The residents of the 40 floors of San Francisco apartments above our heads may live in luxury, but really, they’re just like the rest of us: showering, washing their hands, doing laundry. Normally in the US, all their water would flush out to a treatment facility, and eventually out to a body of water; 34 billion gallons of wastewater is processed this way across the country every day. But with multiple problems for cities now converging—extreme heat, water shortages, and rapid population growth—increasingly scientists are finding clever ways to extract more use from water that’s flushed away.

In this basement, a company called Epic Cleantec intercepts the building’s gray water (dirty water that doesn’t contain human waste or food scraps) and passes it through tanks and a maze of pipes for fine filtration and disinfection with chlorine and UV light. The resulting liquid is then piped back upstairs to fill toilets and urinals, taking at least some of the “waste” out of wastewater.

“By regulation, we’re only reusing the water for nonpotable applications,” says Aaron Tartakovsky, cofounder and CEO of Epic Cleantec. “Scientifically, we can produce drinking-water quality.” Indeed, the company brewed a beer with its recycled water from this building. (A kölsch, if you were curious.) “We’re turning wastewater—which in my opinion, is a term that is in dire need of a rebrand—into clean water, into renewable energy, and into soil,” says Tartakovsky.

Theoretically, the used water that flows out of your home contains 10 times the amount of energy it takes for a treatment facility to process it. It’s also rich in valuable nutrients and minerals, says Peter Grevatt, CEO of the Water Research Foundation, a US nonprofit supported by water utilities. And so as well as recycling water, Epic Cleantec is experimenting with heat exchangers that can extract energy from a building’s wastewater and use it to warm up the water going back upstairs, thus reducing utility bills. The company is also developing a system that processes residents’ black water—which includes human waste and food organics from kitchen sinks and dishwashers—into a soil amendment.

Across the street from this apartment building, in Epic Cleantec’s offices, Tartakovsky grabs a handful of the stuff, which has been treated to remove pathogens. “You can touch it, smell it, whatever you’re comfortable with,” Tartakovsky says. (I do both—it feels and smells like compost.)

This sort of water reuse is happening increasingly at a municipal level, too, with state-of-the-art facilities recycling water instead of releasing it all into nature. What Epic Cleantec has achieved is to essentially shrink down what a water recycling plant does into a system that fits in a high-rise basement, lightening the burden on municipal wastewater treatment and reducing pressure on water supplies.

Turning on the Tap

Whatever the system, water recycling will to need to ramp up massively in the coming decades. Today 56 percent of humanity lives in cities, but that’ll jump to 70 percent by 2050. Cities suck up a whole lot of water, especially as their populations get richer (and therefore more wasteful) and urban industrial activity increases. All the while, climate change is drying out many of the places people are flocking to, like the southwestern US. “You look to the places that are already experiencing the greatest level of water stress, many of those places are the places that are most rapidly growing,” says Grevatt. “Needing to figure out how we recover resources is incredibly important.”

A recent study laid out the surprising dynamics of how this urban growth will unfold. Greenhouse gases scale sublinearly as a city grows, meaning at a slower rate than the population increases, due in part to efficiencies around things like public transportation. Solid waste, which ends up in a landfill, scales linearly, meaning it increases in lockstep with changes to the human population. Wastewater, though, scales superlinearly, so it grows at a faster rate than a growing population.

Put another way: The bigger a city gets, the more wasteful it gets with water, even though its energy usage becomes more efficient. “We had a hard time figuring out why,” says New York University industrial ecologist Mingzhen Lu, coauthor of the study. “The best explanation we came up with is there could be a strong link with wealth creation, which in itself is superlinear with city size. Any dollar we generate as a human society, we consume water. On the other side, when you have more wealth, there might be an argument that you will use more water more lavishly.”

As urban water use grows, the risk is continuing with wastewater treatment as usual: pumping the stuff into the environment. “I think that will be one of the things future societies think is most crazy about the last few hundred years, is that we just dumped wastewater into the ocean rather than pumping it back into the farmland,” says Santa Fe Institute theoretical physical biologist Chris Kempes, coauthor of the paper.

The technology to extract fresh water from wastewater has existed for decades. In San Diego, which has been recycling water since 1981, two water reclamation plants together produce 21 million gallons of water every day (on a yearly average), with more capacity being added in the coming years. Technically, that water isn’t considered potable, so it’s used for agriculture and industry. But in 2026, San Diego will start delivering drinking water, thanks to even more advanced purification techniques: Wastewater is hit with ozone, killing bacteria and viruses, then passes through filters and then through ultrafine membranes with pores so small, basically only water molecules can get through. They’ll eventually ramp up to produce 30 million gallons of water daily, aiming to provide half the city’s drinking water this way by 2035.

