Orange County tap water is "as clean as water can be," the activist Seth Siegel writes in his new book, "Troubled Water."

Things weren't always that way: For decades, seawater was seeping into the community's freshwater supply, threatening to expose residents to excess sodium in their taps.

But in 2008, the county revealed a new system that filtered sewage water through microscopic holes and disinfected it with ultraviolet light to zap contaminants.

Siegel said the system could be replicated all across the US, even in low-income communities.

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Whenever I visit my hometown of Orange County, California, I get to sip some of the purest drinking water in the US.

The quality is sometimes hard to spot, since many drinking-water contaminants are odorless, tasteless, and invisible to the human eye. Even in cities where the water is contaminated with lead, residents have reported that their taps are crystal clear.

But in Orange County, the water is actually as clean as it looks.

It wasn't always that way. In his new book, "Troubled Water," the activist Seth Siegel explains how Orange County's taps went from having too much saltwater to spouting the purest drinking water in the US.

Saltwater was seeping into Orange County's freshwater supply

Orange County is just 35 miles from Los Angeles, but it relies on a different water system to serve its nearly 3.2 million residents. About a decade ago, that system begin churning out the most pristine water the country had ever seen.

From about the 1930s to the 1970s, farmers overpumped water through Orange County's underground aquifers, the bodies of porous rock that act as a natural filtration system. The process allowed seawater to seep into the county's freshwater supply — something known as saltwater intrusion — and threatened to expose residents to excess sodium in their taps.

Though scientists are still studying the health effects of too much sodium in drinking water, early research suggests it could lead to hypertension and chronic kidney disease.

Orange County prevented this scenario by getting people to drink recycled water instead.

Now, Orange County tap water starts out as sewage

The Groundwater Replenishment System in Fountain Valley, California, converts Orange County's sewage water into drinking water. Mary Knox Merrill/The Christian Science Monitor/Getty Images

In 2008 the county unveiled a Groundwater Replenishment System, which purifies wastewater from the local sewage system and turns it into clean drinking water.

Many cities have struggled to implement such a system because of pushback from local residents who aren't keen on drinking water that originated in their toilets. But more than 4 million Americans — including residents of Dallas, Phoenix, and Atlanta — now get at least some of their drinking water from treated sewage.

Read more: 11 cities with the worst tap water in the US

But Orange County's process is unique because it filters for inorganic contaminants — things like pesticides and industrial chemicals that are hard to detect in water and may still be allowed under federal law.

The US Environmental Protection Agency has drinking-water regulations for more than 90 contaminants, but Siegel said more than 100,000 chemicals and pharmaceutical compounds escaped regulation.

"What makes Orange County so special is they say: 'OK, fine, the federal rules are X. We don't really care. We're going to go so far beyond those rules that we're going to make the purest water flow we can possibly have,'" he told Business Insider.

Water gets filtered through invisible holes and zapped with UV light

Orange County's filtration process begins like most "toilet to tap" systems in the US. Household sewage arrives at local wastewater treatment facilities, where it's filtered by screens. Then friendly bacteria are added to get rid of lingering organic material (i.e., human waste).

Most communities allow this treated water to be discharged into public waterways, but Orange County's process doesn't stop there.

Next, the water heads to the Groundwater Replenishment System, where it passes through another set of filters with holes so tiny that they're invisible to the human eye. Mike Wehner, the assistant general manager at the Orange County Water District, told Siegel the holes were one one-hundred fiftieth the width of a human hair.

Orange County's underground filtration system removes particles, bacteria, and viruses from pretreated sewage water and pumps them through stainless steel pipes. Mary Knox Merrill/The Christian Science Monitor/Getty Images

From there, the water goes through reverse osmosis, a process that extracts salt, minerals, chemicals, and pharmaceutical compounds.

The water that emerges is free of minerals, so it's slightly acidic, which means it can corrode local pipes. So the county adds crushed limestone back into the water supply to neutralize the pH. From there, it disinfects the water by zapping it with ultraviolet light. This is meant to ensure that not a single molecule of waste can survive.

"It's not fair to say that a contaminant could never possibly be in Orange County's water," Siegel said. But the community's taps, he added, are "as pure as pure can be."

The process could be replicated all over the country

Orange County's "toilet to tap" system was expensive — about $480 million to get off the ground. But Siegel argues in his book that almost any city can replicate the process for less.

Water from the Groundwater Replenishment System in Fountain Valley. Mary Knox Merrill/The Christian Science Monitor/Getty Images

In many poor communities, he said, water fees aren't actually used to improve the local water system by investing in water infrastructure and technology. Most of these fees, he said, go toward the municipal budget.

"Flint actually had the highest water fees in the United States when the crisis broke," Siegel said. "What they did wrong was they diverted money from water fees to the general budget."

Based on his conversation with Wehner, Siegel estimates that having water as pure as Orange County's would cost communities an extra $33 a person a year. (That's after repaying any loans used to build the system and not including state and federal subsidies.)

As filtration technologies become more advanced, he said, that cost could drop lower.

"Now that Orange County has led the way and spent fortunes of money to figure it out, everybody can adopt more or less the Orange County system at not a phenomenal expense," Siegel said. "Why isn't everyone doing it? The answer is: because nobody's pushing them to."