Tan-Y-Garn Mine Water Treatment Scheme
Location: Tan-Y-Garn is located adjacent to the River Cathon in an incised valley north east of the village of Garnswilt, nr Ammanford, Carmarthanshire.
The experimental Tan-y-Garn treatment scheme was commissioned in January 2006, using a ‘RAPS’ which is the first of its kind for the Coal Authority in the treatment of mine water. Sustainability is the key to this project as it does not rely upon continuous addition of chemicals or electricity: it relies solely on gravity to pass the mine water from natural outflow point, through the treatment system and into the River Cathon.
|Mine Water Parameters
The mine water discharge at Tan-y-Garn is predominantly net – acidic, therefore there is a requirement for addition of alkali to raise the pH, thus allowing the dissolved iron to oxidize and precipitate. A review of treatment methods was undertaken, and a Reducing Alkalinity Producing System (RAPS) was selected as the best and most sustainable approach.
The RAPS is arranged as a vertical flow system, where influent water passes onto a combined compost/limestone layer within a lagoon, and percolates through to collection ducts at the base.
The RAPS relies upon two key processes: ‘sulphate reduction’, where anaerobic conditions provided by a compost layer cause dissolved iron to form insoluble sulphide minerals; and alkalinity generation, where limestone gravel raises the pH of effluent water.
Above: Diagram of the RAPS system. Mine water passes vertically through the compost and limestone layers, before being collected in an under drain and discharged through an upturned siphon.
Once the water has passed through the RAPS, the iron loading is reduced, and the pH is increased. The water then flows through settlement lagoons and a reedbed, which provides additional passive treatment before discharge to the River Cathan.
The scheme comprises the following units which are arranged in series:
A post project review study undertaken by Atkins in 2007 showed that the system was very effective, removing on average 5.5kg/day of iron in total, with 85% of this being removed by the RAPS unit.