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San Diego relies on importing 85 percent of its water supply each year from the Colorado River and Sacramento Delta. But soon, that won’t be the case.

The city is working on a multibillion-dollar project to purify enough wastewater to provide a third of the city’s drinking water by 2035.

The city’s project, called Pure Water, will soon raise resident’s water bills by $6 to $13 a month. That story prompted another round of reminders that some San Diegans aren’t thrilled about the idea of treated sewage flowing from their taps.

But recycling wastewater for drinking water is nothing new. San Diego’s two main suppliers of water already get treated discharge from sewage plants and farms.

That water is mixed with other sources and then cleaned again before it makes its way to your tap. That means the water you’re drinking today has likely been inside someone or something else before it got to you.

Scientists say purifying wastewater, like San Diego intends to do with its Pure Water project, produces cleaner water than what we currently consume.

Here’s how we’ve described the purification process before:

A toilet flushes. The sewage goes through the pipes to a treatment plant. There, it sits as the solids are filtered out. The gunk goes to a landfill. At this point, the water is already clean enough to be used for irrigation. Then the water heads to the filtration process, the actual method by which sewage gets purified in three steps.

Step one: The already treated sewage passes through a series of tiny fibers, which serve as microscopic filters. This pulls out bacteria and viruses.

Step two: Reverse osmosis. The sewage is forced through thin membranes with holes so small that water molecules are about the only things that get through. This is the same technology used to desalinate seawater. It stops just about everything.

Step three: Hydrogen peroxide gets added to the water, which then gets zapped with ultraviolet light. This eliminates two chemicals that can sneak through the membranes, and also disinfects the water.

Right now, most of San Diego’s wastewater ends up at the city’s major sewage treatment plant in Point Loma, near Cabrillo National Monument. It’s then discharged miles away into the Pacific Ocean.

The city has promised for years to build Pure Water in an attempt to avoid spending $2 billion to upgrade that treatment plant, while also providing a drought-proof source of water.

Megan Wood

Megan is Voice of San Diego’s director of marketing. She is responsible for producing and overseeing strategies that extend the reach of the organization....

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