Remediation of high-strength mine-impacted water with mixed organic substrates containing crab shell and spent mushroom compost

Anaerobic passive treatment systems remediating high-strength mine-impacted water (MIW) have not displayed consistent success. For example, the high iron (140 mg/L) and acidity (380 mg/L as CaCO3) of the Klondike-1 discharge near Ashville, Pennsylvania, caused premature clogging of a vertical flow pond (VFP), which was filled with a traditional 90% spent mushroom compost (SMC) and 10% limestone substrate. In this study, continuous-flow columns were designed to simulate VFPs with a hydraulic residence time of 16 h to evaluate if the treatment of high-strength MIW can be improved using crab shell as a substrate amendment. Columns containing between 50 and 100% crab shell (with the balance SMC) were compared with a column containing the traditional substrate as well as to a sand control. Water from the Klondike-1 site was pumped through the columns continuously for 181 days and effluent samples were analyzed for pH, oxidation-reduction potential (ORP), ammonia, acidity, alkalinity, dissolved organic carbon, anions, and metals. An additional passive aeration step after substrate treatment was included to simulate the settling ponds typically used in practice so that the full extent of metals removal under these substrate conditions could be evaluated. An optimal substrate ratio of 70% crab shell +30% SMC treated double the volume of MIW, removed more than twice the mass of metals, and sustained the pH above 5.0 for almost twice as long as the traditional substrate. A treatment efficiency of 1.2 g substrate per liter MIW was calculated as a design parameter for field-scale systems, compared with 2.3 g per liter for the traditional substrate. Although more expensive than traditional substrates (∼50% more expensive for the 70% crab shell mixture), the efficiency of the crab shell amendment enables a 50% reduction in the areal footprint of the VFP and is significantly (two times) less expensive than active treatment alternatives.