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COASTAL FACILITY Engineering

Feedwater Intake Subsurface Feedwater Facilities

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Summary of Issues | Strategies | Benefits & Costs | Key Uncertainties | Additional Resources

KEY POINT: The construction of subsurface intakes requires appropriate geological conditions including permeable sand formation with adequate transmissivity and depth.

SUMMARY Coastal subsurface intakes can be classified into two categories:

Onshore subsurface intakes, including vertical wells, horizontal wells, and beach infiltration gallery

Offshore subsurface intakes, including horizontal directionally drilled (also called slant-drilled) wells, and seabed infiltration galleries.

Subsurface intake facilities extract seawater from the sand below the beach, or below the seabed near the shore. The sand acts as a natural slow filter to minimize ecological impacts (i.e., impingement and entrainment).

By taking advantage of the natural filtration provided by sediments, subsurface intakes can yield better quality feedwater than open seawater intakes, including reduced suspended solids, turbidity, natural organic matter, pathogens. Subsurface intakes commonly achieve lower silt density index (SDI) in the feedwater.

Because of the higher quality feed water, subsurface seawater intakes can reduce the pretreatment required for membrane-based desal systems, thereby lowering associated operations and maintenance costs.

The construction of subsurface intakes, in particular beach wells, requires appropriate geological conditions including permeable sand formation with adequate transmissivity and depth (Voutchkov 2005a). Shallow beaches that contain a substantial amount of mud/alluvial deposits do not provide favorable conditions for beach well operations.

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Natural wave motions near the ocean floor provide the energy to dissipate separated solids from the beach well source water out into the ocean. If the bay area is not well flushed, and the naturally occurring movement is inadequate to transport the solids away from the beach well collection area, solids will begin to accumulate on the ocean floor. This will ultimately reduce well capacity and source water quality (Voutchkov 2005a).

A feasibility assessment for subsurface intake should include a subsurface geological investigation, evaluation of potential impacts on adjacent fresh water aquifers and assessment of potential adverse effects from dynamic action of the sea such as from natural catastrophes and shoreline erosion.

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STRATEGIES

This section describes current technologies and technical issues associated with the different types of subsurface intakes, including vertical and horizontal beach wells; slant wells (or horizontal directional drilling); and infiltration galleries.

Vertical beach well

Vertical beach wells consist of water collectors that are drilled vertically into a coastal aquifer. Each well consists of a nonmetallic casing, well screen, and submersible vertical turbine pump (Voutchkov 2005a, Wright and Missimer 1997). Figure 1 provides a schematic of a typical vertical beach well system.

Vertical beach wells are generally used for smaller desal systems. At present the largest seawater reverse osmosis (SWRO) facility with vertical beach wells is the 14.3 MGD (54,000 m3/d) Pembroke plant in Malta. This plant has been in operation since 1991. The 11.1 MGD (42,000 m3/d) Bay of Palma plant in Mallorca, Spain, has 16 vertical wells with unit capacity of 1.48 MGD (5,600 m3/d) per well. The third largest plant is the 6.34 MGD (24,000 m3/d) Ghar Lapsi SWRO in Malta. Source water for this facility is supplied by 15 vertical beach wells with unit capacity of 1 MGD (3,800 m3/d) (Voutchkov 2005b).




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Figure 1. Vertical beach well intake system Source: Picture courtesy of Nikolay Voutchkov

Horizontal (Ranney) beach well

A horizontal well is a variation of a vertical beach well. Horizontal wells have multiple horizontal collection arms that extend into the coastal aquifer from a central collection caisson where seawater is collected (Voutchkov 2005a). Because the well screens in the collector wells are placed horizontally, a higher rate of source water collection is possible than with vertical wells. The intake capacity could reach 2.5 MGD (97,000 m3/d) per 12 inches diameter well (Poseidon 2007). Figure 2 shows the design for a typical horizontal beach well system.

