Carlsbad Desalination Project, San Diego, California, USA

The Carlsbad desalination project in San Diego County, California will be the largest desalination plant in the western hemisphere when it becomes operational in 2012. The $320m project is the first large-scale desalination plant on the West Coast, being privately financed and developed by Poseidon Resources Corporation (Poseidon). The project includes a desalination plant, a pumping station, product water storage and finished water conveyance pipelines.

Poseidon has leased a four-acre site adjacent to the existing Encina Power Station site, owned by Cabrillo Power, for 60 years for the desalination plant. On the south of the plant site is the Agua Hedionda Lagoon of the Pacific Ocean.

The plant will produce 50 million gallons of desalinated seawater (MGD) a day and provide 10% of the total drinking water needed by San Diego. It will supply 56,000 acre-feet (approximately 2,440ft³) of water annually, sufficient for about 300,000 people. The water from this plant will be purchased by the Carlsbad water agencies, and be supplemented with imported water from San Diego County Water Authority (SDCWA), the region's water supplier.

In 2008, the plant received approvals from the State Lands Commission and the California Coastal Commission. In May 2009, the Regional Water Quality Control Board approved the plant's construction. In September 2009, the City of Carlsbad approved the desalination project with minor changes to the main plant building and pipeline network to the certified EIR.

Bidding for EPC of the project was made in February 2006 and the contract awarded in 2007.

Construction of the first phase of the Carlsbad desalination plant started in November 2009.

Carlsbad desalination project

The Carlsbad desalination project is being developed as a public-private partnership between local utilities and municipalities in San Diego County and Poseidon Resources. Poseidon has 30-year contracts, with an option for extension, for long-term, fixed-price water purchase from nine public water agencies in San Diego.

The water agencies include Carlsbad Municipal Water District, Rincon del Diablo Municipal Water District, Olivenhain Municipal Water District, Vallecitos Water District, Sweetwater Authority, Valley Centre Municipal Water District, Santa Fe Irrigation District, Rainbow Municipal Water District and the City of Oceanside Municipal Water District.

Engineering, procurement and construction (EPC) contracts were awarded to American Water, JR Filanc Construction Company, PBS&J, Acciona Agua and its partners. Poseidon selected Barclays Capital as financial adviser for the Carlsbad desalination project in June 2009.

Water supply

Approximately 30% of the California state water project (SWP) goes to San Diego, but the county still imports 90% of its water from the Bay-Delta and the Colorado River. California has seen a prolonged drought along with changes in environmental pattern and population growth, meaning that tapering sources and stringent water treatment requirements are increasing the cost of traditional water sources.

The Carlsbad desalination project aims to provide a local source of potable water with an increased quality level (250mg-350mg) and reliability by using economically feasible technology.

In 1998 the Carlsbad officials in San Diego teamed up with Poseidon Resources to secure a sustainable future water supply in the region. The public-private-partnership has since then been working towards construction of the 50MGD plant. The Carlsbad desalination plant underwent ten years of planning and was in California's permitting process for five years, before in September 2004, the Carlsbad Municipal Water District signed a water purchase agreement with Poseidon.

Opposition

The plant was proposed in 1998 and faced several hurdles before it gained all the required official approvals that delayed the construction.

Some of the hurdles included desertion by the San Diego County Water Authority in July 2006, a petition against the project's brine discharge by environmental groups in 2007, and opposition regarding energy, climate and marine impacts.

A lawsuit was filed in April 2010 against the San Diego Regional Water Quality Control Board by the Surfrider Foundation. The foundation challenged the board's approval to withdraw seawater (300 million gallons a day) for the desalination plant.

Desalination plant design

The desalination plant construction involves demolishing the fuel storage tank of the power plant, which is not in use. The building will house the reverse osmosis (RO) system, chemical storage, and pumps for water treatment and product water.

To obtain permission from the City of Carlsbad, the design of the building was rotated, a proposed on-site pipeline will be underground and off-site pipelines routes were finalised. The new pipeline routes will be 1.2 miles shorter than the original proposal pipeline of 17 miles. The pipe diameter of the delivery system will be in the range of 24in in the downstream portions and 48 inches in the upstream portions.

The plant's compact design will reduce its physical footprint. It is being built following LEED programme guidelines and will be fully automated to reduce impact on the environment. The plant will incorporate solar power generation system on its approximately 50,000ft² rooftop, pressure exchanger-based energy recovery system and variable frequency drives and energy efficient motors for all pumps.

About 80% of the pipes for the plant would be made of high-density polyethylene (HDPE) and fibreglass-reinforced plastic (FRP) materials for energy saving seawater conveyance.

Water pre-treatment

The pre-treatment of the water will use either granular media filtration or membrane filtration technology, along with a micro-screen filtration process. A chemically enhanced backwash of the pre-treatment membranes will be conducted every day using acid, chlorine and base conditioning of membrane modules. The solution will be treated before disposal at the Encina Water Pollution Control Facility.

Treatment technology

Seawater from the Encina power plant, used for cooling boilers in operation, will be diverted to the desalination facility through an existing cooling water discharge system. The source water will be pre-treated and processed using RO membranes for producing the drinking water. The treated water is temporarily stored in on-site facilities before transmission to the regional distribution systems.

The RO system will be equipped with 12 operational treatment trains and one auxiliary train. They will be arranged discretely and have a total installed water production capacity of 54MGD. The design production capacity averages 50MGD annually.

Filter effluent transfer pumps will be used for conveying the pre-treatment filter effluent through the RO system cartridge filters. The effluent is forced into high-pressure RO feed pumps that feed the pre-treated seawater to the reverse osmosis membrane treatment trains.

The 13 high-speed RO feed pumps (one for each RO treatment train) enable the water and salt separation. They will also have energy-efficient shafts. The reverse osmosis membrane will be manufactured completely within the San Diego County.

The water from the RO system will be disinfected using chloramination, the process of adding chlorine and ammonia. The biological growth in transmission pipelines and the reservoirs of the distribution system will be controlled by forming chloramines in the water. Further, the water is chloraminated with the application of ammonia and sodium hypochlorite. The water at the delivery level will have a chloramine residual concentration in the range of 2mg/l to 2.5mg/l. The product water is conditioned with carbon dioxide and lime to avoid corrosion of the delivery facilities with the product water. Water from RO system will be pumped to the off-site water distribution system.

One gallon of potable water of water produced in the RO membrane separation process produces a gallon of concentrate. The concentrate will be twice as saline as sea water (67,000ppm). The water will be conveyed to the cooling water discharge canal of the power plant. The concentrate will be blended with the cooling water of the power plant before being discharged in to the Pacific Ocean through power plant's discharge canal. All the membranes of the RO train will be cleaned twice a year to remove the accumulated fouling materials and particulates.

Environmental improvements

The Carlsbad plant will conserve local and regional water and complement the recycling programmes. The project will also dedicate 15 acres of lagoon and oceanfront for improving public access and recreation. The project will enhance, restore and maintain the marine environment.

Poseidon has also developed a voluntary greenhouse gas reduction and energy minimisation plan covering the reduction of carbon emissions as per the California Coastal Commission's coastal development permit.

In May 2006, the American Academy of Environmental Engineers awarded the Carlsbad pilot plant the Grand Prize for Excellence in Environmental Engineering Research. In June 2006, Carlsbad City Council certified the environmental impact report (EIR) of the desalination plant, through the independent preparation of consulting firm, Dudek & Associates.