Chapter 6 - Design and Construction of Subsurface Intakes

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Seawater desalination is a viable alternative for diversifying a water supply portfolio and has been made even more attractive through cost-effective and efficient subsurface feedwater supply systems and water treatment technologies. Properly designed subsurface intake systems reduce or completely eliminate pretreatment requirements for particulate matter—thus diminishing impacts to the ocean environment from entrainment or impingement of marine life. The number of subsurface intakes throughout the world is relatively small compared to open ocean intakes. This is largely due to capacity differences between the two types. Subsurface intakes average approximately 12mgd per facility, as compared to approximately 52mgd per facility for open ocean intakes. However, subsurface feedwater supply systems near and beneath the ocean floor utilize the natural properties of permeable subsea aquifer sediments to filter out organic matter and suspended sediment rather than rely on expensive pretreatment systems. This results in a significant cost savings to the seawater reverse osmosis (SWRO) plant in the form of reduction in capital expenditure for construction, operation, and maintenance of a pretreatment system. Research over the past 11 years has shown that total yield is a function of scale and field tests show that properly designed and constructed slant wells with engineered artificial filter packs allow production of marine groundwater with low turbidity and silt density indexes less than one. Most importantly, subsurface intakes are quickly becoming the favored feedwater supply method due to the reduced environmental impacts they offer. They are even recommended in the State of California’s Water Quality Control Plan for Ocean Waters of California. As a result, subsurface intakes can be much easier to get permitted for SWRO desalination plants as compared to open ocean intakes.