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

Banner image: A powerful green laser helps visualize aerosol jets from a toilet when it is flushed. (Credit: Patrick Campbell/CU Boulder)

Thanks to new research from CU Boulder, scientists are seeing the impact of flushing the toilet 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 quickly ejected into the air when a lidless toilet in a public restroom is flushed. Now published in Scientific reportsthis is the first study to directly visualize the resulting aerosol plume and measure the velocity and distribution of particles within it.

These aerosol particles are known to carry pathogens and may pose an exposure risk to public bathroom patrons. However, this vivid visualization of potential disease exposure also provides a methodology to help reduce it.

“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,” said John Crimaldi, lead author of the study and professor of civil, environmental and architectural engineering. “By making dramatic visual representations of this process, our study can play an important role in public health messaging.”

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

Understanding the trajectories and velocities of these particles—which can transport pathogens such as E. coli, C. difficile, noroviruses, and adenoviruses—is important for reducing the risk of exposure through disinfection and ventilation strategies or improved toilet and flushing design. While the virus that causes COVID-19 (SARS-CoV-2) is present in human waste, there is currently no conclusive evidence that it is effectively spread through toilet aerosols.

“People knew that toilets emitted aerosols, but they couldn’t see them,” Crimaldi said. “We’re showing that this thing is a much more energetic and fast-spreading cloud than even people who knew about it understood.”

The study found that these airborne particles shot out quickly, at a speed of 6.6 feet (2 meters) per second, reaching 4.9 feet (1.5 meters) above the toilet within 8 seconds. While the largest droplets tend to settle on surfaces within seconds, smaller particles (aerosols below 5 microns, or one millionth of a meter) can remain in the air for minutes or more.

Bathroom visitors don’t just have to worry about their own waste. Many other studies have shown that pathogens can linger in the bowl for dozens of washes, increasing the potential risk of exposure.

“The purpose of the toilet is to effectively remove waste from the plate, but it also does the opposite, which flushes a lot of contents upwards,” Crimaldi said. “Our lab has created a methodology that provides a basis for improving and mitigating this problem.”

A powerful green laser helps visualize aerosol jets from the toilet

Above: Aaron True, postdoctoral researcher (left) and John Crimaldi pose for a photo with the equipment. Below: A powerful green laser helps visualize aerosol jets from a toilet when it is flushed. (Credit: Patrick Campbell/CU Boulder)

It’s not a waste of time

Crimaldi rules Environmental Fluid Dynamics Laboratory at CU Boulder, which specializes in using laser instruments, dyes and giant fluid tanks to study everything from how smells reach our nostrils on how chemicals move in turbulent bodies of water. The idea to use the lab’s technology to track what happens in the air after a toilet is flushed was the result 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 students from Crimaldi’s lab joined him to set up and run the experiment. Aaron True, the study’s second author and research associate in Crimaldi’s lab, was instrumental in conducting and recording the laser-based measurements for the study.

They used two lasers: one shone continuously on and above the toilet, while the other sent rapid pulses of light over the same area. A steady-state laser reveals where airborne particles are in space, while a pulsed laser can measure their speed and direction. Meanwhile, two cameras took high-resolution images.

The toilet itself was of the same type commonly seen in public toilets in North America: a coverless unit accompanied by a cylindrical flushing mechanism – whether manual or automatic – that protruded from the back near the wall, known as a style valve flowmeter. The brand new, clean toilet was filled with only tap water.

They knew that this sudden experiment might be a waste of time, but instead the research made a big splash.

“We expected these aerosol particles to just float away, but they took off like a rocket,” Crimaldi said.

The energetic water particles in the air mostly went up and back towards the back wall, but their movement was unpredictable. The plume also rose to the ceiling of the lab, and with nowhere else to go, moved out of the wall and spread forward into the room.

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

They also measure particles in the air with an optical particle counter, a device that sucks a sample of air through a small tube and illuminates it with light, allowing it to count and measure the particles. Smaller particles not only stay airborne longer, but can escape the nose hairs and reach deeper into the lungs, making them more dangerous to human health, so you know how many particles and what kind size is also important.

While these results may be disconcerting, the study provides plumbing and public health experts with a consistent way to test improved plumbing design and disinfection and ventilation strategies to reduce the risk of pathogen exposure in public restrooms.

“None of these improvements can be made effectively without knowing how the aerosol plume develops and moves,” Crimaldi said. “Being able to see that invisible feather is a game changer.”

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

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