Microbial community analysis of sulfate-reducing passive bioreactor for treating acid mine drainage under failure conditions after long-term continuous operation

Abstract Sulfate-reducing passive bioreactor is an option for treating acid mine drainage (AMD) for a long period of time without maintenance. A 35-L packed-bed sulfate-reducing bioreactor containing rice bran was continuously operated for over 800 days, however, reactor performance became progressively worsened after day 871, resulting in the effluent zinc concentration increasing above the national effluent standard. Both microbial communities and chemical parameters along the height of the bioreactor were analyzed by using Illumina sequencing of 16S rRNA genes and analytical instruments, respectively, in order to compare the data before and after day 871. During the monitoring period (day 800 to day 884), the total organic carbon concentration remained low (5.3–8.1 mg L –1 ), indicating that the substrates for sulfate-reducing bacteria (SRB) in the bioreactor were nearly depleted. On day 872, oxidation-reduction potential (ORP) values increased, especially in the upper portion of the bioreactor, resulting in dominance of aerobic Alishewanella - and Mitsuaria -related microorganisms. In the lower portion of the bioreactor, relative abundance of SRB decreased significantly on day 872, whereas the candidate phylum OP11 bacteria increased. These results suggested that low organic concentration, high ORP, and low AMD temperature conditions after day 871 decreased the relative abundance of SRB in the bioreactor, leading to the deterioration of the reactor performance.