In this system, a photosynthetic biocathode has been coupled to a MFC. This biocathode consists of a pure culture of Chlorella vulgaris which provides the oxygen (electron acceptor). Some experiments were conducted in order to determine the best flow-mode for feeding the anode chamber. It was found that working under continuous-flow mode (with biofilm and in-suspension microorganisms) the system reaches quicker and steadier output power in comparison with sequencing-batch-flow mode. The maximum power density (at 17:00 h, when steady state is reached) is higher in continuous mode, 23.97 vs 22.85 mW/m² in sequencing-batch mode. Also, a substrate-load characterization was performed. It was determined that COD concentration up to 1066 mgCOD/L can be successfully treated in the system. However, when COD concentration of synthetic wastewater was increased from 343 to 1066 mgCOD/L, the cell voltage decreased from 15 to 10 mV. In addition, two different inorganic carbon (IC) sources for feeding the biocathode were studied.