*7.1. Feedstock*

In general, wastewater strength is related to the degree of contamination of the water and determines the duration of treatment, reactor size, and quantity of energy needed and produced. This is because high-strength wastewater is rich in organic content; consequently, more energy may be recovered by treatment procedures. However, in the case of wastewater that has high strength, treating it will take a longer period, thereby increasing the amount of hydraulic retention time (HRT). An attempt should be made to

enhance the HRT for a lower reactor size, thereby optimizing energy output or utilization. The loading levels and, hence, the organic loading rate (OLR) are substantially influenced by the source of wastewater.

With advancements in MEC cost, a system's sustainable organic loading rate (OLR) varies within 800–1400 mg·L−<sup>1</sup> [88]. As a result of the lower concentration of COD in urban wastewater (300–500 mg·L−1), it necessitates a smaller HRT (5–9 h); swine wastewater, on the other hand, requires a substantially longer HRT of 314 h due to the higher concentration of COD in the waste (18,300 mg·L−1). Longer HRTs provide a substantial obstacle to industrial adoption since they reduce the volume of waste which can be processed each day and necessitate the construction of larger reactors. High-strength wastewaters, on the other hand, offer more energy and, as a consequence, are more economically viable in terms of energy production, since larger organic loads have a higher energy potential. Moreover, wastewater of low strength has a low COD (total dissolved solidity (TDS) <250 mg·L−1), while the HRT is low; however, it is still difficult to economically justify treatment because of the extremely low energy output linked to its lower organic content. When wastewater strength (COD) increases even a little (by 360–400 mg·L−1), treatment is possible economically, since higher energy generation offsets the higher cost of treatment.

A hydrogen-producing MEC with an OLR ranging between 1000 and 2000 mg COD·L−1·day−1, according to Gil-Carrera et al., is a viable alternative when compared to activated sludge in terms of treatment efficacy. For wastewater treatments with OLRs more than 2000 mg COD·L−1·day−1, a possible HRT for diverse waste streams has been estimated [89]. The addition of electrodes, a wider bacterial adhesion surface, and the voltage delivered all contribute to enhanced performance. Based on the 1400 mg COD·L−1·day−<sup>1</sup> OLR, both crude glycerol and cheese wheat, have extremely high HRT. In these circumstances, reactor design is critical to decreasing HRT while increasing solid retention to minimize reactor size. Traditional wastewater treatment requires a lot of energy, particularly activated sludge treatment, which accounts for approximately 60% of the total energy required. Compared to AD, MEC-ADs for the treatment of wastewater have led to a 1.7-fold increase in energy generation [90] and substrate removal, demonstrating the possibility for extremely effectual treatment for commercial waste streams previously employed. In addition, minimizing post-treatment needs would result in a significant reduction in the energy usage of the treatment procedure. The energy demands for full water treatment were estimated to be 0.057 kWh·m<sup>−</sup><sup>3</sup> (or 0.087 kWh·m<sup>−</sup><sup>3</sup> if ultraviolet treatment is chosen), which is about 85% less than the electrical energy consumption of a typical activated sludge process [91]. Utilizing MECs to reduce wastewater treatment energy consumption would have far-reaching global implications. There is an increase in the amount of research being done on the effectiveness of MECs in treating a variety of waste streams, which is aiding in the development of a better knowledge of how different microbial populations interact with different substrates. Because of its abundant availability and chemical composition, the waste feedstock is seen as an appealing and cost-effective substitute to pure chemicals in MECs.

#### 7.1.1. Domestic or Residential Wastewater

The H2 produced by MECs is less expensive than the projected commercial value of hydrogen (\$6·kg−<sup>1</sup> H2), with a cost of \$3.01·kg−<sup>1</sup> H2 for domestic wastewater [92]. Heidrich et al. ran a 120 L MEC on site for the treatment of domestic wastewater. According to their findings, over more than 3 months, with a Coulombic efficiency of 55%, the MEC was capable of creating pure H2 (100% ± 6.4% purity). The reactor generated approximately 0.015 L H2·L−1·day−<sup>1</sup> and retrieved around 70% of the electric power input [82,83]. According to Zhen et al. (2016), the utilization of the liquid portion of pressed municipal solid waste (LPW) for H2 production was investigated. The maximum H2 production (0.38 ± 0.09 <sup>m</sup>3·m<sup>−</sup>3·day−<sup>1</sup> and 30.94 ± 7.03 mmol·g COD−<sup>1</sup> added) was obtained at an applied voltage of 3.0 V and a pH of 5.5. Acetate, propionate, and butyrate, following their acetification, were used to achieve electrohydrogenesis, which resulted in an overall H2 recovery of 49.5% ± 11.3% of the COD provided in the experiment [93].

## 7.1.2. Industrial/Food Processing Wastewater

Montpart et al. investigated the usage of glycerol, milk, and starch in varied concentrations and levels of complexity in synthetic wastewater using a single-chamber MEC. It was discovered that only milk was capable of sustaining hydrogen synthesis for a prolonged period of time. The introduction of glycerol and starch in MEC did not inhibit the total multiplication of H2 scavengers, even under circumstances of short H2 retention time caused by frequent nitrogen sparging [94]. Shen et al. used a continuous up-flow fixed-bed MEC to treat recalcitrant wastewater generated from hydrothermal liquefaction of cornstalks while simultaneously producing hydrogen. At 1.0 V in the cathode, a hydrogen generation rate of 3.92 mL·L−1·day−<sup>1</sup> was attained, although the highest power density (305.02 mW·m<sup>−</sup>3) was achieved at 0.6 V [95]. Guo et al. studied the effect of several cathode/anode ratios in membrane-less MECs using beer wastewater. With an improved cathode/anode ratio of 4 cm2·cm<sup>−</sup><sup>3</sup> and an applied voltage of 0.9 V, methane production of 0.14 <sup>m</sup>3·m<sup>−</sup>3·day−<sup>1</sup> was achieved [95]. Furthermore, wastewater from a soybean edible oil refinery was used to generate bioelectricity and biomethane through the utilization of MFCs and MECs [89]. In comparison to conventional anaerobic digestion, the methane yield was 45.4 ± 1.1 <sup>L</sup>·kg−<sup>1</sup> COD, and the generation rate of MECs was 0.133 ± 0.005 <sup>m</sup>3·m<sup>−</sup>3·day−1.
