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As San Diego’s City Council prepares Tuesday to consider spending $6.6 million on a purified sewage demonstration plant, new analysis from a nonpartisan local nonprofit concludes the new drinking water source would be “a strong, viable addition” to the region’s water portfolio.

The report from the Encinitas-based Equinox Center reinforces several key points about the often-politicized water source — including how much it costs versus other sources:

• The cost of getting drinking water from purified sewage is generally lower ($1,200 to $1,800 per acre-foot) than from expanding the purple-pipe system ($1,600-$2,600), which uses treated sewage for irrigation. Why? Because installing purple pipes throughout the city is expensive: $2 million per mile, the report notes. Purifying sewage is also cheaper than desalinating seawater ($1,800-$2,800).

By 2030, purifying sewage will cost as much as importing water, the report says. (An acre-foot is about 326,000 gallons, enough for two households for a year.) Today, most of our water is imported from hundreds of miles away; the energy and increasing amounts of infrastructure needed to get it here adds cost.

• Purifying sewage produces water that’s cleaner than what we’re already drinking. Adding water from purified sewage into the city’s current untreated supply would make our existing supplies cleaner. We’re already drinking treated sewage — about 350 sewage plants dump into our existing supplies, the report notes. The Colorado River, a major source, is estimated to be about 10 percent to 17 percent treated sewage.

• Purified sewage is less vulnerable to interruption than imported water supplies from our major sources, the Colorado River and Sacramento Delta (though the report doesn’t call it “drought-proof,” the desirable label given to seawater desalination).

• Diverting sewage from the city’s main treatment plant in Point Loma would reduce ocean pollution, but only by a fraction (about 6 percent) unless the city’s current plans for a sewage recycling plant were greatly expanded.

The report notes two downsides to recycled sewage: Public perception challenges and the source’s high energy consumption. It’s more energy-intense to make sewage drinkable than it is to treat it for the purple pipe system. But both use less electricity than importing the same amount of water or desalinating seawater.

The council’s action Tuesday aims to address the perception issue. If approved, the million-gallon-per-day plant in University City would be used in tours to show the public how sewage gets recycled. And lab results from daily operations would be used to obtain state permits (and reassure the public it can be done safely). The water the plant creates would not be put in the city’s drinking water supplies. That can’t happen until the city gets those state permits.

An evolving council majority has supported other aspects of the city’s recycled sewage study, so how the council votes next week will be another test of that shaky majority.

— ROB DAVIS

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