Old Meadows Minewater Treatment Scheme

Site History - The outflow of minewater at Old Meadows is a drainage tunnel into workings from a shaft mine, which emits from an adit on the banks of the River Irwell. The minewater treatment scheme at Old Meadows was constructed in 1998. There was no suitable land available near to the discharge except for a 3ha plot of land. The plot was privately owned agricultural land and was on the steep slopes on the side of the valley. The steep gradients of the land were to provide design and construction difficulties, it also made it harder to reduce the visual impact of the plant and scheme. The adit at Old Meadows used to have to be cleaned by boat, but it has now been sealed up so cleaning is no longer possible, this gives rise to ochre flushing from the underground workings periodically.

Water Chemistry - Prior to the scheme being undertaken there was a major visual impact on the River Irwell due to ochre staining, for a distance of 5 to 6km downstream.

The minewater contains average concentrations of 30-mg/l iron and the average flow from the adit is 49 l/sec. The average inflow pH value is 6.3.

Site Design - Due to restricted space on the site the treatment requirements are to add alkalinity and precipitate out the iron and manganese.

Water flows along two gravity pipes from the adit to a storage sump approximately 300m downstream. This storage sump has a storage capacity of approximately 8000 litres. From here the water is pumped up the steep slope via two centrifugal pipes of 40-l/sec capacity. The water enters a sodium hydroxide (caustic soda) dosing plant at the top of the site. Here the caustic soda (20% solution) is added from the bulk tank. Once the water has been pH corrected it passes into the two parallel settling ponds, with most of the water entering into pond 1 (which is twice the size of pond 2 at 11640m³). Some water does pass into pond 2 (3030m³) but at present this is only to ensure that the pipes do not become blocked.

From the settling ponds the water enters the surface flow aerobic reed bed via a concrete distribution channel. The reed bed is clay lined and is planted with Typha, Phragmites, Scirpus and Iris. Outflow from the reed bed is into a concrete channel, from where the water flows down a pipe to be discharged just below the pumphouse. The cost of the scheme was approximately £1m