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/m(2) 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. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.