Expanding San Diego's water supply

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Water officials throughout San Diego are exploring ways to expand the region’s water supply through purple piping and water purification.

Some of that will come through water purification systems now in the works, while the rest already courses through miles of purple pipes buried under streets in the region.

As officials labor to generate drought-proof, local water supplies, they’re looking at sewage flows as a valuable asset.

“That’s water that we don’t have to import anymore,” said Kimberly Thorner, general manager of Olivenhain Water District in Encinitas, which uses recycled water for golf courses and parks. “It’s utilizing wastewater that is currently unused.”

Purple piping refers to lines designated for recycled water, which is treated to a level suitable for irrigation and industrial use, but not for drinking. Potable reuse employs reverse osmosis filters to purify sources to a level that exceeds drinking water standards, and can be blended with potable supplies.

In the effort to capture and clean those flows, purple pipes and potable reuse are twin parts of the equation.

Environmental groups and water managers say, however, that potable reuse may ultimately be more useful and cost-effective, since it doesn’t require extensive, separate piping, and meets a year-round demand for drinking water.

“They’re not mutually exclusive, but because purple piping is more expensive and opportunities to use it are limited, we’d rather see the resource and uses go into potable reuse,” said Julia Chun-Heer campaign coordinator for Surfrider Foundation San Diego.

PURPLE PIPING

Recycled or reclaimed water has been employed in the region since the 1990s, and has gained steam amid the recent drought, particularly during summer months when irrigation demands are high.

Under the streets of San Diego are 91 miles of purple pipe, which funnel recycled wastewater to parks, medians and golf courses throughout the city.

In the Chula Vista, 88 miles of purple pipe snake through newer neighborhoods in the eastern part of the city, where new developments are required to install the lines.

Purple piping also wends its way through North County communities, watering greenery throughout the region. Those systems could be expanded through the North San Diego Water Reuse Coalition, a coalition of 10 local water agencies that aim to create a network to connect the supply of recycled water with areas that demand it.

Existing wastewater treatments systems produce 30,000 acre-feet of water per year, enough to supply 60,000 households. That’s about four percent of the county’s total water supply, a figure that is expected to rise to at least 6 percent by 2020, said Maria Mariscal, a senior water resources specialist for the San Diego County Water Authority.

In the city of San Diego, recycled water costs 80 cents per 100 cubic feet – about 20 percent the rate for potable water, which costs $4 per 100 cubic feet for irrigation customers, according to the city’s website.

Although the costs vary between districts based on the exact treatment process used and the extent of pipelines needed to deliver it, recycled water typically offers substantial savings over potable supplies.

“The majority of recycled water is used for landscape irrigation purposes,” said Maris Steirer, deputy director for planning and water resources with the City of San Diego. “That is why we have a pretty big differential in the use in the summer versus the wintertime. When it’s cool or it’s raining, the demands really drop.”

POTABLE REUSE

What if you didn’t need separate pipelines, and the water that left homes and businesses through sewer lines could come back clean and pure out of faucets?

That’s the goal of water purification systems such as San Diego’s Pure Water project.

The process would use several stages of treatment. Micro-filters would remove solids from wastewater. Then reverse osmosis would filter out organic material and salts. In the final step, ultraviolet light and hydrogen peroxide would obliterate any potential contaminants.

San Diego’s plan involves three treatment plants that could produce 83 million gallons per day by 2035, to provide about a third of San Diego’s supplies.

It’s projected to cost about $3.5 billion, but could save $1.8 billion by eliminating the need for overdue upgrades to the Point Loma Wastewater Treatment Plant. At completion, it would be one of the nation’s two largest water purification systems.

Other water districts are headed in the same direction.

Padre Dam Water District in Santee broke ground in October on a test project for a potable reuse system that could eventually supply 15 percent of its drinking water.

The city of Escondido is exploring plans to produce up to 12 million gallons of potable water to add to Lake Dixon. And a half dozen other agencies are considering water purification as well, Mariscal said.

DESIGNER WATER

As purple piping and water purification systems proceed along parallel tracks, some water officials say that will provide the flexibility to fine-tune water treatment for a wide range of uses.

Tertiary treatment, required for recycled water, is fine for irrigating parks, medians and lawns. Some industrial uses may require more specialized processes.

And agricultural crops, which benefit from the nitrates and other nutrients remaining in recycled water, can’t tolerate the high amounts of salts in those flows, said Toby Roy, a water resources manager for the county water authority.

So some water districts run part of their recycled water through reverse osmosis in order to bring salt levels down.

“I heard someone call it designer water,” said Thorner, of Olivenhain. “You treat to the need of the end use.”

The North San Diego Water Reuse Coalition is considering a range of options that could include recycled and purified water and various blends in between.

“At the end of the day I think both,” Thorner said. “If you look at San Diego County 20 years from now, you’ll have desalination and water recycling and indirect potable reuse and possibly even direct potable reuse. I don’t think it is an either or. I think it is a both.

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