Scientific Reports: How high can toilet water be flushed?

On the left, nothing is visible
to the naked eye.
On the right, a powerful green laser helps people see the aerosols
that spray when the toilet flushes.

Engineers at the University of Colorado Boulder confirmed what those of us with bacteriophobia have long suspected: Commercial toilets flush and release tiny clouds of droplets and aerosol particles from Vesuvius eruptions that are taller than 5 feet (more than 1.
5 meters)
above the toilet seat.

Although invisible to the naked eye, when illuminated by a green laser, these plumes look like a tiny bunch of confetti thrown at the world's most disgusting party, made up of tiny droplets of
water and anything in a bowl.

The study, published in the journal Scientific Reports in December, is strictly an exploration
in the field of fluid mechanics.
The team only flushed toilets containing clean water and did not investigate the infectivity
of any particles that might be present in the plume.

But their tools confirm that each flush goes far further than most of us would like to believe
.

"We were all surprised," said
John Crimaldi, the study's lead author.
And I said, "Oh my God, that's what happened?"

Crimaldi is a professor of hydrology who specializes in fluid mechanics—specifically, how air and water transport other substances
that flow with it.
He studied how ocean currents distribute sperm and eggs, fertilize corals, and how odor particles travel through the air, sending messages
to animals.

At the urging of his colleague and co-author Karl Linden, he turned his attention to the toilet
.
Linden is an environmental engineer studying the disinfection properties
of ultraviolet light.

While brainstorming the ideal test case for UV-based surface disinfectants, Linden's mind first had to go somewhere dirty
.

"Where do we come into contact with the virus, where do we come into contact with pathogens?" Linden said
.
One of my thoughts was, "So, what's going on in the toilet?"

Linden specifically depicts commercial toilets: tankless, durable toilets
without lids found in public toilet cubicles.
Most public toilets in North America are equipped with flush valves, which rely on pressure rather than gravity to force water through the bowl
.

The result is a high-powered flush that leaves a tiny cloud
of water vapor in its wake.

Previous studies have confirmed that surfaces around public toilets are often breeding grounds
for fecal bacteria.
Linden thinks UV may be an effective disinfectant — but first, he needs to better understand how microscopic pathogens
move through space.

He turned to Crimaldi's lab to use lasers to visualize fluid motion that would otherwise be imperceptible
to the human eye.
Crimaldi's fluid mechanics lab has a tradition every summer of devoting a week to solving a small scientific challenge, with no funding and no pressure
to publish.
The question of toilets
could not be more appropriate.

"We said, 'Maybe there's nothing, maybe we're going to get something really cool
,'" Crimaldi said.
”

Instead of moving their device to the nearest bathroom, the team installed a usable toilet in the lab on a metal frame that aligns
with their laser.
They then calibrated the pipe's water pressure to match
the water pressure of a typical commercial toilet.

They know that their laser will make some aerosols visible
.
But they weren't ready for their first little bang
.

"It's like a volcanic eruption," Crimaldi said
.
Some of us were stunned and silent
.
Some of us just laughed in disbelief and were kind of, 'Oh my God, we've
really been sprayed here.
'" ’”

The team then measured the velocity
of its individual water particles with pulsed lasers and a pair of scientific cameras against the water mist.
The speed of flushing the toilet is not as fast as sneezing, and sneezing droplets can spray to 100 miles per hour, or even without coughing, coughing droplets can reach 50 miles per hour
.

The study's authors report that in plumes of "surprising energy and chaos," the aerosol reached a maximum velocity of 2 meters per second, or less than 4.
5 miles
per hour.
However, once liftoff, it takes a while for them to land
.
Nearly 8 seconds after flushing, the pellet was still hovering more than 5 feet (1.
54 meters) above the edge — for most people, that's well beyond the height of
the nose.
Many stayed in the air for more than a minute
.

"After watching these videos, of course I am more inclined to wear a mask
in public toilets than before," Crimaldi said.

Although these experiments were conducted in toilets, there was nothing in the toilets except clean water, and Crimaldi strongly suspected that mixing toilet paper with human feces would only inject more chaos and energy
into the flush.

"I have an intuitive feeling that the presence of solids can exacerbate the problem because the water hits more things, creating more opportunities
for high-energy mixing of liquids," he said.

Linden hopes to use this experiment as a starting point for future research to track how far bacteria and other pathogens travel through these aerosol clouds, and how long
they remain infectious.
Based on these findings, he said, "we can start thinking, what interventions can we use?" What redesigns might we consider for toilets?"

Proponents of flush toilet alternatives say the study strengthens the argument
for seeking better ways to dispose of human feces.

Brin Nelson, a microbiologist and author of Flushing: The Extraordinary Science of an Impossible Treasure, said: "This new study provides some dramatic visual evidence
for another drawback of traditional Western toilets and our strong desire to forget when we flush.
Many composting toilets use vacuum flushing with little to no water, so this may be another reason
to consider the merits of these eco-friendly models.
”