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Water & Wastewater News
November 11, 2008


Carlsbad project develops plan to mitigate its carbon footprint

Over the past five years, desalination has gained significant momentum in
California. With more than 10 projects in various stages of environmental
review, design, and construction, desalination is planned to provide 1,500 to
2,000 million liters per day (ml/d) of new fresh drinking water supplies for the
state by 2015. One of the largest and most advanced projects under development
is the 189 ml/d (50 million gallons per day or mgd) Carlsbad seawater
desalination plant (Figure 1). This project is collocated with the Encina
coastal power generation station, which uses seawater from the Pacific Ocean for
once-through cooling. The Carlsbad project is a public-private partnership
between Poseidon Resources and eight local utilities and municipalities.

The Carlsbad project received a “green light” from all regulatory agencies in
California in August. Construction should begin in the spring. When completed in
2011, this project will supply 6 to 8 percent of the drinking water in San Diego
County and will be the largest seawater desalination plant in the United States.
In 2006, California lawmakers approved the AB 32 Global Warming Solutions Act
that aims to reduce greenhouse gas (GHG) emissions to 1990 levels by 2020. In
response of this initiative, Poseidon Resources has committed to mitigating the
carbon footprint associated with the desalination plant operations. When
constructed, this will be the first desalination project in the United States
that will have a net zero carbon footprint. Gross Carbon Footprint.

A desalination plant's carbon footprint depends on how much electricity is used
by the plant and what sources (fossil fuels, wind, sunlight, etc.) generate the
supply. The total carbon footprint for this project is estimated at 61,000
metric tons of carbon dioxide per year (CO2/yr). This estimate is based on
desalination plant power use of 3.57 KWh per cubic meter of produced drinking
water and on a greenhouse gas (GHG) emission factor of 248.4 kg of CO2 per MWh
of electricity. San Diego County imports 90 percent of its water from Sacramento
Bay Delta and Colorado River (more than 800 km) via the State Water Project. The
amount of electricity needed to deliver and treat this water is 2.748 KWh/m3.
Because the desalination project will eliminate the need to import 189 ml/d of
water, it will save 189,800 MWh/yr, or 47,240 metric tons CO2/yr of GHG
emissions from the energy used to pump this water to the county.

The California Coastal Commission gives the net carbon emission reduction credit to the
Carlsbad project. In the future, if the Municipal Water District of Southern
California decide to import an extra 189 ml/d, it will be responsible for
mitigating the carbon footprint of this water. The credit recognizes the fact
that the desalination plant will not increase the total carbon footprint
associated with the water supply of San Diego County. Thus the Carlsbad
desalination plant’s net carbon footprint is estimated to be reduced from 61,000
to 13,760 metric tons CO2/yr. The Carlsbad project aims to eliminate the plant’s
net carbon footprint by energy efficient design and operations; green building
design; use of carbon dioxide for water production; on-site solar power
generation; carbon dioxide sequestration by creation of coastal wetlands and
reforestation; funding renewable power generation projects; and acquisition of
renewable energy credits. Carbon neutrality would be achieved by a balanced
combination of these measures. Efficient Design and Operations.


The Carlsbad plant will reuse energy from the seawater concentrate generated in
the salt separation process by a state-of-the-art energy recovery system (see
Figure 2). After membrane separation, most of the energy applied for
desalination is retained in the concentrated stream (brine) that also contains
the salts removed from the seawater. This energy-bearing stream (shown with red
arrows on Figure 2) is applied to the back side of pistons of cylindrical
isobaric chambers, also known as “pressure exchangers” (shown as yellow
cylinders on Figure 2). These pistons pump approximately 45 to 50 percent of the
seawater fed into the reverse osmosis membranes for desalination. Warm Seawater

Source

An average of 5 percent of additional energy savings and respective carbon
footprint reduction (12,308 MWh/yr and 3,057 tons/CO2 per year) are projected to
be achieved by using warm cooling water from the Encina Power Generation Station
as source seawater rather than collecting cold ambient seawater via a separate
ocean intake. The osmotic pressure that has to be overcome during the salt
separation process decreases with the increase of seawater temperature. Because
the power plant discharge has approximately 5°C higher temperature than the
ambient seawater, the collocation technology used for the project (see Figure 3)
will yield both energy and GHG reduction savings.

