The kinetics of ferrous iron Fe(II) oxidation by bacteria attached to rotating surfaces were studied in bench scale rotating biological contactors (RBC). Ferrous iron oxidation efficacies of better than 80 percent were obtained over the range of Fe(II) mass loadings tested (0.14 to 2.34 mg Fe(II)/cm('2)/hr). The amount of Fe(II) oxidized, and rate at which Fe(II) was oxidized depended on a combination of influent and bulk fluid Fe(II) concentration as well as flow rate. The density of iron-oxidizing bacteria associated with RBC disc solids increased when the concentration of Fe(II) oxidized increased. Steady state accessory nutrient utilization per mg of Fe(II) oxidized ranged from 0.001 to 0.003 mg PO(,4)-P and 0.002 to 0.007 mg NH(,3)-N. A model describing growth and substrate utilization for bacteria attached to discs of the RBC units was developed and tested. The model describes attached bacterial growth as a saturation function, where the rate of substrate utilization is determined by a maximum substrate oxidation rate constant, a half saturation constant, and the concentration of substrate within the RBC. The maximum oxidation rate constant was proportional to flow rate and the substrate concentration in the reactor varied with influent substrate concentration. The model allowed prediction of metabolic constants and included terms for both constant and growth rate dependent maintenance energies. Inorganic solids produced on the discs of RBC units during biological oxidation of the Fe(II) present in natural and synthetic acidic mine drainage were compared by X-ray energy dispersive microanalysis, X-ray diffraction, and scanning electron microscopy (SEM). The predominant mineral forms were X-ray amorphous ferric oxyhydroxides. In addition, jarosites were detected on RBC surfaces when natural mine drainage was treated and magnetite was detected in solids during treatment of a synthetic acidic mine drainage. A macroscopically visible slime layer appeared on the discs of some field RBC units seemingly in connection with the source of the drainage undergoing treatment. Bacteria were observed on the surface of the inorganic solids and in association with natural slime layers made visible following chemical dissolution of absorbed ferric iron.
