  In the context of natural-based wastewater treatment technologies (such as
constructed wetlands - CW) the use of a low-cost, continuous-like biosensor
tool for the assessment of operational conditions is of key importance for
plant management optimization. The objective of the present study was to assess
the potential use of constructed wetland microbial fuel cells (CW-MFC) as a
domestic wastewater COD assessment tool. For the purpose of this work four
lab-scale CW-MFCs were set up and fed with pre-settled domestic wastewater at
different COD concentrations. Under laboratory conditions two different anodic
materials were tested (graphite rods and gravel). Furthermore, a pilot-plant
based experiment was also conducted to confirm the findings previously recorded
for lab-scale experiments. Results showed that in spite of the low coulombic
efficiencies recorded, either gravel or graphite-based anodes were suitable for
the purposes of domestic wastewater COD assessment. Significant linear
relationships could be established between inlet COD concentrations and CW-MFC
Ecell whenever contact time was above 10 hours. Results also showed that the
accuracy of the CW-MFC was greatly compromised after several weeks of
operation. Pilot experiments showed that CW-MFC presents a good bio-indication
response between week 3 and 7 of operation (equivalent to an accumulated
organic loading between 100 and 200 g COD/m2, respectively). Main conclusion of
this work is that of CW-MFC could be used as an "alarm-tool" for qualitative
continuous influent water quality assessment rather than a precise COD
assessment tool due to a loss of precision after several weeks of operation.
