Considerations for application of granular activated carbon as capacitive bioanode in bioelectrochemical systems

In the last decades, the research in Microbial Fuel Cells (MFCs) has expanded from electricity production and wastewater treatment to remediation technologies, chemicals production and low power applications. More recently, capacitors have been implemented to boost the power output of these systems when applied as wastewater treatment technology. Specifically, the use of granular capacitive materials (e.g. activated carbon granules) as bioanodes has opened up new opportunities for reactor designs and upscaling of the technology. One of the main features of these systems is that charge and discharge processes can be separated, which offers multiple advantages over more conventional reactor types. In this manuscript, we discuss several aspects to consider for the application of capacitive granules as bioanodes in MFCs and other bioelectrochemical systems, as well as the recent advances that have been made in applying these granules in various reactor systems. Similarly, we discuss the granule properties that are key to determine system operation and performance, and show that biofilm growth is highly dependent on the efficiency of discharge.</p

Real-time monitoring of biofilm thickness allows for determination of acetate limitations in bio-anodes

Several studies have reported that current produced by electro-active bacteria (EAB) is dependent on anode potential and substrate concentration. However, information about the relation between biofilm growth and current density is scarce. In this study, biofilm thickness was monitored in-situ and this relation explored at three anode potentials and three acetate concentrations. The highest current densities of 3.7 A·m−2 were obtained for biofilms thinner than 40 μm, even though thicknesses up to 88 μm were measured. Fick's law was used to estimate the acetate penetration depth in the biofilm, acetate diffusion rates in the biofilm, and specific acetate utilization rates. A maximum biofilm thickness of a non-acetate limited biofilm of 55 μm and an acetate diffusion rate of 2.68 × 10−10 m2·s−1 were estimated at −0.2 V vs Ag/AgCl. The results provide information on the target biofilm thickness for which no acetate limitations occur and provide data for modeling works with bio-anodes

Combination of bioelectrochemical systems and electrochemical capacitors: Principles, analysis and opportunities

© 2019 The Authors Bioelectrochemical systems combine electrodes and reactions driven by microorganisms for many different applications. The conversion of organic material in wastewater into electricity occurs in microbial fuel cells (MFCs). The power densities produced by MFCs are still too low for application. One way of increasing their performance is to combine them with electrochemical capacitors, widely used for charge storage purposes. Capacitive MFCs, i.e. the combination of capacitors and MFCs, allow for energy harvesting and storage and have shown to result in improved power densities, which facilitates the up scaling and application of the technology. This manuscript summarizes the state-of-the-art of combining capacitors with MFCs, starting with the theory and working principle of electrochemical capacitors. We address how different electrochemical measurements can be used to determine (bio)electrochemical capacitance and show how the measurement data can be interpreted. In addition, we present examples of the combination of electrochemical capacitors, both internal and external, that have been used to enhance MFC performance. Finally, we discuss the most promising applications and the main existing challenges for capacitive MFCs

In situ Biofilm Quantification in Bioelectrochemical Systems by using Optical Coherence Tomography

Detailed studies of microbial growth in bioelectrochemical systems (BESs) are required for their suitable design and operation. Here, we report the use of optical coherence tomography (OCT) as a tool for in situ and noninvasive quantification of biofilm growth on electrodes (bioanodes). An experimental platform is designed and described in which transparent electrodes are used to allow real‐time, 3D biofilm imaging. The accuracy and precision of the developed method is assessed by relating the OCT results to well‐established standards for biofilm quantification (chemical oxygen demand (COD) and total N content) and show high correspondence to these standards. Biofilm thickness observed by OCT ranged between 3 and 90 μm for experimental durations ranging from 1 to 24 days. This translated to growth yields between 38 and 42 mgurn:x-wiley:18645631:media:cssc201800589:cssc201800589-math-0001  gurn:x-wiley:18645631:media:cssc201800589:cssc201800589-math-0002 −1 at an anode potential of −0.35 V versus Ag/AgCl. Time‐lapse observations of an experimental run performed in duplicate show high reproducibility in obtained microbial growth yield by the developed method. As such, we identify OCT as a powerful tool for conducting in‐depth characterizations of microbial growth dynamics in BESs. Additionally, the presented platform allows concomitant application of this method with various optical and electrochemical techniques

