The cheapest way to remove ...

The cheapest way to remove any pollution is usually to avoid it in the first place. I'm not expert on bauxite mining but the questions I would start with are things like:

How is the bauxite mining causing pollution?

Is there any way the processes can be changed to prevent the pollution?

What would the cost of implementing mitigation measures be compared to the cost of removing the pollution from the river?

Is there anyway to turn this problem into an opportunity?

Hope this helps