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The effect of the oxygen availability over the performance of an air-breathing microbial fuel cell (MFC) was studied by limiting the oxygen supply to the cathode. It was found that anodic reaction was the limiting stage in the performance of the MFC while oxygen was fully available at cathode. As the cathode was depleted of oxygen, the current density becomes limited by oxygen transport to the electrode surface. The exerted current density was maintained when oxygen mole fraction was higher than 10% due to the very good performance of the cathodic catalysts. However, the current density drastically falls when working at lower concentrations because of mass transfer limitations. In this sense it must be highlighted that the maximum exerted power, when oxygen mole fraction was higher than 10%, was almost three times higher than that obtained when oxygen mole fraction was 5%. Regarding to the wastewater treatment, a significant decrease in the COD removal was obtained when the MFC performance was reduced due to the limited availability of oxygen, which indicates the significant role of the electrogenic microorganisms in the COD removal in MFC. In addition, the low availability of oxygen at the cathode leads to a lower presence of oxygen at the anode, resulting in an increase in the coulombic efficiency.

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In this study, the feasibility and engineering aspects of acidophilic ferrous iron oxidation in a continuous biofilm airlift reactor inoculated with a mixed culture of Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans bacteria were investigated. Specific attention was paid to biofilm formation, competition between both types of bacteria, ferrous iron oxidation rate, and gas liquid mass transfer limitations. The reactor was operated at a constant temperature of 30 degrees C and at pH values of 0-1.8. Startup of the reactor was performed with basalt carrier material. During the experiments the basalt was slowly removed and the ferric iron precipitates formed served as a biofilm carrier. These precipitates have highly suitable characteristics as a carrier material for the immobilization of ferrous iron-oxidizing bacteria and dense conglomerates were observed. Lowering the pH (0.6-1) resulted in dissolution of the ferric precipitates and induced granular sludge formation. The maximum ferrous iron oxidation rate achieved in this study was about 145 molFe(2+)/m(3).h at a hydraulic residence time of 0.25 h. Optimal treatment performance was obtained at a loading rate of 100 mol/m(3).h at a conversion efficiency as high as 98%. Fluorescent in situ hybridization (FISH) studies showed that when the reactor was operated at high ferrous iron conversion (>85%) for 1 month, the desirable L. ferrooxidans species could out-compete A. ferrooxidans due to the low Fe(2+) and high Fe(3+) concentrations.