Desalination plant would go deep to protect marine life

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Map of proposed Deep Water Desal desalination plant

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A desalination project proposed for California's central coast would draw water from one of the world's deepest submarine canyons, making it potentially less harmful to ocean life. The Deep Water Desal facility would require substantially less energy to operate than typical desalination plants, use renewable energy sources, and provide cooling for a data center that would be built in tandem. It's projected to produce 25,000 acre-feet (30 million cubic meters) of freshwater per year, serving 50,000 homes.

The Monterey Bay region suffers from long-term water issues, such as overuse of groundwater, lack of storage reservoirs and diminishing surface water. There's also no access to the State Water Project, which provides imported water to much of the state.

Monterey Canyon is the deepest submarine canyon on North America's west coast at more than 10,000 ft (3 km) deep, twice the depth of the Grand Canyon. Within just 2 miles (3. 2km) from shore it plummets to a depth of 1,600 ft (488 m). Deep Water Desal would draw its seawater from an intake pipe at about 1,000 ft (300 m) offshore at 130-ft (40 m) deep, an area with a quarter of the sea life compared to shallows near shore.

Typical desalination projects pull in water close to shore, exacerbating the harmful effects on sea life. At the same time, those murkier shoreline waters require two energy-intensive filtering stages to clean the water before it can be desalinated through reverse osmosis. The deeper water to be drawn by Deep Water Desal is cleaner and would require only one filtering step prior to desalination. Thus, the energy needed to run the facility would be reduced by an estimated 40 percent.

Intake water at the proposed depths is also colder and more dense, which requires more energy to push it through the micropores in filters for reverse osmosis. Since the region is also in need of more data storage and faster internet infrastructure, developers have proposed building a large data storage facility to be cooled by the cold seawater before it's sent to the desalination plant. The seawater would then be warmed, thus requiring less energy to desalinate.

Planners are also hoping to make the project carbon-neutral or carbon-negative. To achieve this, they're working on creating a municipal power authority that would acquire all its electricity from renewable sources, such as wind or solar, and at wholesale prices.

A portion of the facility would be located inside the Monterey Bay National Marine Sanctuary, which is managed by the National Oceanic and Atmospheric Administration (NOAA), meaning the agency would have final approval over any proposals. To mitigate harm to sea life, such as plankton and fish larvae, the Deep Water Desal would use an open-water intake with 1-mm openings to screen out as many creatures as possible.

To more safely discharge the highly concentrated brine that results from desalination, the NOAA advises to mix it with treated wastewater or power plant cooling water. For now, Deep Water Desal plans to build a diffuser pipeline to disperse the brine 2,000 ft (600 m) offshore.

An environmental impact study is currently being drafted, with construction to possibly begin in 2018. The cost of construction is estimated at around US$300 million, with the final water cost to consumers estimated at less than $2,000 per acre-foot (1,233.5 cu m).

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