Bio-electrochemical systems for treatment of petroleum hydrocarbon contaminated wastewater and sediment

Abstract

Bio-electrochemical systems (BES), such as microbial fuel cells, can produce electric power as well as expedite anaerobic microbial biodegradation of complex contaminants such as petroleum hydrocarbons in water or sediment by using the BES anode to offset the limitation of insufficient electron acceptors. Robust bio-electrocatalytic anodic biofilms and sustained cathodic oxygen reduction reaction are imperative to achieve superior performance in BES. Petroleum hydrocarbons in sediment-water system can be removed using biological methods such as bioremediation. In this work we investigated if bio-electrocatalytic activity of anodic biofilm can be used to supplement and enhance the removal efficiency of petroleum hydrocarbon from wastewater and contaminated sediment while also producing electricity. Bioanodes of double chamber microbial fuel cells (MFCs) were enriched with inoculum from two different sources (MFC effluent and activated sludge) and employed for removal of polyaromatic hydrocarbons (PAHs) from hydrocarbon wastewater under high and low external resistance, Rext (100 and 1000 Ω). Anodic biofilms of all MFCs studied were able to tolerate low concentration of Tween 80 surfactant (100 ppm). Inoculum source influenced current density and charge output in MFCs at start-up. Remarkably, MFCs inoculated with activated sludge started up faster and achieved higher current density (59.44 ± 0.98 mA/m2 ) than MFCs inoculated with MFC effluent (1.65 ± 0.03 mA/m2 ). Inoculum source did not appear to have any influence on overall PAH removal efficiency. Under low Rext (100Ω), similar PAH removal efficiencies were obtained (at 50 ppm PAH) in MFCs inoculated with MFC effluent (86 ± 4 %) and MFCs inoculated with activated sludge (84 ± 3 %). Higher electrochemical PAH removal and coulombic efficiency (CE) was achieved in AS under low Rext (PAH removal = 84 ± 3 %; CE = 27.5 ± 2.9 %) than high Rext (PAH removal = 72 ± 5 %; CE = 14.8 ± 2.6 %) at PAH concentration of 50 ppm. Cyclic voltammetry showed that stable anodic biofilm with higher bio-electrocatalytic properties were developed under low Rext (100Ω). Double and single chamber sediment microbial fuel cells (SMFCs) as well as wicking and submerged cathode configurations were investigated for enhanced removal of petroleum hydrocarbons from hydrocarbon-contaminated sediment. Wicking cathode configuration was utilized in single chamber SMFCs to evaluate trade-off between oxygen and ion transport on SMFC performance (including its PAH removal efficiency). Current density and charge output in single chamber SMFC with vertical electrode arrangement was 3 times higher using wicking cathode, CW (166 ± 78 mA/m2 , 298 ± 140 C) compared with submerged cathode, CS (54 ± 21 mA/m2 , 97 ± 38 C). Comparable PAH removal efficiencies were achieved in CW (51 ± 3 %) and CS (46 ± 2 %). Wicking cathode was shown to enhance current/charge output in single chamber sediment microbial fuel cells using vertical electrode arrangement with marginal enhancement of PAH removal. Microbial community analysis of bioanodes of MFCs (Rext = 100Ω) and single chamber SMFC (vertical electrode arrangement) showed the presence of similar bacterial phyla (Proteobacteria, Actinobacteria, Firmicutes and Bacteroidetes) in varying proportions. In addition, Chloroflexi, Planctomycetes, and Epsilonbacteraeota were found in SMFC bioanodes. Individual members of these phyla found in both MFCs and single chamber SMFCs were distinct, and had potentially diverse roles within the bioreactors. Putative electroactive bacteria (EAB) Geobacter and Acidovorax were enriched in closed circuit MFCs relative to open circuit MFCs. The presence of Pseudomonas (some members of which are putative EAB) in both closed and open circuit MFCs was indicative of its versatile role in MFCs. This new finding showed that bacterial enrichment in each MFC was linked to their specific function in the bioreactor. SMFC anode comprised high diversity of species with more evolutionarily divergent taxa compared with MFC anode communities. Enrichment of putative fermenters Anaerolineaceae, Bacteroidetes and Clostridiaceae relative to other families correlated with low coulombic efficiency (< 2 %) obtained in closed circuit SMFCs. Significant enrichment (145 to 317 %) of putative fermentative anaerobe Anaerolineaceae in single chamber SMFCs within 35 days of operation was a novel finding in this study. In summary, this project demonstrated that electrochemical removal of PAH from hydrocarbon wastewater could be marginally improved using activated sludge inoculum and low external resistance (Rext) due to higher bio-electrocatalytic activity in anodic biofilms under low Rext. Application of wicking cathode enhanced current/charge output in single chamber sediment microbial fuel cell using vertical electrode arrangement with marginal enhancement of electrochemical oxidation of PAH. Microbial community composition showed relative abundance of members with known fermentative metabolic capacities. High microbial diversity was a key factor in enhancement of MFC/SMFC performance due to synergistic interactions between various members of each community.