While this process is expensive—it costs a lot to build out the facilities to process the water and takes a lot of energy to shove liquid through such fine membranes—the technology is maturing and costs are falling. “What’s really wild is we’ve had visitors from other agencies and areas that are water-rich,” says Juan Guerreiro, San Diego’s director of public utilities. “You wouldn’t think they’d want to push towards these projects. But what they’re realizing is that recycling the water that we already have contained within our wastewater systems, from an environmental stewardship perspective, is really beneficial.” Recycling can help reduce demand for river water, for example, thus protecting the fish species there.

Dirty Work

The trickier half of wastewater recycling is the solid human waste that facilities accumulate as biosolids, or sludge. In the US, 56 percent of sludge produced is applied to the land, 27 percent is dumped in landfills, and 16 percent is incinerated. In addition to all the carbon from the food we eat, sludge is infused with chemicals that we (and industries) flush down the drain.

In 2022, Maine banned the use of sludge as fertilizer due to contamination with PFAS, a group of chemicals linked to cancers and hormonal problems. Sludge is also notoriously loaded with microplastics: When we do a load of laundry, millions of synthetic fibers break off and wash into a wastewater facility. Sludge applied to fields turns out to be a major source of microplastics corrupting the environment.

The industry is researching ways to isolate these contaminants, Grevatt says, both so it can keep them from the environment and to safely unlock the potential of our waste carbon and nutrients. “It’s extraordinarily challenging,” says Grevatt. “Wastewater treatment operations are not the producers, but they are recipients of PFAS from all kinds of different sources.”

An alternative option to sludge is biochar. If you heat that organic matter in a special chamber, a process known as pyrolysis, it turns to concentrated carbon. Startups have been doing this with agricultural waste, like corn stalks, to create charcoal and oil that they bury underground. (As those plants grew, they sequestered carbon, so in this case you’d actually be removing carbon from the atmosphere by putting it back in the earth.) Farmers are also sprinkling biochar on their fields, which can improve crop yields and add carbon to soils.

Researchers are experimenting with using the same technique for wastewater solids, basically turning sludge into a solid product. “If you do pyrolysis—because it’s thermochemical, it’s a heated process—you kill these bacteria, kill these pathogens, kill these viruses. It’s much cleaner,” says engineer Fengqi You, who studies wastewater at Cornell University. In addition, sludge is a heavy, unwieldy liquid to ship from facility to farm. “You transport a lot of water in that, and the density is low. But biochar, it’s light—you can put it in bags—making transport easier.” So producers could ship it off more easily to faraway farms, but also distribute it more locally, to urban farms closer to the source of wastewater.

A wastewater facility can also create fuel in oxygen-free chambers, where microbes eat the solid waste and release methane “biogas” as a byproduct. “This biogas can be burned to generate heat,” says You. In Ithaca, New York, that can fully power a wastewater facility itself, but You has also been experimenting with using biogas to heat nearby buildings, including a medical center. Heating a building with natural gas adds carbon emissions to the atmosphere, but as biogas comes from the crops we eat and poop into the sewer system, which grew by drawing down carbon from the atmosphere, so burning it forms a carbon loop.

Before those microbes create biogas, they also generate volatile fatty acids. These could be made into jet fuel, or maybe even a fuel for fleets of city vehicles, says environmental engineer Sybil Sharvelle, who studies wastewater at Colorado State University. “There’s a lot of value in all sorts of those volatile fatty acids,” says Sharvelle.

In addition to using the waste solids as compost, like Epic Cleantec is experimenting with, Sharvelle notes that urban farms could benefit from using recycled wastewater that’s been disinfected for use on crops, but with the nitrogen and phosphorus left in. Those are essential nutrients for plants, but are actually difficult to remove from water. “If you can leave nitrogen and phosphorus in the system, that’s a much more energy-efficient way to just make use of those nutrients directly,” says Sharvelle.

All told, the linear path of water—from source to city to sea—is starting to curve. The future of wastewater is circular, recycling back into drinking water, compost for urban farms, and energy. Far from being unnatural, drinking repurposed toilet water is the kind of resourcefulness that nature intended. “Recycling is ubiquitous in nature,” says Kempes. “If there’s an untapped source of energy or nutrients, someone finds a way to use it. If you can create a fertilizer, find a way to clean water, and produce heat and electricity at the same time, that mirrors what we’ve seen biology evolve to do over billions of years.”