Improving the discharge of capacitive granules in a moving bed reactor

Abstract Bioanodes can be used to recover the energy and nutrients from wastewater in bioelectrochemical systems. The use of capacitive electrodes can improve the current density produced by these bioanodes. Moving bed reactors are studied to produce high current densities, by using capacitive granules for charge storage and with a high bioanode surface area. The bioanodes store charge in the granules, which are intermittently discharged at a current collector. In principle, this allows for more current density in the electrochemical cell, because both faradaic and capacitive currents are harvested. One of the limiting factors of this technology is the capacitive discharge rate. In abiotic tests, the capacitive discharging was most improved by changes in potential difference between the current collector and charged granules (ΔE 0.3 and 0.5 V). Increasing the bulk electrolyte conductivity also increased the transferred charge, which could originate from the increased capacitance – as measured in a separate setup. Discharging from both sides of the granular bed, as compared to discharging from one side, reduced the maximum distance to the current collector, which increased the transferred charge, irrespective of an increase in bulk electrolyte conductivity. This showed the electrical resistance was more important in determining the transferred charge than the ionic resistance. Further analysis of the discharging process showed that discharging increased the local conductivity through the release of ions from the granules. This offers opportunities for the treatment of low conductivity wastewaters. These results provide new insights to further improve capacitive bioanodes.