Some time ago (back when I didn't know any better?), I let someone
convince me to apply greensand because of its ability to release potassium
(potash) slowly. I have an area that gets flooded with each heavy rain and
potassium leaches out quickly. I'm of a cautious nature when it comes to my
garden so I spread the greensand over only a small area. When I got my next
soil test back, I learned what the greensand seller didn't tell me; had I
had any sense, I would have investigated it thoroughly before using it.
That area now has 2000+ ppm of manganese, about 100 times more than
recommended. What's more is that it is in an ionic form (available to
plants), unlike the potassium that is chemically bound. The 'book' would
say that anything growing there would be adversely affected, but no, I don't
see any major, on going problems.
As for spreading sludge, that is a definite no - no. Don't do it!(!)
Whether municipal or industrial, sludge has a concentrated amount of heavy
metals. Many (most?) plants absorb heavy metals and you could get an unsafe
amount in your diet, not unlike the many warnings to not eat fish because of
mercury build-up within the bodies of the fish. >The solution to pollution
is dilution< Around here, sludge, and septic tank waste, is spread on
agricultural lands, hay fields mostly. The cows eat the hay and we eat the
cows; European farmers have been doing this for 'eons' with no (apparent)
ill effect.