There is abundant energy in organic matter in wastewaters, and conventional wastewater treatment practices such as activated sludge process require large amount of energy to remove them. Microbial fuel cells (MFC) is an emerging and innovative technology for wastewater treatment and renewable energy recovery, which has been drawn great attention in the past decade. In microbial fuel cells, exoelectrogenic microorganisms grow on an electrode (the anode), treat wastewater by oxidizing organic and inorganic matter and generate electricity by releasing the electrons to the anode. The competition for substrates between exoelectrogenic bacteria and other microorganisms can affect removal rates and current production. Dr. Xiaoyuan Zhang at Tsinghua University, Prof. Bruce Logan at Penn State University, and their colleagues have systematically studied COD removal characteristics in air-cathode microbial fuel cells to better understand this competition and its impact on substrate removal and current generation. The findings in their study show that for simple organic molecules, such as acetate, and soluble organics in filtered wastewater, organic matter removal rates followed first-order kinetics and therefore there was removal in proportion to the concentration of organic matter by both exoelectrogens and by other microbes. Using a relatively low external resistance allowed microbial fuel cells to generate high current. A high current directed more of the organic matter into electrical current, and it accelerated the rate of organic matter removal so treatment could be accomplished faster. Surprisingly, the rate of removal of complex organic matter in wastewater was even faster than that of acetate. The findings in the study are helpful for understanding factors that will affect the use of microbial fuel cells for wastewater treatment. By designing the system to maximize current, it should be possible to improve renewable energy recovery and reduce the time needed for wastewater treatment.
Bioresource Technology, Volume 176, 2015, Pages 23-31.
Xiaoyuan Zhang1,2, Weihua He3, Lijiao Ren2, Jennifer Stager2, Patrick J. Evans4, Bruce E. Logan2Show Affiliations
1 State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, PR China.
2 Department of Civil & Environmental Engineering, Penn State University, 231Q Sackett Building, University Park, PA 16802, USA.
3 State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 73 Huanghe Road, Nangang District, Harbin 150090, PR China.
4 CDM Smith, 14432 S.E. Eastgate Way, Suite 100, Bellevue, WA 98007, USA.
Exoelectrogenic microorganisms in microbial fuel cells (MFCs) compete with other microorganisms for substrate. In order to understand how this affects removal rates, current generation, and coulombic efficiencies (CEs), substrate removal rates were compared in microbial fuel cells fed a single, readily biodegradable compound (acetate) or domestic wastewater (WW). Removal rates based on initial test conditions fit first-order kinetics, but rate constants varied with circuit resistance. With filtered WW (100 Ω), the rate constant was 0.18 h−1, which was higher than acetate or filtered WW with an open circuit (0.10 h−1), but CEs were much lower (15–24%) than acetate. With raw WW (100 Ω), COD removal proceeded in two stages: a fast removal stage with high current production, followed by a slower removal with little current. While using microbial fuel cells increased COD removal rate due to current generation, secondary processes will be needed to reduce COD to levels suitable for discharge.