The purity of tap water is based on two things: the source it originates from and the level of filtration it goes through before it gets to our taps. Some countries like Switzerland receive plenty of rain and have melt from its glaciers to have one of the cleanest tap waters in the world. Other countries like Singapore uses a network of reservoirs and state-of-the-art filtration processes to achieve high level of clean drinking water.  

It is not a surprise to say that tap water is safest in the developed world, such as UK, Canada, U.S., Australia, New Zealand, Northern and Western Europe, and Japan. On the other hand, in some countries in Central America, Africa, Asia and the Middle East are considered unsuitable for visitors to drink tap water. This doesn’t mean their tap water is dirty but unsuitable for visitors because the pathogens in the water are foreign to their immune systems. So, when you’re travelling, be sure to check where you can and cannot drink tap water.

As for Korea, it is safe for people to drink tap water. The water infrastructure across the country is reliable and purification processes are well maintained. According to ‘Water and Sanitation Report’ by EPI(Environmental Performance Index), South Korea is ranked 20^th in the world, 2^nd in Asia after Japan. Also, Seoul’s tap water “Arisu” obtained the ISO 22000 certification on food safety management systems. When you’re travelling in Seoul, you’ll generally find water purifying machines in public buildings like museums, restaurants, and government buildings. It is smart to bring a water bottle and fill up your bottle with either tap water or water from the purifying machine.

Reference

Arisu is < Arisu < Seoul's tap water, Arisu | The Office Of Waterworks Seoul Metropolitan Government

Environmental Performance Index (yale.edu)

The 187 countries where you can NOT drink tap water because it's not safe  | Daily Mail Online

Which Country Has The Cleanest Tap Water? | Castle Water