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| It's not what people are eating that causes sludge to test high in heavy metals. It's the industrial waste that flows through the same sewer systems as human waste that is the problem with metals.
Heavy metals bioaccumulate which is why one should be concerned about spreading it on crops. Sludge is often ammended with lime to keep the pH high enough to sequester the metals and keep them from being taken up by plants.
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Pelletized Human Waste
