Remediation of High-Strength Mine Influenced Water with Crab Shell Substrate Mixtures: Laboratory Column and Field Pilot Tests

Abstract

Anaerobic passive treatment systems remediating high-strength mine influenced water (MIW) have failed to consistently treat sources with high metals loads and flow rates. For example, the high iron (140 mg/L) and acidity (390 mg/L as CaCO3) of the Klondike-1 discharge near Ashville, PA, caused premature clogging of a vertical flow pond which was filled with a limestone buffered organic substrate (90% spent mushroom compost (SMC) and 10% limestone chips). In this study, continuous-flow columns and pilot-scale field reactors were used to evaluate if treatment of high-strength MIW can be improved using crab shell as a substrate amendment. For the lab study, eight columns were wet-packed with substrate mixtures ranging from 0 - 100% crab shell (with the balance SMC), and fed Klondike-1 water to produce a 16 hr hydraulic residence time (HRT). After determining the best performing substrate mixture in the column test, a pilot-scale field study was initiated in August 2010, in which 1,000 gallon (3,785 L) tanks were filled with a limestone underdrain and an upper substrate layer of: 1) 100% crab shell; 2) 70% crab shell + 30% SMC; or 3) 90% SMC + 10% limestone. A fourth tank containing a sandstone underdrain with a 70% crab shell + 30% SMC substrate layer was installed to determine if similar performance could be achieved without the limestone underdrain. All columns and field reactors also contained sand as a proppant to ensure hydraulic conductivity. Aqueous samples were collected from the columns/reactors over time and analyzed for pH, ammonium, acidity, alkalinity, DOC, anions, and metals. In the column study, an optimum ratio of 70% crab shell + 30% SMC sustained complete iron removal, pH above 5.0, and excess alkalinity generation for twice as long as the traditional SMC and limestone substrate. To date, the field study is still achieving complete iron removal, and other results generally mirror the laboratory findings.