Desalination planning: Green seawater desalination in Carlsbad

Nikolay Voutchkov takes us through one of California’s most advanced desalination projects – the Carlsbad seawater desalination plant – and explains the associated climate action plan in order to keep the plant as carbon neutral as possible.

Figure 1: The Carlsbad seawater desalination project.
Figure 1: The Carlsbad seawater desalination project.
Figure 2: The energy recovery system of the Kwinana seawater desalination plant.
Figure 2: The energy recovery system of the Kwinana seawater desalination plant.
Figure 3: The Tampa Bay desalination plant pelton wheel energy recovery system.
Figure 3: The Tampa Bay desalination plant pelton wheel energy recovery system.
Figure 4: The solar panel rooftop system.
Figure 4: The solar panel rooftop system.

Over the past five years desalination has gained a significant momentum in California. With more than ten projects in various stages of environmental review, design and construction, desalination is planned to provide 1,500,000 to 2,000,000 m3/day of new fresh drinking water supplies for the state by the year 2015.

One of the largest and most advanced projects under development today is the 189,000 m3/day (50 MGD) Carlsbad seawater desalination plant (Figure 1). This project is co-located with the Encina coastal power generation station which currently uses seawater for once-through cooling. The Carlsbad seawater desalination project is developed as a public-private partnership between Poseidon Resources and eight local utilities and municipalities in San Diego County, California.

Project environmental review is planned to be completed by the second quarter of 2008 and construction is expected to begin by the end of this year. The Carlsbad project is targeted to be in operation by mid-2011. This project would supply 6 to 8 % of the drinking water in San Diego County and when completed it would be the largest seawater desalination plant in the USA.

In 2006 California legislation introduced the AB 32 Global Warming Solutions Act which aims to reduce the greenhouse gas (GHG) emissions of the state to 1990 levels by year 2020. In proactive response to this legislation, project proponent, Poseidon Resources has voluntarily taken upon the commitment to completely offset the carbon footprint associated with desalination plant operations. The project proponent has developed a Climate Action Plan which outlines a portfolio of operational and design technologies and measures; green energy supply alternatives and carbon emission offset initiatives. The key components of the Climate Action Plan are described below.

Project carbon footprint

The carbon footprint of the seawater desalination plant is the amount of carbon dioxide that would be released into the air from the power generation sources that will supply electricity for the plant. Usually, a carbon footprint is measured in lbs of metric tons of carbon dioxide (CO2) emitted per year. The total plant carbon footprint is dependent on two key factors: (1) how much electricity is used by the desalination plant; and (2) what sources (fossil fuels, wind, sunlight, etc.) are used to generate the electricity supplied to the plant. Both of these factors could be variable over time and therefore, the Climate Action Plan has to have the flexibility to incorporate such changes.

The Carlsbad seawater desalination plant is planned to be operated continuously, 24 hours a day and 365 days per year, and to produce an average annual drinking water flow of 189,000 m3/day. When the plant was originally conceived over five year ago, the total baseline power use for this plant was projected at 31.3 megawatts (MW) or 3.96 KWh/m3 (15.0 KWh/1,000 gallons) of drinking water. This power use incorporates both production of fresh drinking water and conveyance, and delivery to this water to the distribution systems of the individual utilities and municipalities served by the plant.

However over the lengthy period of project permitting, the seawater desalination technology has evolved. By taking advantage of the most recently available state-of-the art technology for energy recovery and by advancing the design to accommodate latest high efficiency reverse osmosis system feed pumps and membranes, the actual project power use was reduced down to 28.1 MW or 3.568 KWh/m3 (13.5 KWh/1,000 gallons) of drinking water. As a result, the total annual energy consumption for the Carlsbad seawater desalination project used to determine the plant carbon footprint is 246,156 MWh/yr. This energy use is determined for an annual average plant production capacity of 189,000 m3/day.

Next, in order to convert the desalination plant annual energy use into carbon footprint (CF), this use is multiplied by the electric grid emission factor (Emission Factor), which is the amount of greenhouse gasses emitted during the production of unit electricity consumed from the power transmission and distribution system:

CF (lbsCO2/yr) = Annual Plant Electricity Use (MWh/yr) x Emission Factor (lbs of CO2/MWh)

The actual value of the Emission Factor is specific to the actual supplier of electricity for the project, which is San Diego Gas and Electric (SDG&E) for the Carlsbad plant. Similar to other power suppliers in California, SDG&E determines their Emission Factor based on a standard protocol developed by the California Climate Action Registry (CCAR). CCAR was created by California Legislature in 2001 as a non-profit voluntary registry for GHG emissions and is the authority in California that sets forth the rules by which GHG emissions are determined and accounted for.

