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 (CH)-based membrane biofilm reactor (MBfR), since the reduction rate was usually limited by electron supply. A low surface loading of SO promoted Cr(VI) reduction. The Cr(VI) removal percentage increased from 60 to 70% when the SO loading increased from 0 to 4.7 mg SO/m-d. After the SO 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 SO (26.6 mg SO/m-d) significantly slowed down the Cr(VI) reduction to 40% removal, which was probably due to competition between Cr(VI) and SO 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 SO. 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 SO, we conclude that Methylocystis, a type II methanotroph, reduced both Cr(VI) and sulfate, Meiothermus only reduced Cr(VI), and Ferruginibacter only reduced SO.