Performance review of an operational Reducing Alkalinity Producing System (RAPS) treating coal mine waters at Tan y Garn, Wales
The Coal Authority's passive coal mine water treatment scheme (MWTS) at Tan y Garn in Wales incorporates a Reducing Alkalinity Producing System (RAPS) A long term performance review has been undertaken after 10 years of continuous operation without any major maintenance. Over the life of the MWTS, raw mine water quality has gradually improved due to flushing. Tan y Garn RAPS continues to generate alkalinity and raise mine water pH but the alkalinity generation efficiency has declined over time. Alkalinity generation within the RAPS is predominantly due to limestone dissolution. Sulfate reducing bacteria are present in the RAPS but net redox transformations of sulfur are interpreted to be of minor significance to the current mine water treatment process. The RAPS removes a significant proportion of iron from the mine water. Short term variations in iron removal efficiency reflect variations in flow rate and iron loading. Longer term decline in iron removal efficiency is also occurring despite decreasing iron concentrations in the raw mine water. The performance of the Tan y Garn RAPS has declined over the past decade but the system continues to achieve the desired treatment goals. RAPS technology is considered a viable option for the long term passive treatment of coal mine waters.