Opportunity
For years, the Tampa Bay area relied solely on groundwater to meet its drinking-water needs. By the 1990s, population growth led to groundwater overdraft, damaging lakes and wetlands and leading to the intrusion of saltwater. As part of a legal agreement, Tampa Bay Water was required to reduce the use of groundwater dramatically or face serious financial penalties.
In 1999, Tampa Bay Water chose a consortium to build a seawater desalination plant adjacent to the Big Bend Power Plant at Apollo Beach in Tampa Bay. The largest plant of its kind in the US, it would use membranes to desalinate seawater through reverse osmosis. Despite financing problems and the bankruptcy of one consortium member, the plant was completed and began testing in 2003.
Problems quickly arose. The plant was plagued with membrane fouling, which were blamed on variations in the temperature of intake water and an infestation of Asian green mussels. Cleaning the membranes required the use of chemicals that could not be disposed of without violating sewage permits. Cost-cutting measures led to rust and corrosion in water pumps.
The plant was shut down in 2005, and Tampa Bay Water selected the American Water and Acciona Agua Joint Venture Team to modify the plant.
Solution
American Water and Acciona Agua retained Mott MacDonald to assist in design-build modifications to the plant. The extensive modifications included the following:
- New pretreatment facilities
- New facilities for diatomaceous-earth filtration
- New chemical storage and feed facilities
- New residuals treatment facilities
- Replacement of membranes
- Modifications to sand filtration system
- Modifications to reverse osmosis system
- Modifications to various pumping systems
- Upgrading of lime and CO2 treatment systems
- Upgrading of instrumentation and controls
- Upgrading of power systems
As the project’s engineer of record, Mott MacDonald was responsible for structural, architectural, mechanical, and site-related design of the planned modifications. We reviewed the pilot testing and process design, provided engineering oversight during construction, and reviewed Florida Department of Environmental Protection permit applications.
Outcome
The upgrade was completed and the plant reopened in 2008. In 2010, it passed two performance tests: producing 25 million gallons (95 million liters) of drinking water a day for 120 consecutive days, and maintaining an average of 20 million gallons (76 million liters) a day for 12 consecutive months.
The plant’s expected lifespan is 30 to 50 years. It supplies about 10% of the drinking water for more than 2.5 million people in the Tampa Bay area.
The plant processes 40 million gallons (150 million liters) of seawater to produce 25 million gallons (95 million liters) of potable water. To avoid increasing the salinity of the bay, the concentrated brine that remains is mixed with seawater at a ratio of 70 to 1 before being discharged.
For several reasons, the plant is expected to produce drinkable water at lower than average cost. Energy costs in the region are relatively low, the salinity of the source water is lower than usual, and the proximity of the Big Bend power plant allows the desalination plant to take advantage of infrastructure, supporting operations, and maintenance. The plant will produce drinking water from seawater at far less than a penny per gallon.