*2.2. Reactor Design*

Two reactors were used in the experiment: (1) AD-MEC: a digester that contained an MEC connected to an electricity source, and (2) AD-only: a digester with an MEC included to provide equal surface area and reactor volume, but the MEC was not connected to an electricity source (substrate was treated using only the digestion process) (Figure 1). Both reactors were made of stainless steel, with each reactor having an inner diameter of 18 cm and length of 46 cm, resulting in a total volume of 11 L (8 L active volume, and 3 L head space) (Auzone BMR-A10L, Shanghai Auzone Bio-engineering Equipment CO., LTD., Shanghai, China). Each digester was digitally programmed for temperature and mixing speed. Heating of the digesters was conducted using water that was heated and pumped into water jackets surrounding the reactors. The temperature within the digester was maintained at 16 ◦C for the first 10 days of digestion, then increased to 35 ◦C. At day 20, the MEC was connected to the power supply in the AD-MEC treatment for 11 days (from days 20–31), while the AD-only treatment operated for 11 additional days (days 20–31) without MEC as an AD-only treatment. A pH probe was inserted into each reactor (Mettler Toledo 52003679, Beijing, China), as well as an Ag/AgCl reference electrode. There was an outlet (1 cm diameter) located 14 cm from the bottom of the reactor for collecting liquid samples.

**Figure 1.** Photograph and schematic diagram of combined AD and MEC (AD-MEC) system compared to AD without MEC (AD-only) treatment.

#### *2.3. Microbial Electrolysis Cell Design*

The MEC was fabricated according to our previous research [4] and included a PVC pipe closed with PVC caps as the MEC sleeve, with a diameter of 3.49 cm and length of 15.50 cm, resulting in a total MEC volume of 0.153 L. To guarantee the flow of liquid between the inside and the outside of the MEC, ten vertical holes of approximately 5.5 cm in length and 0.35 cm in width were created on the sides of the PVC sleeve [4]. An additional hole with a diameter of 1.6 cm was created in the center of each PVC cap to allow gas produced within the MEC to escape (Figure 1).

Each anode was made of eleven graphite plates composed of ≥99.95% pure graphite with high electric resistivity (≤1000 μΩ·cm), with dimensions of 15.0 cm (length) × 1.50 cm (width) × 0.1 cm (thickness) per plate. Rubber bands were used to separate each graphite plate to create a space between the plates, which formed the first part of the anode [4]. The second part of the anode was made of a stainless-steel cylinder (grade 201) with 15.0 cm length, 1.85 cm diameter, and 0.05 cm thickness and low electric resistivity (≤68.5 μΩ·cm) [4]. The graphite plates (first part of the anode) were inserted into a stainless-steel cylinder (second part of the anode). The integration of a stainless-steel cylinder and graphite plates was used to increase the electric conductivity between the graphite plates and to reduce the ohmic resistance compared to a graphite-only anode [4,19,20]. Furthermore, the utilization of grade 201 stainless steel in the anode helped to decrease the distance between the electrode plates and increase the conductivity. Moreover, the stainless steel grade 201 contains manganese (5.5–7.5%), which has been shown to increase the electrical energy produced by a factor of 10 [21,22].

The anode was positioned inside the cathode (stainless-steel cylinder with a 3.13 cm diameter, 15.0 cm length, and 0.05 cm thickness) and rubber bands were used to isolate the anode and cathode to avoid short circuiting (Figure 1) [4]. Insulated wires were used to connect the anode and cathode to the circuit.
