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Desalination Feasibility Study in the Monterey Bay Region


Accepted by the AMBAG Board of Directors on November 8, 2006

This report was prepared for the Association of Monterey Bay Area Governments under a
grant awarded by the Department of Water Resources under the Proposition 50
Desalination Grant Program.

Prepared by:

Brad Damitz, Monterey Bay National Marine Sanctuary
David Furukawa, Separation Consultants, Inc.
Jon Toal, Kinnetic Laboratories

Thanks to the following people for their contributions and review of this report: Linette
Almond, Holly Alpert, Joyce Ambrosius, Tom Luster, Michael Stottlemeyre, Nikolay
Voutchkov, Sabine Latteman, Nick Papadakis, Peter von Langen, Robert Johnson, Andy
Bell, Steven Leonard, Steve Lonhart, and Mark Lucca.

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example high relief habitat, which is particularly sensitive to disturbances, should be
avoided and moorings and anchors should be placed in areas free of sensitive organisms
or habitats.


5.d.i Overview of Intake Related Impacts
While environmental impacts related to the discharge of desalination brine were long
considered to be of most concern, in California intake related impacts now are considered
the aspect of desalination plant operation with the potentially most severe environmental
impacts. A 2003 California Coastal Commission report on seawater desalination states:
“the most significant direct adverse impacts of a desalination facility are likely to be
caused by its intake” (California Coastal Commission, 2004). There is very little
information available about the intake related impacts of seawater desalination plants,
and especially when compared to the body of literature regarding discharge-related
impacts. Much of what is known about these impacts is from studies based on coastal
power plants, which tend to draw in significantly higher volumes of water. Desalination
plant intake volumes and velocities are much less than power plant intakes; therefore the
impacts are expected to be significantly less. Even so, because these are impacts that in
many cases can be avoided entirely or mitigated, this issue will likely receive significant
scrutiny during permit review. While potentially acute, there are a number of ways to
mitigate entrainment and impingement impacts and even to completely eliminate impacts
through the use of alternative designs and practices. These are discussed in the next
section on mitigation measures for intake-related impacts. While it is relatively simple to
assess levels of entrainment and impingement, it is very difficult and complex to estimate
the actual impacts to the ecosystem that results.

All seawater desalination plants require a feedwater source, which is then treated to
produce fresh water as a final product. In plants with larger production capacities,
substantial volumes of seawater are required; the pumping of the feedwater into the plant
can result in significant environmental impacts from entrainment and impingement.
Impingement occurs when organisms become trapped on intake screens due to suction
from the seawater intake velocity, whereas entrainment occurs when organisms too small
to be excluded by intake screens get drawn into the plant with the feedwater.
Impingement and entrainment are regulated under Section 316(b) of the Federal Clean
Water Act, and are often referred to as 316(b) impacts.

The coastal waters of the Monterey Bay are highly productive, due to extensive
upwelling of nutrient rich waters from the Monterey Canyon. The seawater from the Bay
contains a wide array of tiny photosynthetic plants and animals that drift freely in the
water column, collectively referred to as plankton. This phytoplankton represents the
foundation of the food web, providing sustenance for filter feeding species, which in turn
are then preyed upon by larger animals. The animals (zooplankton) that make up the
other “living” portion of the feedwater include both animals such as fishes and crabs that
spend early life stages as plankton in the form of eggs or larvae (meroplankton), as well
as other animals such as copepods that spend their entire lives as plankton

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(holoplankton). In the Monterey Bay this can include fish eggs and larvae, larval crabs,
mollusks such as abalone, clams, and mussels, and echinoderms including sea urchins
and sea cucumbers. In addition to the phytoplankton found in the seawater, spores and
seeds from various species of algae, seagrass, and potentially marsh plants are also
present; while often overlooked, abundances can be significant: in the waters around a
kelp forest levels as high as 1010 giant kelp spores/1,000 m3 can occur (California Energy
Commission, 2005).

Similar to negative environmental impacts from other aspects of desalination plant
operation, the magnitude of impacts due to entrainment and impingement vary
enormously among desalination plants. Therefore, when assessing the impacts caused by
the intake of a desalination facility, it is essential to consider the technology and
operational practices used, the actual volumes and velocity of water being drawn into to
desalination plants, and the species composition and abundance of the surrounding water.

