Sulfate Reducing Bioreactors
Some information adapted from "A handbook of Technologies for Avoidance and Remediation of Acid Mine Drainage." prepared by J. Skousen, A. Rose, G. Geidel, J. Foreman, R. Evans, W. Hellier, and Members of the Avoidance and Remediation Working Group of the Acid Drainage (ADTI)
Sulfate-reducing bioreactors use special forms of bacteria to remove metals of AMD and add alkalinity. These bacteria convert sulfate (an ion present in AMD) into both sulfide and bicarbonate. The sulfide then combines with dissolved metals to create an insoluble compound that drops out of the water. Meanwhile, the bicarbonates add alkalinity to the water, increasing its pH.
Usually, AMD flows through a layer of these bacteria in a pond after it leaves the mine, but before it enters a stream. In some cases, however, it is possible to introduce the bacteria into the mine itself, called "in situ" treatment, so that the water emanating from the mine is relatively harmless.
Operations & Maintenance Considerations
It may be necessary to replace the compost layer because compost may become compacted or clogged, reducing its permeability. Also, flows of water may create preferential paths through the compost over time. This results in short-circuiting and decreased retention time. Compost replacement will probably take place once during the life of a system.
Of course, like most passive treatment systems, basic maintenance including mowing grass around a system and removing leaves that may clog conduits is necessary. Also, animals may damage a system, especially muskrats, whose tunnels must be blocked with dirt and rocks.
Passive Treatment of Aluminum-Bearing Acid Rock Drainage by James J. Gusek, P.E., Thomas R. Wildeman, Ph.D.