Figure 2. Horizontal beach well intake system Source: Picture courtesy of Nikolay Voutchkov




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Slant well (or horizontal directional drilling)

Slant wells are a variation of vertical and horizontal well intake structures. Slant wells are drilled at an angle of between 20o and 25o. They extend under the seabed to maximize the collection of seawater. The construction of slant wells may be advantageous in areas with limited beach construction access and poor seawater quality (such as at port basins, dredging areas and other permanent or seasonal problematic seawater conditions). Figure 3 provides a schematic of a typical slant well intake system.

Figure 3. Slant well intake system Source: MWDOC presentation

The Municipal Water Management District of Orange County (MWDOC) recently completed a two-phased, two year hydrogeological study to investigate the feasibility of developing a full-scale seawater intake system using slant well technology for the proposed 15 MGD (56,775 m3/d) Dana Point desal plant (MWDOC 2007). The test slant well was drilled and constructed at an angle of 23° below horizontal, using a dual rotary drilling rig. Extensive groundwater modeling showed that 9 slant wells (7 wells would be operational with two in rotational mode for system reliability) could provide a total volume of 30 MGD (113,550 m3/d) (MWDOC 2007). A configuration of one of the alternative slant well intake systems is shown in Figure 4 (MWDOC 2007). For more details, see the MWDOC case study developed as part of this project.


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Figure 4. One alternative configuration of slant well intake system for Dana Point Desalination Plant Source: MWDOC 2007

One specific type of slant well is known as Neodren technology. This technology is based on horizontal drains consisting of patented special porous filter pipes acting as wells. As shown in Figure 5, Neodren systems are installed in bore holes drilled by the horizontal directional drilling method in the stratum below the seabed. They are set a few meters below the ocean floor and can be several hundred meters in length.

Seawater is extracted by the wells indirectly through the sub-seabed area that acts as a natural filter. The Neodren technology can be operated in sandy and karstic seabeds as an ecological and economical alternative for conventional open seawater intake systems. There are currently 10 Neodren installations with a total capacity of nearly 79 MGD (300,000 m3/d) in operation (Peters and Pinto 2008, Farinas and Lopez 2007).




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Figure 5. Schematic of the drains of the Neodren system Source: Catalana de Perforacions, from Peters and Pinto 2008.

Onshore beach infiltration gallery

Engineered infiltration galleries are seashore intake systems that are used where geologic conditions are relatively impermeable or have insufficient thickness and depth to support groundwater extraction (Pankratz 2004, Voutchkov 2005a). An infiltration gallery intake is a variation of the radial collector well arrangement where the radial arms and screens are installed in a trench that is then backfilled with a gravel pack and/or selected filter materials.

The infiltration galleries can be designed similar to conventional rapid sand filters if the natural source water movement (e.g., ocean water wave motion) can provide adequate flushing of the infiltration gallery media contact surface with the water body. They can also be designed as slow sand filtration systems, which have at least a 30-feet layer of sand overlying the collection well screens (Voutchkov 2005a).

Infiltration galleries usually cost about 15 to 20% more to construct than conventional vertical or horizontal intake wells. The use of infiltration galleries is therefore typically only warranted when the hydrogeological conditions of the intake site are not suitable for conventional intake wells (Voutchkov 2005a). Figure 6 provides a schematic of an onshore beach infiltration gallery.




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Figure 6. Schematic of onshore infiltration gallery Source: Picture courtesy of Nikolay Voutchkov

Offshore seabed filtration intake system

Seabed infiltration systems consist of a submerged slow sand media filtration system (filtration bed) located in the near-shore surf zone of the ocean floor. The filtration bed is connected to a series of onshore intake wells via tunnels or horizontal collector pipes. The filtration bed is sized and configured using design criteria similar to those used for slow sand water treatment plant filters. The surface filtration matt is often removed from the surface of the filtration bed by naturally occurring seasonal scouring events, such as waves and tides (Voutchkov 2005a). When the matt is removed and some of the filtration bed sand is lost over time, the sand media have to be replaced to its original depth in order to maintain the filtration bed’s performance efficiency. Figure 7 provides a schematic of an offshore seabed filtration intake system.