Based on information provided in their most recent emissions report, which is available on the CCAR internet site (, SDG&E emission factor is 546.46 lbs of CO2 per MWh of delivered electricity. At 246,156 MWh/yr of energy use and 546.46 lbs CO2/MWh, the total carbon footprint for the Carlsbad seawater desalination project is calculated at 134.5 million lbs of CO2 per year (61,004 metric tons CO2/yr).

It is important to note that the value of the emission factor is reduced with the increase of the portion of renewable power sources in power supplier’s energy resource portfolio. Because of the statewide initiatives and legislation to expand the use of renewable sources of electricity, the emission factors of all California power suppliers are expected to decrease measurably in the future. For example, currently approximately 10 % of SGD&E’s retail electricity is generated from renewable sources (solar irradiation, wind, geothermal heat, etc.). In their most-recent Long-term Energy Resource Plan, SDG&E has committed to increase energy from renewable sources by 1% each year, reaching 20 % by year 2017. This reduction will certainly reduce the Carlsbad desalination plant carbon footprint over time, especially taking into consideration that the plant will not be fully operational before mid 2011.

Offseting carbon footprint by reduced water imports

Currently, San Diego County imports 90 % of its water from two sources – the Sacramento Bay – San Joaquin River Delta, traditionally known as the “Bay-Delta”, and the Colorado River. This imported water is captured, released and conveyed via a complex system of intakes, dams, reservoirs, aqueducts and pump stations (State Water Project), and treated in conventional water treatment plants prior to its introduction to the water distribution system. The total amount of electricity needed to deliver this water to San Diego County via the State Water Project facilities is 2.748 KWh/m3 (10.4 KWh/1,000 gallons), which includes 2.589 KWh/m3 (9.8 KWh/1,000 gallons for delivery; and 0.079 KWh/m3 (0.3 KWh/1,000 gallons) each for evaporation losses from water reservoirs, and for water treatment.

Over the past decade the availability of imported water from the State Water Project has been in steady decline due to prolonged drought, climate change patterns and environmental, and population growth pressures. One of the key reasons for the development of the Carlsbad seawater desalination project is to replace 189,000 m3/day of the water imported via the State Water Project with fresh drinking water produced locally by tapping the ocean as an alternative drought-proof source of water supply. Since the desalination project will offset the import of 189,000 m3/day of water via the State Water Project, once in operation, this project will also offset the electricity consumption of 2.748 KWh/m3 (10.4 KWh/1,000 gallons), and the GHG emissions associated with pumping, treatment and distribution of this imported water. The annual energy use for importing 189,000 m3/day of State Water Project water is therefore, 189,800 MWh/yr. At 546.46 lbs CO2/MWh, the total carbon footprint of the water imports that will be offset by desalinated water is therefore, 103.7 million lbs of CO2 per year (47,240 metric tons CO2/yr).

Taking under consideration that the gross carbon footprint of the desalination plant is 61,004 metric tons CO2/yr, and that 47,240 metric tons CO2/yr (77.4 %) of these GHG emissions would be offset by reduction of 189,000 m3/day of water imports to San Diego County, the Carlsbad desalination plant’s net carbon footprint is estimated at 13,764 metric tons CO2/yr.

Climate action plan for net carbon footprint reduction

The main purpose of the Climate Action Plan for the Carlsbad seawater desalination project is to eliminate plant’s net carbon footprint by implementing measures for: energy efficient facility 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. Project carbon neutrality would be achieved by a balanced combination of these measures.

The size and priority of the individual projects included in the Climate Action Plan will be determined based on a life-cycle cost-benefit analysis and overall benefit for the local community. Implementation of energy efficiency measures for water production, green building design, and carbon dioxide sequestration projects in the vicinity of the project site will be given the highest priority.

