Scientific Approaches to Assess Impacts Associated with Seawater Desalination

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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

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Outline

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

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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

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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
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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

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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)

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

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Case Study Encina Power Station Regional
Seawater Desalination Project

• Located at Cabrillo Power Plant
• SDCWA is seeking to produce 50 MGD
• Work done in conjunction with RBF Consulting
• SDCWA is not pursuing project

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Southern California Bight Region
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Regional Seawater Desalination ProjectELCOM 328
ft (100 m) Computational Grid
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Model Application

• Encina Desalination Plant

Movie 1 Temperature
Movie 2 Salinity
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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

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Extra Slides
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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.
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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?
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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

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Regional Seawater Desalination Project
Calibration Fall 2004Comparison of Simulated to
Observed Water Temperature
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ELCOM Calibration Temperature Profiles