In 2015, Bill Gates tossed back a glass of drinking water extracted from pure sewage sludge, and the world gulped in horror. “The water tasted as good as any I’ve had out of a bottle,” the billionaire philanthropist blogged. Video of Gates’ courageous quaff drew a chorus of “eeewws” and potty-punning headlines — but few adherents or imitators.

Ten years on, the world is coming around. In late 2023, California water regulators approved rules allowing local water agencies to recycle wastewater directly into tap water.

California joins Colorado as the second state with finalized “pipe-to-pipe” water treatment regulations. Other states are moving in the same direction.

The world is running out of drinking water, and time is not on humanity’s side. Urbanization, overdrawn aquifers and a changing climate inject a sense of urgency into the quest for large-scale water recycling infrastructure deployed worldwide.

With water scarcity expected to reach critical levels in the near future, the world needs to get the scientific, policy and economic dynamics of water reuse sorted out quickly.

Los Angeles has been recycling water since 1982 for irrigation, but not for its drinking water supply. That’s about to change. Under a 2019 citywide initiative, the mayor’s office set the goal of recycling 100% of wastewater and sourcing 70% of water locally by 2035.

Currently, very little of the Southland’s water supply is locally sourced. “Ninety percent is imported from far-off places like the Sierra Nevada and the Colorado River,” said USC Viterbi environmental engineering professor Adam Smith. “And the vast majority of that water is not reused. It’s just discharged into the ocean, where it’s really a lost resource.”

That’s a pity, because “recycled” or “reclaimed” water is perfectly safe, as Bill Gates demonstrated. It’s actually cleaner than traditional water sources, cheaper than desalinated water and less energy intensive, experts agree.

“Historically, we’ve had the mentality that we need to get rid of wastewater,” said Amy Childress, USC Viterbi Dean’s Professor of Civil and Environmental Engineering. “Now we realize that the water within wastewater is a valuable resource.”

 

High-water mark

It’s an exciting time for water engineers like Childress and Smith, both of whom are based at the USC ReWater Center. A hub for USC Viterbi-based water research, the new center in Biegler Hall is flooding the zone with advanced technologies for water treatment, energy reduction, and the detection and removal of contaminants and pathogens.

Other key players at the ReWater Center are colleagues Dan McCurry and Adam Simpson, and documentary filmmaker Daniel Druhora, who spearheads the consortium’s outreach efforts.

Childress is a water-science rock star. Last fall, she was awarded the 2024 Clarke Prize, her field’s highest accolade, by the nonprofit National Water Research Institute. For nearly 30 years, she’s been designing, modeling and testing ways to squeeze value out of wastewater. She’s built complex pilot systems for a proposed NASA Mars mission, for forward operating bases in war zones and for a self-sufficient military installation in the Arizona desert. She’s worked closely with Singapore and Israel, both pioneers in water recycling at the nation-state level.

Now Childress is targeting megapolises like Los Angeles. At the end of the search may lie the holy grail: a menu of “potable water reuse” options wherein the inputs, outputs and byproducts of treatment can be used and reused in a near-perfect feedback loop.

USC is at the forefront of this research, and Childress is academic lead for the Water Reuse Consortium — a thriving partnership with the University of Nevada, Reno (UNR); the University of Arizona (UA); and the U.S. Army Engineer Research and Development Center (ERDC). The collaboration is funded by a major grant from ERDC, the R&D arm of the U.S. Army Corps of Engineers (USACE).

The consortium brings together leading researchers from multiple institutions, each contributing their expertise and operating independently while working toward shared goals.

Childress leads the USC ReWater Center. Launched in 2023.

UA is home to the Water and Energy Sustainable Technology (WEST) Center. Founded in 2016, WEST is co-led by Andrea Achilli, a UA environmental engineering professor.

Also backed by the ERDC grant is the newly created Nevada Center for Water Resiliency at UNR. That project is overseen by chemical and materials engineering professor Sage Hiibel and civil and environmental engineering professor Eric Marchand.

The Water Reuse Consortium’s goal: find ingenious, energy- and cost-efficient ways to recycle as much water as possible. That includes graywater from sinks, showers and bathtubs; blackwater from toilets and urinals; and industrial wastewaters.

Each partner has a somewhat different expertise: The USC Viterbi researchers specialize in next-generation treatment technologies, contaminant and pathogen detection and removal, and energy reduction. Innovations spilling out of the USC ReWater Center involve membrane biofilm reactors to improve biological nutrient removal, new models for integrating seawater desalination and wastewater treatment plants to maximize efficiency, and testing multiple disinfection strategies to optimize reuse streams.

Water scientists at UA are experimenting with decentralized and automated treatment systems for use at the military outpost, small community and consumer levels; and UNR researchers are building a large-scale test bed capable of running high-volume water treatment pilot systems that can then be deployed with confidence at an industrial level.

“It’s a vast program, and I’m really pleased with the work all partners are doing,” said ERDC researcher and government project lead Dawn Morrison, who oversees the Water Reuse Consortium cooperative agreement and grant funding streams.

