Performance of a passive treatment system for net-acidic coal mine drainage over five years of operation.

A full-scale passive treatment system (PTS) was commissioned in 2003 to treat two net-acidic coal mine water discharges in the Durham coalfield, UK. The principal aim of the PTS was to decrease concentrations of iron (<177 mg L(-1)) and aluminium (<85 mg L(-1)) and to increase pH (>3.2) and alkalinity (> or =0 mg L(-1) CaCO(3) eq). Secondary objectives were to decrease zinc (<2.8 mg L(-1)), manganese (<20.5 mg L(-1)) and sulfate (<2120 mg L(-1)). Upon treatment, water qualities were improved by 84% in the case of Fe, 87% Al, 83% acidity, 51% Zn, 23% Mn and 29% SO(4)(2)(-). Alkalinity (74%) and pH (95% as H(+)) were increased. Area adjusted removal rates (Fe=1.49+/-0.66 g d(-1) m(-2); acidity=6.7+/-4.9 g d(-1) m(-2)) were low compared to design criteria, mainly due to load limitation. Disregarding seasonality effects, acidity removal and effluent pH were stable over time. A substantial temporal decrease in calcium and alkalinity generation suggests that limestone is increasingly armoured. Once pH is no longer buffered by the carbonate system, metals could be remobilized, putting treatment efficiency at risk.