Alkalinity Generation and Metals Retention in Vertical-Flow Treatment Wetlands

Abstract

Designs of constructed wetlands for acid mine drainage (AMD) treatment have evolved substantially during the past decade. Current research focuses on the study of vertical-flow treatment systems containing labile organic substrates and limestone. Also known as successive alkalinity producing systems (SAPS), these systems emphasize contact of acidic waters with the substrate, thus maximizing biological alkalinity generation via bacterial sulfate reduction, and abiotic alkalinity generation via carbonate dissolution. In this study, alkalinity generation and metals retention were evaluated during the initial year of operation for a modified successive alkalinity producing system (SAPS). The system consists of four l 85-m 2 in-series cells comprised of alternating vertical-flow anaerobic substrate wetlands (VFs) and surface-flow aerobic settling ponds (SFs). The substrate in the VFs consists of spent mushroom substrate (SMS) and limestone gravel (LS), supplemented with hydrated fly ash (HFA) in a 2: I :0.1 ratio by volume. Approximately 15±4.4 L/min of acid mine drainage (AMD) from an abandoned underground mine in southeastern Oklahoma was directed to the pilot-scale treatment system in October 1998. Mean influent water quality was characterized as follows: 660 mg L-1 net acidity as CaC0 3 eq., pH 3.4, 215 mg L-1 total Fe, 36 mg L-1 Al, 14 mg L-1 Mn, and 1000 mg L-1 so,-2 • Flow through the first vertical-flow wetland cell (VF!) resulted in substantial alkalinity increases, metal concentration decreases and circumneutral pH (6.33±0.34). Alkalinity was produced in VF! by a combination of processes, including LS and HFA dissolution and bacterial sulfate reduction (BSR), to 258±84 mg L-1• Adequate alkalinity was added to the AMD in the VF cells so that final discharge waters were net alkaline on all sampling dates (mean net alkalinity -136 mg L-1 ). Total Fe and Al concentrations decreased significantly in VF! from 216±45 to 44±28 mg L-1 and 36±6.9 to 1.29±4.4 mg L-1 , respectively (p<0.05). Approximately 600 kg of iron and 120 kg of aluminum were retained in VF!. Sequential extractions of VF! substrate samples fractionated metals by form (water-soluble, exchangeable, organically-bound, carbonate, oxide or oxide-bound or residual). Manganese concentrations did not change significantly in the first two cells, but decreased significantly (p<0.05) in the second two cells (from 15.41±3.26 to 5.69±0.91 mg L-1 ). Mean acidity removal rates in VF! (51 g m-2 daf 1 ) were considerably higher than previously reported values. Sustainable acidity removal rates of this magnitude would result in design criteria leading to considerable savings in system construction and land acquisition costs.

Passive Treatment of Low-Ph, Ferric Iron-Dominated Acid Rock Drainage in a Vertical Flow Wetland I: Acidity Neutralization and Alkalinity Generation

“…Although biological alkalinity generation is commonly observed, most RAPS rely solely on limestone dissolution for alkalinity generation (Nairn et al, 2000). However, biological alkalinity generation can effect the amount of limestone dissolution both negatively, by raising the ARD pH before it reaches the limestone drain, and positively, by generating carbon dioxide and effectively raising the pCO 2 of the ARD.…”