The use of a suspended growth system of thiobacillus ferrooxidans attached in a film to individual particles of bentonite and operated as a sequencing batch reactor is shown to be a practical way of oxidizing ferrous iron in acid mine drainage. This component, followed by limestone neutralization, is both a technically and economically feasible approach to treating these wastewaters, compared with existing and other proposed methods. The advantages, in addition to less expense, are greater safety, acceptable treatment at a lower ph, lower solids buildup, less danger of overtreatment, and greater flexibility in operational characteristics. In this paper, data on the continuous bench-scale operation of the system under a variety of conditions show sustained levels of oxidation in excess of 98%. Capital and operating costs are projected to be $80/1,000 m3 ($80 per 35,000 cu ft) of wastewater.