Drinking Water Purification

It is true that water treatment plants can stop biotoxins, but the flaw is in your logic. To infect the water supply all you would need to do is overcome the pressure at a water hydrant and intorduce the biotoxin at the hydrant. While this would not result is a wide spread outbreak, it would cause fear in the safety of the water supply. Most attacks are aimed at creating fear and not just the casulties of the attack. A charcoal filter wouldn’t be of much use against one of these attacks. The filters at a water plant are used for getting solids out of the water. A biotoxin would more tham likely need chlorine or other disinfectant to render the water safe. So you don’t have to get it through the water plant plant to create a problem. You can cause more problems by bypassing the water plant and injecting your toxin at any fire hydrant.

You are right, it is a logic flaw; HOWEVER, water treatment plants are designed according to the KNOWN hazards of the water to be treated, so it IS possible.

In some cases, yes and some cases no.
Most depend on filters and activated charcoal which will NOT catch everything.

I would be more worried about airborne attack though, as it is much simpler to arrange and harder to defend against.