On-site performance evaluation of a 1,000-litre microbial fuel cell system using submergible multi-electrode modules with air-cathodes for sustainable municipal wastewater treatment and electricity generation

Microbial fuel cells (MFCs) are bio-electrochemical systems with the ability to utilise microbial metabolism for the direct conversion of chemical energy to electrical energy. The model idea of biofilms harbouring exoelectrogenic bacteria can be used to describe the oxidation of organic substances under anaerobic conditions. During this biotransformation, electrons are delivered to the anode, while protons and by-products are released into the solution. Due to a theoretical maximal potential gradient of ≤1.105 V between an anaerobic anode (−0.30 V vs. standard hydrogen electrode, SHE) and an aerobic cathode (+0.805 V vs. SHE), the electrons are transported through an external circuit with an applied electrical load resistance to the cathode. With the help of a catalyst, they are reduced to the water together with oxygen and protons (Logan 2008).

When using municipal wastewater as feedstock, mixed microbial cultures are present in the anaerobic environment of the anodes as well as in locally aerobic zones near the air-cathode. Predictions of a whole-cell dynamic simulation model suggest not only the conversion of organic substrate. The total ammonia nitrogen (TAN) chiefly takes place via two processes: biological nitrification-denitrification and electrochemical ammonia stripping associated with a pH increase (up to 12) caused by the proton consumption at the cathode (Littfinski et al. 2021a). Besides the high pH near the cathode surface, the electric field favours salt precipitation at the catalyst and the gas diffusion layer. Its prevention is one of the key challenges for achieving stable long-term microbial fuel cell (MFC) performance. Rossi et al. (2019a) and Hiegemann et al. (2019) reported an energetic performance decrease of 78 and 50% after 1 month of operation using similarly activated carbon catalyst air-cathodes. The reduced area of open pores and coating of the catalyst leads to an ongoing decrease in the overall mass transfer coefficient (Littfinski et al. 2022).

The addressed high potential of MFCs as a sustainable wastewater treatment technology reported in the literature is mostly based on bench-scale experiments. To date, only a few published scientific research studies have been found that have investigated a MFC system with more anolyte liquid volume than 500 L (Bird et al. 2022; Rossi et al. 2022). Each of them is different in design, indicating that the MFC technology for full-stream treatment is still in the ‘proof of concept’ phase for municipal wastewater treatment. Among these six studies, there are only two pilot studies (including this study, see Table 1) in which large-scale air-cathodes were used. Thus, further pilot studies like this presented one are essential to evaluate and demonstrate the design feasibility with deeper submerged hydrophilic air-membranes, their performance and influence on following biological treatment steps depending on the readily biodegradable organic substrate, as well as specific scaling-up challenges before a reliable full-scale evaluation of the technical and economic viability can be done (Wang & He 2020; Bird et al. 2022).

When scaling-up MFCs, the need for a high electrode packing density is one of the major tasks to achieve a comparable energetic and treatment performance as in bench-scale. For example, the characteristic polarisation curve (cell voltage and/or electrode potential as a function of current density) of an ideal MFC is not only a result of the installed materials and the designed system but is also linked to the electrode's surface area and electric capacitance. Electrochemical losses are also dynamically dominated during operation by thermodynamic, side reaction, activation, ohmic, concentration, and turnover kinetic losses (Harnisch & Schröder 2010). For instance, substrate concentration, liquid temperature, or electrical conductivity (EC) influenced by rainfall events in mixed sewer systems also affect cell performance due to changes in cell internal parameters (Hiegemann et al. 2016; Adekunle et al. 2019; Littfinski et al. 2022). In municipal wastewater, these operational parameters can be subject to high fluctuations, which is why the application of real-time maximum power point tracking (Littfinski et al. 2021b) methods (e.g., perturbation and observation, P/O, algorithm) is inevitable to adjust the external resistance to the actual cell internal resistance of the system. According to Woodward et al. (2010), more than 50% of power may be lost if the external resistance does not match the internal one.

Considering the status of MFC development, a full-scale integration in municipal wastewater systems upstream from the biological wastewater treatment seems a promising challenge (Tan et al. 2021). Single-chamber modules, enclosing the air side of the cathodes in a watertight housing, can be submersed in wastewater tanks or for example, close to the effluent channel of primary settling tanks. To gain a more active surface, the module size can be increased by increasing the submersion depth into the settling tank. The further presented study addresses this module concept of parallel connected cells. In addition, the organic substrate degradation of the MFC is described in terms of the chemical oxygen demand (COD) fractions. The influence of the MFC system on the essential downstream activated sludge units using enhanced biological phosphorus removal (EBPR) and upstream denitrification can be determined in the context of the reduced fast-degradable COD fraction.

