Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources

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Wesley McGee
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1 Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources KEY POINT: Although the use of an open intake is conceptually very straightforward, the construction of new infrastructure can be very expensive and difficult to permit. SUMMARY OF ISSUES Inland desal plants typically use groundwater wells to extract water from aquifers. Some plants however, divert water from rivers or draw water from lakes and reservoirs. A pipe is submerged into the surface water source and the source water is conveyed to the desal facility. Different types of screens are used at the intake to help prevent weeds, algae, and suspended solids from entering the pipeline. Intake screens also reduce impingement and entrainment (I&E). Well-developed technology. Inland surface water desal plants use intake technology that is no different from traditional water treatment plants dependent on surface water. This technology is well developed and relatively straightforward to implement (NRC 2008, Xu et. al. 2009). Water quality. Source water quality, and the surface water intakes themselves, may be adversely affected by wastewater and other waste discharges (point and non-point), oil and hydrocarbon residues, urban runoff, algae blooms, and agricultural pollution (e.g. animal wastes, fertilizers and pesticides). Appropriate siting of the intake can help to avoid adverse impacts related to source water quality. Design. The use of an open intake for the collection of saline surface water involves either the construction of new infrastructure (i.e., piping, pumps, screens, etc.) or the reuse of existing infrastructure that is not currently being utilized. Although the use of an open intake is conceptually very straightforward, the construction of new infrastructure can be very expensive and difficult to permit. An appropriate intake design must consider potential effects of continuous or intermittent pollution, navigation, and membrane fouling, and take necessary steps to mitigate these environmental and operational risks. STRATEGIES Source water characterization A thorough raw water characterization should be conducted at the proposed intake site(s). This should include the evaluation of all constituents that may impact plant operation and process 1

2 performance (e.g. physical, microbial, and chemical characteristics, meteorological and oceanographic data, and aquatic biology). Like most process systems, desal plants operate most efficiently and predictably when feedwater characteristics remain relatively constant and are not subject to rapid or dramatic water quality fluctuations. Therefore, the water quality review should consider both seasonal and diurnal fluctuations. Water quality data should be collected over a long period of time, in order to capture any seasonal variations. Design and operational considerations For rivers under tidal influence, source water should be extracted when the salinity of the source water is at its lowest in order to minimize desal treatment costs. For example, the Thames Gateway Desalination plant in Beckton, east London, UK, will produce freshwater from the brackish saltwater of the Thames Estuary, which is a mix of fresh water from the river and seawater. Water will be taken from the tidal river during the hours of low tide, when water is flowing out of London and the salt content is at its lowest. A salinity buffering tank will be constructed to guarantee the supply of appropriate quality of raw water to the process. For low quality surface water, riverbank filtration (RBF) may be employed to reduce the influence of wastewater discharges, oil and grease, storms, and algae blooms. RBF has been recognized as a proven method for drinking water treatment process in Europe. RBF is utilized as a natural water treatment process that induces surface water flow downward, through sediment and into a vertical or horizontal well. It is a relatively low-cost and efficient technology, and it reduces the level of pretreatment for the membrane desal process. The ability of RBF to serve as stand-alone pretreatment to reverse osmosis (RO) is dependent upon site-specific water quality and aquifer conditions (Carollo 2006). The RBF process would not be feasible for aerobic aquifers producing waters high in iron and/or manganese, which can cause severe membrane scaling. The design and operation of surface intake facilities should consider erosion, icing, and siltation. To minimize impacts, intake screens can be backflushed or have provisions for routine cleaning. The siting and design of surface water intakes should include a site plan showing the horizontal and vertical location of potential and actual sources of pollution or contamination. This includes, but is not limited to, all proposed or existing wastewater treatment and disposal system components, sewage pump stations, sewer line manholes and cleanouts, petroleum storage tanks and lines, and potential sources of urban and agricultural runoff. 2

3 Source water monitoring and control Desal facilities require reliable sources of water with a reasonably consistent quality. Groundwater usually has a more consistent water quality than surface water, which can be affected by naturally occurring elements, storms, spills, and other sources of contamination. Source water monitoring and control is a preventive approach to ensure the quality of source water and to protect intake and treatment processes. In addition, the presence of contaminants may affect product water quality if the source water is used in blending processes. A desal plant needs to take immediate measures or even shut down the facilities if the source water monitoring system detects large changes in key source water quality constituents including salinity, algae, oil and grease, silts, or elevated concentration of other contaminants such as silica, which can cause severe membrane fouling. Intake Protection Program (SWIPP) Same as other conventional surface water sources, a SWIPP program should be developed to protect the source water for a desal plant. The following is an example of the guidelines developed by the Michigan Department of Environmental Quality: Designating a source water protection area for each water supply source based on the state s defined source water area. Identifying potential contaminant sources within each source water protection area. Utilizing management approaches for protection of source water, including but not limited to education and regulatory approaches. Creating contingency plans for public water supply sources including the location of alternate drinking water sources. Assuring proper siting on new water sources to minimize potential contamination. Encouraging public participation. 3

4 BENEFITS & COSTS Benefits Surface water desal plants often use open intake structures. The benefits of open intake include: Easy to design, construct, and operate Flexible capacity Long useful life (30-50 years) Reliable in providing required water quantity to desal plant Costs The costs of open intakes can increase significantly if advanced screen technologies are necessary and employed to control I&E. Advanced screen technologies can include barrier nets, fine mesh screens, and fine slot wedge wire screens. KEY UNCERTAINTIES Open surface water intakes are susceptible to increased pollution loads due to urban wastewater effluents, uncontrolled discharge, contaminated surface runoff, agricultural pollution, and algal blooms. ADDITIONAL RESOURCES Carollo Engineers The Role of Riverbank Filtration in Reducing the Costs of Impaired Water Desalination. Desalination and Water Purification Research and Development Program Report No Available: Beckton Desalination Plant. Thames Water website: 4

5 Michigan Department of Environmental Quality. Guidance for Intake Protection Program (SWIPP). Water Bureau. Drinking Water and Environmental Health Section. Available: NRC (National Research Council), Committee on Advancing Desalination Technology Desalination: A National Perspective. Washington, D.C.: National Academy Press. Available: < WHO Desalination for Safe Water Supply: Guidance for the Health and Environmental Aspects Applicable to Desalination. Geneva: WHO. Available: Xu, P., Cath, T, Wang, G., Drewes, J.E. and Dolnicar, S Critical assessment of implementing desalination technology. AwwaRF Project Published by American Water Works Association Research Foundation, Denver, CO. 5

Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources

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.