Well, if the metal content is high, you can’t use it to fertilize crops, either. What do you suggest to do with the sludge?
Not so. Ferrous sulfate, selenium, iron, magnesium, copper, zinc, etc... are trace metals needed by plants (and people).
You just don’t want it as burned ash. That’s like say 500 tons of sludge burned down to 100 pounds of ash. Much of the metals go airborne in the smoke stack. The rest are 500 tons worth in 100 pounds of ash. That has to go to a hazmat landfill.