The most important potential contaminant of food produced on sewage sludge amended soil is

Cadmium is the natural element found in tiny amounts in air, water, soil and food.
Dumping of sewage sludge into the soil leads to soil pollution and various heavy metals like cadmium, lead, etc present in waste water amend in soil and cause contamination of soil also to the foods and crops grown on that soil. This makes soil potentially toxic to the plants.