Though they can’t be measured in gallons per minute, “deliverables” are gushing out of the consortium at high flow rates. Less than two years into the grant, Morrison of ERDC counts 34 major projects underway.

 

Potable reuse builds steam

Nevada and Arizona face somewhat different water challenges from coastal California. The Colorado River can’t supply their growing needs, and ocean desalination isn’t a realistic option for these inland states. A far-fetched scenario involves piping purified seawater from the Sea of Cortez. Nor is it a safe bet to rely on captured stormwater or snowmelt. In the high desert, normal weather change can bring drought or sudden deluge.

“Water is not a fixed quantity,” UNR’s Marchand said. “It’s a boom-or-bust-type situation. In 1997, we had 20,000 cubic-feet-per-second flows going through the middle of downtown Reno after a warm weather pattern suddenly melted winter snow in the Sierra Nevada range.”

In Arizona, agriculture consumes 72% of the water supply. With enough water, even the desert can become bountiful farmland, “because when you have sun, you can grow crops very efficiently and very fast,” UA’s Achilli explained. “And that’s been exploited in many parts of Arizona.”

Historic lack of regulation has left aquifers severely depleted, making smart water policy and creative reuse solutions urgent priorities in Arizona. Outside big cities like Tucson and Phoenix, landowners with drill rights can siphon massive amounts of groundwater.

This has led to the absurdity of Saudi Arabian agribusinesses growing hay and alfalfa in the Butler Valley to feed dairy cows in the Gulf Kingdom. Meanwhile the Yuma area is known as America’s “Winter Salad Bowl,” producing 80% to 90% of the nation’s leafy vegetables in the wintertime, according to Achilli.

Nevada’s and Arizona’s water woes are exacerbated by rapid urbanization and growing economies. Both states are major producers of mineral resources essential to the tech industry: Arizona is the nation’s leading copper producer, and Nevada holds an estimated 25% of the world’s supply of lithium.

 

What’s in it for the Army?

From the ERDC’s perspective, an important goal of the grant is to move the nation’s largest service branch toward smarter water-reuse practices.

Morrison, the ERDC project lead, heads up her own water research group in Champaign, Illinois, at the Construction Engineering Research Laboratory (CERL), part of ERDC.

As a first step, Morrison’s multidisciplinary team of environmental engineers, materials scientists, urban planners, biologists, geographic information system analysts and social scientists surveyed and inventoried all potable reuse systems operating across the Army — an institution comprised of nearly 1 million uniformed personnel and more than 250,000 civilians.

“Before, if you were to ask anyone at any level, ‘How much water reuse is actually going on?’ — nobody knew the answer,” Morrison said.

Now they know: The global Army enterprise, she reports, currently reuses 4.51 million gallons of effluent water a day. Morrison’s group at CERL, however, has identified more than 30.4 million gallons per day that could be reused — nearly a sevenfold potential increase. Her team is now focused on boosting that ratio by targeting specific units poised to take advantage of water reuse technologies.

Year three of the USACE grant was recently reauthorized by Congress, and Morrison is already thinking about spin-offs to drive continued cross-collaboration among consortium partners once funding winds down in 2027.

 

Family reunion

Childress has deep ties to the consortium partners. She mentored many of them at UNR, where she was on faculty for 16 years before joining USC Viterbi in 2013.

She’s married to an environmental engineer. Husband Brian Dela Barre is a manager with the U.S. Army Corps of Engineers, specializing in dam and levee safety. Both of their daughters are undergraduates at USC. Sabrina is a freshman in biomedical engineering, and Jillian is a junior in political science.

Longtime colleagues, mentors and mentees, students past and present, family and friends came out to cheer Childress as she accepted the 2024 Clarke Prize at an award ceremony in Irvine last October. She acknowledged them all in her talk summarizing three decades of research.

Her slide deck highlighted the opening scene from “Waterworld,” the 1995 blockbuster that’s set in an apocalyptic future resembling Noah’s flood. The hero, played by Kevin Costner, relieves himself in a container, hand-cranks his wastewater through the sailboat’s primitive purification contraption, and casually swigs the newly recycled water. “My takeaway from this,” Childress quipped (see video at 32-minute mark), “is that Kevin decided it was more efficient to do potable reuse than seawater desalination.”

It’s too soon to think about a one-size-fits-all solution. Many new systems will come online in the next few years. Waves of technical and regulatory innovation are percolating up across the drought-stricken Southwest. Centralized and decentralized. Membrane and non-membrane filtration. The only constant will be the imperative to recycle as much water as humanly possible.

 

The myth of ‘fresh water’

Hesitation around recycled water is rooted in the mistaken belief that there is such a thing as fresh water.

“We have this wonderful picture of snow melting and coming into our pipes, like we’re drinking this magnificent glacier water, like it’s Evian,” said USC ReWater Center’s Simpson, an assistant professor of civil and environmental engineering at USC Viterbi. That’s a deeply flawed idea.

“Our water has always undergone some de facto reuse,” Childress explained. “Unless you live at the top of a mountain or by a spring, the water you drink has previously been used by a community upstream. They probably treated their wastewater and sent it back to a river.”

