Inorganic residues produced on the disks of rotating biological contactor (RBC) wastewater treatment units during microbiological oxidation of the ferrous iron in both natural and synthetic acidic mine drainages were analyzed using x‐ray energy‐dispersive microanalysis, x‐ray diffraction, and scanning electron microscopy. The predominant mineral forms were x‐ray amorphous ferric hydroxysulfates. In addition, jarosites were detected in RBC units which treated natural mine drainages, and magnetite was detected in solids generated during treatment of a synthetic acidic mine drainage. Bacteria were observed on the surface of the inorganic residues and in gelatinous films seen microscopically following chemical dissolution of the iron crust.