- [Narrator] Moving water from areas of abundance to areas of scarcity was the kind of mega engineering that made the modern state of California possible.

Over 1500 dams and about 4,000 miles of canals had been rerouting water across the state for decades.

But the population had exploded, more than tripling since the 1950s.

With the system nearing a breaking point, one thing was clear.

- The old engineering solutions of the past just weren't gonna sustain us going forward.

- [Narrator] So they focused on a source flowing right under their noses, 200 million gallons of waste water literally going down the drain every day.

But how do you turn the county's dirty water into something people can actually drink?

It starts at the Orange County Sewage Treatment Plant.

Here, the largest solids are separated from the raw sewage and smaller particles are allowed to settle to the bottom of enormous tanks.

Air rating the remaining waste water provides oxygen to pollutant eating bacteria, which process the water to be clean enough to release into the environment.

For decades, that meant all of the treated water was pumped several miles out into the Pacific Ocean.

- Literally tens of millions of gallons a day just going out to the ocean being lost, essentially.

- [Narrator] Could those millions of gallons be the solution to Orange County's dwindling aquifer and also make their system essentially drought proof?

- We knew that people had done water recycling.

They just hadn't done it to a point where that water could be really efficiently reintroduced into a groundwater basin, but we knew it was possible.

- [Narrator] Today, Mehul's plant finishes the job.

It's a computer operated system of airtight tubes and underground pipes.

At the heart of the operation is a process called reverse osmosis, where 1000 horsepower engines push the water through tightly wrapped membranes, filtering out impurities like pharmaceuticals, viruses, and salts.

- So this is the end of the process.

We can see how the water started out.

You can see everything that was removed and then you can see the purified water here that's safe enough to drink but we'll still go through a couple more steps after this.