Nitrate effects on chromate reduction in a methane-based biofilm

The effects of nitrate (NO₃ ⁻) on chromate (Cr(VI)) reduction in a membrane biofilm reactor (MBfR) were studied when CH₄ was the sole electron donor supplied with a non-limiting delivery capacity. A high surface loading of NO₃ ⁻ gave significant and irreversible inhibition of Cr(VI) reduction. At a surface loading of 500 mg Cr/m²-d, the Cr(VI)-removal percentage was 100% when NO₃ ⁻ was absent (Stage 1), but was dramatically lowered to < 25% with introduction of 280 mg N m⁻²-d NO₃ ⁻ (Stage 2). After ∼50 days operation in Stage 2, the Cr(VI) reduction recovered to only ∼70% in Stage 3, when NO₃ ⁻ was removed from the influent; thus, NO₃ ⁻ had a significant long-term inhibition effect on Cr(VI) reduction. Weighted PCoA and UniFrac analyses proved that the introduction of NO₃ ⁻ had a strong impact on the microbial community in the biofilms, and the changes possibly were linked to the irreversible inhibition of Cr(VI) reduction. For example, Meiothermus, the main genus involved in Cr(VI) reduction at first, declined with introduction of NO₃ ⁻. The denitrifier Chitinophagaceae was enriched after the addition of NO₃ ⁻, while Pelomonas became important when nitrate was removed, suggesting its potential role as a Cr(VI) reducer. Moreover, introducing NO₃ ⁻ led to a decrease in the number of genes predicted (by PICRUSt) to be related to chromate reduction, but genes predicted to be related to denitrification, methane oxidation, and fermentation increased.