Bioelectrochemical systems serve anaerobic digestion process for process monitoring and biogas upgrading

Abstract

Bioelectrochemical systems (BES), which employ microbes as catalysts to convert chemical energy stored in organic matter into sustainable electricity and high-value chemicals, is an emerging and promising technology. BES have broad applications including wastewater treatment, chemical production, resource recovery and waste remediation. Recently, new concepts of been proposed. The purpose of this work was to optimize the AD process using BES in two aspects: developing a new volatile fatty acid (VFA) monitoring system which can be used as the AD process indicator, and for improving biogas quality by removing CO2. In this thesis, a microbial desalination cell (MDC) was developed for measuring VFAs concentrations. The MDC was composed of three chambers, namely an anode, a cathode and a middle chamber. The samples were measured in the middle chamber, which was separated from the anode by an in their ionized form contained in the sample, diffused through AEM to the anode where they were microbially oxidized and produced current signals. The effect of operating parameters such as ionic strength and external resistance on the performance of the MDC-typed biosensor were assessed. High ionic strength and small external resistance were advantageous for current signal amplification. Two linear relationships between current outputs and VFA concentrations were observed. The response time was approx. 5 h and the detection range was 1 to 200 mM. The selectivity of the biosensor was demonstrated since organic matter such as protein and lipids were retained by the AEM and their interference was eliminated. The reliability was proved by real AD effluents. In order to reduce the construction cost and simplify the VFA biosensor, a new configuration was developed. The number of chambers was reduced from three to two. The new configuration was a microbial electrolysis cell (MEC). The anode and cathode chambers were separated by an AEM and a small additional voltage was supplied to the cell. The samples were measured in the cathode. The effect of different parameters such as external voltage, ionic strength and VFA composition ratio on the MEC-typed biosensor performance was evaluated. Higher current signals were observed under larger external voltage and higher ionic strengths. The current output was mainly contributed by acetate which was always dominant in AD reactors. The current density increased linearly along with VFAs concentrations ranging from 5 to 100 mM. The response of the biosensor was now only 1 h due to the faster transfer of VFAs supported by the external voltage. The interference from other non-ionic organic matter (glucose, cellulose, lipids and protein) could be eliminated since they were retained by the membrane. During the process, hydrogen (H2) was generated from water hydrolysis. The produced H2 could potentially contribute to the energy needs for operating the biosensor and thereby to a self-sustaining system. Moreover, the biosensor was successfully validated both with synthetic and real AD effluents. To improve biogas quality, a microbial electrolytic capture, separation and regeneration cell (MESC) was developed. The effects of external voltage and inlet gas flow rate were elucidated. The current output increased along with the gas flow rate, while cathodic pH and upgrading performance showed opposite trends. The current output, cathodic pH and upgrading performance increased with the increasing external voltage supply. In MESC, acid and alkaline generation, CO2 capture, biogas upgrading and COD removal were simultaneously achieved. Under the optimum condition at 1.2 V external voltage and 19.6 mL/h gas flow rate, pH in the regeneration and cathode  chambers could reach 1.34±0.04 and 9.19±0.11, respectively; the maximum methane content was up to 97.0±0.2% and COD removal efficiency reached 98.2±2.6%. The energy consumption for biogas upgrading was around 0.17 kWh/m3 raw biogas. Moreover, the generated H2 from water hydrolysis could potentially compensate for 23.4% of the energy consumption. It has been proved that the development of efficient, cheap, fast and reliable VFA monitoring with a wide detection range can be realized in BES which is sustainable and environmental friendly. The development technology could easily be installed as online monitoring system for optimizing the AD process. Moreover, BES could be a sustainable economic technology to upgrade biogas to biomethane and thereby increase the value of biogas. The proof-of-concept study in lab-scale offers ideas for expanding BES application.