Biochemical reactors (BCRs) biologically reduce sulfate to treat mine-impacted water. Generally, the BCR is a pondlike system containing a permeable bed including organic material. That organic material is used as an electron donor and sulfate in the metal-containing water is transformed during electron acceptance to sulfide, which precipitates many metals in an insoluble form. Adsorption and coprecipitation are other possible mechanisms of metals removal in BCRs. The key design parameter is the rate of sulfate reduction, which is determined by the rate at which the organic material, usually a woody material, is consumed. A wide range of sulfate reduction rates have been reported, presumably due to differences in degradability of the organic substrate. The hydraulics of a BCR is another design consideration, particularly because operational problems are most often related to flow patterns in the bed.