This study analyzes the biofilm growth and long‐term current production of mixed‐culture, electrochemically active biofilms (EABs) on macrostructured electrodes under low‐shear‐force conditions. The channel dimensions were altered systematically in the range 400 μm to 2 mm, and the channel heights were varied between 1 and 4 mm to simulate macrostructures of different scales. Electrodes with finer‐structured surfaces produced higher current densities in the short term owing to their large surface area but were outperformed in the long term because the accumulation of biomass led to limitations of mass transfer into the structures. The best long‐term performance was observed for electrodes with channel dimensions of 1×4 mm, which showed no significant decrease in performance in the long term. Channels with a diameter of 400 μm were overgrown by the biofilm, which led to a transition from 3 D to 2 D behavior, indicating that structures of this scale might not be suitable for long‐term operation under low‐shear‐stress conditions.

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