A £300,000 scheme to treat water coming from Deerplay Colliery, above Weir, should be complete in 13 weeks.
The scheme is being carried out by Bolton Firm A & E Yates on behalf of the Coal Authority.
Consultant Alistair Byfield said: "We dropped a pumping station 140 metres into the former colliery last March.
"We have got sludge beds and we are installing two treatment ponds and a reed bed which is 50m by 30m and will take the iron out of the water before it goes back into the rivers."
He said there was also a PH imbalance with water filtering through the former colliery site before going into the river and that water was also going to be treated.
The project is smaller but similar to a scheme further down Burnley Road which treats water flowing from the former Old Meadows Colliery and into the River Irwell.
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