Anoxic limestone drains
Anoxic limestone drains are buried trenches or channels containing crushed limestone. Acidic AMD flows through the limestone, dissolving it, and thereby adding alkalinity and increasing pH.
The channels are covered with clay or soil to reduce or eliminate oxygen in the channel. Oxygen is necessary for iron oxide to develop (from ferrous iron), so without it, iron oxide will not coat ("armor") the limestone or clog the channel. However, other metals, such as aluminum and even ferric iron, will precipitate without oxygen, so anoxic limestone drains should not be used to treat AMD containing metals other than ferrous iron.
An anoxic limestone drain not only keeps oxygen out, but it also traps carbon dioxide inside. The carbon dioxide in the drain helps the limestone dissolve into the water.
Once water exits the drain and is exposed to the air, it should be captured in a settling pond. This is because the oxygen, combined with the higher pH acquired in the drain, will cause iron to precipitate very quickly.
Operations & Maintenance Considerations
As limestone dissolves over time, it may need to be replaced if water still has a low pH at its effluent. Also, anoxic limestone drains intend to prevent ferrous iron from oxidizing into ferric iron (which in turn becomes iron oxide), but in the event that the AMD already contains ferric iron or high dissolved oxygen before entering the anoxic limestone drain, iron oxide will form inside, armoring the limestone and clogging the drain. And whatever the composition of the AMD, iron oxide is likely to form at the output of an anoxic limestone drain where it is exposed to air; the discharge should be inspected periodically and accumulated iron removed.
External link: Schematic cross-sections of anoxic and oxic limestone drains (Bucknell University)