Energy balance and microbial fuel cells experimentation at wastewater treatment plant Milano-Nosedo

The wastewater treatment at the Milano-Nosedo plant is the last phase of a complex sewerage system that collects water from the central and eastern areas of the city of Milan dominated, in its southern part, by the Roggia Vettabbia irrigation network A scaling trial of Microbial Fuel Cell (MFC) technology, from laboratory to real plant condition, is currently underway at this plant. This is a first step of the experimentation of MFCs in the industrial process. Floating MFCs with a simple planar configuration were placed in operation into a denitrification tank of the plant. Performances of thirty MFCs with three different size (20 × 15 cm, 30 × 20 cm, 40 × 30 cm) were monitored. The current density through two different loads (100 ω and 10 ω) was measured for each MFC. Power curves were drawn on MFCs with a different output. The results of more than six months operation indicated that all the tested MFCs were able to supply power, with a density rather inversely proportional to the electrode surface (maxima of 15.5,13, 7.35 mW/m2, respectively). Given the very low concentration of dissolved COD (∼20 mg/L) and the relative low conductivity (∼600 mS/cm) of the wastewater, the current densities were generally low and increased strongly switching the external load from 100 to 10 ω. A maximum of about 750 mA/m2 was detected in MFCs with the smaller electrode surface; the medium size MFCs showed a maximum of about 500 mA/m2; the maximum current density decreased to less than 150 mA/m2 for the largest MFCs. Weather events and water flow variations significantly affected the MFCs output. On the contrary, vegetation growing on the air facing cathodes did not influenced negatively the performances of MFCs up to when they were able to float well balanced on the water.