So putting those metals on your garden to grow food is somehow a good idea?
My understanding is that most, at least, of the metal content is not due to human feces, but to "other" stuff that gets dumped down the drain.
The metals in sludge are dispersed over a huge area when used as fertilizer. As a byproduct of combustion they become super concentrated as ash. The metals are mostly iron oxide and in older cities, lead from the sewer hook-up pipes used prior to 1970.