Bacterial diversity in petroleum coke based biofilters treating Oil Sands process water

Abstract Adopting nature-based solutions for the bioremediation of oil sands process water (OSPW) is of significant interest, which requires a thorough understanding of how bacterial communities behave within systems operated under natural conditions. This study investigates the OSPW remediation potential of delayed petroleum-coke (PC), which is a byproduct of bitumen upgrading process and is readily available at oil refining sites, in fixed-bed biofilters particularly for the degradation of naphthenic acids (NAs) and aromatics. The biofilters were operated continuously and total and active bacterial communities were studied by DNA and RNA-based amplicon sequencing in a metataxonomic fashion to extrapolate the underlying degradation mechanisms. The results of total community structure indicated a high abundance of aerobic bacteria at all depths of the biofilter, e.g., Porphyrobacter, Legionella, Pseudomonas, Planctomyces. However, redox conditions within the biofilters were anoxic (−153 to −182 mV) that allowed selected anaerobic bacteria to actively participate in the remediation of OSPW, i.e., Ruminicoccus, Eubacterium, Faecalibacterium, Dorea. The removal of classical NAs was recorded up to 20% whereas oxidized NAs species were poorly removed, i.e., O3-NAs: 4.8%, O4-NAs: 1.2%, O5-NAs: 1.7%, and O6-NAs: 0.5%. Accordingly, monoaromatics, diaromatics, and triaromatics were removed up to 16%, 22%, and 15%, respectively. The physiology of the identified genera suggested that the degradation in the PC-based biofilters was most likely proceeded in a scheme similar to beta-oxidation during anaerobic digestion processes. The presence of hydrogenotrophic methanogens namely Methanobrevibacter and Methanomassiliicoccus and quantification of mcrA gene (2.4 × 102 to 8.7 × 102 copies/mg of PC) revealed that methane production was likely occurring in a syntrophic mechanism during the OSPW remediation. A slight reduction in toxicity was also observed. This study suggests that PC-based biofilters may offer some advantages in the remediation of OSPW; however, the production of methane could be of future concerns if operated at field-scale.