*2.5. Analytical Methods*

The dairy manure and inoculum TS and VS concentrations were measured using Standard Methods [26]. The COD was measured using colorimetry, using Method 410.4 [27]. Wet gas meters (W-NK-0.5, Shinagawa Co., Tokyo, Japan) were used to measure biogas production from both treatments (AD-MEC and AD). A gas bag was attached to the gas meter outlet to ensure air could not enter the gas meter while collecting the gas sample. A gas chromatograph (GC) (GC2014C, Shimadzu Co, Chiyoda-ku, Japan) was used to analyze the gas samples, which were collected from the reactor using a gas-tight syringe.

Scanning electron microscopy (SEM) (TM3000 Tabletop, HITACHI, Fukuoka, Japan) was used for scanning the electrodes, the biofilm, and the liquid between the electrodes. Prior to SEM, the samples were air dried for approximately 20 min inside a closed incubator, and then coated with gold for 20 s.
