Earlier quoted context omitted.
I'm an environmental engineer with some experience in drinking water treatment. I doubt it's in the drinking water. One of the oldest drinking water treatment methods would remove the virus: slow-sand filtration. Most places in China would at least use this, and South Korea is likely using more advanced technology.
I understand that is an older method, but I would be surprised if it is the method of choice for these cities. What do you consider ‘advanced technology’?
Improvements on slow-sand filtration are from additional pre-treatment methods, disinfection, more advanced filtration media like activated charcoal, and additional stages of filtration using a variety of media. Some high tech treatment plants are using biological membranes for wastewater-to-drinking-water treatment, but that's quite rare as far as I know.
Here's a decent primer of the commonly used processes: https://www.cdc.gov/healthywater/drinking/public/water_treat...