Making the best use of capacitive current: Comparison between fixed and moving granular bioanodes

Abstract

Capacitive bioanodes can be used to improve current production in bioelectrochemical systems for combination of energy recovery and wastewater treatment. Here, we compared current production in fixed and moving bed capacitive bioanodes. For fixed bed bioanodes, the recovered charge was studied as a function of the discharge current collector position and the thickness of the granule bed. The most capacitive charge was recovered from the current collector closest to the membrane. Increasing bed thickness from 5 mm to 10 mm resulted in a 1.6 times higher current density per membrane area. These findings were used to improve the design of a moving bed reactor, where granules moved through a discharge cell and were recirculated using a gas lift. The moving bed produced a current of 43 A/m2, about 2 times the fixed bed current over the full charging and discharging cycles. The relatively short discharge time and long charging time of the moving bed as compared to the fixed bed bioanodes led to higher capacitive currents. The design of the discharge cell and the ratio between charge and discharge times can be further optimized to make better use of stored charge of the granular capacitive bioanodes.