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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. 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

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

slide2

Outline

  • Scientific Approaches to Address Key Management Issues:
    • Source Water Quality Issues
    • Entrainment/Impingement Issues
    • Receiving Water Quality Issues
  • Evaluation of Impacts Through Modeling
evaluation of possible desal configurations
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
flow schematic co location
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

intake issues source water
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
intake impingement and entrainment
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)
alternatives to ocean intakes test slant well section
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

slant well intake system concept
Slant Well Intake System Concept

Desalination Plant Site

Subsurface Slant Wells & Buried Collector Intake System

SOCWA Outfall

Thanks to MWDOC

receiving water issues
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

use of modeling to assess impacts
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
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

Case Study: Encina Power Station – Regional Seawater Desalination Project
model application
Model Application
  • Encina Desalination Plant

Movie 1: Temperature

Movie 2: Salinity

conclusions
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
inflow intrusion
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.

receiving water regulations
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?

evaluating water quality model overview
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
slide24
Regional Seawater Desalination Project Calibration Fall 2004Comparison of Simulated to Observed Water Temperature
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