The effect of electron competition on chromate reduction using methane as electron donor

We studied the effect of electron competition on chromate (Cr(VI)) reduction in a methane (CH4)-based membrane biofilm reactor (MBfR), since the reduction rate was usually limited by electron supply. A low surface loading of SO4 2− promoted Cr(VI) reduction. The Cr(VI) removal percentage increased from 60 to 70% when the SO4 2− loading increased from 0 to 4.7 mg SO4 2−/m2-d. After the SO4 2− loading decreased back to zero, the Cr(VI) removal further increased to 90%, suggesting that some sulfate-reducing bacteria (SRB) stayed in the reactor to reduce Cr(VI). However, a high surface loading of SO4 2− (26.6 mg SO4 2−/m2-d) significantly slowed down the Cr(VI) reduction to 40% removal, which was probably due to competition between Cr(VI) and SO4 2− reduction. Similarly, when 0.5 mg/L of Se(VI) was introduced into the MBfR, Cr(VI) removal percentage slightly decreased to 60% and then increased to 80% when input Se(VI) was removed again. The microbial community strongly depended on the loadings of Cr(VI) and SO4 2−. In the sulfate effect experiment, three genera were dominant. Based on the correlation between the abundances of the three genera and the loadings of Cr(VI) and SO4 2−, we conclude that Methylocystis, a type II methanotroph, reduced both Cr(VI) and sulfate, Meiothermus only reduced Cr(VI), and Ferruginibacter only reduced SO4 2−.