Comparative column testing of three reactive mixtures for the bio-chemical treatment of iron-rich acid mine drainage

Optimization of the hydraulic properties of reactive mixtures in passive systems can improve treatment efficiency of iron-rich acid mine drainage (Fe-rich AMD). The use of highly permeable and porous substrates could limit clogging and flow-related issues. The efficiency and evolution of hydraulic conductivity (kat) of three reactive mixtures - two types of dispersed alkaline substrate (DAS), composed of wood ash (50% v/v - WA50) or calcite (20% v/v - C20), and one mixture consisting mainly of organic matter (70% w/w), typically used in passive biochemical reactors (PBRs) - were tested in 1.5 L columns. The reactors were operated with hydraulic retention times (HRTs) of 1 to 5 d over a period of 16-63 days. In the WA-DAS reactors, results showed that a minimum HRT of 2 d was required to remove 33-62% of Fe in AMD with 2500 mg/L Fe. The calcite-DAS showed limited Fe removal (< 10%) in AMD at > 1500 mg/L Fe, but was around 40% at < 1500 mg/L Fe, over a 7-day period, at 2 d of HRT. Slight increase of the PBRs efficiency was found (77% and 91%), at initial Fe concentration of 500 mg/L, when the HRT was doubled (from 2.5 d to 5 d). All reactors removed other metals (37-99.9% for Al, Zn, and Pb; 20-98% for Ni, except in C20) and SO42- (5-37%). Evolution of the hydraulic parameters of all reactors showed insignificant variation of the initial porosity of 0.68-0.74 and k(sat) around 10(-2) cm/s, indicating no evidence of clogging throughout the testing period, even at 5 d of HRT. Nonetheless, Fe removal was HRT-dependent. Therefore, water quality, especially Fe concentration, should be among the design criteria for a long-term satisfactory treatment of Fe-rich AMD.