Integration of membrane contactors and bioelectrochemical systems for CO 2 conversion to CH 4

Abstract

Anaerobic digestion of sewage sludge produces large amounts of CO 2 which contribute to global CO 2 emissions. Capture and conversion of CO 2 into valuable products is a novel way to reduce CO 2 emissions and valorize it. Membrane contactors can be used for CO 2 capture in liquid media, while bioelectrochemical systems (BES) can valorize dissolved CO 2 converting it to CH 4 , through electromethanogenesis (EMG). At the same time, EMG process, which requires electricity to drive the conversion, can be utilized to store electrical energy (eventually coming from renewables surplus) as methane. The study aims integrating the two technologies at a laboratory scale, using for the first time real wastewater as CO 2 capture medium. Five replicate EMG-BES cells were built and operated individually at 0.7 V. They were fed with both synthetic and real wastewater, saturated with CO 2 by membrane contactors. In a subsequent experimental step, four EMG-BES cells were electrical stacked in series while one was kept as reference. CH 4 production reached 4.6 L CH 4 m −2 d −1 , in line with available literature data, at a specific energy consumption of 16–18 kWh m −3 CH 4 (65% energy efficiency). Organic matter was removed from wastewater at approximately 80% efficiency. CO 2 conversion efficiency was limited (0.3–3.7%), depending on the amount of CO 2 injected in wastewater. Even though achieved performances are not yet competitive with other mature methanation technologies, key knowledge was gained on the integrated operation of membrane contactors and EMG-BES cells, setting the base for upscaling and future implementation of the technology.