**3. Results and Discussion**

#### *3.1. Organic Matter Reduction: COD, TS, and VS Removal*

The COD concentration after 20 days of digestion and before MEC use was 46.7 g/L. The AD-MEC had 38.3% further reduction in COD (28.8 g/L) after the 272 h of the combined MEC and AD digestion (days 20–31) compared to 19.1% additional COD reduction in the AD-only treatment (37.8 g/L). The greater increase in COD reduction (50.1% higher) in the AD-MEC reactor compared to AD-only was higher than previous studies with AD-MECs, which showed increased COD reductions of 5% to 15% compared to AD-only [4,28,29]. Asztalos and Kim (2015) included three bioanodes and a stainless-steel mesh cathode to treat waste activated sludge at ambient temperature and found the VS and COD removal were only 5–10% higher than AD-only [30]. In our work, not only was the overall COD reduction higher, but the COD conversion efficiency to renewable energy was higher in the AD-MEC (7.09 kJ/g COD removed) compared to AD-only (6.19 kJ/g COD removed). The TS and VS concentrations in the reactors before MEC use were 6.89% and 4.70%, respectively. The additional TS reduction in the last 10 days of digestion was also higher in the AD-MEC treatment (35.2%) compared to AD-only (13.4%), and the additional VS reduction was 41.9% for AD-MEC treatment compared to 19.0% for AD-only. The results showed that the AD-MEC treatment increased the reduction of COD, TS, and VS concentrations during digestion compared to AD-only, with the additional organic matter removal likely occurring through oxidization of the organic material using exoelectrogenic bacteria attached to the anode and H2 production at the cathode [30].

## *3.2. Anode Bacterial Attachment*

The scanning electron microscopy (SEM) images on the anode surface showed bacterial cell colonization and growth. These cells were likely exoelectrogens, electricigens, and anodophilic bacteria that promoted substrate breakdown and electricity production (Figure 2) [31,32]. The anode (graphite) surface was completely covered with attached biofilm after the 11 days of the MEC inclusion, as confirmed by the SEM analysis. Coccoid and rod-shaped bacteria dominated the biofilm. It appears that 11 days were sufficient for the bacteria to colonize and acclimate to the anode graphite plates to form the biofilm needed to degrade the organic material (dairy manure). The SEM analysis also showed a denser microbial adherence on the graphite compared to stainless steel due to the porous structure of graphite materials [33].

**Figure 2.** Morphological characteristics of the anode and cathode as visualized by the scanning electron microscope (SEM).
