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Scientific Approaches to Assess Impacts Associated with Seawater Desalination

Scientific Approaches to Assess Impacts Associated with Seawater Desalination. Desal Conference October 5, 2006. Susan C. Paulsen, Ph.D., P.E. Vice President and Senior Scientist. Outline. Scientific Approaches to Address Key Management Issues: Source Water Quality Issues

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Scientific Approaches to Assess Impacts Associated with Seawater Desalination

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  1. Scientific Approaches to Assess Impacts Associated with Seawater Desalination Desal Conference October 5, 2006 Susan C. Paulsen, Ph.D., P.E. Vice President and Senior Scientist

  2. Outline • Scientific Approaches to Address Key Management Issues: • Source Water Quality Issues • Entrainment/Impingement Issues • Receiving Water Quality Issues • Evaluation of Impacts Through Modeling

  3. Evaluation of Possible Desal Configurations • Intake • Co-located with power plant • Separate intake • Beach or subsurface wells • Discharge • With power plant effluent • With treated wastewater • Surface discharge • Diffuser discharge • Dynamics are well understood, and can be accurately modeled

  4. Flow Schematic – Co-Location Source Water Heated Water with Concentrate Heated Water To Receiving Water Body 100-800 MGD Power Plant 50-750 MGD 100 MGD Desalination Plant Brine Concentrate 50 MGD Drinking Water 50 MGD

  5. Intake Issues: Source Water • Contaminants may enter the plant and may or may not be removed by the desalination processes • Bacteria • Heavy metals • Etc. • Sources of Contamination • Wastewater treatment plant discharges • Storm flows, urban runoff • Recirculation • Other • Sanitary Surveys & Source Water Analyses are Conducted • DHS Approval is Required

  6. Intake: Impingement and Entrainment • Function of velocity, volume, location • Biology! • Time of year • Duration • Local Dynamics • Effects can be quantified, including cumulative impacts (studies by others – MBC, Tenera)

  7. Alternatives to Ocean IntakesTest Slant Well - Section Drill Rig Ocean Surface Land Surface 23o 350 feet ± Ocean Bottom Main Aquifer 40 to 130 feet ± Infiltration Fresh Water Salt Water Test Slant Well 200 to 250 feet ± Thanks to MWDOC

  8. Slant Well Intake System Concept Desalination Plant Site Subsurface Slant Wells & Buried Collector Intake System SOCWA Outfall Thanks to MWDOC

  9. Receiving Water Issues • Typically, desalination of seawater yields 50% brine (68 ppt) • Mixing in ocean is a function of density (temperature, salinity) • Unless diluted, the brine may cause an environmental impact • Besides a few added chemicals, brine is concentrated seawater Seawater Brine Residue Desalination Plant To Disposal Fresh Water

  10. Use of modeling to assess impacts • Model must evaluate • Near-field mixing • Far-field mixing • Stratification • Meteorological and oceanic processes • Validate model against existing data • Apply model to predict future conditions • Used ELCOM (Estuary and Lake Computer Model) to evaluate Encina discharge

  11. Located at Cabrillo Power Plant SDCWA is seeking to produce 50 MGD Work done in conjunction with RBF Consulting SDCWA is not pursuing project Case Study: Encina Power Station – Regional Seawater Desalination Project

  12. Southern California Bight Region

  13. Regional Seawater Desalination ProjectELCOM 328 ft (100 m) Computational Grid

  14. Model Application • Encina Desalination Plant Movie 1: Temperature Movie 2: Salinity

  15. Conclusions • Source and receiving water issues must be quantified • Multiple configurations can be simulated • Modeling needs to consider all relevant physical processes • Analysis must consider hydrodynamic (physical), chemical, and biological processes • Science can and should be used to quantify impacts

  16. Extra Slides

  17. Inflow Intrusion Source: Textbook “Mixing in Inland and Coastal Waters” by N.H. Brooks, Hugo Fischer,Bob Koh, Jorg Imberger, and John List. Pergamon Press 1979. Entrainment flow arrows added to original.

  18. Receiving Water Regulations • Temperature: Thermal plan for new coastal discharges says that a plume cannot exceed 4o F at the shoreline, the surface of any ocean substrate (including bottom) or 1,000 ft away on sea surface for more than 50% of any tidal cycle. Older plants generally have exceptions, but not all. • Salinity: There are no clear regulations for salinity. However, there are some concerns: • If maximum salinity outside of the immediate area of the discharge exceeds a ppt in the low to mid 40s, then there may be biological concerns if the exposure time is in the range of hours to days. • If the maximum possible increase is about 37 to about 40 ppt, then there may be biological concerns if the exposure is in the range of days to a week. How do we evaluate and quantify these potential impacts?

  19. Evaluating Water Quality: Model Overview • Used Estuary and Lake Computer Model (ELCOM) • Developed at Centre for Water Research at University of Western Australia • In use in 60 countries • State-of-the-art code with continuous development • Applied in both research and practical applications • 3-Dimensional • Solves approximate flow equations in stratified environments • Included tides, meteorological forcing, and currents

  20. Regional Seawater Desalination Project Calibration Fall 2004Comparison of Simulated to Observed Water Temperature

  21. ELCOM Calibration: Temperature Profiles

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