Aerobic naphthenic acid-degrading bacteria in petroleum-coke improve oil sands process water remediation in biofilters: DNA-stable isotope probing reveals methylotrophy in Schmutzdecke

Abstract There is an increasing interest in treatment of oil sands process water (OSPW) via biofiltration with petroleum coke (PC) as a substratum. In fixed bed biofilters (FBBs) with PC, the dominance of anaerobic digestion of dissolved organics results in poor removal of naphthenic acids (NAs) along with a high degree of methanogenesis. In this study, operation of FBBs was modified to improve OSPW remediation by supporting the filtering bed with aerobic naphthenic acid-degrading bacteria treating aerated OSPW (FBBbioaugmentation). The results were compared with a biofilter operated under controlled conditions (FBBcontrol). To this end, a consortium of three aerobic NAs-degrading bacterial strains was immobilized on PC as a top layer (10 cm). These bacteria were pre-screened for growth on 15 different NAs surrogates as a sole carbon source, and for presence of catabolic genes coding alkane hydroxylase (CYP153) and alkane monooxygenase (alkB) enzymes. The results illustrated that biofiltration in FBBbioaugmentation removed 32% of classical NAs in 15 days; while in the FBBcontrol, degradation was limited to 19%. The degradation of fluorophore (aromatic) compounds was also improved from 16% to 39% for single ring (OI), 22% to 29% for double ring (OII), and 15% to 23% for three rings (OIII) compounds. DNA-Stable Isotope Probing revealed that potential hydrocarbons degraders such as Pseudomonas (inoculated), Pseudoxanthomonas (indigenous) were present up to 5.0% in the 13C-labelled DNA fraction. Furthermore, a high abundance of methylotrophs was observed in the Schmutzdecke, with Methylobacillus comprising more than two-third of the total community. This study shows that bioaugmentation rapidly improved OSPW remediation. Aeration mostly contributed to methane consumption in the top layer, thus minimize its release into the environment.