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Passive treatment of mine water uses chemical and biological processes to decrease metal concentrations and neutralize acidity. Compared to conventional chemical treatment, passive methods generally require more land area, but use less costly reagents, and require less operational attention and maintenance. Currently, the three most common types of passive technologies are aerobic ponds and wetlands, anoxic limestone drains (ALDs), and reducing and alkalinity-producing systems (RAPS). Aerobic wetlands promote mixed oxidation and hydrolysis reactions, and are effective when the raw mine water is net alkaline. Anoxic limestone drains generate bicarbonate alkalinity and can be used to convert water that is net acidic into netalkaline water for treatment in aerobic ponds and wetlands. RAPS promote reducing conditions and limestone dissolution. They extend the concept of ALDs by pre-treating the water before it contacts the limestone, to eliminate dissolved oxygen and reduce dissolved ferric iron to ferrous iron. These systems can generally be used to treat more acidic water than ALDs, and can better treat water with significant aluminum concentrations. In passive treatment systems, rates of metal and acidity removal and alkalinity generation have been developed empirically. Aerobic wetlands remove iron from alkaline water at rates of 10 to 20 g m &2 d &1. Anoxic limestone drains add 150 to 300 mg/L of alkalinity in about 15 hours of contact, imparting 5 to 20 mg/L of alkalinity per hour of contact. Reducing and alkalinityproducing systems add 15 to 60 g m &2 d &1 of alkalinity, depending on influent water quality and contact time. Selection and sizing criteria for the design of passive treatment systems are presented in this report.