A combined passive system for the treatment of acid mine drainage

Acid mine drainage (AMD) mitigation at Canadian mine sites using passive treatment technologies requires a combined strategy aimed at minimizing the impact of climatic variability of system performance. In this research, a vertical-flow combined passive system was developed, which consists of four components with specific treatment functions: an oxidation/precipitation stage for excess iron removal; a peat biofilter for metal sorption and the establishment of anoxic conditions; sulphate reducing bacteria for alkalinity generation and sulphate reduction; and an anoxic limestone drain for alkalinity addition. A substantial portion of the research focused upon the peat biofilter component, because the removal mechanisms involved in this type of application and the impact of operational parameters on its performance were least understood.; Four laboratory investigations aimed at understanding AMD mitigation were undertaken in this research. (1) Physical/chemical characterization of the peat. (2) Kinetic and equilibrium studies to determine the sorption capacity of the peat. (3) Peat column experiments to assess the impact of substrate/amendment addition, constituent loading, hydraulic loading, water table elevation and ambient temperature on AMD mitigation. (4) Evaluation of a bench-scale combined passive system.; The peat column sorption study conducted at 0°C demonstrated that constituent and hydraulic loading had an impact on metal retention where: LQ-L[ ]<HQ-L[ ]<LQ-H[ ]<HQ-H[ ], with a general metal breakthrough order of Cd<Mn<Zn<Al<Ni<Cu<Fe. At 20°C, sulphate-reducing conditions developed in the HQ columns leading to an increase in effluent pH and metal removal. Sequential extraction indicated that Fe, Cu and Al were typically bound to the organic matter fraction, while Mn, Zn, Ni and Cd were primarily associated with the exchangeable fraction. Substrate/amendment addition improved metal retention in the peat columns when a more readily available source of alkalinity was provided. A bench-scale combined system dosed with 95 L/m2/d moderate-strength synthetic AMD was operated for 10 months under continuous flow conditions. Removal efficiencies of 99.7%, 99.9%, 99.9%, 98.6% and 99.9% were observed for Fe, Al, Zn, Mn, Ni, and Cu, respectively, while Cd remained fairly mobile through the system. Sulphate concentrations reduced by 73.1% and the highly acidic drainage was neutralized with an effluent pH of 7.2 and alkalinity of 1350 mg/L.