*3.4. Energy Recovery*

A relatively constant current, averaging at 111 mA, was recorded during the duration of the experiment, with a maximum of 383 mA recorded in the first 24 h. With an AD-MEC liquid volume of 8 L and a cathode surface area of 294 cm2, the volumetric current and current density were calculated to be 13 <sup>A</sup>/m3, and 7.4 <sup>A</sup>/m2, respectively. These results were 70.9% higher than the current density (4.33 <sup>A</sup>/m2) in a previous study [34] that resulted in 112% additional biogas with MEC inclusion. Biofilmontheelectrodesalsoplaysanimportantrole(Figure2)inincreasingthecurrentdensity[45,46].

Higher energy production was observed for the AD-MEC treatment (WAD-MEC = 964 kJ) compared to the AD-only treatment (WAD-only = 434 kJ). The total energy needed (electricity) to run the MEC (We(AD-MEC) = 107 kJ) was only 20.2% of the extra energy produced by the AD-MEC treatment, compared to the AD-only treatment (WAD-MEC – WAD-only), and 11.1% of the AD-MEC's total energy production (WAD-MEC) (Figure 5).

**Figure 5.** Energy from the CH4 and H2 produced by the AD-MEC and AD-only treatments from days 20–31 (272 cumulative h) after MEC introduction (days 0–20 not shown, as reactors acted as duplicate reactors).

The energy recovery e fficiency, based on the extra energy produced (WAD-MEC – WAD-only) from the AD-MEC over the electrical energy needed to operate the MEC, ranged from 73.1% (ηE min.) to 324% (ηE max.), with an average increase of 170% over time (Figure 6). Huang et al. (2020) showed that coupling AD and MEC in the same chamber to treat food waste resulted in a 238% energy recovery efficiency when operated under negative pressure [47]. In the current study, the energy return was also greater than the input energy required to operate the MEC.

**Figure 6.** Electric energy recovery efficiency for MEC inclusion.

#### *3.5. Biogas Utilization in a Fuel Cell or Combined Heat and Power (CHP) Generator*

A combined heat and power (CHP) generator has a 30% electric conversion efficiency and 45% heat conversion efficiency [48], while a fuel cell has a 70% electric conversion efficiency [49]. The expected electricity production per m<sup>3</sup> digester volume using the produced biogas from the AD-MEC (during the last 11 days of digestion) in a fuel cell is 1.5 kwh/m<sup>3</sup>/d, which is more than double the electricity production than AD-only (0.7 kwh/m<sup>3</sup>/d), due to the 149% higher daily CH4 and H2 production in the AD-MEC system (Table 1). Similarly, the calculated electricity and heat production from a CHP generator was more than double with MEC inclusion compared to AD-only. Previous studies have found the potential electricity production for dairy manure digestion to be between 0.08 and 1.8 kWh/m<sup>3</sup>/<sup>d</sup> over the entire digestion period [25,50]. By adding an MEC system to just the last 11 days of digestion, the dairy manure electric output went from the lower part of this range to the upper part of this range, while accounting for the electric input needed to operate the MEC. Overall, combining AD with an MEC greatly improved the biogas output during the last 11 days of digestion, allowing for a high electricity production when the biogas is used in a CHP or fuel cell (0.7 and 1.5 kWh/m<sup>3</sup>/d, respectively).

**Table 1.** Biogas utilization in a fuel cell and combined heat and power (CHP) generator based on daily H2 and CH4 production per m<sup>3</sup> digester.

