The black is likely sulphide salts such as FeS. Most heavy metals precipate with sulphide.

There is no general fix for this. If you could avoid that the redox goes so low that you produce sulphide it might help. Avoid/reduce the input concentration of sulphate and metals. Extended aeration of the black water might oxidise the sulphide salts, but this is usually kinetically difficult (long reaction time).

The black is likely sulphide salts such as FeS. Most heavy metals precipate with sulphide.

There is no general fix for this. If you could avoid that the redox goes so low that you produce sulphide it might help. Avoid/reduce the input concentration of sulphate and metals. Extended aeration of the black water might oxidise the sulphide salts, but this is usually kinetically difficult (long reaction time).

You could test adding hydrogen peroxide to increase dissolved oxygen level and to oxidize sulphides. Should remove the black color relatively quickly.

There is a picture showing visually the effect of hydrogen peroxide addition to anoxic wastewater. The required dose will depend on many factors and the only way to find it out by testing.

I have found that blackness is generated due to extensive bacterial decomposition of  organic matter in water. can any one suggest me solution about it.