Simultaneous reduction of perchlorate and nitrate using fast-settling anoxic sludge

Abstract

Fast-settling, anoxic sludge (FAS) was cultivated and utilized in this study to simultaneously reduce elevated levels of perchlorate and nitrate in an anaerobic sequencing batch reactor (AnSBR). Average perchlorate and nitrate removal efficiencies of 96.5 ± 8.44 % and 99.8 ± 0.32 %, respectively, were achieved from an average perchlorate and nitrate loading rate of 159 ± 101 g ClO4−/m3·d and 10.8 ± 7.25 g NO3−-N/m3·d, respectively, throughout long-term operation (>500-d). Batch activity tests revealed a preferential utilization of nitrate over perchlorate, where significant perchlorate reduction inhibition occurred when nitrate was present as a competing electron acceptor under carbon-limiting conditions. Specific perchlorate and nitrate reduction rates were shown to increase as the hydraulic retention time (HRT) of the AnSBR was step-wise decreased and subsequently the perchlorate and nitrate loading rates were step-wise increased. Functional, mRNA-based expression of the nitrite reductase (nirS and nirK), nitrous oxide reductase (nosZ), perchlorate reductase subunit A (pcrA), and the chlorite dismutase (cld) genes illustrated the simultaneous activity of heterotrophic denitrification and perchlorate reduction occurring throughout a complete standard reactor operational cycle, and allowed for expression trends to be documented as the HRT of the AnSBR was reduced from 5-d to 1.25-d. Nitrous oxide (N2O) production was detected as a result of incomplete denitrification, where the largest N2O production occurred at the highest nitrate loading rates investigated in this study. Thauera species were heavily enriched at a longer HRT of 5-d, but were out-competed by Dechloromonas species as the HRT of the AnSBR was step-wise reduced to 1.25-d.