The project Climate Action Plan is a living document that has to be updated periodically in order to reflect the dynamics of development of desalination and green energy generation technologies; and the efficiency and cost-effectiveness of various carbon footprint reduction and offset alternatives. Once the Carlsbad seawater desalination plant is operational, the actual carbon footprint will be verified at the time of plant startup and will be updated periodically to account for changes in power supplier’s Emission Factor, and for the actual performance of the already implemented carbon footprint reduction initiatives. Periodic assessment and re-prioritisation of activities that keep the desalination plant operations “green” is a very essential component of the Climate Action Plan because both desalination technology and green power generation (i.e., solar, wind and bio-fuel-based power) are expected to undergo accelerated development over the next decade as they evolve from marginal to mainstream sources of water supply and power supply, respectively. The specific carbon footprint reduction measures incorporated in the Carlsbad Climate Action Plan, and their key benefits and constraints are discussed below.

Energy efficient design and operations

Over 50 % of the energy used at the Carlsbad seawater desalination plant is applied for salt-fresh water separation by reverse osmosis. The seawater desalination project design incorporates a number of features minimising plant energy consumption. One of them is the use of state-of-the art pressure exchanger-based energy recovery system that allows recovering and reusing 33.9 % of the total initial energy applied for salt separation. 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 is applied to the back side of pistons of cylindrical isobaric chambers, also known as “pressure exchangers”. These pistons pump approximately 45 to 50 % of the seawater fed into the reverse osmosis membranes for desalination. Since a small amount of energy (4 to 6 %) is lost during the energy transfer from the concentrate to the feed water, this energy is added back to feed flow by small booster pumps. The reminder (45 to 50 %) of the feed flow is pumped by high-pressure centrifugal pumps equipped with high-efficiency motors. Figure 2, shows similar pressure exchanger system installed at the Kwinana seawater desalination plant in Perth, Australia.

The pressure exchanger energy recovery system is projected to recover 10,200 hp (7.6 MW) of power and yield 2,650 hp (1.98 MW) of additional power savings as compared to the energy that could be recovered using standard energy recovery equipment (pelton wheels). Pelton wheels are presently employed at most large seawater desalination plants worldwide, including at the 95,000 m3/day (25 MGD) seawater desalination plant in Tampa, Florida (see Figure 3).

In addition to the state-of-the-art pressure exchanger energy recovery technology, the Carlsbad desalination plant design incorporates variable frequency drives on seawater intake pumps, filter effluent transfer pumps, and product water pumps as well as premium efficiency motors for all pumps in continuous operation that use electricity of 500 hp-hr or more. Installation of premium -efficiency motors and variable frequency drives on large pumps would result in additional 1.26 MW (4%) power savings. Harnessing, transferring and reusing the energy applied for salt separation at very high efficiency by the pressure exchangers allows reducing the overall amount of electric power used for seawater desalination with over 11.5 % (3.24 MW) as compared to standard designs of similar facilities. These savings correspond to a total annual electricity use reduction of 28,382 MWh/yr and a carbon footprint reduction of 7,050 tons of CO2/yr and are already accounted for in the net carbon footprint of the desalination plant (i.e., 13,764 tons of CO2 per year).

Up to 5 % 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 for the desalination plant. Osmotic pressure that has to be overcome in order to produce fresh drinking water decreases with the increase of seawater temperature – as a result desalination of warmer seawater requires less energy. The Carlsbad seawater desalination plant will be co-located with the Encina power plant (see Figure 1) and its intake will be connected to the cooling water canal to take advantage of the warmer seawater discharged by the desalination plant.

Over 80 % of the desalination plant piping would be made of low-friction fiberglass reinforced plastic (FRP) and high-density polyethylene (HDPE) materials, which in turns would yield additional energy savings for seawater conveyance. The desalination plant operations will be fully automated, which would allow to reduce plant staff requirements and associated GHG emissions for staff transportation and services.

Green Building Design

The desalination plant will be located on a site currently occupied by a dilapidated fuel oil storage tank which in no longer used by the power plant. This tank and its content will be removed and the site will be reclaimed and reused to construct the desalination plant. Reclaiming the land will reduce project imprint on the environment as compared to using new undisturbed site for the desalination plant.

