Microbial fuel cells (MFCs) have the outstanding ability to transform the chemical energy contained in organic matter directly to electrical energy. Unfortunately, they give only low cell voltage at maximum power. Connecting several MFCs electrically in series inside the same reactor may be a way to increase the cell voltage, but experimental attempts have shown poor efficiency for such single-electrolyte stacks. The present study uses numerical modelling to understand the behaviour of single-electrolyte MFC stacks and to assess possible ways to improve it. The numerical model was validated by comparison with two experimental MFCs that produced 0.85 +/- 0.05 mW each at 0.23 V cell voltage. Connected in series in a common electrolyte, the stack produced only 0.7 mW at 0.21 V, while, in theory, 1.7 mW could be reached at 0.47 V. The model showed that the drastic power loss was due to ionic short-circuiting, which may, however, be an interesting phenomenon to be exploited for designing an electro-microbial snorkel. The model also showed that decreasing the anode-cathode distance, increasing the distance between the MFCs or using baffles between them could optimize the single-electrolyte stack to produce up to 80% of the theoretical maximum power. Nevertheless, such designs are appropriate only for specific applications, e.g. biosensing. The model further suggests that benthic MFCs could be effectively connected in series. (C) 2017 Elsevier Ltd. All rights reserved.