SAN DIEGO -- The city of San Diego is in the midst of a one-year study to see if it can treat and purify sewage into potable, drinkable water.

Opponents to the concept called it "toilet to tap" in the past. However, the process has been used successfully in Orange County since 2008.

At its groundwater replenishment system, the Orange County Water District cleans and purifies 70 million gallons of drinking water, which is enough water for about 550,000 people.

It takes about three days to turn raw sewage – which is more than 90 percent water – into clean water.

10News received a guided tour of the system from general manager Mike Markus, who carefully described the process.

After the Orange County Sanitation District treats the water, the water goes through three different filtration processes at the groundwater replenishment system.

The first is a microfiltration process, followed by reverse osmosis. Finally, the water is hit with an ultraviolet irradiation system. The result is perfect water.

"We stripped out all the pharmaceuticals [and] viruses in the water," said Markus. "There's nothing left. It's the highest quality water that we have in the region."

The Orange County Water District regularly tests the water for about 400 chemicals and compounds.

"When we test for these compounds, we cannot detect them," said Markus.

Visitors to the groundwater replenishment system can sample the water directly out of the plant.

10News reporter Joe Little tested the water and said, matter-of-factly, "It's water."

Markus said, "Anyone in northern or central Orange County ultimately will be drinking this water and that area encompasses about 2.4 million people."

The recycled water is also cheaper than water imported through the Metropolitan Water District. It costs Orange County about $800 per acre foot to buy water imported from the Sacramento Bay Delta and the Colorado River, while the recycled water only costs about $480 per acre foot.

The city of San Diego is studying the same process Orange County is using to take advantage of those savings.

"It's a huge additional amount that we would have locally controlled that we wouldn't have to buy from [the Metropolitan Water District] who sticks it to us every time they get a chance," said San Diego Mayor Jerry Sanders.

The process has proved so effective in Orange County that plans have already been approved to expand the system to recycle 100 million gallons of water per day by 2014.