I’ve been reading up on biological weapons and there’s a lot of talk about the possible use as an areosol, but very little about putting botolism in the water supply for a city and then just waiting for people to start getting sick.
Is it not mentioned because it’s not a particular threat, or because they don’t want to give the terrorist ideas? I know that some dieases live in the water, but I don’t know if any of the one usally considered possible bio-weapons(anthrax, smallpox, etc) can.
I don’t think it works that way. The pros will be along shortly to elaborate, but from what I remember from my coursework and stuff, aerosol is the prefered method of distribution. Botulism can’t survive in the water supply. It is anaerobic, and the oxygen and chlorine in tap water kills it. The same thing goes for the rest of the items on your list. They are either anaerobic, or require a larger dosage to be effective. I also remember that to get a concentration high enough to kill would take pumping a ton of chemicals into the water supply…not something easily accomplished unless you are a US polluter. 
A little chlorine is used in many water processing systems.
Other technologies are used elsewhere.
These would have to be de-activated before the bioweapon could be introduced.
As these plants tend to have large staffs, spread out over several buildings, & over a fairly large area, such an operation would be rather prohibitive in terms of the number of terrorists needed to pull it off.
Also there may be cutoff valves & pumping stations throughout the city, & if the plant seizure isn’t 100% secret, if one call to the cops gets out, the whole thing falls apart.
I’d call it tactically impractical.
Bioweapons are pretty crappy in general. If you look at the anthrax attacks, you’ll notice that only a handful of people died, and it didn’t take much effort to contain the “outbreak.”
Even if someone did manage to get a bioweapon in the water supply, the CDC and other agencies would have the affected city locked down tight in next to no time. The source of the agent could be identified, and people could buy bottled water or boil tap water to avoid exposure while they waited for vaccines or the water system to be sterilized.
The SARS outbreak a few years ago shows you how fast an outbreak can be contained, and remember, the Chinese weren’t very forthcoming about information in the beginning so it spread faster and farther than it would have if the outbreak had occured in a more open nation like the US, UK, France, Germany, etc.