2.2. MFC construction and operation
Single chamber air-cathode MFCs without a membrane separator were constructed with a 4 cm length and 28 mL liquid volume (24 mL effective volume) (Fig. 1). The anode was made from a graphite brush electrode pretreated with ammonia gas (Logan et al., 2007) and the air-cathode was a 30% wet-proof carbon cloth containing 0.5 mg/cm2 of the Pt catalyst. Titanium wires were used to connect the anode and cathode with an elec- trical circuit.
The MFCs were operated in two modes, batch and sequencing batch. In batch mode, the substrate (Table 1) was spiked (0.5 mL) into the reactor to produce a constant initial condition (1.3 g SCOD/L-slug). Six different resistors (22–980 X) were used to char- acterize the electrical production and polarization curves. In addi- tion, the intermediates of the organic acids were monitored during a cycle of voltage production by high performance liquid chro- matography (HPLC). Based on the result of the batch experiment (Rext = 680 X for EEmax and Rext = 56 X for CEmax), the maximum power density and energy efficiency were determined for the sequencing batch mode experiment. The OLR of the MFCs was var- ied from 0.75 to 6.17 g TCOD/L-d, resulting in HRTs that changed from 92 to 11 d. MFCs with different organic loading rates (OLR, 1.5, 3.1 and 6.17 g COD/L-d) were examined under 56 X (repre- senting Pmax, and CEmax), whereas 0.75 and 1.51 g COD/L-d were applied to examine the energy efficiency using a 680 X resistor (representing EEmax).