Just a quick glance, but it would seem that the biggest problem with biosolids is additional contamination in the sludge, not the sludge itself. Having said that, once you mix human waste with toxins it is difficult to unmix them.
In an ideal world, sludge would be disposed of on-site and not mixed with anything toxic. I believe it is used that way in some third world countries and is not a problem for them.
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