Proper treatment of wastewater is essential to protecting water resources, ecosystems, and wildlife. Unfortunately, we have failed to achieve this globally, with more than 50% of wastewaters not being treated at all (1). The cost of treating wastewaters, along with the costs of the infrastructure required for transport and treatment, have limited the implementation of wastewater treatment in much of the Global South. Furthermore, wastewater treatment in the Global North is expensive and a non-trivial source of greenhouse gases. Sadly, wastewater treatment often is viewed as something to be “tacked on” only when required and when financial resources are available.

An irony is that wastewater contains energy, nutrients, and water that represent resources of significant economic value (2); recovering them can promote a circular economy and reduce greenhouse gas emissions. To achieve this, we need new, dependable technologies that shift the paradigm from mere treatment towards recovery for value. Then, wastewater treatment can stop being an economic and environmental liability and become a profitable source of energy, nutrients, and clean water. Wastewater treatment can thereby become more affordable in all regions and for all industries.

Microbial electrochemical technologies (METs) have the potential to help meet both goals—resource recovery and economic feasibility. METs are based on the metabolism of electroactive bacteria, which oxidize organic compounds and deliver the electrons to an anode of an electrochemical cell (3). The anode is the bacteria’s respiratory electron acceptor, as electrons are transferred through a conductive biofilm that employs cytochrome-based nanowires. This anode-respiring metabolism converts the chemical energy contained in the wastewater’s organic compounds into electrical energy that can be directed towards processes yielding economic value.

The electrons generated at the anode pass through an electrical circuit and end up at the cathode, where they reduce oxidized species to produce something of value. Using the anode as the bacteria’s respiratory electron acceptor avoids direct reliance on oxygen (O₂). This avoids two disadvantages of normal aerobic wastewater treatment, namely the limited rate of O₂ transfer to water, which prevents having a very compact process, and the high biomass yield, which generates a large amount of excess, wasted biomass. In addition, aeration results in the release of gaseous contaminants, such as nitrous oxide (N₂O), into the atmosphere (4). METs can decrease these emissions owing to their anaerobic means of metabolism.

The early development of METs focused on generating electrical power by harvesting the potential difference between the anode and O₂ reduction at the cathode; this is called a microbial fuel cell (MFC). Generating electrical power in an MFC became less interesting because the net harvest of electrical potential is small due to losses (called over-potentials) at both electrodes. Today, the focus has shifted to investing the electrons (and their embedded energy) to generate valuable products at the cathode, e.g., hydrogen gas (H₂) by reducing hydrogen ions (H⁺), hydrogen peroxide (H₂O₂) by partially reducing O₂, and organic biomolecules by reducing carbon dioxide (CO₂). This shift is due in part to the inherent low-voltage efficiency of MFCs, stemming from their construction and the treatment of low conductivity wastewaters. Because H₂, H₂O₂, and organic biomolecules generally have considerably greater economic value than electrical power, their production at the cathode drives much of the current applications-oriented research on METs.

In their Frontiers in Science lead article, Schröder et al. (5) outline many engineering applications that take advantage of converting chemical energy to electrical energy. The new applications can provide wastewater treatment and important economic value, such as energy recovery (direct or indirect), nutrient recovery, sensing, and bioproducts. For example, our studies on H₂O₂ production estimated that a small wastewater treatment plant (10⁴ m³ wastewater/day) could produce nearly 2 tons of H₂O₂ per day using an MET to treat their waste sludges (6). The volume of the MET would be only approximately 50% of an anaerobic digester, and the economic value of H₂O₂ would be four-fold more than the value of methane (CH₄). Thus, MET applications that focus on high treatment rates are particularly attractive.

So far, the major large-scale successes in the wastewater industry, such as the METlands^® and the Aquacycl’s BioElectrochemical Treatment Technology (BETT^®) reactors, have focused not on energy or product recovery, but on treatment, particularly the removal of biochemical oxygen demand. Even when the MET is not generating a valuable output, its energy conversion approach using an electroactive biofilm allows faster organic fluxes than aerobic biofilms. Thus, METs take advantage of their ability to generate high currents, even when the harvestable potential is limited. The high current provides an opportunity for high-rate oxidation that intensifies the treatment process, making it more compact.

Another set of applications aims to improve current wastewater treatment processes: for example, assisted methanogenesis improves digestibility in anaerobic digestors. In this case, the MET’s value is gauged by the extra organic loading rate that the digester can handle, which increases methane production. These “add-on” MET technologies can provide a straightforward way to introduce METs into the wastewater industry.

METs also have applications outside of directly treating the wastewater. For example, sensors based on microbial electrochemistry exploit the simplicity of electrical measurements to predict the metabolic health of wastewaters (7). By tracking current, electroactive bacteria make real-time monitoring of wastewater processes possible and effective. These monitors can then rapidly predict changes in the oxidation/reduction status in activated sludge and organic acid accumulation in anaerobic digesters. Sensing applications may be among the first MET technologies to reach full commercialization. Microbial electrochemistry also can be used to aid in situ remediation of groundwater by promoting the transport ionized contaminants [e.g., ammonium (NH₄⁺)].

The future of METs in the wastewater industry will depend in good measure on the success of these early technologies, but long-term success will not necessarily target the same goals. Like Schröder et al., we envision a future for METs in the wastewater field that implements resource recovery (direct or indirect) and automation through sensing technologies. Advances will be needed in MET materials, design configurations, and control of factors such as anode and cathode potentials and pH, as well as efficient recovery of high-value outputs. Through these improvements, METs could provide an attractive business model for wastewater treatment, allowing automated energy, nutrient, and water recovery to be readily implemented in industries and municipalities, including those with limited treatment capacity today.