Microbial community dynamics in a feedback-redox controlled bioreactor process that enabled sequential selenate, nitrate and sulfate removal, and elemental selenium recovery

Oxidation reduction potential (ORP) feedback-control by ethanol-dosing was recently proved effective in facilitating a two-stage fluidized bed reactor (FBR) process to remove selenate, nitrate, and sulfate from wastewater, and recover high-purity elemental Se without the formation of carcinogenic SeS. However, further optimization requires knowledge on the relationship between microbial community composition and process performance. Here, changes of microbial communities during the reduction of selenate and nitrate in the first FBR (FBR1) under various ORP conditions (−240 to −520 mV vs. Ag/AgCl), sulfate reduction in subsquent FBR (FBR2) and sulfide oxidation in sulfide oxidation reactor (SOR) were examined. Based on 16S rRNA gene sequencing, high ORP (stage i without sulfate: >−360 mV; stage ii with sulfate: >−440 mV) and elevated sulfate at ORP ≤ −480 mV (stage iii) in FBR1 altered the microbial communities both on fluidized carrier and bulk liquor. Selenate reducers (Geoalkalibacter and Geovibrio genera) were impacted at ORP > −360 mV (stage i) or >−440 mV (stage ii), when selenate removal in FBR1 declined. However, nitrate reducers (Caenispirillum and Paracoccus genera) were not affected in the FBR1, and facilitated ~100 % nitrate removal. At ORP ≤ −480 mV, selenate reducers were readily enriched in FBR1. In the FBR2, the major sulfate reducing genera were Desulfomicrobium, Desulfovibrio and Desulfuromonas, whereas the dominant sulfide-oxidating genera in SOR were Paracoccus, Thiobacillus and Acholeplasma. This study suggested that both nitrate- and selenate-reducing communities could remain active under elevated sulfate concentrations, enabling sequential oxyanions removal and elemental Se recovery from wastewater.