  Twelve single chambered, air-cathode Tubular Microbial Fuel Cells (TMFCs)
have been filled up with fruit and vegetable residues. The anodes were realized
by means of a carbon fiber brush, while the cathodes were realized through a
graphite-based porous ceramic disk with Nafion membranes (117 Dupont). The
performances in terms of polarization curves and power production were assessed
according to different operating conditions: percentage of solid substrate
water dilution, adoption of freshwater and a 35mg/L NaCl water solution and,
finally, the effect of an initial potentiostatic growth.
  All TMFCs operated at low pH (pH$=3.0 \pm 0.5$), as no pH amendment was
carried out. Despite the harsh environmental conditions, our TMFCs showed a
Power Density (PD) ranging from 20 to 55~mW/m$^2 \cdot$kg$_{\text{waste}}$ and
a maximum CD of 20~mA/m$^2 \cdot$kg$_{\text{waste}}$, referred to the cathodic
surface. COD removal after a $28-$day period was about $45 \%$.
  The remarkably low pH values as well as the fouling of Nafion membrane very
likely limited TMFC performances. However, a scale-up estimation of our
reactors provides interesting values in terms of power production, compared to
actual anaerobic digestion plants. These results encourage further studies to
characterize the graphite-based porous ceramic cathodes and to optimize the
global TMFC performances, as they may provide a valid and sustainable
alternative to anaerobic digestion technologies.
