Can UV-C be Sparkling Clean? Testing the effectiveness of Ultraviolet light to clean stainless steel water bottles

By Alysha Lee
Intermediate Category (Grades 9-10)
Experiment | Biology

BCVSF Note: The required ethics forms have been submitted for this project.

In last year’s science experiment, I discovered that compared to glass and plastic, stainless steel water bottles created the best environment for bacteria to thrive in. As an extension of my project, I wanted to know which method of cleaning would be the most effective for stainless steel water bottles.

The purpose of this experiment is to find out if UV-C light will eliminate more bacteria than the common method of handwashing with a brush, soap, and water since many popular stainless steel bottles are not dishwasher safe. I hypothesized that… if I handwash a stainless-steel water bottle with soap and water and another with 10 minutes of exposure to UV-C light, then the UV-C light will eliminate more bacteria (CFU) than the hand-washed one because UV light disrupts the DNA in bacteria by creating thymine dimers, which eventually kills the bacteria.

For this experiment, I used three 500 mL stainless steel water bottles, labelled A (control group, no washing), B (brush, soap, and water), and C (UV-C exposure). Each bottle was filled with 450mL of water and over the course of three days, I drank from each bottle three times on a rotational basis. Samples were taken from the waterline inside of each bottle before cleaning and was swabbed onto a petri dish. For Bottle A the water was poured out of the bottle with no further cleaning, Bottle B was scrubbed with a brush, dish soap, and water for ten seconds, and Bottle C was exposed to a UV-C (185-254nm) Sterilization Lamp for ten minutes. Samples were taken again after each cleaning method. All petri dishes were incubated at room temperature for five days. I observed and recorded the number of colony forming units (CFU) and the shape/size/colour of the bacteria during the incubation period. This experiment was repeated two more times.

For my results, I calculated the average of all three trials and found the bacterial elimination percentage. Bottle A (control) only removed half of its bacterial content after pouring out all the water. Bottle B (brush, soap, and water) removed 98% of its bacteria. Bottle C (UV-C exposure) was not as effective as Bottle B, as it eliminated an average of 83% of its bacteria. Washing the bottle with soap and water created a slippery solution to prevent the bacteria from adhering to the walls of the bottle. The brush-scrubbing dislodged the remaining bacteria, which was a feature the UV-C light did not have. Another factor of the UV-C light’s bacterial elimination percentage was that the intensity of the UV rays was not the most effective wavelength, which is around 270nm. Other sources of error include the number of bacteria in my mouth before drinking the bottles and the cleaning technique between each trial.

From this experiment, I have concluded that my hypothesis was not supported. UV-C light is not as effective as cleaning your water bottle with a brush, soap, and water. However, if you are looking for an environmentally-friendly option, UV-C light is reusable and eliminates the majority of the bacteria in your bottle. Make sure to clean your water bottle every day; rinsing the bottle and refilling it will only remove half of the bacterial content.

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