swamp cooler blows black stuff

I have a new side draft swamp cooler that blows a very fine black substance, almost like a powder, that makes me think the belt is somehow involved. It isn't mold....the pads aren't black, the water is clean and clear and there is no musty smell. There's no burning or hot smell either. I started using filters to catch the black stuff and I have to replace them every 4-5 days as they become completely black. What could be causing this?
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