Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources

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1 Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources KEY POINT: In some cases, subsurface intakes can be less expensive than surface water intakes at stand-alone facilities. SUMMARY OF ISSUES Currently, large seawater desal plants almost exclusively use open intake structures due to site-specific limitations associated with subsurface facilities (Xu et. al. 2009). However, when feasible, subsurface intakes can be less expensive than stand-alone surface water intakes. By taking advantage of the natural filtration provided by sand and substrate, subsurface seawater intakes can reduce the amount of total organic carbon and total suspended solids in the feed water, thereby reducing pretreatment and associated O&M costs. In some cases, the source water for subsurface intakes may be diluted with less saline groundwater from adjacent aquifers, reducing the total dissolved solids (TDS) in the intake water. This results in lower energy requirements (and associated costs) for the RO desal process. One of the potential disadvantages is that deep wells may result in lower water temperature and thus higher viscosity. In these cases, higher pressure will be required to pump the water through the RO membranes (i.e., thus increasing energy consumption and associated costs). The feasibility of subsurface intakes depends largely on the characteristics of the associated hydrogeology and substrates, and often may not be practical for large desal plants. Subsurface intake systems have been proven economically justifiable for seawater RO desal plants with a capacity of up to 13 mgd (49,000 m3/d) (CDWR 2003). A large area is required to generate water from 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 2005). Thus, land costs can be significant. The construction costs of subsurface intakes are very site specific. However, Wright and Missimer (1997) compared the relative costs of various seawater intake and pretreatment systems serving relatively small RO desal plants. Their analysis showed that, when feasible, beach well systems are the least expensive among the alternatives, and seabed 1

2 infiltration galleries are the most expensive for small desal systems. Surface water intakes are often more expensive because of high pretreatment requirements (Wright and Missimer 1997). The results of this analysis, adapted from Xu et. al. (2009) are presented in Table 1, with the relative cost of beach wells set equal to 1. No absolute numbers for capital vs. operating costs were provided. Table 1. Relative cost comparisons of intake types serving SWRO desal plants Water Supply System Capacity m 3 /d 2,000 4,000 7,500 15,000 30,000 MGD Beach wells: Infiltration Gallery: Seabed Filtration: Surface Water*: Source: Wright and Missimer (1997) from Xu et. al. (2009) *Including pretreatment 100 μm self-cleaning filter, and mixed-media, high rate, pressure filter. STRATEGIES In the case of aquifers of high porosity and transmissivity, which easily facilitate underground seawater transport such as the limestone formations of many Caribbean islands and Malta, seawater of high quality and large quantity may be collected using intake wells located inland rather than at the shore. This allows reducing the distance for seawater collection, and thus the costs of conveyance by locating the desal plant closer to the main users rather than at the shore. Beach wells are typically only economically justifiable for small systems (<5 MGD) where the local hydrogeology (e.g., aquifer transmissivity) will permit it. Beach wells have been used effectively in the Caribbean and Mediterranean and are the intakes of choice for proposed plants 2

3 in Hawaii. They pose minimal environmental concerns because benthic communities remain undisturbed and impingement and entrainment (I&E)of marine organisms are eliminated. This helps to reduce costs, relative to surface intakes, associated with environmental impact studies and for I&E minimizing technologies (NRC 2008). Horizontal directionally drilled (or slant-drilled) wells are increasingly being considered for use in larger seawater desal facilities. Although more expensive to construct than beach wells, they can minimize shoreline structures. Slant-drilled wells are under study in Dana Point, California. They are also currently in use at several seawater RO plants in Spain, including the facility at San Pedro del Pinatar, which has a capacity of over 170,000 m3/day (NRC 2008 from Peters et. al. 2006). In 2007, facility planners for the Carlsbad Desalination Plant prepared cost estimates for several different types of subsurface intakes with a capacity of 304 MGD. The use of a subsurface Intake system was being considered as a strategy to minimize I&E. As shown in Table 2, estimated costs ranged from about $418 million 1 for slant wells to almost $650 million for vertical beach wells. In comparison, the estimated cost for a 304 MGD capacity surface water intake at the plant amounted to $150 million (Xu et. al. 2009). 1 The year in which these costs are reported is not clear in the source document 3

