Abstract
Passive treatment of As-rich neutral drainage is less documented relative to acid mine drainage (AMD). In passive biofilters, sorption is the main mechanism of contaminant removal for neutral mine drainage (NMD), whereas precipitation is dominant for AMD. Regardless of the type of mine drainage to be treated, passive treatment systems show a deterioration in efficiency in cold climates and non-iced waters.
The objective of the study is to evaluate the mechanisms of As removal from NMD-As and to propose design criteria using underutilized organic (peat) and Fe-rich inorganic materials (AMD active treatment sludge) for a full-scale passive biofilter. The efficiency of peat and AMD treatment sludge, alone or in mixtures, was evaluated in batch (150 cm3) and in a laboratory-scale pilot biofilter (80 dm3), under continuous flow. Based on historical monitoring data, batch tests were conducted at 22°C vs 5°C, using synthetic NMD-As (2 mg/L As, pH 7) to determine the maximum sorption capacity of solids. A preliminary laboratory-scale pilot experiment was further set-up and operated for 1 month at 22°C to optimize the hydraulic retention time (HRT) as a critical design parameter.
Batch test results showed that As removal efficiency was higher than 83%, while As leaching was better at 22°C, at the onset of the tests. Laboratory-scale pilot results also revealed that final As and Fe concentrations of ~1.67 mg/L and ~4.40 mg/L, respectively, can be reached at 1-day of HRT.
The findings could improve the knowledge of underutilized materials in NMD-As treatment in passive biofilters. This study is also expected to provide additional information on design criteria for the long‑term efficiency of passive biofilters for NMD-As treatment in cold climates and unfrozen waters.