A key “green” feature of the Carlsbad seawater desalination plant design is its compactness. The desalination plant facilities will be configured as series of structures sharing common walls, roofs and equipment which would allow significant reduction of its physical footprint. The total area occupied by the desalination plant facilities would be less than 5 acres. When built, this would be the smallest footprint desalination plant in the world per unit production capacity (5 acres per 189,000 m3/day). For comparison, the 95,000 m3/day Tampa Bay seawater desalination plant occupies 8 acres; the 273,000 m3/day Orange County Groundwater Recharge Project, which also uses reverse osmosis system, occupies approximately 40 acres; and the 325 m3/day Ashkelon seawater desalination plant, which currently is the largest seawater reverse osmosis facility in the world, occupies 24 acres. A plant with a smaller physical footprint would also yield a smaller construction-related carbon footprint: lower construction material expenditures and GHG emissions from construction equipment due to smaller volume of excavation and concrete works. Reduced construction site footprint also generates less dust emissions and requires less water for dust control.

A large portion of the desalination plant facilities and equipment will be located in several buildings. Building design will follow the principles of the Leadership in Energy and Environmental Design (LEED) program. This is a program of the United States Green Building Council and is developed to promote construction of sustainable buildings that reduce the overall impact of building construction and functions on the environment by: (1) sustainable site selection and development; (2) energy efficiency; (3) materials selection; (4) indoor environmental quality, and (5) water savings.

Consistent with the principles of the LEED program, the desalination plant buildings will include features and materials that allow minimising energy use for lighting, air conditioning and ventilation. For example, a portion of the walls of the main desalination plant building will be equipped with translucent panels to maximise daylight use and views to the outside. Non-emergency interior lighting will be automatically controlled to turn-off in unoccupied rooms and facilities. A monitoring system will ensure that the ventilation in the individual working areas in the building is maintained at its design minimum requirements. In addition, building design will incorporate water conserving fixtures (lavatory faucets, showers, water closets, urinals, etc.) for plant staff service facilities and for landscape irrigation.

The green desalination plant buildings will incorporate low-emitting materials and thus pose less risk to the natural environment and building’s occupants. Low emitting paints, coatings, adhesives, sealants and carpet systems are planned to be used on the interior of the buildings. Building design team will include professional engineers that have achieved the LEED Accredited Professional designation and are well experienced with the design and construction of green buildings.

The additional costs associated with the implementation of the green building design as compared to the costs for a standard building are estimated at US$5 million and the potential energy savings are in a range of 300 MWh/yr to 500 MWh/yr. The potential carbon footprint reduction associated with this design is between 75 and 124 tons of CO2 per year (0.5 to 0.9 % of the net power plant footprint). The unit cost of carbon footprint reduction for green building design is estimated at US$3,000 to US$5,000/ ton of CO2.

The total actual energy reduction that would result from green building design will be verified during plant commissioning, which will incorporate a LEED-compliance review process (i.e. by mid-2011). The LEED-review process will be completed by an independent third party consultant certified to complete such reviews.

On-site solar power generation

One enhancement of the green building design is the installation of rooftop photovoltaic (PV) system for solar power generation (see Figure 4). The main desalination plant building would have a roof surface of approximately 50,000 square feet, which would be adequate to house a solar panel system that could generate approximately 777 MWh/yr of electricity and reduce the net carbon footprint of the desalination plant with 193 metric tons of CO2 per year, which is approximately 1.4 % of the net desalination plant carbon footprint of 13,764 tons of CO2 per year.

The construction cost of the rooftop solar power system is estimated at US$4.1 million. The net present worth cost of power generation using this alternative is US$366,700/yr which corresponds to unit cost of generated electricity of 47.2 cents/kWh. This unit cost is approximately five times higher than the cost of power supply from the electric grid. The unit cost of carbon footprint reduction for this alternative is US$1,900/ton of CO2.

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 post-treatment of the fresh water (permeate) produced by the reverse osmosis (RO) system. Carbon dioxide in a gaseous form will be added to the RO permeate in combination with calcium hydroxide or calcium carbonate in order to form soluble calcium bicarbonate which adds hardness and alkalinity to the drinking water for distribution system corrosion protection. In this post-treatment process of RO permeate stabilisation, gaseous carbon dioxide is sequestered into soluble form of calcium bicarbonate. Because the pH of the drinking water distributed for potable use is in a range of 8.3 to 8.5 at which CO2 in a soluble bicarbonate form, the carbon dioxide introduced in the RO permeate would remain permanently sequestered in this form and ultimately would be consumed with the drinking water.

