By Nikolay Voutchkov
Over the past five years, desalination has gained momentum in California. With more than ten projects in various stages of planning, environmental review, design and construction, brackish and seawater desalination plants are planned to provide 400 mgd to 500 mgd of new drinking water supplies for the state by 2015.
One of the largest and most advanced projects under development today is the 50 mgd Carlsbad seawater desalination plant (Figure 1). This project is collocated with the Encina coastal power generation station, which currently uses seawater for once-through cooling. The Carlsbad project is developed as a public-private partnership between project proponent Poseidon Resources and eight local utilities and municipalities.
Figure 1 – Carlsbad seawater desalination project
Since 1999 a team of planners, scientists, engineers, equipment manufacturers and environmental experts have been working on the development and evaluation of the desalination project. The environmental impact assessment and local land use permit for the Carlsbad desalination project were approved in the first half of 2006.
Also in 2006, the project was granted an ocean discharge permit for disposal of high-salinity concentrate generated during the RO membrane separation process and in 2007 the California Coastal Commission confirmed the project’s viability. Project permitting is scheduled for completion in the spring of 2009, and plant construction is expected to begin by the second quarter of 2009.
The Carlsbad project is planned to be operational by the end of 2011 and to supply six to eight percent of the drinking water in San Diego County. When completed, this will be the largest seawater desalination plant in the US and one of the five largest in the world.
Assessing gross carbon footprint
In 2006 the 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, the project proponent voluntarily undertook the commitment to completely offset the carbon footprint associated with desalination plant operations.
In order to make this challenging task a reality, the project proponent developed a GHG Reduction Plan that outlines a portfolio of operational and design technologies and measures, as well as green energy supply alternatives and carbon emission offset initiatives. Key components of the GHG Reduction Plan include two primary carbon footprint elements.
The carbon footprint of the 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, carbon footprint is measured in pounds of metric tons of carbon dioxide (CO2) emitted per year.
The total plant carbon footprint is dependent upon how much electricity the plant uses and 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 GHG Reduction Plan has to have the flexibility to incorporate such changes.
The Carlsbad plant will be operated continuously, 24 hours a day, 365 days per year and produce an average annual drinking water flow of 50 mgd. When the plant was originally conceived over five years ago, the total baseline power use was projected at 31.3 megawatts (MW) or 15.0 kWh/1,000 gallons of drinking water. This power use incorporates both production of fresh drinking water and conveyance and delivery, of water to the distribution systems of the individual utilities and municipalities served by the plant.
However, over the lengthy period of project permitting, 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 RO system feed pumps and membranes, the actual project power use was reduced to 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 50 mgd. As actual production capacity may vary from year to year, as would the total energy use.
In order to convert annual energy use into carbon footprint (CF), this use is multiplied by the electric grid emission factor (emission factor). This is the amount of greenhouse gases emitted during the production of unit electricity consumed from the power transmission and distribution system:
CF (lbs. CO2/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, 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 legislation (SB 1771) in 2001 as a non-profit voluntary registry for GHG emissions and is the state authority that sets forth the rules by which GHG emissions are determined and accounted for.
Based on SDG&E emissions report information available on the CCAR Internet site (www.climateregistry.org/CARROT/Public/Reports.aspx) the power plant 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 project is calculated at 134.5 million lbs of CO2 per year (61,004 metric tons CO2/yr).
Therefore, the production of one gallon of desalinated water would have a carbon footprint of 3.34 grams of CO2/gallon. This compares favorably to the carbon footprint associated with the production of one gallon of milk (over 1,100 grams of CO2/gallon) and the preparation of one hamburger (between 766 and 3,000 grams of CO2/hamburger).
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 percent 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 one percent each year, reaching 20 percent by year 2017. This reduction will certainly reduce the plant‘s carbon footprint over time, especially taking into consideration that the plant will not be fully operational before mid-2011.
Reduced water imports
Currently, San Diego County imports over 80 percent 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 State Water Project facilities is 10.4 kWh/1,000 gallons. This includes 9.8 kWh/1,000 gallons for delivery and 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 key reason for the development of the desalination project is to replace 50 mgd of imported water with fresh drinking water produced locally by tapping the ocean as an alternative, drought-proof source.
Since the project will offset the import of 50 mgd of water via the State Water Project, once in operation, it will also offset electricity consumption of 10.4 kWh/1,000 gallons and GHG emissions associated with pumping, treatment and distribution of this imported water. The annual energy use for importing 50 mgd of water is 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 103.7 million lbs. of CO2 per year (47,240 metric tons CO2/yr).
Taking into consideration that the gross carbon footprint of the plant is 61,004 metric tons CO2/yr and that 47,240 metric tons CO2/yr (77.4 percent) of these GHG emissions would be offset by reduction of 50 mgd of water imports to San Diego County, the plant’s net carbon footprint is estimated at 13,764 metric tons CO2/yr.