While the intake related impacts of a large desalination plant might be considerable, the
impacts from coastal power plants using once-through cooling are typically several
orders of magnitude more severe. Since much of the academic research regarding the
impacts of seawater intakes is based on coastal power plants rather than desalination
plants, misconceptions about the impact of desalination plant intakes can exist. Power
plants in California typically take in volumes of cooling water that are exponentially
larger then the volume of feedwater required for desalination plants. CalAm’s proposed
desalination plant at Moss Landing would require up to 24 MGD of feedwater to produce
volumes as high as 10 MGD of product water. On the other hand, the Moss Landing
Power Plant is permitted to draw in as much as 1.226 billion gallons per day (CalAm,
2005). Still, for desalination intakes that would entrain sensitive or listed marine species
or would result in further reductions of already depleted fish stocks, the impacts, while
smaller, may still be considered significant. Not surprisingly, intake volumes (and intake
technologies) vary to a large extent among the proposed plants in the Monterey Bay area.

Another indirect consequence resulting from the intake of large volumes of seawater is
the dead impinged and entrained organisms that are ultimately discharged along with the
brine effluent, potentially resulting in impacts such as a decreased oxygen levels and
addition of nutrients. Since this issue is associated with the desalination plant outfalls, it
is examined in a subsequent section on discharge-related impacts.

Entrainment and impingement of special status species may also be an issue with some
desalination plants. In the Monterey Bay area some of the species of special concern
potentially impacted by a desalination plant intake include abalone and certain species of
rockfish. Entrainment or impingement of threatened and endangered fish species is not
expected to occur in the Monterey Bay area, since chinook and coho salmon eggs and
larvae are not present in the marine environment and adults are strong enough swimmers
to be able to avoid being impinged against the intake screen (EDAW, 2005).

Due to technical and cost issues current entrainment studies do not account for plankton
smaller than approximately 0.3mm. The result of this is that only impacts to fish and crab

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(e) Compensating for the impact by replacing or providing substitute resources or

NEPA: National Environmental Policy Act

Nanofiltration: A membrane used to desalinate water. The membrane has a molecular
weight cutoff of about 100, and rejects ions with greater than 100 molecular weight at
about 90 percent. The membrane operates by overcoming osmotic pressure.

MBNMS: Monterey Bay National Marine Sanctuary

NMFS: National Marine Fisheries Service

NOAA: National Oceanic and Atmospheric Administration

Post-treatment: The processes, such as pH adjustment and chlorination that may be
employed on the product water from a desalting unit.

Pretreatment: The processes such as chlorination, clarification, coagulation, scale
inhibition, acidification, and de-aeration that may be employed on the feedwater to a
desalting unit to minimize algae growth, scaling and corrosion.

Reverse Osmosis (RO): A process of desalination where pressure is applied
continuously to the feedwater, forcing water molecules through a semipermeable
membrane. Water that passes through the membrane leaves the unit as product water;
most of the dissolved impurities remain behind and are discharged in a waste stream.

Semipermeable Membrane: A membrane that is permeable for certain molecules or
ions only. RO membranes, for example, ideally will pass water but not salt.

Subsurface intakes: These include various types of systems that pull in water through an
overlying substrate, such as sand or fractured rock. Names for such systems include
"beach well", "infiltration gallery", and "Ranney well." The feasibility of these types of
systems depends on the geologic and hydrologic characteristics of a site. These types of
structures are often environmentally beneficial since they avoid or reduce effects on
marine organisms, including entrainment impacts.

Total Dissolved Solids (tds): Total amount of matter, typically salts and calcium
carbonate, in solution in a sample of water, usually expressed in milligrams per liter
(mg/L) or parts per million (ppm). The state-recommended Maximum Contaminant Level
(MCL) drinking water standard for total dissolved solids is 500 mg/L, the upper MCL is
1,000 mg/L, and the short-term permitted level is 1,500 mg/L. Seawater contains roughly
30,000 mg/L.

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Turbidity: Opaqueness or cloudiness caused by the presence of suspended particles in
water, usually stirred-up sediments. The turbidity of water is measured by its capacity for
absorbing or scattering light.

USBR: U.S. Bureau of Reclamation

Ultrafiltration: A membrane used to treat water with about a 10,000-300,000 molecular
weight cutoff. The membrane rejects organic macromolecules, viruses, and asbestos. The
membrane operates by sieving.

Most definitions compiled from:
Seawater Desalination and the California Coastal Act, California Coastal Commission,

Desalting Handbook, United States Bureau of Reclamation, 2003

Common Conversions:
1 acre foot = 325,851 gallons
1 million gallons = 3.07 acre-feet
1 MGD = 1,120 AFY

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