Which contaminant's presence would be the strongest indicator that municipal sewage sludge is unfit for use as fertilizer?

Heavy metal is known to be highly toxic especially when it is exposed to the environment. One example of heavy metal is arsenic which is known to be very toxic especially when consumed by living things.

There are a lot of deaths that have occurred because of arsenic poisoning. The appearance of heavy metals will contaminate the sewage sludge and make the mud unfit to become fertilizer. One can only imagine how hard it is going to be when it is used to grow plants. The plants may start to grow and still have traces of the contaminants. It may only hurt those who would consume the plants in the long run.