|| Acid mine drainage (AMD) is acidic and generally enriched with iron, aluminum, sulfate and heavy metals, such as lead and cadmium. AMD is a growing problem of emerging concern that cause detrimental effects to the environment and living organisms. Yanahara mine in Misaki Town, Okayama, Japan, had mainly produced pyrite for sulfuric acid manufacture. Although it was closed in 1991, AMD is being generated from the mine now. A passive treatment based on the biological oxidation of ferrous iron is a promising strategy for AMD remediation. AMD from Yanahara mine is treated in a plant using iron-oxidizing bacteria, Acidithiobacillus ferrooxidans and Ferrovum spp. The AMD generation continues for several centuries with dramatic consequences on the receiving environments. Therefore, the development of sustainable and cost effective treatment process is required. A development of the effective biological treatment process with an iron oxidation reactor operated at pH 3.5 is described in this report. Economic aspects are also discussed.