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Most water systems are required to run a "free chlorine residual" of about 1 to 2 parts per million at the fathest reaches of their system. To achieve this they may need to run as high as 6 parts per million in their tanks.

Free Chlorine is that chlorine which is still active after "demand dosages' have been met. Technically it is assumed that all water has some organic contaminants. Chlorine "burns" up those contaminants. If only enough is used to do this, the water can form trihalomethanes. (which is what you smell when you think drinking water has been overchlorinated, actually it has not enough) At this point (break even) you add additional chlorine to prevent this.

Now then I am about five years out of date, due to retirement,and the methods of ultraviolet and reverse osmosis, are newer than my education.

The only point I was trying to make is that it takes a lot more effort than peeing or taking a dump in the holding tank to make water a water system unsafe.

Senator Bedfellows numbers are more descriptive than my own.

Now for an "Urban Legend" that is alluded to in water classes throughout the State of Texas. Near a large University somewhere East of Austin, during an inspection of the interior of an elevated storage tank belonging to a municpality, it was found one of the interior walkways was furnished with a small sofa and several cases of empty beer cans. Draw your own conclusions, and remember it's only a rumor from a long time ago.

Near a large University somewhere East of Austin, during an inspection of the interior of an elevated storage tank belonging to a municpality, it was found one of the interior walkways was furnished with a small sofa and several cases of empty beer cans.

I conclude that I'm happy that I didn't attend this university. Mainly for the "ick" factor, if not heath concerns ;)

Anyway, you could probably engineer a biological agent that would survive chlorinated water systems - you'd probably start with a chlorine resistant strain of E. coli or Legionalla, or protozoans like Giardia or Cryptosporidium, and tweak them to increase their effects on humans. But again, this is beyond the ability of some guy working out of his basement - while it's probably doable, we're talking about large investments of time, money, equipment, and expertise to pull it off. Comparatively speaking, biological agents are much, much harder to produce than chemical agents. Hell, even nuclear weapons are probably somewhat easier to produce, since you at least have a sort of blueprint to work from with nukes.