Transcript Use of Models to Assess Water Quality Associated with

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
 Entrainment/Impingement Issues
 Receiving Water Quality Issues
 Evaluation of Impacts Through Modeling
Figure 2
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
Figure 3
Flow Schematic – Co-Location
Source
Water
100-800 MGD
Power
Plant
Heated
Water
Heated
Water with
Concentrate
100 MGD
Desalination
Plant
Drinking
Water
50 MGD
Figure 4
Brine
Concentrate
50 MGD
To Receiving
Water Body
50-750 MGD
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
Figure 5
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)
Figure 6
Alternatives to Ocean Intakes
Test Slant Well - Section
Ocean Surface
Land Surface
23o
Ocean Bottom
Main Aquifer
40 to 130 feet
±
Infiltration
Test Slant Well
200 to 250 feet ±
Figure 7
Thanks to MWDOC
Slant Well Intake
System Concept
Subsurface Slant Wells
& Buried Collector
Intake System
Desalination Plant
Site
SOCWA Outfall
Figure 8
Thanks to MWDOC
Receiving Water Issues
Seawater
Desalination
Plant
Brine
Residue
To Disposal
Fresh
Water




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
Figure 9
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
Figure 10
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
Figure 11
Southern California Bight Region
Figure 12
Figure 13
Figure 14
Figure 15
Regional Seawater Desalination Project
328 ft (100 m)
Computational Grid
Regional ELCOM
Seawater
Desalination
Project
View from the west, Showing Vertical Grid
ELCOM 328 ft (100 m) Computational Grid
Agua Hedionda Lagoon
depth (ft)
-300.0 -200.0 -120.0 -90.0
-60.0
Figure 16
-30.0
0.0
Calsbad Desalination Plant - Case C
Salinity at the Bottom
4
Jul 24 1h 00m --- Year 1996
1000
Salinity on the Cross Section A - A'
Depth (m)
2
800
0
South - North
-2
600
-4
500
A - A'
600
700
West - East
800
900
Discharge
4
400
Grid on the Cross Section A - A'
Depth (m)
2
200
0
-2
0
500
600
700
800
900
-4
West - East
Salinity
(psu)
500
33.7 34 35 36 37 38 39 40 41 42 43 44
Figure 17
600
700
West - East
800
900
Model Application
 Encina Desalination Plant
Movie 1:
Temperature
Movie 2:
Salinity
Figure 18
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
Figure 19
Extra Slides
Figure 20
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.
21
Entrainment flow arrows added to Figure
original.
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?
Figure 22
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
Figure 23
Regional
Seawater
Desalination
Project
Regional Seawater
Desalination
Project - Calibration
Fall 2004
Comparison of Simulated to Observed Water Temperature
Calibration FallSimulated
2004 Temperature Contours with
ELCOM
Comparison of Simulated to Observed
Water
Temperature
Oct 12 14h 00m --- Year 2004
Digitized Temperature Contours from
Receiving Water Monitoring
Oct 12 14:05 - 14:40 --- Year 2004
latitude (UTM Zone 11, meters)
3669000
3664000
Agua Hedionda
Lagoon
Encina Power Plant
discharge
+2 °F
+2 °C
+4 °F
+6 °F
+8 °F
monitoring stations
467000
470000
longitude (UTM Zone 11, meters)
Figure 24
ELCOM Calibration: Temperature Profiles
Water Temperature Comparison
Receiving Water Monitoring Data Collected Oct. 12, 2004 ~ noon
Station D-20
Station D-30
Station D-50
0
0
10
10
10
10
20
20
20
20
30
30
30
depth (ft)
0
depth (ft)
0
depth (ft)
depth (ft)
Station D-10
30
40
40
40
40
50
50
50
50
60
60.0 65.0 70.0 75.0 80.0
temperature (°F)
60
60.0
60
59.0 63.0 67.0 71.0 75.0
temperature (°F)
60
59.0 63.0 67.0 71.0 75.0
temperature (°F)
65.0 70.0 75.0
temperature (°F)
80.0
data
simulation
Figure 25