In this study, one of the largest ever operated pilot-scale MFC systems using multi-electrode submergible MFC modules was retrofitted between the mechanical treatment (screens, aerated grit, and grease chambers) and the activated sludge unit of an existing 40,000 peoples equivalents municipal wastewater treatment plant (WWTP) situated in Hecklingen, Saxony-Anhalt (Germany). The MFC was continuously operated with real municipal wastewater (effluent of the primary sedimentation) over a 6-month period. The feed water volume followed a fixed dry weather diurnal variation pattern. Storm-water events are considered only by reduced wastewater concentrations. Throughout the experiments, the long-term performance stability of the MFC system was evaluated using energetic and pollutant removal efficiency measures. To restore cathode performance, the effects of acid treatment were evaluated.

Six anode assemblies, each consisting of 10 graphite fibre brushes (MILL-ROSE Company, USA; Ø 6 cm; 80 cm), were placed parallel to the cathodes. Due to the small distance, the anode assemblies were electrically separated by a glass fibre mat (300 g m⁻²) as an additional layer on the current collector of the cathodes. All brushes were heat-treated at 450 °C for 30 min (Feng et al. 2010). The brushes were installed in 8 cm distance in the wastewater flow direction. Because the brushes touched the glass fibre separator on each side, the installation height between each brush was alternately shifted by 5 cm within the aluminium support. This created an open flow channel (Figure 1(b)) in case the anode biofilm overgrew the brushes. The total cylinder-equivalent surface area of the six anodes was 9.36 m²_An. One anode assembly (1.56 m²_An) and one cathode (0.515 m²_Cat) formed the outer electrical Circuits 1 (MFC-1) and 6 (MFC-6). The remaining inner electrical Circuits 2–5 (MFC-2, MFC-3, MFC-4, and MFC-5) shared one anode assembly (1.56 m²_An) and two parallel connected cathodes (1.03 m²_Cat). This configuration allowed for six separately installed parallel electrical circuits (Figure 1(c)).

The cell voltage (U_MFC in V) across the adjusted external electrical load resistance (R_load in Ω) of each electrical circuit (MFC-1 to 6) was continuously recorded with a controlling and monitoring unit (C&M unit; AWITE Bioenergie GmbH, Germany). The C&M unit was equipped with a digital potentiometer and provided a resistor variation range from 0.5 to 50 Ω with a time interval of 1 min between resistor changes and a perturbation, ΔR_load, of 0.5 Ω (50–30 Ω), 0.3 Ω (30–10 Ω), and 0.1 Ω (10–0.5 Ω). Thus, the duration to record the polarisation curves was set to approximately 3 h and 20 min. For real-time optimisation, the P/O algorithm introduced by Woodward et al. (2010) was used to adjust the external resistance to react immediately to changes in operating conditions. Thus, each MFC was operated at its maximum power point. Before installation, the implemented P/O algorithm was verified on bench-scale single-chamber MFCs described elsewhere (Littfinski et al. 2021b). The flow rate was set to follow an empirical, typical diurnal pattern and was automatically controlled via the C&M unit. Accordingly, the mean hydraulic retention time (HRT) was 11 h (Days: 0–39) and 20 h (Days: 39–180), respectively. During rainfall events, the diurnal pattern was maintained. Rainfall events were only monitored in terms of reduced wastewater concentration and respective EC. Furthermore, the C&M unit was equipped with diverse sensors to monitor and record the actual bulk-liquid temperature, pH (pH + T; SE 101-MS, Knick, Germany), and EC (SE 615, Knick, Germany). Physically, the sensors were placed in a flow-through fitting of the internal recirculation of the reactor. Influent and effluent of volumetric proportional wastewater samples were analysed twice per week for the COD (LCK 114, LCK 314, LCK 1414, Hach, Germany), total nitrogen (TN; LCK 238, LCK 338, Hach, Germany), nitrite (NO₂⁻; LCK 341, Hach, Germany), nitrate (NO₃⁻; LCK 339, Hach, Germany), TAN (LCK 302, LCK 303, Hach, Germany), and BOD₁₀ (biological oxygen demand) (Hach Lange BOD Direct).

The dissolved inert fraction S_i was estimated based on filtrated COD samples of the effluent (S_{COD, eff}) from the secondary clarifiers (DWA-A 131 2016) of the WWTP Hecklingen (aerobic stabilisation). For the differentiation of the particulate fractions, long-term BOD measurements over 10 days were carried out respirometrically, considering the intermediate result for each day of the test duration. Using the model of a first-order degradation reaction, the limit value for the BOD_∞ was determined by fitting the k-exponent with the method of least squares for each BOD₁₀ test. For the final calculation of the total degradable substrate concentration BOD_tot (S_S + X_S), an additional fraction of 0.15 (f_BOD) was considered for the biomass formed to include the share used for cell growth (Roeleveld & Loosdrecht 2002; Gillot & Choubert 2010).

The generated electrical current, I_MFC (in A), was estimated based on Ohm's law (I_MFC = U_MFC/R_load). Power (PD_Cat in W m⁻²_Cat) and current densities (j_Cat in A m⁻²_Cat) were normalised to the cathode surface area (A_Cat; Circuits 1 and 6: 0.515 m²_Cat; Circuits 2–5: 1.031 m²_Cat) and calculated with and ⁠.

