Selection of reactive mixture for biochemical passive treatment of acid mine drainage

Acid mine drainage, characterized by low pH and high concentrations of sulfate and metals, is a worldwide problem. Passive bioreactors under sulfate reduction conditions are a sustainable technology for the treatment of waters impacted with acid mine drainage. Effectiveness and sustainability of the treatment depends on the selection of a balanced reactive mixture composed of an organic carbon and nitrogen source for sulfate-reducing bacteria. Different mixtures of organic and inorganic materials were tested searching which could be used for the treatment of acid drainage produced in the Zipaquirá Mining District of Colombia. Six organic materials that are readily available in this zone were studied for their ability to serve as substrate for sulfate-reducing bacteria. Five reactive mixtures were then tested for 45 days in eighteen batch bioreactors under anaerobic conditions for their efficiency to increase pH and alkalinity, as well as enhancing sulfate reduction and metal removal. The hydraulic conductivity and porosity of the mixtures were also measured. The results showed that all mixtures were efficient for increasing pH and alkalinity, reducing sulfate (>70 %), and removing Fe2+ and Zn2+ (>99 %), but only two mixtures removed Mn2+ (>69 %). One reactive mixture, containing 15 % cow manure, 10 % mushroom compost, 25 % sajo sawdust, 20 % gravel, 15 % limestone, and 15 % sediment, with permeability of 4.5 × 10−2 cm s−1 and porosity of 0.49–0.53, was the most promising option for use in passive bioreactors for treating acid mine drainage in the Zipaquirá Mining District.