*2.4. MEC Voltage*

A programmed direct current power-supply (YH-305D, Yi Hua Inc., Shanghai, China) was used to apply the voltage (1.2 V) across the anode and the cathode. The anode with a 10 Ω resistor was connected serially to the power supply positive lead, while the cathode was connected to the negative lead. The voltage drops across the 10 Ω resistor were measured using a multi-meter (model ATW9205L; ATTEN Instruments Inc., Shanghai, China). Measurements of voltage drop were conducted approximately 7 times per day.

Ohm's law was used to calculate the current (I = V/R, where V is the voltage drop measured across the resistor (R)). The energy recovery e fficiency was based on electricity (ηe) (%) input based on the measured current compared to the energy di fference in the usable gas production between the AD-MEC and AD-only reactors. The volume of CH4 and H2 produced daily were normalized to the reactor active volume (m<sup>3</sup> gas/m<sup>3</sup>/d). The volumetric current density (IV, <sup>A</sup>/m3) was normalized by the MEC liquid volume (0.150 L), and the total volume of manure + inoculum (8 L). Following Call and Logan (2008), the energy dissipation in the 10 Ω resistor (WE) was accounted for in order to determine the actual energy supplied [23]. Gibbs' free energy equation was used to calculate the energy production from CH4 (Δ GCH4 = 890.4 kJ/mol) and H2 (Δ GH2 = 237.1 kJ/mol) [4,24,25].