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Figure 7. Seabed infiltration gallery Source: Picture courtesy of Nikolay Voutchkov

The largest seawater desal plant with a seabed intake system currently in operation is the 13.2 MGD (50,000 m3/d) Fukuoka District RO facility in Japan (Matsumoto et al. 2001). The Fukuoka Plant’s seawater intake and pretreatment system has been performing as designed since it was constructed in 2005. The sea water is drawn up at a very slow rate, less than the critical velocity of flow, without pumping sand. A small amount of fine sand was observed on the filters during the summer but was cleaned by the ocean current during winter. There has been no increase in head loss across the sand filters during the 4 years of operation. Sand filter effluent has a Silt Density Index of 2.0. The downstream UF is technically not needed but is used to minimize biological fouling of RO membranes (LBWD 2009a). Figure 8 provides a sketch of the seawater intake at the Fukuoka District RO facility.


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Figure 8. Seawater intake of Fukuoka District RO facility Source: Fukuoka District RO facility

Long Beach Water District (LBWD) is currently experimenting with the Under Ocean Floor Seawater Intake and Discharge Demonstration System designed by LBWD and the U.S. Bureau of Reclamation (LBWD 2009b). The filtration system is located just below the seabed and is designed to draw seawater for desal feedwater through beach sand over a large enough area that the intake velocity can be low, thereby eliminating impingement and entrainment (I&E) commonly associated with open ocean (surface water) intakes. In addition, the slow sand filtration (loading rate of less than 0.1 gallons per day per square-foot) effectively reduces organic and suspended solids in the feedwater without the use of pre-treatment chemicals. Thus, the process functions as both an intake and pretreatment system.

The same sand filtering concept applies to the discharge of the brine concentrate stream, minimizing the environmental impacts of the brine plume as well. Another advantage of this “sandbox” approach is that flow rates and system operation are not affected by waves and tides.


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In fact, the action of waves and tides functions as a natural cleaning agent for the beach sand. The system is essentially maintenance-free, requiring no backwashing, cleaning, treatment, recharging, and/or rehabilitation, so there are no operating and maintenance costs. Figure 9 provides a sketch of the LBWD demonstration system. For additional information on this project, see the LBWD case study, developed as part of this project.

Figure 9. Under Ocean Floor Seawater Intake and Discharge Demonstration System Source: Long Beach Water District

As depicted in Figure 10, Jones (2008) presented a synthetic infiltration gallery designed to for all coastal types. It incorporates directional drilling and microtunneling, utilizing geotextile fabrics, and relocating the subsurface reservoir onshore. In addition to the removal of suspended solids, the offshore filter media is designed to foster an environment for microbial communities to effectively remove biologically available nutrients such as phosphorus and nitrogen compounds as well as assimilated organic carbon. The system can be backwashed by isolating sections and flushing with up to twice the infiltration rate.




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Figure 10. Synthetic infiltration gallery of seawater intake system Source: Jones 2008.


BENEFITS & COSTS

Extremely low filtering velocities have negligible influence on marine flora and fauna; thus eliminating impingement and entrainment of aquatic organisms.

Benefits

Yields high quality feed water, thereby reducing or eliminating costly RO pretreatment.

Can protect desal processes from shock loads of contaminants associated with unusual events such as algal blooms and oil spills.

The flow rate and operation of the under ocean floor intake system is unaffected by wave action and tidal forces. These forces actually improve operation as they act as a natural cleaning agent for the beach sand.

Systems can be virtually maintenance free, eliminating operation and maintenance costs. They require no backwashing, cleaning, treatment, recharging, and/or rehabilitation.




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The construction costs of subsurface intakes are very site specific. Wright and Missimer (1997) compared the system costs of various seawater intake and pretreatment systems serving RO desal plants. They found beach well systems to be the least expensive among the alternatives evaluated, and seabed infiltration galleries to be the most expensive for small desal systems (See PIM cell discussion on

Costs

economic and financial issues related to subsurface intakes).

