What recycled sewage water reveals about human psychology

The technology now exists to recycle sewage water safely, but would you drink it?

By guest blogger Sam McNerney

Each year around one million people die from water-related diseases. In most cases, the causes are painfully obvious. Without access to a modern sewage system, people dump their bodily waste into the nearest river or street, which funnels their filthy excrement and urine back into the water supply. It’s a catastrophic problem without a cheap solution.

Until now. A few years ago Bill Gates teamed up with an engineer named Peter Janicki to create an ingenious machine that uses the same ingredient that taints water supplies—human waste—to clean them. The “Janicki Omniprocessor”, which looks something like a miniature power plant, can turn waste from 100,000 people into 86,000 liters of clean water a day while generating enough electricity to power itself. “The water tasted as good as any I’ve had out of a bottle,” Gates wrote on his blog.

The Janicki Omniprocessor is a major technological breakthrough, but a psychological barrier remains. We know recycled water is clean, we trust the science, and it’s exciting to think about how many lives it will save. And yet, that nasty “Yuk!” feeling persists.

Part of our deeply rooted aversion for tainted water has clear evolutionary origins. Like many animals, we instinctually avoid food and liquid that has touched nasty substances for health reasons. Think of this instinct as Paleolithic germ theory. It wasn’t exactly modern epidemiology, but for the most part, it worked.

Unfortunately, our instincts occasionally play tricks on our judgment. In a somewhat grotesque but captivating area of study, researchers have shown that people refuse to drink orange juice from unused urine collection bottles, eat soup served in a brand-new bedpan, or touch delicious fudge baked in the shape of dog feces. It’s as if there’s some sort of immaterial essence that tarnishes these perfectly edible items.

This brings me to a new survey of over 2,600 Americans conducted by disgust guru Paul Rozin and his colleagues. Participants first read a short passage explaining how recycled water is certified safe and indicated their willingness to drink it. Next, they scored how comfortable they were drinking different types of water, from commercial bottled water, to tap water, to sewage water that had been boiled, evaporated, and condensed into pure water. Finally, they rated how comfortable they felt drinking recycled water if it had spent a certain amount of time in a reservoir or aquifer before it was fed back into the water supply. The purpose of this question was to see if the contagion heuristic—“Once in contact, always in contact”—wears off over time.

Although 13 percent of the sample indicated that they would never drink recycled water, almost half said that they would, while 38 percent remained uncertain. Disgust sensitivity, as measured by a short disgust test, correlated inversely with a willingness to drink recycled water. The more easily someone is grossed out, the less likely he or she will sip water from the Janicki Omniprocessor.

The next finding revealed something important about how the human mind perceives purity. Even though sewage water that is boiled, evaporated, and condensed is purer than tap water, participants overwhelmingly preferred tap water. Furthermore, compared to tap water, participants were more willing to drink bottled water that was filtered from tap water, even though the two are equally pure. It’s like running your clothes through the wash twice—the second wash doesn’t make anything cleaner; it just makes you feel a little bit better.

The scientists also found that participants were more likely to drink recycled water the longer it remained in a reservoir or aquifer, even though feeding recycled water back into a natural system actually decreases its purity. Rozin attributes this quirk in perception to the idea of “spiritual purification.” Just like we’re more willing to wear Hitler’s sweater if it supposedly came in contact with Mother Theresa, we’re more likely to drink recycled water if it was reintroduced into “natural systems.” (Distance also mattered. The further the water travelled, the cleaner it was perceived by the subjects.)

Rozin and his team didn’t stop there. To tease out some of these initial findings, they created a second survey for about 400 undergraduates at the University of Pennsylvania. This time, participants read about four contaminants—a harmless amount of sodium cyanide, a drop containing active HIV (AIDS) virus, a heat sterilised cockroach, and a convicted murderer—then imagined drinking water that had recently come in contact with each contaminant. Not surprisingly, the undergrads vehemently rejected the idea of drinking any of this water. The cyanide provoked the least discomfort, while the HIV virus provoked the most.

So far, so obvious: when deciding to drink water or not, the most important variable to consider is its perceived purity. The most intriguing part of the paper came when Rozin asked the undergrads if they would drink clean water from the same glass, if the contaminated water was poured out and the glass thoroughly sterilised. It’s worth pausing to think about how you would answer this question. Would you drink from a glass if its previous user were a convicted murderer—even if the glass was completely clean?

Rozin found that the undergrads’ behaviour depended on a few key variables. Similar to the first study, those who were more willing to drink recycled water, and who scored low on the disgust scale, were more likely to drink from a recently contaminated glass. On the other hand, the undergrads who tended to distrust standard purification techniques to clean contaminated water were more likely to refuse. “This is particularly striking,” Rozin writes, “because these same people readily consume tap or bottled spring water, which usually have the very same contamination history as the water they reject as contaminated. We can describe these individuals as responding to what we call spiritual contamination.” For these people, even the most thorough sterilisation processes cannot remove the perceived impurities.

And that’s why I find this topic so interesting. Humans pay special attention to the history of objects—where they have been, what they have touched, and who has touched them—because we subscribe to the notion that objects have an underlying reality, an essence. Normally, this piece of mental software is helpful, but sometimes it can lead us astray.  The Janicki Omniprocessor represents a major breakthrough for producing clean water and improving health in the developing world at a low cost. But before we can put it to use, we will have to overcome a bigger obstacle: ourselves.


Paul Rozin, Brent Haddad, Carol Nemeroff, & Paul Slovic (2015). Psychological aspects of the rejection of recycled water: Contamination, purification and disgust Judgment and Decision Making

further reading
Yuk! Unfairness really does leave a bad taste in the mouth

Post written by Sam McNerney (@sammcnerney) for the BPS Research Digest. McNerney is a US writer with a focus on cognitive psychology, philosophy and business. He’s written for Scientific American, Scientific American Mind, Fortune, Fast Company, TechCrunch and BBC Focus and maintained a blog on BigThink.com called Moments of Genius. He currently blogs at his website: sammcnerney.com.