Terrorism
In the United States, most treatment plants are of the public domain, providing a public service. As they collect the effluent from hospitals, industry, and households, they may be subject to an easy terrorist attack - especially damaging given the fact that the result sludge is typically not well policed and will be dispersed far and wide across wide areas of farm land.
Some have made claim that sludge in itself, especially the Class B variety, is already weapons of mass destruction as the use of sludge near rural homes has brought claim that people have gotten sick or died due to the presence of sludge on farm land.
Read more about this topic: Sludge