Supplemental Table 1 Comparison of the most widely used AMD treatment

Treatment method / Used materials / Type of treatment / Principle / Advantages / Disadvantages / References
Active treatment (addition of alkaline chemicals to raise pH and precipitate metals) / Limestone, hydrated lime, pebble quicklime, caustic soda, soda ash, ammonia, potassium hydroxide, magnesium hydroxide, etc. / Direct addition of chemicals (e.g., limestone) into the AMD stream, lime treatment plant with a mixer/aerator, basket-hopper system, etc. / Active treatment systems collect AMD into pond and apply alkaline chemicals which raise water pH, neutralize acidity and cause precipitation of metals / Some materials (e.g., limestone) are cheap; calcium sulphate and metal hydroxide precipitate; limestone is the safest and easiest to handle of used AMD chemicals; effective treatment / Require continual additions of chemicals; some materials (e.g., caustic soda, ammonia) have high cost and are danger in handling; limestone has low solubility and is usually coated by ferric hydroxide; hydrated lime powder is hydrophobic and extensive mechanical mixing is required; formation of bulky sludge; needs vast aria for sludge storage in ponds; long term stability of neutralization sludge is poorly understood; treated effluent has high pH; scaling; sometimes needs flocculants and oxidants; expensive when the cost of equipment, chemicals, and manpower are considered / GARD/INAP; Skousen and Ziemkiewicz 1995; Johnson and Hallberg 2005; Taylor et al. 2005; Gaikwad and Gupta 2008
Passive treatment / Limestone, clay, natural soil, peat, plant residue, composted organic matter, etc. / Open/Oxic limestone drains, anoxic limestone drain, aerobic and anaerobic wetlands, vertical flow systems, permeable reactive barriers, etc. / Chemical, biological and physical removal processes occurring naturally in the environment / Acid neutralization; remove metals from AMD by physical, chemical and biological processes (uptake by plants, adsorption on organic substrates, oxidation and precipitation, microbial reduction); improve water quality; wetland are valuable ecological system; cost effective; constant operations are not required / Long retention time and large surface areas*; residence time in the limestone layer is critical factor governing the limestone dissolution rate; weather and streamflow are important factors; clogging; construction cost can be high initially depending on the size and specific design of the system; regular maintenance is required / GARD/INAP; Johnson and Hallberg 2005; Taylor et al. 2005; Gaikwad and Gupta 2008; Zipper et al. 2011

*Vertical flow system generally requires shorter residence times and smaller surface areas comparing aerobic and anaerobic wetlands (Zipper et al. 2011)