Effect of UV Irradiation for Remaining Water from Drinking

Background
In recent years, especially with the onset of the COVID-19 pandemic, there has been an increased reliance on bottled water due to concerns about contamination from shared water sources like public water dispensers. Many people, particularly children, often drink from plastic water bottles but leave the water unfinished, resulting in the storage of these bottles for later consumption. Previous studies have shown that bacteria can multiply rapidly in these bottles within just a few hours after they are opened, even if they are stored in a refrigerator. Such bacterial growth can lead to water contamination, potentially causing illnesses like food poisoning. To combat bacterial growth in water, various methods are used, including heating, UV sterilization, filtering, and chemical treatments. Among these, UV sterilization is a relatively simple and practical method that can be employed at home. UV-C light, particularly in the 200–280 nm wavelength range, is known to be effective at killing microorganisms by damaging their DNA. This study aims to evaluate the effect of UV sterilization on the water quality of unfinished bottled water, especially in terms of total dissolved solids (TDS) and bacterial contamination, when stored under different conditions.

Purpose of the Study
The primary objective of this study is to investigate how UV-C sterilization impacts the quality of water left in plastic cups after drinking, comparing sterilized and non-sterilized water stored under room temperature and refrigeration conditions. The study seeks to determine whether UV-C sterilization can effectively slow down the degradation of water quality over time and identify which combination of treatment and storage condition best preserves the quality of the remaining water.

Method
In this experiment, disposable plastic cups are used to simulate common conditions under which water is stored after drinking. The cups are first sterilized using a UV-C sterilizer (UP602-GR), which exposes the cups to UV light for 10 minutes to eliminate any pre-existing bacteria. Drinking water from a water purifier is then added to the cups (120ml per cup), and a sip is taken from each cup after eating a snack to introduce a realistic level of contamination (i.e., saliva, food particles). The water quality is measured using a portable water quality meter (COM-300, HM Digital), which records TDS and water temperature. The TDS represents the total amount of dissolved solids in the water and serves as an indirect measure of contamination or bacterial growth.

The experiment is divided into eight test groups based on two variables: whether the water is UV-sterilized or not, and whether it is stored at room temperature or in a refrigerator. The groups are as follows:

• Room temperature, non-sterilized (6HUV-)
• Room temperature, sterilized (6HUV+)
• Refrigerated, non-sterilized (6HRUV-)
• Refrigerated, sterilized (6HRUV+)
• 1-day room temperature, non-sterilized (1DUV-)
• 1-day room temperature, sterilized (1DUV+)
• 1-day refrigerated, non-sterilized (1DRUV-)
• 1-day refrigerated, sterilized (1DRUV+)

Each condition is tested twice for accuracy, and the results are averaged. TDS and water temperature are measured immediately after drinking, 6 hours later, and 1 day later. The differences in TDS between these time points are compared to assess the rate of water quality deterioration.

Results
The initial TDS of the water measured immediately after filling the sterilized cups was 129.2 ppm, with an average water temperature of 28.6°C. After consuming a snack and taking a sip from the water, the TDS slightly increased to 133.3 ppm, likely due to the introduction of food particles and saliva.

After 6 hours of storage at room temperature, the non-sterilized water (6HUV-) showed a significant increase in TDS to 149.0 ppm, representing an 11.2% increase from the initial TDS. In contrast, the UV-sterilized water (6HUV+) stored under the same conditions only showed a 4.3% increase in TDS to 144.0 ppm. Refrigerated water showed less deterioration, with the non-sterilized group (6HRUV-) experiencing a 5.7% increase in TDS, while the sterilized group (6HRUV+) saw a 13.4% decrease in TDS, indicating an improvement in water quality due to UV sterilization.

After 1 day, the differences were even more pronounced. The non-sterilized water stored at room temperature (1DUV-) had the highest TDS increase, doubling to 255.0 ppm, indicating a 100% increase. On the other hand, the UV-sterilized water stored at room temperature (1DUV+) showed a much smaller increase to 150.5 ppm, only an 8.3% rise in TDS. In the refrigerated groups, non-sterilized water (1DRUV-) saw a 14.4% increase in TDS, while UV-sterilized water (1DRUV+) experienced a 12.3% reduction in TDS, similar to the 6-hour measurements.

Discussion
The results clearly show that UV sterilization significantly slows down the degradation of water quality compared to non-sterilized water, particularly when combined with refrigeration. Room temperature storage without sterilization leads to a rapid decline in water quality, as evidenced by the sharp increase in TDS. However, UV sterilization mitigates this effect by reducing bacterial growth. The most effective preservation method is UV sterilization combined with refrigeration, which not only slows down contamination but also seems to actively improve water quality by killing existing bacteria.

The observed reduction in TDS in UV-sterilized, refrigerated water suggests that the sterilization process inhibits bacterial growth more effectively when combined with cold storage. This could be due to a synergistic effect between the bactericidal properties of UV light and the slowed bacterial metabolism at lower temperatures. Further studies involving bacterial culture tests could provide more detailed insights into the exact mechanisms at play and confirm the reduction in bacterial load.

Conclusion
In conclusion, UV sterilization is an effective method for maintaining the quality of water after consumption. Water stored in non-sterilized conditions, especially at room temperature, deteriorates rapidly due to bacterial growth, leading to a significant increase in TDS. However, UV sterilization, particularly when combined with refrigeration, significantly reduces the rate of water quality deterioration, even showing improvements in water quality in some cases. This method not only helps to maintain safer drinking water but also has the potential to reduce the environmental impact of discarded plastic bottles by allowing people to safely store and reuse unfinished water. The results of this study suggest that households and individuals can adopt UV sterilization as an accessible and effective solution for preserving drinking water quality, particularly in situations where water may be consumed over extended periods.