Moving bed capacitive bioanodes

Wastewater treatment is required to remove pollutants such as organics. The chemical energy in organics can be recovered using anaerobic treatment: anaerobic digestion (AD) to produce biogas, or bioanodes to directly produce electrical current. In practice 26% of the energy can recovered as electricity using AD. Bioanodes are theoretically able to convert 100% (except for some biomass growth) of the organics to electrical current. The bioanode current is used to generate electrical power in Microbial Fuel Cells (MFCs), or to produce products, such as hydrogen and NaOH, in Microbial Electrolysis Cells (MECs) (which require an applied cell voltage). Traditional designs require space to prevent clogging, by solids in the wastewater or biomass growth in the cell. The wide spacing results in voltage losses due to low conductivity of the wastewater, thus decreasing the energy recovered from the organics. Bioanodes have yet to produce high current densities, especially at larger scale, which is a limitation compared to AD. Fluidized capacitive bioanodes promise to tackle the challenges.The use of activated carbon granules in fluidized capacitive bioanodes allows for intermittent charge storage and large surface area for the bioanode per volume. Electrons, from the oxidation, and cations, from the bulk solution, are stored in electrical double layers on the surface of the pores in the porous granules. During discharging of the electrons to the anode, the cations are released again, increasing the conductivity of the wastewater, reducing voltage losses. Fluidized bed reactors in literature have yet to reach high current densities, therefore this thesis was focused on studying the discharge of the capacitive granules. We studied single granules as capacitive bioanodes, in a 1 mL cell, to discover the maximum achievable current density per granule. Activated carbon granules, with volumes below 1 mm3, stored and released large amounts of charge: on average 73 C/cm3, compared to 18 C/cm3 from a non-capacitive graphite granule, resulting in a maximum of 77 mA/cm3 granule during discharging.The moving bed reactor increased the contact time of granules, compared to existing fluidized bed systems, by creating a moving bed of capacitive granules at the anode in the discharge cell. Instead of fluidization for contacting, a gas lift was used to transport the granules from below the discharge cell, to the top of the reactor, which allowed the granules to settle into a packed bed like formation inside the discharge cell. The granules, charged when not in the discharge cell, were discharged as electrical charge was transferred from the granules to the anode. Because charged granules, passing through the cell, were continuously discharged, a continuous current was generated. This is in contrast with fixed granules, where the discharging needs to be alternated with charging: an intermittent current is generated.Two moving bed reactor versions were developed. The largest bioanode reactor had tubular construction with a cylindrical discharge cell (total volume 7.7 L, containing 1.2 L granules, and a 163mL discharge cell). The maximum daily current was 1.9 mA/cm3cell (257 A/m3granules). This current was mainly produced by the passing granules. The activity of the granules, measured in an external cell as the current produced by a sample of granules from the reactor, increased over time, but eventually stagnated. The same pattern of growth and stagnation (or rather stabilization) of the reactor current was observed. The stagnation in the current likely resulted from the biofilm, which only grew in the larger pores on the surface of the granules. Slow discharging, seen during discharging without charging (stopped influent), indicated the discharging should be further improved to reach the results of the single granule study.The discharging was further studied in a smaller reactor (total volume 1.5 L, with 0.4 L granules, with a planar discharge cell of 22 mL). The performance of the moving bed bioanode was compared to a fixed bed bioanode, with a similar cell construction. Investigating in the discharge cell configuration in the fixed bed showed discharging using the anode closest to the membrane produced the highest discharge current. At a maximum current of 4.3 mA/cm3cell, the moving bed bioanode produced double the current of the fixed bed, resulting from discharging fully charged granules, resulting from a long discharge time compared to charging time in the moving bed. Under abiotic conditions, with the granules charged via a cathodic current in the discharge cell, the discharging increased: with higher potential difference ΔE (between the anode and the charged granules), with higher bulk electrolyte conductivity, with decreased maximum distance to the anode (by discharging from both sides of the granular bed) and for a shorter residence time of the granules in the cell. The experiments showed the discharge process was affected more by the electrical resistance than the ionic resistance, although both influenced the transferred charge. The discharge resistance was reduced at higher transferred charge, resulting from release of ions during discharging, which increased the local conductivity in the cell on average by 40% (depending on the experimental conditions).&nbsp;The four cell configurations were discussed in relation to: 1) the contact time (moving bed), or discharge time (fixed bioanodes), 2) contact resistance, 3) ratio between charging and discharging, and 4) the faradaic and capacitive contributions to the discharge current. Recommendations were given for improving 1) the discharge cell for a tubular design, 2) the charging and discharging volumes in a multi discharge cell reactor for large scale implementation, and 3) the granules in relation to electrical resistance, ion transport resistance, and the biofilm presence on the granules.The bioanode experiments showed the moving bed produced 3 – 20 times higher current density in the discharge cell compared to previous fluidized bed reactors from literature. Although the aim of 3.5 mA/cm3, for competition with anaerobic digestion, was not reached in the reactor, the moving bed bioanode did produce 4.3 mA/cm3 in the discharge cell. The findings show the moving bed reactor has great promise as an alternative for scaling up bioanodes. If improvements are implemented, this current density can be increased and bring the reactor performance closer to the desired goal.&nbsp;&nbsp

