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SOURCE Summer 2012

Elimination of Desalination Pre-Treatment Using a Slant Well Feedwater Supply By Dennis E. Williams, Ph.D., GEOSCIENCE Support Services, Inc. Cas a potential source of water water Supply System -Description of a SlantWell Desalination Feedoastal communities are increasinglyconsidering seawater desalination for municipal supply due to limited or A slant well desalina- poor inland ground water quality and a tion feed water supply decreasing reliability of imported water. system typically consists As such, seawater desalination is a viable of shallow angled wells alternative, which has been made even (slant wells) in a beach more attractive through more cost effective or near-coastal environ- and efficient subsurface intake systems and ment. A slant well intake water treatment technologies. Desalination system may consist of a intake systems using low angled wells (slant single slant well, an array wells) produce ground water from offshore of slant wells or multiple aquifer systems and provide a number of arrays of slant wells advantages over open ocean intake systems originating on shore but including: avoidance of impingement and completed beneath the entrainment to marine life, elimination of ocean. Slant well systems costly reverse osmosis (RO) pretreatment, are also environmentally Figure 1: Slant Well Desalination Feedwater Supply no ocean construction impacts, and no sensitive as they are permanent visual impacts. buried below the land surface with no visual impacts. Tried and Slant wells near the coast pump ocean water Elimination of Pretreatment Using true well design and construction methods from permeable subsea aquifers inducing a Slant Well Intake System include an engineered artificial filter pack, vertical recharge through the sea floor due Permeable materials (i.e. sand and gravel), which eliminates the need for pretreatment to the hydraulic head difference between which extend offshore near the mouths of by removing particulate matter. Slant well the slant well pumping level and the level rivers and streams, form offshore subsea construction is the same as vertical wells of the ocean (see Figure 1). As the supply aquifer systems. These offshore aquifer except the wells are drilled on an angle source (ocean) is constant, the water supply systems provide natural filtration from (typically 10 degrees to 30 degrees below to a slant well system provides a long-term suspended organic matter and sediment horizontal; see Figure 1). The same design sustainable source. In order to prevent particularly during storm surges and heavy criteria apply to slant wells and vertical movement of sand from the subsea aquifer precipitation. Field tests show the engi- wells alike (e.g. engineered filter packs, well into the well (which clogs the well screen, neered artificial filter pack surrounding the casing and screen design) 2,3,4. However, near-well zone as well as the pumping screened portion of a slant well completed slant wells pose more construction chal- system), an artificial filter pack is placed in an offshore aquifer system results in low lenges than vertical wells due to shallow between the outside of the well screen and turbidity and silt density indices. The Silt construction angles and reduced vertical the open borehole. Artificial filter packed Density Index (SDI) is a design parameter gravitational forces. These drilling and well technology has been perfected since used in reverse osmosis (RO) systems to completion challenges have been overcome the 1940s with excellent success leading to measure the membrane fouling potential using the dual-rotary method of construc- well designs with sand free water and high 1. When Silt Density Indices are less than tion whereby a telescoping well design productivity. It is this proven technology, 3 to 5, costly pre-treatment can be elimi- allows slant wells to extend to lineal lengths which is being adapted to subsurface intake nated. Properly designed and constructed of 1,000 ft or more offshore into subsea systems to provide sand and silt free water wells typically have SDIs less than one. aquifer systems. to desalination plants. 24 SOURCE summer 2012


SOURCE Summer 2012
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