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Acid mine drainage (AMD) pumped from an underground mine can contain dissolved carbonates. When hydrated 1 ime is added in the treatment process to neutralize the AMO, the dissolved carbonates react with the lime to form insoluble calcium carbonate. The presence of dissolved carbonates in the untreated water increases both the amount of 1 ime required to neutralize the AMO and the amount of sludge formed during the treatment process. Laboratory tests show that dissolved carbonates can be removed as gaseous carbon dioxide by aerating the AMD prior to the addition of 1 i me (pre-aeration). Fie 1 d tests were conducted at three AMO treatment plants. The tests demonstrated that pre-aeration is an effective method of reducing treatment costs for an AMO containing dissolved carbonates. Pre-aeration has been permanently applied at each of the sites, and treatment costs have been substantially reduced, while strict compliance with environmental permits has been maintained. The chemistry of dissolved carbonates in the AMD treatment process, the results of the three field tests, and the specific modifications that were made to the treatment plants are discussed.

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