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). 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.
| Main Author: | |
|---|---|
| Other Authors: | |
| Format: | Doctoral thesis biblioteca |
| Language: | English |
| Published: |
Wageningen University
|
| Subjects: | Life Science, |
| Online Access: | https://research.wur.nl/en/publications/moving-bed-capacitive-bioanodes |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|