looking to supplement aquifer and surface water sources to serve 2.5 million customers, the wholesale drinking water provider decided to tap the largest and most local source: 398-square-mile Tampa Bay. Managers located the reverse-osmosis facility, which at full capacity leaves 19 mgd of twice-as-salty seawater behind, next to a local electricity provider to dilute the filtered seawater with water the plant uses as a cooling agent.

In 2005, Tampa Bay Water shut the plant down and entered a public-private partnership with American Water and Acciona Agua to enhance and operate the facility.

The district required the plant to produce 25 mgd for 120 consecutive days and an average 20 mgd for 12 consecutive months. The plant had already satisfied the first two requirements: acceptance by the district, which occurred in 2007, and producing an average 12.5 mgd for 12 months.

Today, the facility provides 10% of the region's drinking water supply at much less than a penny a gallon, according to Mark Strauss, president of American Water Enterprises. Its lifespan is 30 to 50 years.