Subsurface intake systems have been proven economically justifiable for SWRO desal plants with a capacity of up to 13 MGD (49,000 m3/d) (CDWR 2003). The costs of subsurface intakes for large desal plants can be very expensive. For example, cost estimates for the 304 MGD intake facilities at the Carlsbad desal plant showed subsurface intakes would cost 2 to 3 times more than a new open ocean intake (Poseidon 2009)

A significant area is required for beach wells. It is estimated that for a 10 MGD plant, 4.2 acres of beach shore may be needed for horizontal beach wells, infiltration galleries or seabed infiltration galleries, as opposed to 2 acres for open surface intake (Voutchkov 2005b).


KEY UNCERTAINTIES

The quality of source water for subsurface intake systems can be affected by adjacent groundwater aquifers. For example, water abstracted from beach wells for seawater desal in Morro Bay and in Salne Cruz, Mexico, exhibited high concentrations of manganese and/or iron caused by water contributed by adjacent aquifers (Voutchkov 2005b). The high-iron concentration problem was resolved by the installation of a pretreatment filter designed for a loading rate of 2.5 gpm/ft2.

The useful life of a well-designed and operated seawater beach wells is estimated 15-20 years without major refurbishment. Beach well yield may diminish due to naturally occurring scaling of the well collectors caused by chemical precipitates or/and bacterial growth. Beach erosion may also damage the well collectors and impact the useful life of the wells (Voutchkov 2005b). In the worst-case scenario, two sets of beach wells may need to be constructed over the useful life of the SWRO plant. The need to replace some or all of the original beach wells after the first 10 to 20 years of operation would magnify the shoreline impacts of the beach wells and increase the overall cost of water production (Voutchkov 2005b).




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The knowledge and experience in designing subsurface intake facilities is far less than for surface water supply systems. This may result in poorly designed (and even failure of) some intake systems.


ADDITIONAL RESOURCES

CDWR (California Department of Water Resources). 2003. Water Desalination Task Force, Feedwater Intake Working Paper. Revised Draft – September 12, 2003. California Department of Water Resources. Office of Water Use Efficiency and Transfers.

EPRI (Electric Power Research Institute). 2007. Fish Protection at Cooling Water Intake Structures: a Technical Reference Manual. Report No. 1014934.

Farinãs, M. and L.A. López, L.A. 2007. New and innovative sea water intake system for the desalination plant at San Pedro del Pinatar. Desalination, 203(1-3): 199-217.

Gille, D. 2003. Seawater intakes for desalination plants. Desalination, 156:249-256.

GWI (Global Water Intelligence). 2006a. Desalination Markets 2007: A Global Forecast. Oxford, UK: Media Analytics Ltd.

Jones, A.T. 2008. Can we reposition the preferred geological conditions necessary for an infiltration gallery The development of a synthetic infiltration gallery. Desalination, 221(1-3):598-601.

LBWD (Long Beach Water Department). 2009a. Uminonakamichi Nata Sea Water Desalination Plant, Fukuoka, Japan. Seawater Intake and Pretreatment System Update. Available: <http://lbwd-desal.org/presentations/FukuokaRpt.pdf>. [cited September 2, 2009]

LBWD (Long Beach Water Department). 2009b. Under-Ocean Floor Seawater Intake and Discharge Test Plan. Updated: April 1, 2009. Available: <http://lbwd-desal.org/presentations/Under%20OceanFloorTestPlan.pdf>. [cited September 2, 2009].

Matsumoto, Y., T. Kajiwara, K. Funayama, M. Sekino, T. Tanaka and H. Iwahori. 2001. 50,000 m3/day Fukuoka Sea Water RO Desalination Plant by a Recovery Ratio of 60%. In Proc. of the 2001 International Desalination Association Conference.