A small quantity of carbon dioxide used in the desalination plant post-treatment process is sequestered directly from the air when the pH of the source seawater is adjusted by addition of sulfuric acid in order to prevent RO membrane scaling. However, a large amount is typically delivered to the desalination plant site by commercial supplier. Depending on the supplier, carbon dioxide is of one of two origins: (1) a CO2 Generating Plant or (2) a CO2 Recovery Plant. CO2 Generating Plants use various fossil fuels (natural gas, kerosene, diesel oil, etc.) to produce this gas by fuel combustion. CO2 Recovery Plants produce carbon dioxide by recovering it from the waste streams of other industrial production facilities which emit CO2-rich gasses: breweries, commercial alcohol (i.e., ethanol) plants; hydrogen and ammonia plants, etc. Typically, if these gases are not collected via CO2 Recovery Plant and used in other facilities, such as the desalination plant, they are emitted to the atmosphere and therefore, constitute a GHG release.

The Carlsbad desalination plant is planned to use only carbon dioxide produced in a CO2 Recovery Plant. This requirement will be enforced by requiring the commercial supplier of carbon dioxide for the desalination plant operations to provide a certificate of origin of each load of this water treatment chemical delivered to the plant site. This would encourage and incentivise the commercial suppliers and manufacturers of CO2 to recover this gas from industrial waste streams rather than to generate new gas by combustion, and thereby to prevent its release to the atmosphere. Sequestration of CO2 at the desalination plant by its conversion from gaseous to chemically bounded soluble form would is therefore considered a desalination plant carbon footprint reduction alternative. By sequestering 2,100 tons of CO2 per year in the desalination plant post-treatment process, the net carbon footprint of the plant (13,764 tons of CO2 /yr would be reduced by 15.3 %). At annual expenditure for carbon dioxide supply of approximately US$147,000/yr, this carbon footprint reduction alternative is very cost-competitive (US$70/ton CO2).

Carbon dioxide sequestration by reforestation

Almost every year parts of San Diego County are exposed to measurable loss of forest, urban and suburban trees due to large wildfires. For example, in 2007 San Diego wildfires burned over 35,000 acres, including forests, tree farms, urban forestry. A specific annual carbon offset program required by the California Coastal Commission is the revegetation of areas in the San Diego region impacted by the wildfires that occurred during the fall of 2007. In response to this program Poseidon has committed to investing US$1.0 million in reforestation activities.

The average tree planting cost is estimated at US$26.7/tree and the average annual maintenance cost is US$5.5/tree. Assuming tree maintenance costs for 25 years @ US$5.5/tree, the total lifecycle expenditure per tree is US$137.5. When added to the three planting cost of US$26.7, the total cost for planting and maintaining of the trees included in the reforestation project would be US$164.2/tree. At commitment of US$1.0 MM and total costs of US$164.2/tree, the total amount of trees planned to be replanted is 6,090. At an annual three sequestration rate of 60 lbs/tree over the 25-year period of the desalination plant operations, the total annual carbon footprint reduction associated with the tree sequestration project is estimated at 365,400 lbs (166 metric tons) of CO2 per year. This is approximately 1.2 % reduction of the net desalination plant footprint. At annual expenditure per tree of approximately US$33,500/yr, the unit carbon footprint reduction cost for this alternative would be US$202/ton of CO2.

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 these found in the Agua Hedionda Lagoon (see Figure 1), from which source seawater is collected for the power plant and for desalination plant operations. Once the wetlands are fully developed, they will be maintained and monitored over the life of the desalination plant operations. The cost of the wetland restoration project is estimated at US$3.0 MM.

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 the 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.

Based on a detailed study completed in a coastal lagoon in Southern California ( the average annual rate of sequestration of carbon in coastal wetland soils is estimated at 0.03 kg of C/m2.yr. Other source (

indicates that in addition to accumulating CO2 in the soils, central and southern California tidal marshes could also sequester 0.45 kg of C/m2.yr in the macrophytes growing in the marshes and 0.34 to 0.63 kg of C/m2.yr in the algal biomass. Taking under consideration that the total area of the proposed wetland project is 37 acres (149,739 square meters) and the maximum sequestration capacity of the coastal wetlands could be 1.11 kg of C/m2.yr, than the wetland carbon sequestration capacity would be up to 83 tons of C/m2.yr. With a conversion factor from carbon to carbon dioxide of 3.664 the estimated offset of the desalination plant carbon footprint by the wetland project is estimated at 304 tons of CO2/year (a 2.2 % reduction of the net carbon footprint). At total present worth costs for wetland development and maintenance of approximately US$120,000/yr, the unit carbon footprint reduction cost for this alternative would be US$395/ton of CO2).