Green Building Design

The plant will be located on a site currently occupied by a dilapidated fuel oil
storage tank. Reclaiming the land will reduce project imprint on the environment
as compared to using an undisturbed site. A key “green” feature of the plant
design is its compactness. A number of desalination plant facilities (i.e., RO
Building, Administration Building and Pretreatment Filters) will share common
walls, roofs, and equipment. The plant will occupy less than 5 acres. When
built, this desalination plant will have the smallest footprint in the world per
unit production capacity (5 acres per 189 ml/d). Building design will follow the
principles of the Leadership in Energy and Environmental Design (LEED) program
that aims to reduce the overall impact on the environment of building
construction and functions. Minimizing energy use for lighting, air
conditioning, and ventilation will be achieved by, for example, translucent
panels to maximize daylight use and views to the outside, automatic light
switching, monitored ventilation in the individual working areas, and water
conserving fixtures for service facilities and landscape irrigation.

The energy savings associated with the implementation of the green building design are in a
range of 300 MWh/yr to 500 MWh/yr. The carbon footprint reduction associated
with this design is between 75 and 124 tons of CO2 per year. On-site Solar Power Generation

One enhancement of the green building design is the installation of a rooftop
photovoltaic (PV) system for solar power generation. The main desalination plant
building will have approximately 50,000 square feet of roof surface, which can
accommodate a solar panel system that could generate approximately 777 MWh/yr of
electricity and reduce the net carbon footprint of the desalination plant by 193
metric tons of CO2 per year. Use of Carbon Dioxide for Water Production
Approximately 2,100 tons of CO2 per year are planned to be used at the
desalination plant for final conditioning of the fresh water for corrosion
protection. The drinking water ion balance (pH) will be maintained in a range
(8.3 to 8.5), which allows the CO2 to remain sequestered with the drinking
water. The desalination plant is designed to use only carbon dioxide collected
from industrial operations that generate this gas as a waste stream (i.e.,
breweries, ethanol plants, etc.), which otherwise would be released in the air.
Re-vegetation of Wildfire Zones

Almost every year parts of San Diego County experience large wildfires. For
example, in 2007 San Diego wildfires burned over 35,000 acres. In response to an
ongoing wildfire zone re-vegetation program spearheaded by the California
Coastal Commission, Poseidon Resources has committed to invest $1 million in
reforestation activities. Total annual carbon footprint reduction associated
with the tree sequestration project is estimated at 166 metric tons of CO2 per
year. Carbon Dioxide Sequestration in Coastal Wetlands

As a part of the Carlsbad seawater desalination project, Poseidon Resources is
planning to develop 37 acres of new coastal wetlands in San Diego County. These
wetlands will be designed to create habitat for marine species similar to those
found in the Agua Hedionda Lagoon (see Figure 1), from which source seawater is
collected for the power plant and for desalination plant operations. In addition
to the benefit of marine habitat restoration and enhancement, coastal wetlands
also act as a “sink” of carbon dioxide. Tidal wetlands are very productive
habitats that remove significant amounts of carbon from the atmosphere, a large
portion of which is stored in wetland soils. While freshwater wetlands also
sequester CO2, they are often a measurable source of methane emissions. For
comparison, coastal wetlands and salt marshes release negligible amounts of
greenhouse gases and therefore, their carbon sequestration capacity is not
measurably reduced by methane production. The offset of the desalination plant
carbon footprint by the wetland project is estimated at 304 tons of CO2/year.
Other Offset Projects and Renewable Credits

For the remainder of the project’s GHG emissions, Poseidon Resources will
purchase a combination of carbon offset projects and Renewable Energy Credits
(RECs). Contracts for offset projects provide more price stability and are
typically established for longer terms (10-20 years) than RECs (1-3 years).
Offsets or RECs will be used as the swing mitigation option to “true-up” annual
changes to the project’s net carbon footprint. The Carbon Action Plan for the
Carlsbad project defines a roadmap for carbon-neutral seawater desalination for
future projects in the United States and worldwide.




 

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