Black Sludge comes from sulfur that has been reduced and reacted with iron and other minerals in the water to form a coating of sulfides. It can be easily eliminated with proper oxidation provided by aeration.
A higher dissolved oxygen (DO) content removes black sludge in two ways. The first is that it discourages sulfur reducing bacteria, which cannot survive in a high oxygen environment. These bacteria are responsible for creating sulfides in the first place.
The second way that high DO removes black sludge is by oxidizing any sulfur compounds present to highly water soluble sulfates. These can be passed on to any subsequent water treatment, and are not toxic in any way.
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