News Feature | January 23, 2015

Purple Pipes Vs. Indirect Potable Reuse

Sara Jerome

By Sara Jerome,

Which is more essential to San Diego's water future: purple pipes or indirect potable reuse?

San Diego Coastkeeper pitted them against each other in a blog post, trying to figure out which is more important.

"In the first corner, the purple pipe system is looking to continue its reign in San Diego," the post said. "In the second corner, the up-and-coming Indirect Potable Reuse (IPR) is looking to solve San Diego’s water problems."

The winner of this theoretical showdown? "After four hard fought rounds, IPR has dominated the ring, proving that it would be a strong, viable addition to San Diego’s arsenal for fighting the water crisis," the blog post said.

In practice, potable reuse and purple piping are not mutually exclusive and both are essential to San Diego's water future.

"Purple pipes and potable reuse are twin parts of the equation," U-T San Diego recently reported. Purple pipe carries recycled water that can be used for irrigation and industrial use, while potable reuse "employs reverse osmosis filters to purify sources to a level that exceeds drinking water standards, and can be blended with potable supplies."

Purple pipes are already in use. The city has 91 miles of purple piping to water golf courses and parks. "Existing wastewater treatment 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," according to the report.

This water is largely used in the summertime when homeowner associations and businesses are seeking cheap water. Maris Steirer, deputy director for planning and water resources in San Diego, explained the seasonal shifts in demand.

“The majority of recycled water is used for landscape irrigation purposes,” she said, per the report. “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 is expected to become more important to San Diego in the future. The city'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," the report said.

"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," the report said.

San Diego took steps last year to expand its potable reuse potential. In May the San Diego County Water Authority approved a measure supporting the city's "proposed large-scale water recycling project Pure Water San Diego, which the Water Authority has identified as the region’s most likely next source of local supply," according to the water authority.

For more information on water scarcity, check out Water Online's Water Scarcity Solution Center.