Greenhouse gas reduction plan
The main purpose of the GHG Reduction Plan for the project is to eliminate the 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; 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 GHG Reduction 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‘s GHG Reduction Plan is a living document that must be updated periodically in order to reflect the dynamics of development of desalination and green energy generation technologies. It also must note the efficiency and cost-effectiveness of various carbon footprint reduction measures and offset alternatives. Once the plant is operational, the actual carbon footprint will be verified at the time of startup. It will be updated periodically to account for changes in the power supplier’s Emission Factor and for the actual performance of the already implemented carbon footprint reduction initiatives.
Periodic assessment and re-prioritization of activities that keep plant operations ‘green’ is a very essential component of the GHG Reduction 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. Specific carbon footprint reduction measures incorporated in the GHG Reduction Plan have key benefits and constraints.
Energy efficient design and operations
Over 50 percent of the energy used at the facility is applied for salt-fresh water separation by RO. The project design incorporates a number of features minimizing energy consumption. One of them is the use of state-of-the art pressure exchanger-based energy recovery systems that allow recovery and reuse of 33.9 percent of the total initial energy applied for salt separation.
After membrane separation, most energy applied for desalination is retained in the concentrate stream (brine) that also contains salts removed from seawater. This energy-bearing stream is applied to the backside of the pistons of cylindrical isobaric chambers, also known as pressure exchangers (see yellow cylinders on Figure 2).
These pistons pump approximately 45 to 50 percent of the seawater fed into the RO membranes for desalination. Since a small amount of energy (four to six percent) is lost during the energy transfer from concentrate to feed water, this energy is added back to the feed flow by small booster pumps. The reminding (45 to 50 percent) feed flow is pumped by high-pressure centrifugal pumps equipped with high-efficiency motors.
Figure 2 – Energy recovery system at the Perth seawater desalination plant in 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 25-mgd seawater desalination plant in Tampa, Florida (see Figure 3).
Figure 3 – Tampa Bay desalination plant’s Pelton wheel energy recovery system
In addition to state-of-the-art pressure exchanger energy recovery technology, the plant design incorporates variable frequency drives on seawater intake pumps, filter effluent transfer pumps product water pumps and 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 (four percent) 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 desalination of over 11.5 percent (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, a carbon footprint reduction of 7,050 tons of CO2/yr and are already accounted for in the net carbon footprint of the plant (i.e., 13,764 tons of CO2 per year).
Approximately five 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. 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 facility will be collocated with the Encina power plant (see Figure 1). Its intake will be connected to the cooling water canal to take advantage of the warmer seawater discharged by the plant.
Over 80 percent of plant piping will be made of low-friction fiberglass reinforced plastic (FRP) and high-density polyethylene (HDPE) materials, which will yield additional energy savings for seawater conveyance. Plant operations will be fully automated, allowing reductions in staff requirements and associated GHG emissions for staff transportation and services.
Green building design
The plant will be located on a site currently occupied by a dilapidated fuel oil storage tank that is no longer used by the power plant. This tank and its contents 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.
A key ‘green’ feature of the design is its compactness. Plant facilities will be configured as series of structures sharing common walls, roofs and equipment that will allow significant reduction of its physical footprint.
The total area occupied by the facilities will be less than five acres. When built, this would be the smallest footprint desalination plant in the world per unit production capacity (five acres per 50 mgd).
For comparison, the 25 mgd Tampa Bay plant occupies eight acres. The 73 mgd Orange County Groundwater Recharge Project, which also uses a reverse osmosis system, occupies approximately 40 acres, while the 86 mgd Ashkelon plant, currently the largest seawater RO 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 will be achieved due to smaller volumes of excavation and concrete works. A reduced construction site footprint also generates less dust emissions and requires less water for dust control.
A large portion of facilities and equipment will be located in several buildings. Building design will follow the principles of the US Green Building Council Leadership in Energy and Environmental Design (LEED) program, developed to promote construction of sustainable buildings that reduce the overall impact of building construction on the environment.
Consistent with LEED principles, plant buildings will include features and materials that allow minimizing energy use for lighting, air conditioning and ventilation. For example, a portion of the walls of the main plant building will be equipped with translucent panels to maximize 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 ventilation in individual working areas 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.
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 for use on the interior of the buildings. The building design team will include professional engineers who have achieved the LEED Accredited Professional designation and are well experienced with design and construction of green buildings.
Additional costs associated with implementation of green building design as compared to the costs for a standard building are estimated at $5 million (USD) 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 percent of the net power plant footprint). The unit cost of carbon footprint reduction for green building design is estimated at $3,000 to$5,000 (USD)/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 green building design is the installation of rooftop photovoltaic (PV) systems for solar power generation (see Figure 4). The main 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 with 193 metric tons of CO2 per year, approximately 1.4 percent of the net plant carbon footprint of 13,764 tons of CO2 per year.
Figure 4 – Solar panel rooftop system
Construction cost of the rooftop solar power system is estimated at $4.1 million (USD). The net present worth cost of power generation using this alternative is $366,700(USD)/yr, which corresponds to a unit cost of generated electricity of 47.2 cents/kWh. This unit cost is approximately five times higher than the cost of power from the electric grid. The unit cost of carbon footprint reduction for this alternative is $1,900(USD)/ton of CO2.
