CU scientists explain what happens when you flow

Banner photo: A powerful green laser helps visualize the aerosol spraying from a toilet as it flushes. (Credit: Patrick Campbell/CU Boulder)

Thanks to new CU Boulder research, scientists are looking at the effects of flushing toilets in a whole new way — and now, the world can too.

Using a bright green laser and camera equipment, CU Boulder’s team of engineers conducted an experiment to reveal how tiny water droplets, invisible to the naked eye, are quickly ejected into the air when an uncovered public toilet is flushed. Now published in Scientific reportThis is the first study to directly visualize the resulting aerosol column and measure the velocity and diffusion of the particles within it.

These volatile particles are known to transmit pathogens and can pose a hazard to visitors to public baths. However, clear visualization of potential exposure to these diseases also provides a methodology to help reduce them.

“If it’s something you can’t see, it’s easy to pretend it doesn’t exist. But once you watch this video, you will never think of flushing the toilet the same way again.” John Krimaldi, lead author of the study and professor of civil, environmental, and architectural engineering. “By creating compelling visuals of this process, our research can play an important role in public health messaging.”

Researchers have known for more than 60 years that when a toilet flushes, solids and liquids escape by design, but tiny invisible particles are also released into the air. Previous studies have used scientific tools to detect the presence of these airborne particles over flushing toilets and have shown that larger particles can land on surrounding surfaces, but until now, no one understood what these clumps looked like or how they got there. .

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Understanding the trajectory and velocity of these particles—which can transmit pathogens such as Escherichia coli, Clostridium difficile, norovirus and adenovirus—is important for reducing the risk of exposure through disinfection and ventilation strategies, or better toilet and flush designs. Although the virus that causes COVID-19 (SARS-CoV-2) is present in human feces, there is currently no conclusive evidence that it is spread efficiently via toilet spray.

“People know the toilet sprays, but they can’t see it,” said Crimaldi. “We’re showing that this is a much more active and pervasive column than anyone who knows about the concept.”

The study found that these airborne particles move quickly, at a speed of 6.6 feet (2 meters) per second, reaching 4.9 feet (1.5 meters) over the toilet in 8 seconds. While larger droplets tend to settle to surfaces within seconds, smaller particles (aerosols less than 5 microns, or one millionth of a meter) can linger in the air for several minutes or more.

It’s not just their own waste that bathroom visitors have to worry about. Several other studies have shown that pathogens can survive in containers for dozens of flashes, increasing the potential risk of exposure.

“The purpose of a toilet is to effectively remove waste from the bowl, but also the other way around, i.e. to spray a lot of its contents upwards,” says Crimaldi. “Our laboratory has created a methodology that provides the basis for improving and mitigating this problem.”

A powerful green laser helps visualize the aerosol spray from the toilet

Above: Postdoctoral researcher Aaron True (left) and John Crimaldi pose with the device. Below: A powerful green laser helps visualize the aerosol shooting out of the toilet as it flushes. (Credit: Patrick Campbell/CU Boulder)

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Don’t waste time

Crimaldi runs the file Environmental Fluid Dynamics Lab at CU Boulder, specializing in the use of laser-based devices, dyes and giant liquid tanks to study everything from How does smell reach our nose? about how chemicals move in turbulent bodies of water. The idea of ​​using lab technology to track what happens in the air after a toilet flush is one of convenience, curiosity, and circumstance.

During a free week last June, fellow professors Carl Linden And Mark Hernandez from the Environmental Engineering program, and several graduate students from the Crimaldi lab joined him to set up and run the experiment. Aaron True, co-author of both studies and research partner in the Crimaldi lab, was instrumental in executing and recording the laser-based measurements for the study.

They used two lasers: one that shone continuously over the toilet, and the other that sent a fast beam of light over the same area. A stationary laser detects where airborne particles are in space, while a pulsed laser can measure their speed and direction. Meanwhile, two cameras take high-resolution photos.

The toilet itself was of the same type typically seen in public restrooms in North America: a lidless unit accompanied by a cylindrical flush mechanism — either manual or automatic — hidden behind the wall, known as a flushmeter-style valve. Clean toilets are just filled with tap water.

They knew this sudden experiment could be a waste of time, but instead, the research created great impetus.

“We expected these aerosols to float, but they came out like rockets,” said Crimaldi.

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The energetic water molecules in the air mostly head up and back towards the back wall, but their movement is unpredictable. The shaft also rose to the ceiling of the laboratory, and with no other purpose, it moved out of the wall and spread forward, into the room.

The experimental setup did not include any solid waste or toilet paper in the bowl, and there were no moving stalls or people. All of these real-world variables can exacerbate the problem, says Crimaldi.

They also measure airborne particles using an optical particle counter, a device that draws a sample of air through a small tube and shines a light on it, allowing it to count and measure the particles. Smaller particles not only stay in the air longer, but they can also escape from the hairs of the nose and reach deeper into a person’s lungs — making them more hazardous to human health — so knowing the number and size of the particles is also important.

While these findings may be concerning, this study provides plumbing and public health experts with a consistent way to test better plumbing designs, disinfection and ventilation strategies, to reduce the risk of exposure to pathogens in public restrooms.

“None of these improvements can be made effectively without knowing how the aerosol column evolves and how it moves,” says Crimaldi. “Being able to see these invisible pillars is a game changer.”

Additional authors on this publication include: Aaron True, Carl Linden, Mark Hernandez, Lars Larsson, and Anna Pauls of the Department of Civil, Environmental, and Architectural Engineering.

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