According to Simpson, recent research shows that recycled water is actually safer than raw water. An authority on food disinfection and toxicity, he investigates screening and treatment for micropollutants in water concentrates and potable reuse systems.

What sneering critics mistakenly disparage as toilet-to-tap actually involves a cascade of processing steps: from zapping wastewater with ozone to exposing it to pathogen-devouring bacteria, filtering it through activated carbon, pressing it repeatedly through reverse osmosis membranes, oxidizing it with a disinfectant like hydrogen peroxide and beaming it with high-intensity UV light. Any valuable minerals that get filtered out, such as calcium, can be restored. As a final step, recycled water undergoes all the standard treatments of any drinking water source.

Many in the industry want to jettison the very idea that there are different waters, opting instead for the concept of “one water.”

“Water is water,” said Kerri Hickenbottom, an AU associate professor of chemical and environmental engineering. “It’s not wastewater or drinking water or even seawater. It goes through different phases.”

Discarding the clean-dirty, new-used hierarchy will help “close the loop,” she believes, so that water is no longer seen as something that could be imported or dumped. According to the UN Environment Program, today only 11% of the world’s treated wastewater is reused.

Hickenbottom’s research looks for hybrid technologies that can be coupled for maximum efficiency. In her lab, a concentrated solar power trough integrated with photovoltaics produces both heat and electricity. Either power source can drive her desalination system.

People underestimate the inherent synergies in water and power that make hybrid systems so attractive. “It takes a lot of energy to make clean, potable water,” she said, “and it takes a lot of water to make energy.”

Indeed, all energy applications require water.

Hydroelectric facilities harness energy from the water itself. Coal-fired and gas-fired power plants use water for cooling. Geothermal power plants pull water from deep in the earth to spin turbines. Even nuclear power needs water to cool the reactor, and solar panels need to be washed regularly to stay clean.

Hickenbottom’s hybrid solar desalination system would work well in off-grid disaster relief camps and war zones, where water and power are precious resources. More than half of all casualties on forward operating bases, she notes, are related to hauling water and other supplies.

Meanwhile Achilli, her UA colleague, envisions a future where every home has a self-sufficient potable reuse system. The Italian-born researcher investigates how machine learning and artificial intelligence can take treatment technology to the consumer level: He wants the water recycler to be a household appliance, like the washing machine.

Purifying water is far more complicated than laundering clothes, however, and the stakes are much higher. Any mistake or malfunction could result in serious illness.

Achilli spells out the problem: “In our lab, we have advanced water-treatment processes that work extremely well. We also have postdocs and grad students attending to them and basically operating them 24/7.”

Here’s where machine learning and data science enter the picture. Achilli’s research team is partnering with system engineers to develop a mini recycling treatment plant for the consumer market using an unsupervised autonomous system. His multidisciplinary team includes process engineers, optical scientists, microbiologists with expertise in trace organics, and public health experts.

 

A brief history of water

Outbreaks of deadly cholera and typhoid were once commonplace in American cities. In 1908, the first experimental water treatment plants simultaneously sprang up in Columbus, Ohio, and in Jersey City, New Jersey, on the Hudson River across from Lower Manhattan. Waterborne disease fell precipitously because of new water purification systems; the technology spread like wildfire.

“By the mid-20th century, essentially every public tap water supply was disinfected,” said USC ReWater Center researcher McCurry, Shiao-Ping Siao Yen Early Career Chair in Civil and Environmental Engineering and associate professor of civil and environmental engineering. Those early treatment systems involved filtering, softening and injecting chlorine. The technology has come a long way since then.

“Penicillin, seat belts, better nutrition, modern medicine — these lifestyle interventions collectively add up to about half the life expectancy gains over the 20th century. The other half is just water treatment,” McCurry said.

With passage of the Clean Water Act in 1972, about 95 percent of the contaminants polluting America’s waterways were targeted for elimination. “Now the remaining 5 percent becomes the challenge,” UNR’s Marchand said. Removing microscopic elements—like nitrogen, which can cause “eutrophication,” leading to algae blooms and tainted drinking water, or “forever chemicals” like PFAS, associated with cancer—requires the most expensive and energy-intensive filtration. The challenge is knowing exactly when and how to treat each water source with maximum efficiency.

In the water treatment industry, “you never know what you’re going to get coming in the front door to a facility,” Marchand added. “But what you send out the back door into the environment or other applications needs to be compliant with EPA and state regulations.”

Water scientists know the margin of error in potable reuse has to be zero. Happily, after 50 years of global experience, that’s exactly where the record stands.

“Namibia has been doing potable reuse since the early ’70s,” said Smith of USC ReWater Center. “Singapore has been doing it since the early 2000s. There haven’t been any public health emergencies. As engineers, we’ve put so many safeguards in place that there’s no chance of one occurring.”

Which is why Bill Gates wasn’t really worried when he boldly took that sip of sewage water in 2015. “Having studied the engineering behind it,” he said, “I would happily drink it every day. It’s that safe.”