My guess is that once you add the pills to the water they need time to dissolve and if they are effervescent then they are like alka seltzer, the chemical reaction would probably purify the water by releasing the harmful chemicals into the atmosphere when the water fizzes. Just consuming the pills may have a very nauseating effect so just be careful. Imagine swallowing a couple of alka seltzer without dissolving them first. The key ingredient here looks to be sodium and my guess has similar properties to baking soda.