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Initial Evaluation of the Subfloor Water Intake Structure System (SWISS) vs. Conventional Multimedia Pretreatment Techniques

Pacific Research Group; Ventura CA.
May 2001

Report #66 - Executive Summary -

Open ocean intakes allow impurities (biological organisms-both macro and micro, colloidal solids, suspended solids such as silt and clay, etc.) access to the inner working of a seawater RO facility (SWRO). The dissolved oxygen levels associated with open ocean intake systems also support the growth of these organisms and cause long-term deterioration of polymeric RO membranes due to oxidation. The facility must then be equipped with an array of pretreatment components including chemical additions, which add significant capital and O&M cost to the production of the potable water. Overall the capital and O&M costs associated with these complex pretreatment systems can range from 20-30% of the total SWRO facility costs. In addition, capital and O&M costs associated with the intake subsystem can range from 10-20% of the total SWRO facility costs.

One solution to these high costs is to utilize the natural filtering properties and the low dissolved oxygen levels of the seafloor. This can be done by locating an intake system within the porous strata of the seafloor. The following two techniques are the focus of this research:

  • - vertically jetting an intake structure beneath the subfloor of a body of water, and
  • - utilizing horizontal direction drilling (HDD) techniques to position a horizontal well within the seafloor.

    The objective was to ultimately demonstrate that a reduction of 20-32% in initial capital expenditure costs as well as a reduction of O&M of SWRO facilities could be achieved using an advanced intake/pretreatment concept termed "subfloor water intake structure system" (SWISS). Results of this initial research have been highly successful. The level of filtration that is created with the SWISS design is equivalent to a deep bed slow sand filter and, therefore, provides filtration of particles down to the 1 to 10 micron size range. The quality of raw water directly exiting the SWISS is, in many ways, superior to RO feed water pretreated with conventional direct filtration systems. It effectively eliminates the need for costly media pretreatment and associated capital and O&M costs for water treatment facilities.


    For more information about the DWPR program, contact Kevin Price at: Bureau of Reclamation, 86-69000, PO Box 25007, Denver CO 80225; phone (303) 445-2260; or e-mail a message to MPrice@usbr.gov.