Seoul's tap water "Arisu" suppresses microbial growth even under conditions of repeated drinking and prolonged storage, according to the city government, as concerns grow over the hygiene of water carried in tumblers or PET bottles and consumed over multiple sittings during summer.
The Seoul Metropolitan Government announced Thursday that a recent study on microbial changes in Arisu after repeated drinking and over time confirmed that residual chlorine suppresses the growth of general bacteria introduced from external sources, ensuring a level of hygienic safety that makes the water safe to drink even in summer.
During summer, rising temperatures and increased water intake often lead people to carry water for long periods or drink it in multiple sittings. In particular, when drinking repeatedly directly from a container, microorganisms from the mouth or the external environment can enter the water, and the likelihood of bacterial growth increases at high temperatures.
To address this, the Seoul Water Institute conducted an experiment reflecting actual drinking conditions. Nine participants drank Arisu from tumblers repeatedly, and samples were collected and analyzed for changes in general bacteria at one hour and three hours after the first drinking, and at five, seven, and 24 hours after the second drinking.
The results showed that no general bacteria were detected in Arisu for 24 hours, even after repeated drinking. The analysis attributed this to residual chlorine (0.2 mg/L) maintained during the water supply process, which kills externally introduced bacteria within one hour and suppresses their growth. While residual chlorine is sometimes cited as the cause of a distinctive odor, it plays an important role in maintaining hygiene by suppressing microbial growth.
In contrast, commercially sold bottled water, which contains no residual chlorine, showed a tendency for bacteria to increase under the same conditions. The study found an average of 41 colony-forming units (CFU)/mL of general bacteria after the first drinking and an average of 85 CFU/mL after the second drinking, with some cases showing up to a 60-fold increase within three hours. The city explained that bottled water should be consumed as quickly as possible after opening.
Arisu also showed competitiveness in mineral content analysis. Arisu's mineral content stood at 42 mg per liter, higher than the average of 29.8 mg/L for the five commercial bottled water brands surveyed.
The Seoul Water Institute advised that using tumblers filled with tap water during summer can be helpful for hygiene management. However, it cautioned that since water consumed directly from a container can become contaminated with bacteria through saliva, it should be drunk as quickly as possible and not left at room temperature for extended periods. If the smell of tap water is bothersome, pouring it into a clean container and refrigerating it can reduce the odor.
"Arisu is drinking water whose safety has been confirmed through water quality tests covering 362 items," said Yoon Hee-cheon, director of the Seoul Water Institute. "Since microbial growth is effectively suppressed even in summer environments where water is shared over long periods, using tumblers will help both with hygienic hydration and with reducing single-use products."
