Although similar in appearance to aerobic wetlands, anaerobic wetlands are used to treat AMD that is acidic or that has high sulfate concentrations.
In an anaerobic wetland (also called a "compost wetland"), water flows horizontally through organic compost and limestone (either in layers or mixed together). Bacteria in the compost remove oxygen and sulfate from the water. The bacteria also help to reverse the normal chemical reactions by which AMD creates iron oxide, and in doing so, some of the acid in the water is neutralized (see Sulfate-Reducing Bioreactors). The limestone adds even more alkalinity to neutralize acid.