Abstract
Open-ocean intake systems require extensive and advanced pretreatment unit operation to produce feed water with low membrane fouling potential in seawater reverse osmosis (SWRO) facilities. Alternatively, subsurface intake systems tend to produce high quality raw seawater even before pretreatment. Subsurface intakes extract seawater indirectly through the geological structure of shoreline or nearshore sediments. Water percolation through geological units provides physical and biological treatment, so that the raw seawater is microbiologically stable with relatively low particulate and organics content. Overall, utilization of subsurface intakes will reduce the intensity of pretreatment, which reduces operating cost, lowers chemical and energy consumption, and reduces environmental impacts. An important aspect in the feasibility of a subsurface intake is the compatibility of the local geological environment. In this study, a field investigation was conducted at Ras Abu Ali Island in the Arabian Gulf. This location currently contains an of existing oil company facilities and a proposed governmental marine fish hatchery facility. Recreational, commercial, and domestic potable water uses require the need to use the SWRO process to meet demands. Characterization of the shoreline and marine offshore bottom were performed as well as observation of tidal fluctuations and wave heights. A specific grid area was chosen where 35 sediment samples were collected from the seabed floor for laboratory analysis of grain size distribution, sediment porosity, and hydraulic conductivity. Onsite observation showed that the marine bottom has a low slope creating a wide intertidal area. The lowest tidal zone is more than 150m from the shoreline defining a far seaward boundary for the intake construction point. A relatively thin layer of mixed-type sediment (carbonate and siliciclastic) covers the marine hardground bottom. The unlithified bottom sediment contains a low mud percentage (less than 1%) with porosity ranging between 0.29 and 0.41 and hydraulic conductivities up to 22.5m(3)/d. It was determined that seabed gallery development is suitable at this location. Preliminary design for a seabed gallery filter was developed using a series of cells. Each gallery cell represents a unit that can be simply duplicated to meet the overall intake capacity requirement. Each gallery cell is designed to have minimum 8 m/d infiltration rate through five layers of engineered sand and gravel. The total thickness of the filter bed is 4.5m (2m top layer). The dimensions of the proposed cells are 100 x 30m and each cell will conservatively provide 24,000m(3)/d of filtered water. The design is flexible to meet the required capacity. For example, a SWRO desalination plant which produces 54,000m(3)/d product water from 38,000 mg/L salinity seawater at a 45% conversion rate will require a minimum of 6 cells using the preliminary design.