Water from the tap in any country may pose a problem. In Bangkok, the water is technically fine. However, there are ways to test the water to determine if there are excess amounts of iron or chloride in the water. Even if there are more elements in the water than usual, then it may still be fine to drink.
The World Health Organization determines whether the water is safe to drink and use. Sometimes, the pipes are the problem instead of the water itself. With that said, the pipes should be tested and it would depend on each structure whether the water pipes are corroded. Bangkok has safe drinking water, but the pipes should be checked throughout any city to determine if the water is flowing safely.