Site-specific research is planned to be completed in order to quantify the actual carbon sequestration capacity of the new wetland system proposed to be developed as a part of the Carlsbad seawater desalination project, once the wetland project is completed and is fully functional. Typically it takes three to five years for a coastal wetland project to be fully functional and to begin to yield enhanced habitat and GHG sequestration benefits.

Carbon emissions offset by reducing energy needs for water reclamation

The Carlsbad Municipal Water District owns and operates a 15,000 m3/day (4 MGD) water reclamation plant which consists of advanced tertiary treatment facilities for the entire flow and of 3,750 m3/day (1 MGD) brackish reverse osmosis water desalination system, which at present uses 1,950 MWh of electricity per year. The purpose of the brackish water desalination plant is to reduce the salinity of the treated effluent from 1,400 mg/L to below 1,000 mg/l in order to make the effluent suitable for irrigation. The current high level of salinity of the reclaimed water is mainly due to the relatively high salinity of the City’s drinking water which could reach 1,000 mg/l at times.

Once the Carlsbad seawater desalination plant is in operation and completely replaces the existing high-salinity drinking water, the salinity of the City’s reclaimed water is projected to be reduced by half. Therefore, the replacement of the existing City high-salinity imported water supply with desalinated water would eliminate the need for operation of the 1.0 MGD brackish water desalination plant at the Carlsbad Water Recycling Facility. This in turns would reduce the carbon footprint of the Carlsbad Water Reclamation Facility with 1,950 MWh x 546.46 lbs of CO2 /MWh = 1,065,957 lbs of CO2/yr (484 tons of CO2/yr). Since this GHG reduction is directly credited to the seawater desalination plant operations, the Carlsbad desalination plant’s carbon footprint could be reduced by 3.5 %.

Other carbon emission offsets

Poseidon plans to invest in a number of green power projects with its public partners who will be receiving desalinated water from the Carlsbad seawater desalination Plant. The total carbon footprint offset for the desalination plant is projected at 561 tons of CO2/year (4.1 %). For the remainder of the Project’s carbon emissions, Poseidon 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) then RECs (1-3 years).

Projected annual net-zero carbon emission balance

Analysis of data presented indicates that up to 40 % of the GHG emissions associated with seawater desalination and drinking water delivery will be reduced by on-site reduction measures and the remainder will be mitigated by off-site mitigation projects and purchase of renewable energy credits. It should be noted that the contribution of on-site GHG reduction activities is expected to increase over the useful life (i.e, in the next 30 years) of the project because of the following key reasons:

•           The power supplier (SDG&E) is planning to increase significantly the percentage of green power sources in its electricity supply portfolio, which in turns will reduce its emission factor and the net desalination plant carbon footprint.

•           Advances in seawater desalination technology are expected to yield further energy savings and carbon footprint reductions.

The lowest unit cost of carbon footprint reduction can be achieved by using carbon dioxide for post-treatment of the desalinated water (US$70/ton CO2). The most costly carbon footprint reduction options are green building design (US$3,000 to 5,000/ton CO2) and installation of rooftop solar power generation system (US$1,900/ton CO2). Development of new coastal wetlands is a very promising carbon footprint reduction option (US$395/ton CO2), which could be several times less costly than the construction of solar panel generation system of the carbon footprint reduction capacity. Similarly, reforestation could also be a cost-competitive GHG reduction alternative (US$202/ton CO2). As compared to green power generation alternatives (solar and wind power) reforestation and wetland mitigation have added environmental benefits. For example, the new coastal wetlands developed in relation to seawater desalination project could be designed to create habitat for species that are impacted by the intake operations of the desalination plant via impingement and entrainment of these species on the intake screens.

Summary and conclusions

Greenhouse gas emissions associated with production of desalinated seawater at the Carlsbad project in Southern California are planned be mitigated by a portfolio of alternative technologies and measures: from the use of carbon dioxide for water production, green building design and of state-of-the-art technology; to the development of on-site and off-site green energy projects and carbon dioxide sequestration by reforestation and new coastal wetlands. The mix of GHG reduction alternatives will be prioritised and implemented under a Carbon Action Plan which defines a roadmap for carbon-neutral seawater desalination.


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