Melbourne Water. 2007. Melbourne Water: Seawater Desalination Feasibility Study. June 2007. www.melbournewater.com.au/./publications/fact_sheets/water/seawater_desalination_feasibility_study.pdf. [Cited October 3, 2007].


http://lbwd-desal.org/presentations/FukuokaRpt.pdf%3e�

http://lbwd-desal.org/presentations/Under%20OceanFloorTestPlan.pdf%3e�

http://lbwd-desal.org/presentations/Under%20OceanFloorTestPlan.pdf%3e�

http://www.melbournewater.com.au/.../publications/fact_sheets/water/seawater_desalination_feasibility_study.pdf�

http://www.melbournewater.com.au/.../publications/fact_sheets/water/seawater_desalination_feasibility_study.pdf�



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MMWD (Marin Municipal Water District). 2007a. Engineering Report MMWD Seawater Desalination Pilot Program. Prepared by Kennedy/Jenks Consultants in association with CH2M Hill. January 26, 2007a. Available: <http://www.marinwater.org/controller?action = menuclick&id = 413>. [Cited May 12, 2007].

MWDOC (Municipal Water District of Orange County). 2007. Final Draft of Engineering Feasibility Report: Dana Point Ocean Desalination Project, March 2007. Available: <http://www.mwdoc.com/documents/FinalDraftReport4-6-07.pdf>. [cited July 1, 2007].

NRC (National Research Council). 2008. Desalination: A National Perspective. Washington, D.C.: National Academy Press. Available: < http://www.nap.edu/catalog/12184.html>.

Pankratz, T. 2004. An overview of seawater intake facilities for seawater desalination. The Future of Desalination in Texas, Volume II: Technical papers, case studies, and desalination Technology Resources. Available: Texas Water Development Board. < http://www.twdb.state.tx.us/iwt/desal/docs/The%20Future%20of%20Desalination%20in%20Texas%20-%20Volume%202/documents/C3.pdf>. [cited October 1, 2007].

Peters, T. and D. Pintó. 2008 Seawater intake and pre-treatment/brine discharge-environmental issues. Desalination, 221:576–584.

Poseidon Resources. 2005. Poseidon Resources Carlsbad Desalination Project Environmental Impact Report. Carlsbad, CA. Available: <http://www.carlsbad-desal.com/EIR.asp>. [Cited April 1, 2007].

Poseidon (Poseidon Resources). 2007. Response to the State Lands Commission’s Letter Dated December 19, 2007. Draft Lease Amendment to PRC 8727.1 for the Proposed Use of the Existing Intake and Outfall Channels and Jetties Located at the Pacific Ocean And Aqua Hedionda Lagoon, Adjacent to 4600 Carlsbad Avenue, Carlsbad, San Diego County. January 22, 2008.

Poseidon (Poseidon Resources). 2009. Carlsbad Seawater Desalination Project. San Diego Regional Water Quality Control Board Region 9, San Diego Region. Flow Entrainment and Impingement Minimization Plan. Attachment 2: Cost Estimate of Subsurface Intake Alternatives.

Voutchkov, N. 2005a. Groundwater intake wells – types and applications. Asian Water, 2005(January/February):31-34.

Voutchkov, N. 2005b. SWRO desalination process: on the beach – seawater intakes. Filtration & Separation, 42(8):24-27.


http://www.mwdoc.com/documents/FinalDraftReport4-6-07.pdf%3e�

http://www.nap.edu/catalog/12184.html�

http://www.twdb.state.tx.us/iwt/desal/docs/The%20Future%20of%20Desalination%20in%20Texas%20-%20Volume%202/documents/C3.pdf%3e�

http://www.twdb.state.tx.us/iwt/desal/docs/The%20Future%20of%20Desalination%20in%20Texas%20-%20Volume%202/documents/C3.pdf%3e�



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Wright R.R., and T.M. Missimer. 1997. Alternative Intake Systems for Seawater Membrane Water Treatment Plant. In Proceedings of the 1997 International Desalination Association, Madrid, Spain. IDA



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