Biological co-reduction of perchlorate and nitrate in wastewater: Insights into the competitive inhibition mechanisms in UASB reactors

The pollution caused by wastewater with high perchlorate concentrations should not be underestimated. Efficient biodegradation is a promising treatment method. Nitrate, which often coexists with perchlorate in wastewater, has a great influence on its degradation. Therefore, in-depth studies on the combined degradation mechanisms of perchlorate and nitrate under coexisting conditions are essential. This study developed two heterotrophic up-flow anaerobic sludge blanket (UASB) reactors. In R1, perchlorate was efficiently removed, with a load of 927 ± 7.72 gClO₄^-/(m³·d), and removal efficiency exceeded 99.4 %. In R2, perchlorate and nitrate were removed simultaneously, with loadings of 927 ± 3.52 gClO₄^-/(m³·d) and 81 ± 7.25 gNO₃^--N/(m³·d), respectively. Removal efficiencies were 95.9 ± 3.24 % for perchlorate and 99.8 ± 0.09 % for nitrate. A modified Monod competition model indicated that nitrate concentration exerts a more significant inhibitory effect at nitrate-to-perchlorate concentration ratios of 1:1 or higher. Batch experiments identified the optimal CH₃COO^-: ClO₄^-: NO₃^--N molar ratio as 5.4:1:3.6 and the ideal environmental ORP as -200 ± 20 mV for co-removal of perchlorate and nitrate. Metagenomic analysis revealed the dynamics of microbial communities and functional genes associated with perchlorate and nitrate reduction. Nitrate presence regulates the abundance and activity of NapC and cyt c transporter enzymes, which indirectly weaken the perchlorate electron transport pathway and directly affect terminal electron receptor coupling efficiency. This study provided valuable insights for optimizing biological treatment strategies for wastewater contaminated with perchlorate and nitrate.