**1. Introduction**

Microbial electrolysis cell (MEC) is a bioelectrochemical technology that uses concepts from microbial fuel cell (MFC) research. While MFCs use microbial decomposition of organic compounds to produce an electric current, in an MEC, an electric current is applied to reverse the reaction to convert organic material to hydrogen (H2) and/or methane (CH4). Recently, MECs have been explored as a clean energy source and a promising innovative technology for H2 production using bioelectrochemical properties. Hydrogen gas is formed in an MEC from two sources of energy: (1) bacterial oxidization of organic matter and (2) electric input [1–4]. Converting organic matter into H2 in an MEC requires: (1) exo-electrogenic anodal microbes to release electrons and protons from organic material (oxidation

reaction), and (2) an external electricity input (voltage > 0.114 V) to push the reaction to be favorable, as the reaction may not be thermodynamically favorable without the electrical input [5].

Methane-enriched biogas is produced during anaerobic digestion (AD) by anaerobic bacteria decomposing organic material, such as manure, sewage, municipal waste, and/or food waste [6]. During AD, the waste is converted into biogas, a renewable energy source that consists of 55–75% CH4, 45–25% CO2, and small amounts of hydrogen sulfide (H2S), hydrogen (H2), and other gases [7,8]. The microbial process in the AD process is divided into two main phases: acidogenic and methanogenic [6,9].

In order to enrich H2, methanogenic bacterial growth should be decreased, as methanogens can use H2 as a pathway for CH4 production. While methanogenic production can be limited by low pH conditions, slightly acidified conditions (pH 5–6) were not e ffective in controlling methanogenesis in an MEC-only treatment in which H2 production was the desired product [10,11]. In most MEC studies, methanogens have been found to use the produced H2 for CH4 generation [11–15], with CH4 production continuing when the voltage was no longer applied. In an MEC study that washed methanogens from the MEC reactor using a low hydraulic retention time (5.3 h), CH4 production was still detected [15]. A recent experiment confirmed that methanogens are a preventive factor for H2 production from wastewater treatment plants using MEC [16], as CH4 production reduces H2 concentration and purity. Few MEC studies have been conducted using actual waste material [17,18]. Furthermore, MEC treatment volumes are often small (<0.3 L), with a high electrode surface area in the MEC, which would make scaling expensive. Liu et al. (2012) studied the e ffect of feeding an MEC with waste activated sludge fermentation liquid using bi-frequency ultrasonic and alkaline addition as a pre-treatment step to suppress methanogenic activity and increase H2 production, which eliminated CH4 production in favor of H2, but greatly increased the process complexity [18].

In our previous work [4], we evaluated incorporating an MEC with AD in the same reactor to digest food waste to first increase H2 production, and then use the H2 substrate to further increase the CH4 concentration. This work increased the energy production output, with >90% CH4, without an increase in the process complexity. In this previous work, three treatments were tested for 23 days: (1) a merged AD and MEC system with the MEC operating for the duration of the experiment; (2) a merged AD and MEC system with the MEC operating for only the first five days, followed by the AD for the remaining 18 days; and (3) an AD-only system operating for the entire 23-day experiment. Our previous results showed that, incorporating our unique MEC design within the AD reactor enhanced the biogas quantity and quality, total energy production, and food waste treatment. At this point, there has not been a study that incorporated an MEC with AD treatment at the end of dairy manure digestion to determine the e ffect of MEC inclusion to increase residual energy potential, nor has there been an MEC study focused on dairy manure, which is a readily available substrate used in AD systems worldwide.

In this research, an AD and MEC were combined to demonstrate the performance of using MEC during the last 11 days of digestion when biogas production had decreased but solids and chemical oxygen demand (COD) still persisted and could be further reduced given enough time. The current study aim was to determine the e ffect of a combined AD-MEC to enhance the treatment performance (i.e., COD and solids removal) and energy production (CH4 + H2) during the last 11 days of digestion of dairy manure. The MEC was designed to comprise a small volume compared to digester volume (0.0191 m<sup>3</sup>/m3). This study demonstrated a new application of MEC to increase both organic waste treatment and energy production performance.

## **2. Materials and Methods**

## *2.1. Substrate and Inoculum*

The dairy manure substrate was obtained from the Northwest Agriculture and Forestry University Dairy Farm, Yangling, Shaanxi, China. Prior to use, the dairy manure was stored for seven days at −4 ◦C. The total solids (TS) and volatile solids (VS) content of the dairy manure were 13.3% and 10.9% on a wet weight basis, respectively. An inoculum to substrate (ISR) VS ratio of 1:2 was used for the experiment. The inoculum was collected from the e ffluent of a previous AD-MEC experiment, with 5.6% TS and 3.3% VS, on a wet weight basis. After 20 days of digestion (before starting up the MEC), the contents of both treatment reactors were mixed together and distributed back to the digesters. The COD, TS, and VS concentrations of dairy manure and inoculum mixture after 20 days of digestion (before starting up the MEC) were 46.7 g/L, 6.89%, and 4.70%, respectively. The pH, ORP, and conductivity were 7.88, −374 mV, and 11.12 ms/cm, respectively. The total active volume of each digester (AD-MEC and AD-only) was 8 L.
