AERATION TREATMENT EFFECTIVENESS FOR NET ALKALINE MINE DRAINAGE

The Operation Scarlift Site 21 abandoned mine discharge in Ranshaw, Pennsylvania is a significant contributor of metal loading (4% of total) to the Shamokin Creek watershed. A study was conducted to determine the feasibility and effectiveness of treating the discharge with aeration and settling ponds.

This low Mn, low Al, net alkaline discharge has a pH of ≈ 5.6, an Fe(II) concentration of 16 mg/L, and a flow rate of 3000 L/min. Net alkaline discharges are able to self-treat based on their chemistry if they are retained long enough in a treatment pond system. The discharge water was aerated in the field in a small flow-through reactor to promote CO₂ degassing, which caused a pH increase and Fe(II) oxidation rate to increase.

Within 3 hours, pH increased from 5.7 to 7.7, temperature increased 12.2 to 19.1 ºC, and Fe(II) concentration fell from 16 mg/L to less than 0.05 mg/L. The same experiment repeated with a 12-hour run time with no active aeration caused pH to change from ≈ 6.1 to 6.3 and Fe(II) concentration to decrease from 16.3 mg/L to 13.8 mg/L. Results from an Fe(II) oxidation model written in the differential equation solver Stella™ matched the field experiments very well. An aeration experiment at the Packer 5 discharge in Girardville, Pennsylvania produced similar results. In 3 hours, the pH increased from 6.1 to 7.3, temperature increased from 13.9 to 24.1 ºC, and Fe(II) concentration fell from 16.5 mg/L to less than 0.05 mg/L. The land surrounding Site 21 was surveyed to determine the position of the discharge and elevation of the area where a treatment pond would be installed and a natural neighbor method contour map was produced. The system was found to have a head difference of only 1.5 m so pumping would be required for any treatment system to be feasible.

Using a published Fe removal rate of 20 g m^-2 day^-1 and Fe loading from field data, a 3600 m² passive treatment pond size was calculated for Site 21. The Stella™ Fe(II) oxidation model predicted a 400 m² by 1 m deep pond would be adequate to lower Fe(II) concentration to less than 1 mg/L. The use of active aeration for high-CO₂, net alkaline discharges can result in considerably reduced treatment area - nearly a factor of 10 reduction in this case – and may lower treatment costs. The reduced capital cost for earthmoving will need to be compared to energy and maintenance costs for aeration