Seawater desalination plants typically consist of three major unitoperations: seawater intake, treatment (pretreatment and/or desalination), and brine discharge. Traditionally, seawater intake is carried out using open ocean intakes. These open ocean intakes draw seawater through the meshed intake screen and then convey the seawater to the desalination plant. Use of these open ocean intakes often has negative impacts on the environment, particularly when marine microorganism become entrained or impinged in the screens.
While the intake screens prevent debris and most aquatic life from entering, elevated levels of suspended solids and other constituents still have to be removed. Prior to the desalination process, the raw seawater collected through the open ocean intake undergoes pretreatment typically using either conventional or membrane pretreatment processes to prevent particle/colloidal fouling of RO/NF membranes. Use of these pretreatment processes increases the capital cost of the facility and also escalates the operational costs as these facilities require additional chemicals and energy. In addition, these facilities produce a waste stream that while small, is difficult to handle because of the large solids loading associated with the removal of suspended particles and chemicals.
With the proposed "Under Ocean Floor Seawater Intake and Discharge Demonstration" system, the negative environmental impacts typically associated with open ocean intakes are minimized. By drawing seawater through the beach sand, the under ocean floor intake system avoids the ecological impacts of entrainment and impingement associated with open water intakes. Similarly, the concept would also be applied to the discharge of the brine concentrate stream in order to minimize the environmental impacts of the brine plume. The under ocean floor intake system also provides synergistic benefits as it acts as both an intake and pretreatment system.
This intake system is based on the design criteria associated with slow sand filtration systems. The century-old slow sand filtration concept has been utilized around the world and now offers the opportunity to be applied in an innovative manner for seawater desalination systems. By incorporating slow sand filtration (loading rate of less than 0.1 gallons per day per square-foot) into the seawater collection process, a natural, biological filtration process reduces organic and suspended solids loading on the desalination plant. Therefore, additional pretreatment is not required, reducing costs, and improving the desalination process.
The advantages of the under ocean floor seawater intake system over open ocean intakes or desalination pretreatment processes are: