Ynysarwed Minewater Treatment Scheme

Site History - The polluting discharge is associated with the Lower Ynysarwed Mine and flows from the Ynysarwed adit near Neath in South Wales.

Although there has been a discharge from the adit for many years, the flow increased substantially and the quality worsened in spring 1993.

This water discharged into the Neath Canal and initially affected a 12km length of the Canal. This was reduced to 7km before release into the River Neath.

Water Chemistry - The flow from the Ynysarwed Adit is net acidic. This can result in pH levels as low as 3.5 following oxidisation.

Peak flows of 36l/sec were discharged with over 400mg/l Fe in summer 1994. These concentrations of Iron had reduced to 150mg/l in Early 2000 and the long-term value of the recovered water is predicted to be 30mg/l although it is likely to be up to 20 years before these levels are reached.

Site Design - The treatment required is to raise the pH level and to precipitate the Iron out of the water. After a process of balancing costs, risks and benefits of the various options, Neath Port Talbot Borough Council and the project partners (Env. Agency, Welsh office, and the Coal Authority, with technical input from SRK (UK) Ltd) identified the preferred treatment option as a combined active and passive system.

The active treatment system was designed by Hyder Consulting Ltd, consisting of mechanical aeration with lime and flocculent dosing and lamella plate separators, the plant also mechanically dewaters the ochre sludge for disposal off site. This was supplemented with a constructed wetland to provide final polishing of the treated minewater prior to its discharge to the River Neath.

The design of the treatment scheme took into account the long term predicted decline in Iron concentration in the minewater discharge. From its peak of 400 mg/l to 30mg/l. it is anticipated that when the iron concentration has fallen to its predicted residual level, precipitation of iron may be achieved without generating excess acidity, thereby reducing the necessity to lime dose.

It may therefore be possible to discontinue the active treatment after about a decade, and achieve treatment in the aerobic wetland.

The scheme at Ynysarwed Cost in the region of £1.5 million and was partly funded by National and European grants.