The total internal resistance of each MFC was estimated based on the slope of the recorded polarisation curve by using the measured data points near the maximum power point. For a more advanced electrochemical analysis of the system, for the first time, the pulse-width modulated electrical load resistance (R-PWM) technique (Littfinski et al. 2021b) was applied to the pilot-scale MFC. The ohmic resistance was calculated from the immediate voltage transient response of the MFCs caused by switching frequencies of the external electrical circuit (from open-circuit to closed-circuit or vice versa) of 100 Hz (duty cycle: 80%).

The hardly stable, measurable TN and TAN removal efficiencies (see Table 2) are caused by several factors mainly related to the cathodic conversion processes: (i) small specific cathode surface area (A_Cat/V_MFC) required for both nitrification and ammonia volatilisation, (ii) insufficient gas exchange inside the gas-filled inner chamber of the cathode resulting in gaseous ammonia accumulation and oxygen depletion, (iii) operational conditions (low temperature, low TAN concentration), (iv) insufficient current density to increase the pH near the cathode surface, (v) HRT of wastewater, and (vi) ongoing salt precipitation (presented below) leading to a reduced mass transfer. Nonetheless, chemical analysis of the accumulated – almost clear – water on the air-facing side of the gas diffusion cathode could provide indications regarding possible ammonia stripping. Results revealed that the pH and TAN concentrations of 9.7 and 68 mg_NH4-N L⁻¹, respectively, were far above the anolyte level (7.2 and 40 mg_NH4-N L⁻¹).

In contrast to the nitrogen elimination, the COD elimination was significant and constant throughout the operating time. Together with the low CE, this indicates the presence of additional non-exoelectrogenic microbial consortia such as heterotrophs and methanogens that are not influenced by electricity generation (Littfinski et al. 2022). Lower CE in Stage I compared to that in Stages II and III might be a result of the shorter HRT, which leads to a higher organic loading rate. The correlation between CE and organic loading rate was also reported by Choi & Ahn (2013), who observed a nearly exponential decrease in CE with an increasing organic loading rate due to microbial side reactions consuming COD without releasing current.

A clear and direct correlation between conductivity and resistance control only by ohmic resistance can therefore be excluded. A separate recording of the individual losses (activation-, ohmic-, concentration-related) was not carried out continuously. The interpretation of the influence of concentration gradients, biofilm adaptions, capacitive effects, etc., even in the diurnal cycle of the municipal WWTP, was therefore difficult.

Figure 8 shows the influence of the cathode area, providing the exchange surface for diffusive mass transport and determining the diffusive losses. Consequently, a lower total resistance R was achieved by the P/O algorithm for the 1.03 m² cathode area modules compared to the 0.515 m² cathodes (see Table 2).

The observed long-term performance reduction (see Figure 4(c)) is mainly associated with cathode contaminations. In general, two major mechanisms can contribute to internal and surface cathode contamination: (i) biofilm formation on the water-facing side of the cathode and (ii) inorganic scaling as a result of salt deposits on the catalyst and the gas diffusion layer. Biofilms, especially when using separators on the cathode, function as a diffusion barrier, reducing the mass ion transport to the cathode interface. Salt crystallisation in the meso- and micropores of the catalyst layer reduces the reactive three-phase interface.

One of the world's largest air-cathode MFC systems (1,000 L) equipped with five submergible multi-electrode modules was operated at a municipal WWTP over a 6-month period. The system, fed with mechanically treated wastewater, generated a maximum power of 0.57 W (2.60 A) or 113 mW m⁻²_Cat, at a current density of 0.51 A m⁻²_Cat. The COD removal efficiency of the flow reactor was 34% (T = 14 °C). In addition to the reduction of biodegradable COD, in average, 20% of the particulate inert material was also retained from the wastewater stream. The distribution of the different COD fractions due to bio-electrochemical wastewater treatment was subject to only minor changes. Nevertheless, the elimination rate of 34 ± 7% COD, in particular, 41 ± 10 mg L⁻¹ of the readily biodegradable fraction, results in a reduced EBPR or upstream denitrification rate. In contrast, the nitrogen removal efficiency (13 ± 6% during operation Stage I at 14 °C) proved to be volatile, mainly caused by cathode-limiting processes. If it can be stabilised, the envisaged application scenario appears to be possible upstream of an activated sludge tank, without affecting TN effluent limits. The technology was able to demonstrate its ability to regenerate after rain events. The reduced loads to the activated sludge stage may result in potential savings in terms of energy consumption and (excess) sludge quantities. In addition to the stabilisation of nitrogen decomposition, the full-scale application of the MFC technology still essentially depends on the long-term stability of the process, which is currently limited by the deposition of salts and biofilm formation in the electrocatalytic structure of the used air-cathode with activated carbon catalyst. Accordingly, further investigations are still required concerning applicable materials and methods of operation.

The authors of this work gratefully acknowledge the financial support of the German Federal Ministry of Education and Research (BMBF), funding code: 02WQ1466A-C, and the project support provided by the Project Management Agency Karlsruhe (PTKA).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.