Abstract
Subsurface intakes can be used as part of the pretreatment system for seawater reverse osmosis facilities. Wells of various designs and galleries are being used for intakes at many sites globally to reduce pretreatment costs, chemical usage, biofouling potential, and environmental impacts (entrainment and impingement). The goal of using a subsurface intake is to reduce or replace conventional pretreatment processes. A field and laboratory investigation was undertaken to assess the feasibility of developing a seabed gallery intake offshore near the Shoaiba seawater RO (SWRO) plant site (150,000 m(3)/d treatment capacity) that could be used to replace the current intake used for only the RO capacity. A survey of the beach and offshore area was made and 51 sediment samples were collected from the seabed for laboratory analysis of grain size distribution, porosity, and hydraulic conductivity. Field observations showed that the marine bottom has a low slope from the shoreline seaward a distance of about 100 m to a depth of about 1.0 m before it steepens to a depth of over 2 m at 150 m from shore. The site has a relativity thin cover of unlithified carbonate sands, 2-5 cm thick, sitting on a soft coralline limestone of Pleistocene age. The sediments investigated were found to be clean carbonate or slightly muddy carbonate sands with mean grain diameters ranging mostly between 0.25 and 0.5 mm. Most of the area investigated contained a mud percentage between 0 and 5%. The measured porosity values range between 0.33 and 0.45 and measured hydraulic conductivity values from about 1.6 to 79.5 m/d with 64.6 m/d being the maximum offshore value. A preliminary design was developed to meet the full operational capacity of the Shoaiba SWRO treatment plant which would require 375,000 m(3)/d of raw water to produce 150,000 m(3)/d of permeate (40% conversion assumed for Red Sea water with a TDS of 41,000 mg/L). The design of the RO plant uses 10 trains to produce the 150,000 m(3)/d of product, but at full operational efficiency, only nine trains would produce the required capacity. The engineered gallery design included five layers with a total thickness of 3 m. The proposed infiltration rate is 7 m/d with the possibility of increasing it to 10 m/d. The gallery design consists of 10 cells (one for each train) producing 42,000 m(3)/d each. The cells have dimensions of 30 x 200 m and an aggregate area of 60,000 m(2). It is believed that use of the seabed gallery at this site is feasible and would reduce environmental impacts by eliminating impingement and entrainment of marine organisms, the use of chlorine in the process, the use of coagulants in the pretreatment filtration process, the disposal of marine debris and coagulant sludge, and may eliminate the need for use of any additional pretreatment before the cartridge filters.