Biogas upgrading, CO2 valorisation and economic revaluation of bioelectrochemical systems through anodic chlorine production in the framework of wastewater treatment plants.

Biogas production in wastewater treatment plants (WWTPs) plays a decisive role in the reduction of CO2 emissions and energy needs in the context of the water-energy nexus. The biogas obtained from sewage sludge digestion can be converted into biomethane by the use of biogas upgrading technologies. In this regard, an innovative water scrubbing based technology, known as ABAD Bioenergy® is presented and considered in this work. The effluents resulting from this system consist of biomethane and treated wastewater with a high CO2 concentration. Therefore, the study explores the feasibility of using this CO2-containing effluent in the cathode of a bioelectrochemical system (BES) for the transformation of CO2 into methane. Techno-economic assessment of the process is presented, including the valorisation of anode reactions through the production of chlorine compounds. Finally, the potential impacts of applying this technology in a WWTP operated by FCC Aqualia are (i) increasing biomethane production by 17.4%, (ii) decreasing CO2 content by 42.8% and (iii) producing over 60 ppm of chlorine compounds to disinfect all the treated wastewater of the plant.

Mixed Culture Biocathodes for Production of Hydrogen, Methane, and Carboxylates.

Formation of hydrogen, methane, and organics at biocathodes is an attractive new application of bioelectrochemical systems (BESs). Using mixed cultures, these products can be formed at certain cathode potentials using specific operating conditions, of which pH is important. Thermodynamically, the reduction of CO2 to methane is the most favorable reaction, followed by reduction of CO2 to acetate and ethanol, and hydrogen. In practice, however, the cathode potential at which these reactions occur is more negative, meaning that more energy is required to drive the reaction. Therefore, hydrogen is often found as a second product or intermediate in the conversion of CO2 to both methane and carboxylates. In this chapter we summarize the inocula used for biocathode processes and discuss the achieved conversion rates and cathode potentials for formation of hydrogen, methane, and carboxylates. Although this overview reveals that BESs offer new opportunities for the bioproduction of different compounds, there are still challenges that need to be overcome before these systems can be applied on a larger scale. Graphical Abstract.

Microbial electrosynthesis of butyrate from carbon dioxide: Production and extraction.

To date acetate is the main product of microbial electrosynthesis (MES) from carbon dioxide (CO2). In this work a tubular bioelectrochemical system was used to carry out MES and enhance butyrate production over the other organic products. Batch tests were performed at a fixed cathode potential of -0.8V vs SHE. The reproducibility of the results according to previous experiments was validated in a preliminary test. According to the literature butyrate production could take place by chain elongation reactions at low pH and high hydrogen partial pressure (pH2). During the experiment, CO2 supply was limited to build up pH2 and trigger the production of compounds with a higher degree of reduction. In test 1 butyrate became the predominant end-product, with a concentration of 59.7mMC versus 20.3mMC of acetate, but limitation on CO2 supply resulted in low product titers. CO2 limitation was relaxed in test 2 to increase the bioelectrochemical activity but increase pH2 and promote the production of butyrate, what resulted in the production of 87.5mMC of butyrate and 34.7mMC of acetate. The consumption of ethanol, and the presence of other products in the biocathode (i.e. caproate) suggested that butyrate production took place through chain elongation reactions, likely driven by Megasphaera sueciensis (>39% relative abundance). Extraction and concentration of butyrate was performed by liquid membrane extraction. A concentration phase with 252.4mMC of butyrate was obtained, increasing also butyrate/acetate ratio to 16.4. The results are promising for further research on expanding the product portfolio of MES.

On the Edge of Research and Technological Application: A Critical Review of Electromethanogenesis.

The conversion of electrical current into methane (electromethanogenesis) by microbes represents one of the most promising applications of bioelectrochemical systems (BES). Electromethanogenesis provides a novel approach to waste treatment, carbon dioxide fixation and renewable energy storage into a chemically stable compound, such as methane. This has become an important area of research since it was first described, attracting different research groups worldwide. Basics of the process such as microorganisms involved and main reactions are now much better understood, and recent advances in BES configuration and electrode materials in lab-scale enhance the interest in this technology. However, there are still some gaps that need to be filled to move towards its application. Side reactions or scaling-up issues are clearly among the main challenges that need to be overcome to its further development. This review summarizes the recent advances made in the field of electromethanogenesis to address the main future challenges and opportunities of this novel process. In addition, the present fundamental knowledge is critically reviewed and some insights are provided to identify potential niche applications and help researchers to overcome current technological boundaries.

Tracking bio-hydrogen-mediated production of commodity chemicals from carbon dioxide and renewable electricity.

This study reveals that reduction of carbon dioxide (CO2) to commodity chemicals can be functionally compartmentalized in bioelectrochemical systems. In the present example, a syntrophic consortium composed by H2-producers (Rhodobacter sp.) in the biofilm is combined with carboxidotrophic Clostridium species, mainly found in the bulk liquid. The performance of these H2-mediated electricity-driven systems could be tracked by the activity of a biological H2 sensory protein identified at cathode potentials between -0.2V and -0.3V vs SHE. This seems to point out that such signal is not strain specific, but could be detected in any organism containing hydrogenases. Thus, the findings of this work open the door to the development of a biosensor application or soft sensors for monitoring such systems.