4 Table 2. Comparison of cost estimates for subsurface intake systems at the planned Carlsbad Desalination Plant Vertical beach wells Slant wells Horizontal Ranney Wells Subsurface infiltration gallery Individual intake well capacity (MGD) Number of intake wells needed Additional standby intakes needed Total intake wells Minimum distance between wells (ft) Length of beach occupied by wells (miles) Land needed to install wells and support facilities (acres) Direct (construction) costs Individual well (gallery) installation $1,200,000 $2,400,000 $2,500,000 $120,000,000 Total cost of well installation $304,000,000 $182,400,000 $190,000,000 $360,000,000 Seawater conveyance US$500/ft $18,925,000 $11,250,000 $15,000,000 $7,922,606 Intake booster pump stations $30,400,000 $30,400,000 $30,400,000 $12,160,000 Electrical power supply for well pumps $50,160,000 $31,920,000 $33,060,000 $18,608,000 Total construction (direct) costs $403,485,000 $255,970,000 $268,460,000 $398,690,606 Indirect costs Land acquisition $4,304,408 $8,723,600 $8,723,600 $8,956,114 Engineering design and 25% $100,871,250 $63,992,500 $67,115,000 $99,672,652 Environmental 15% $60,522,750 $38,395,500 $40,269,000 $59,803,591 20% $80,697,000 $51,194,000 $53,692,000 $79,738,121 Total indirect costs $246,395,408 $162,305,600 $169,799,600 $248,170,478 Total costs $649,880,408 $418,275,600 $438,259,600 $646,861,084 Source: California State Water Resources Control Board ent_2.pdf (Accessed 9/22/09) 4

5 BENEFITS & COSTS Benefits Subsurface intakes reduce the need for pretreatment of the source water prior to desal. This can substantially lower associated operation and maintenance costs. Subsurface intakes reduce costs associated with I&E of aquatic species relative to open intake facilities. Costs Subsurface intakes have the potential for altering the flows of hydraulically connected freshwater aquifers and could possibly accelerate seawater intrusion into these aquifers. A significant area is required to generate water from beach wells. Thus, land costs can be significant. KEY UNCERTAINTIES The useful life of a well-designed and operated seawater beach wells is estimated 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 damage the well collectors and impact the useful life of the wells (Voutchkov 2007). 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 2007). The knowledge and experience in designing subsurface intake facilities are far less than surface water supply systems. It might result in not-well-designed and even failure of intake systems. 5

6 ADDITIONAL RESOURCES California Department of Water Resources (CDWR) Water Desalination Task Force, Working Paper. Revised Draft September 12, California Department of Water Resources. Office of Water Use Efficiency and Transfers. California State Water Resources Control Board. Comparison of cost estimates for subsurface intake systems at the planned Carlsbad Desalination Plant _3_09_09/attachment_2.pdf (Accessed 9/22/09) Peters, T., Pintó, D Seawater intake and pre-treatment/brine discharge-environmental issues. Desalination, 221: Voutchkov, N SWRO Desalination Process: on the Beach Seawater Intakes. Filtration& Separation, 42(8): Voutchkov, N Budgeting and Implementation of Desalination Projects. In The Guidebook to Membrane Desalination Technology. Edited by Wilf, M. L'Aquila, Italy: Balaban Desalination Publications. Wright R.R., and T.M. Missimer Alternative Intake Systems for Seawater Membrane Water Treatment Plant. In Proceedings of the 1997 International Desalination Association. Madrid. IDA Xu, P., T. Cath, G. Wang, J.E. Drewes and S. Dolnicar Critical Assessment of Implementing Desalination Technology. Denver, Colorado: Water Research Foundation. 6