Anaerobic digestion (AD) has been employed with a long history as an effective and practical technology to degrade organic wastes with biogas produced as a renewable and green energy product for developing circular economy (Wei et al., 2024; Subbarao et al., 2023). While AD owns advantages, such as low operating cost and net energy output, challenges still need to be addressed for improving its efficiency. For example, long time for establishing a stable and robust microbial community is one of them, and extended hydraulic retention time (HRT) is another, which consequently compromise organic loading rate (OLR) for anaerobic digestors to be built with large volume and high capital investment (Wei et al., 2024). On the other hand, raw biogas is a mixture of CH4, a large amount of CO2 up to 40%, and small amounts of other impurities (Calbry-Muzyka et al., 2022). Such a characteristic of raw biogas compromises its energy density, rising a necessity for upgrading with significant cost (Aghel et al., 2022). When AD is integrated with microbial electrolysis cell (MEC), not only is the biogas production enhanced, but also the raw biogas upgraded in situ.

MEC has been developed rather late for degrading organic wastes to produce H2 and other value-added products with electroactive microorganisms (Liu et al., 2005; Kong et al., 2020). A typical MEC is composed of an anode, a cathode, and an external circuit for power supply, and the anode and cathode are separated by an ion-exchange membrane to selectively transport ions (Kong et al., 2022). The anode takes electrons that are released from the oxidization of organic wastes, and these electrons are transported by the external power through the external circuit to the cathode to reduce H+ to H2, which can further reduce CO2 to produce CH4 or other chemicals (Kong et al., 2020).

A notable drawback of MEC is the loss of carbon resources in the form of CO2 at the anode. Meanwhile, the overall volumetric productivity of MEC is much lower, since the electrodes cannot provide enough surface for electrochemical reactions to perform efficiently, needless to say high cost for its construction with two chambers that are separated by an ion-exchange membrane. However, when MEC is combined with AD, those intrinsic disadvantages can be overcome properly (Fig. 1).

When AD and MEC are integrated, electroactive bacteria can be enriched on the electrode surfaces as well as in the bulk solution to make electron transfer more efficient and effective, which consequently enhances the AD process for biogas production (Wang et al., 2022). In addition, with the assistance of electrotrophic and hydrogenotrophic methanogens, more CO2 can be reduced to CH4 for upgrading the raw biogas in situ to save capital investment on upgrading facilities with conventional AD processes (Ning et al., 2021). However, no commercial applications of AD-MEC systems are available at present, although intensive studies have been performed.

This critical review aims at assessing the state-of-the-art progress in AD-MEC systems, focusing on underlying fundamentals with AD and MEC, materials and configurations for fabricating the electrodes, designs for integrating the two units, and potential applications of these integration systems. Meanwhile, challenges and strategies for their solutions are also highlighted.