CASE STUDY OF POT RIDGE PASSIVE TREATMENT SYSTEMS, CAMBRIA COUNTY, PENNSYLVANIA
Journal American Society of Mining and Reclamation
Several combinations of anoxic limestone drains (ALO), vertical flow systems (VFS or SAPS), and open limestone channels with associated ponds were constructed at the Pot Ridge reclaimed surface coal mine in 1995 and 1997, and have been monitored and investigated since then. Acid mine drainage flowing into the systems is strongly acid (pH 2.5-3.5, acidity 500-1000 mg/L, Fe 100-300 mg/L, Al 20-70 mg/L). The TEST system, constructed in 1995, consists of two collection ALO's, a pond, a VFS and a
... al pond Flows from the ALO have decreased over time, apparently because of partial plugging by Al concentrations of 20-30 mg/L. Acidity decreases markedly in the following oxidation-settling pond, probably by precipitation of ferric hydroxysulfates. The TEST VFS reduces acidity by about 400 mg/L, to a pH of 4.5-5, but effluent generally has <50 mg/L alkalinity. During summer and fall, S0 4 decreases through the VFS due to S0 4 reduction, but in winter and spring, S0 4 is released from the VFS, indicating that the 20 cm of compost is unable to maintain reducing conditions, and that precipitated Fe sulfides are being oxidized. This unit has a retention time averaging 10 days. The C system, constructed in 1997, treats AMO by two VFS plus an open limestone channel and several ponds at a flow of 100-300 L/min. Inflow has acidity of 600-900 mg/L at pH 3.5-4, Fe of 200-300 mg/L and Al 20-40 mg/L. About 300 mg/L acidity is removed by hydroxysulfate precipitation in ponds. Effluent from the first VFS, with an average retention time of about 3 days, has acidity of 200-350 mg/L, pH 3.2-3.7, Fe 20-60 mg/L and Al 10-20 mg/L. The outflow during the first two years contained appreciable ferric iron, indicating incomplete reduction in 30 cm of compost, but recently, at somewhat lower flow, the Fe is all ferrous. The second VFS produces water with acidity 5-150 mg/L (mostly as Mn), alkalinity 30-80 mg/L, pH 4.0-6.3, Fe<30 mg/L, Al<lO mg/Land Mn 20-30 mg/L. 1n the A system, two VFS remove about 340 mg/L acidity each from highly acid water (influent acidity 1050 mg/L). The five VFS remove acidity (=increase net alkalinity) at rates of 35-50 g/m 2 /d, or 16-26 g/d per tonne of limestone, in the range of other VFS reported in the literature. Problems encountered have included plugging of pipes with Fe hydroxide, overflow at high flow rates, accumulation of Fe hydroxide precipitate on top of the compost, incomplete reduction in compost layers, and the partial plugging of the ALO with Al hydroxides. Although design improvements are possible, the units greatly improve these highly acid, Fe-rich waters. Additional key words: acid mine drainage, vertical flow systems, SAPS, anoxic limestone drains, sulfate reduction.