The granular capacitive moving bed reactor for the scale up of bioanodes

BACKGROUND: Scaling up bioelectrochemical systems for the treatment of wastewater faces challenges. Material costs, low conductivity of wastewater and clogging are issues that need a novel approach. The granular capacitive moving bed reactor can potentially solve these challenges. In this reactor, capacitive activated carbon granules are used as bioanode material. The charge storage capabilities of these capacitive granules allow for the physical separation of the charging and the discharging process and therefore a separation of the wastewater treatment and energy recovery process. RESULTS: This study investigates the performance of the granular capacitive moving bed reactor. In this reactor, activated granules were transported from the bottom to the top of the reactor using a gas lift and settled on top of the granular bed, which moved downwards through the internal discharge cell. This moving granular bed was applied to increase the contact time with the discharge anode to increase the current density. The capacitive moving bed reactor (total volume 7.7 L) produced a maximum current of 23 A m−2 normalized to membrane area (257 A m−3granules). Without granules, the current was only 1.4 A m−2membrane. The activity of the biofilm on the granules increased over time, from 436 up to 1259 A m−3granules. A second experiment produced similar areal current density and increase in activity over time. CONCLUSION: Whereas the produced current density is promising for further scaling up of bioanodes, the main challenges are to improve the discharge of the charged granules and growth of biofilm on the granules under shear stress.</p

Performance of single carbon granules as perspective for larger scale capacitive bioanodes

The use of high surface area electrodes, like carbon-based felt or granules, in Bioelectrochemical Systems is crucial for high volumetric current production. In case activated carbon granules are used, charge can also be stored in the form of an electric double layer in the pores, which has been shown to improve bioanode performance. So far, it is not known how much current can be generated by a single granule. In this study, we investigate the current production and charge storage behavior of a single carbon granule. Two types of activated carbon granules and one graphite granule are tested to find the untapped potential of granular bioanodes. A single activated carbon granule produces up to 0.6 mA, corresponding to 60 mA cm−3 granule volume at −300 mV vs. Ag/AgCl anode potential. Charge – discharge experiments show that capacitive granules produced 1.3–2.0 times more charge compared to a graphite granule with low surface area. When extrapolated to other granular systems, our study indicates that the current generated by granular bioanodes can be improved with several orders of magnitude, which could form the basis of an economically feasible Microbial Fuel Cell

The granular capacitive moving bed reactor for the scale up of bioanodes

BACKGROUND: Scaling up bioelectrochemical systems for the treatment of wastewater faces challenges. Material costs, low conductivity of wastewater and clogging are issues that need a novel approach. The granular capacitive moving bed reactor can potentially solve these challenges. In this reactor, capacitive activated carbon granules are used as bioanode material. The charge storage capabilities of these capacitive granules allow for the physical separation of the charging and the discharging process and therefore a separation of the wastewater treatment and energy recovery process. RESULTS: This study investigates the performance of the granular capacitive moving bed reactor. In this reactor, activated granules were transported from the bottom to the top of the reactor using a gas lift and settled on top of the granular bed, which moved downwards through the internal discharge cell. This moving granular bed was applied to increase the contact time with the discharge anode to increase the current density. The capacitive moving bed reactor (total volume 7.7 L) produced a maximum current of 23 A m−2 normalized to membrane area (257 A m−3granules). Without granules, the current was only 1.4 A m−2membrane. The activity of the biofilm on the granules increased over time, from 436 up to 1259 A m−3granules. A second experiment produced similar areal current density and increase in activity over time. CONCLUSION: Whereas the produced current density is promising for further scaling up of bioanodes, the main challenges are to improve the discharge of the charged granules and growth of biofilm on the granules under shear stress.</p

Experimental data: Real-time monitoring of biofilm thickness allows for determination of acetate limitations in bio-anodes

The data provided in these files have been generated/collected during the study of the development of electro-active biofilms (EABfs) in real-time. These data contain measured current density, acetate concentrations, and biofilm development over tim