Figure 5 – Solar panel rooftop system
Carbon dioxide for water production
Approximately 2,100 tons of CO2 per year are planned to be used at the plant for post-treatment of fresh water (permeate) produced by the RO system. Carbon dioxide in 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 stabilization, gaseous carbon dioxide is sequestered into a soluble form of calcium bicarbonate. Because the pH of 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 plant’s 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 site by a commercial supplier.
Depending on the supplier, carbon dioxide is produced from one of two origins. These include a CO2 generating plant or a CO2 recovery plant. CO2 generating plants use 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 waste streams of other industrial production facilities that emit CO2. –These include rich gasses from breweries, commercial alcohol (i.e., ethanol) plants, hydrogen and ammonia plants, etc. Typically, if these gases are not collected via the CO2 recovery plant and used in other facilities, they are emitted to the atmosphere and therefore, constitute a GHG release.
The Carlsbad plant will use only carbon dioxide produced in a CO2 recovery plant. This requirement will be enforced by requiring the commercial supplier of carbon dioxide to provide a certificate of origin of each load delivered to the site.
This will encourage and incentivize 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 plant by its conversion from gaseous to chemically bounded soluble form is therefore considered a plant carbon footprint reduction alternative.
By sequestering 2,100 tons of CO2 per year in the post-treatment process, the net carbon footprint would be reduced by 15.3 percent. At an annual expenditure for carbon dioxide supply of approximately $147,000(USD)/yr, this carbon footprint reduction alternative is very cost-competitive at $70(USD)/ton CO2.
Carbon dioxide sequestration
Almost every year parts of San Diego County are exposed to measurable loss of forest, urban and suburban trees due to large wildfires. At annual GHG sequestration rate of 60 lbs/tree, the total annual carbon footprint reduction associated with this green project is estimated at 365,400 lbs (166 metric tons) of CO2 per year. The unit carbon footprint reduction cost for this alternative would be US$200/ton of CO2.
Planned 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 both power plant and desalination plant operations. Once wetlands are fully developed, they will be maintained and monitored over the life of the desalination plant operations.
In addition to 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.
Based on the results of carbon sequestration studies completed in a coastal lagoon in Southern California, the estimated offset of the desalination plant carbon footprint by the 37-acre wetland project is estimated at 304 tons of CO2/year. At total present worth costs for wetland development and maintenance of approximately $120,000 (USD)/yr, the unit carbon footprint reduction cost for this alternative would be $395 USD)/ton of CO2.
Reducing energy needs
The Carlsbad Municipal Water District owns and operates a four mgd water reclamation plant that consists of advanced tertiary treatment facilities for the entire flow and one mgd brackish RO 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 plant is in operation and completely replaces the existing drinking water, salinity of the City’s reclaimed water is projected to be reduced in a half. Therefore, replacement of the existing imported water supply with desalinated water would eliminate the need for operation of the brackish water plant at the Carlsbad Water Recycling Facility. This 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 would be reduced by 3.5 percent.
Investing in carbon offset projects
Poseidon plans to invest in a number of green power projects with its public partners who will be receiving desalinated water from the Carlsbad plant. The total carbon footprint offset from such projects is currently projected at 560 tons of CO2/year and is expected to increase in the next five years.
For the remainder of the project’s carbon emissions, the project proponent will invest in carbon-offset projects in the desalination plant service area and will purchase renewable energy credits (RECs).
Net-zero carbon emission balance
Table 1 presents the total and net carbon footprint estimates of the Carlsbad project and quantifies GHG emission reduction and mitigation options which are planned to be implemented in order to reduce the plant net carbon emission footprint to zero. Analysis of data presented in Table 1 indicates that up to 43 percent 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.
The lowest unit cost of carbon footprint reduction can be achieved by using carbon dioxide for post-treatment of the desalinated water ($70 (USD)/ton CO2). The most costly carbon footprint reduction options are green building design ($3,000 to $5,000 (USD)/ton CO2) and installation of rooftop solar power generation system ($1,900 (USD)/ton CO2).
Development of new coastal wetlands is a very promising option ($395 (USD)/ton CO2), which could be several times less costly than the construction of a solar panel generation system of the carbon footprint reduction capacity. Similarly, reforestation could also be a cost-competitive GHG reduction alternative ($200 (USD)/ton CO2). As compared to green power generation alternatives (solar and wind power) reforestation and wetland mitigation have added environmental benefits.
About the author
Nikolay Voutchkov is Chief Technology Officer for Poseidon Resources Corporation. The company is located at 1055 Washington Boulevard, Stamford, CT 06901. Voutchkov can be contacted by phone at 203-327-7740, ext. 126, or by e-mail at [email protected].
About the company
In contribution to green solution of this challenge, the project proponent has committed to invest $1.0 million (USD) in reforestation activities including planting and care of over 6,000 trees. Poseidon is also planning to develop 37 acres of new coastal wetlands in San Diego County, with the cost of the wetland restoration project estimated at $3.0 (USD) million.