Article pubs.acs.org/EF
Performance of the Bio-electrochemical Anaerobic Digestion of Sewage Sludge at Different Hydraulic Retention Times Young-Chae Song,*,† Qing Feng,† and Yongtae Ahn‡ †
Department of Environmental Engineering, Korea Maritime and Ocean University, Busan 606-791, Korea Department of Energy Engineering, Gyeongnam National University of Science and Technology, Jinju, Gyeongnam 660-768, Korea
‡
ABSTRACT: The performance of bio-electrochemical anaerobic digester, applied with 0.3 V between anode and cathode, for sewage sludge was explored at different hydraulic retention times (HRTs) ranging from 20 to 5 days. The state variables including pH, alkalinity, VFA, and SCOD are quite stable and in suitable ranges for anaerobic digestion. At 20 days of HRT, the performance is considerably high in volatile solids (VS) reduction (70.5%), the specific methane production rate (407 mL L−1 d−1), and the methane content (76.9%) in biogas. The VS reduction and the methane content in biogas slightly deteriorated as the HRT decreased from 20 to 5 days, while the methane production rate increased to 1,339 mL L−1 d−1. The overall energy efficiencies for methane recovery are in the range of 69.1−98.7%, and the maximum energy efficiency appears at 10 days of HRT. These results showed that the bio-electrochemical anaerobic digestion system could obtain much higher VS reduction and energy recovery with shorter HRTs than the conventional anaerobic digestion technologies. biologically relevant condition at a pH 7.0.11 From the relationship (ΔG = −nFE) between the electrode potential and the free energy change, the driving force for the anodic oxidation becomes theoretically larger at a more positive electrode potential than Epa and the cathodic reduction becomes theoretically larger at a lower, more negative value than Epc. The bio-electrochemical methane production from the reduction of proton and carbon dioxide was confirmed at −0.65 V (vs. Ag/AgCl) of the cathode potential, and significantly increased at more negative cathode potentials.12−15 Recently, enhanced methane production was shown to be possible in a bio-electrochemical reactor with a low applied voltage of 0.3 V, but higher applied voltage led to more accumulation of hydrogen.16,17 The optimal electrode potentials for oxidation and reduction in a bio-electrochemical anaerobic digester are still ambiguous. The bio-electrochemical methane production is certainly influenced by the design parameters including electrode materials, electrode size, and arrangement in the digester, as well as the operational parameters including pH, temperature, and organic loading rate. However, the studies on the design and operational parameters for the bio-electrochemical methane production are still limited. In the present study, a laboratory scale anaerobic bioelectrochemical digester for sewage sludge was semicontinuously operated at a low applied voltage of 0.3 V between an anode and a cathode. The performance of the digester including digester stability, organic matter removal, biogas production, and methane content in the biogas were investigated at different HRTs from 20 to 5 dyas.
1. INTRODUCTION Anaerobic digestion is a traditional appropriate technology which has been used for over 100 years for the stabilization of organic waste and energy recovery in the form of methane. However, some concerns remain that need to be resolved, such as low organic matter degradation (e.g.,
