CU scientists shed light on what happens when you flush | CU Boulder today

CU scientists shed light on what happens when you flush |  CU Boulder today
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Poster image: A powerful green laser helps visualize aerosol clouds coming out of the toilet as it is flushed. (Credit: Patrick Campbell/CU Boulder)

Thanks to new CU Boulder research, scientists see the effect of toilet flushing in a whole new light, and now the world can too.

Using bright green lasers and camera equipment, a team of CU Boulder engineers conducted an experiment to reveal how tiny water droplets invisible to the naked eye are rapidly sprayed into the air when a lidless public toilet is flushed. Published now Scientific ReportsIt is the first study to directly visualize the resulting aerosol cloud and measure the velocity and diffusion of particles within it.

These aerosolized particles are known to carry pathogens and may pose an exposure risk for public restroom users. However, this vivid visualization of disease exposure also provides a methodology to help reduce disease.

“If it’s something you can’t see, it’s easy to pretend it doesn’t exist. But once you see these videos, you’ll never think about flushing the toilet the same way again,” he said. John Crimaldi, lead author of the study and professor of civil, environmental and architectural engineering. “By creating dramatic visual images of this process, our work can play an important role in public health messages.”

Researchers have known for over 60 years that when the toilet is flushed, solids and liquids go down as designed, but tiny, invisible particles are also released into the air. Previous research has used scientific tools to detect the presence of these airborne particles above flush toilets and have shown that larger particles can land on surrounding surfaces, but until now no one understood what these plumes looked like or how the particles got there.

Understanding the trajectories and velocities of these particles, which can carry pathogens such as E. coli, C. difficile, noroviruses and adenoviruses, is important to reduce the risk of exposure through disinfection and ventilation strategies or improved toilet and flush design. Although the virus that causes COVID-19 (SARS-CoV-2) is found in human faeces, there is currently no conclusive evidence that it spreads efficiently via toilet aerosols.

“People know that toilets emit aerosols, but they couldn’t see them,” Crimaldi said. “We show that this thing is a fluff that spreads much more energetically and rapidly than even people who know it realize.”

The study found that these airborne particles ejected at 6.6 feet (2 meters) per second, reaching 4.9 feet (1.5 meters) above the toilet in 8 seconds. The largest droplets tend to settle on surfaces within seconds, while smaller particles (aerosols smaller than 5 microns or a millionth of a meter) can remain suspended in the air for minutes or longer.

It’s not just their own waste that bathroom customers have to worry about. Many other studies have shown that pathogens can siphon dozens of times in the bowl, increasing the potential risk of exposure.

“The purpose of the toilet is to effectively remove waste from the toilet bowl, but it also does the opposite, which sprays most of the contents upward,” said Crimaldi. “Our lab has established a methodology that provides a foundation for improving and mitigating this problem.”

Aaron True, Postdoctoral Fellow (left) and John Crimaldi
A powerful green laser helps visualize aerosol clouds coming out of the toilet

Above: Aaron True, Postdoctoral Fellow (left) and John Crimaldi posing for a photo with equipment. Below: A powerful green laser helps visualize aerosol fumes as the toilet is flushed. (Credit: Patrick Campbell/CU Boulder)

not a waste of time

Crimaldi is leading Ecological Fluid Dynamics Laboratory at CU Boulder, which specializes in using laser-based instrumentation, dyes, and giant liquid tanks to examine everything. How do odors reach our nostrils? how chemicals move through turbulent bodies of water. The idea of ​​using the lab’s technology to track what happens in the air after a toilet has been flushed was a matter of convenience, curiosity, and coincidence.

During a free week last June, my fellow professors karl linden and Mark Hernandez Several graduate students from the Environmental Engineering Program and Crimaldi’s lab joined him to set up and run the experiment. Aaron True, second author of the study and a research associate in Crimaldi’s lab, was instrumental in executing and recording the laser-based measurements for the study.

They used two lasers: one continuously shined on and over the toilet, while the other sent rapid pulses of light over the same area. The stationary laser can reveal where airborne particles are in space, while the pulsed laser can measure their velocities and directions. Meanwhile, two cameras took high-definition images.

The toilet itself was the type commonly seen in public restrooms in North America: a manual or automatic, manual or automatic, cylindrical flush mechanism known as a siphonometer-style valve protruding close to the wall, accompanying a lidless unit. Brand new, clean toilet filled with tap water only.

They knew this instant experiment could be a waste of time, but instead, the research took a huge leap forward.

“We expected these aerosol particles to float through the air somehow, but they came off like a rocket,” Crimaldi said.

Energetic, airborne water particles mostly headed up and back against the back wall, but their movements were unpredictable. The smoke also rose to the ceiling of the laboratory and, having nowhere else to go, crept out of the wall and spread into the room.

In the experimental setup, there was no solid waste or toilet paper in the bowl, and no counters or people moving around. All of these real-life variables can exacerbate the problem, Crimaldi said.

They also measured particles in the air with an optical particle counter, a device that sucks a sample of air from a small tube and shines a light on it, allowing it to count and measure particles. Not only can the smaller particles float longer in the air, they can also escape through the nose hairs and reach deep inside the person’s lungs – making them more dangerous to human health – so it was also important to know how many particles and what size they were.

While these results are troubling, the study provides sanitary and public health professionals a consistent way to test disinfection and ventilation strategies with improved plumbing design to reduce the risk of exposure to pathogens in public restrooms.

“None of these improvements can be made effectively without knowing how the aerosol cloud develops and moves,” said Crimaldi. “Being able to see this invisible feather is game-changing.”

Other authors in this publication are Aaron True, Karl Linden, Mark Hernandez, Lars Larson, and Anna Pauls from the Department of Civil, Environmental and Architectural Engineering.

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