*2.3. Photo Fermentation*

The process of photo fermentation involves organic/inorganic substrate oxidation in the presence of O2 to release electrons that end up reducing ferredoxins. These reduced ferredoxins are related directly to the production of H2 and to the fixation of CO2. The following reaction takes place:

$$\text{(CH}\_2\text{O}\_2\text{)}\_2 \overset{\text{NADPH}}{\rightarrow} \text{Fd}\_{\text{(red)}} + \text{ATP} \overset{\text{Niitrogenase}}{\rightarrow} \text{H}\_2 + \text{Fd}\_{\text{Ox}} + \text{ADP} + \text{Pi}$$

Although this process has advantages such as enhanced potential conversion yields and the ability to absorb a wide variety of substrates, there are drawbacks such as continuously regulated area/volume ratio maintenance in photobioreactors, temperature control, and controlled agitation. For the development of bio-H2 from wastewater, various types of reactor systems such as batch and continuous reactors are used, with each configuration providing its advantages and disadvantages [33].

#### **3. Existing Wastewater Treatment Technologies and Their Bottlenecks**

Traditional wastewater treatment usually takes place when wastewater is carried by a sewer to a centralized wastewater treatment plant, where the wastewater is then treated linearly by the end-of-pipe technology. It is time to make the switch to a closed-loop system [34], in which water is treated with simultaneous nutrient and energy recovery. Due to the superior positioning of decentralized wastewater treatment plants, centralized wastewater treatment plants can lead to a reduction in operational and capital expenditure (OPEX). Despite being in their infancy when it comes to deployment and optimization, decentralized solutions are lagging in the adoption phase when related to centralized ones. Clearly, the consequences of decentralization are visible, and developments in wastewater treatment plants are trying to make the transition from centralization to decentralization, where waste resources are reclaimed [35]. Because of a lack of effective methods for garbage disposal, management, and recycling, this problem will inevitably worsen [36]. Although centralized water systems are not relevant in many regions of the globe despite the drastic expansion in population and people favoring metropolitan areas, that trend is becoming even more prevalent as time progresses. For several reasons, from the changing demographics to building codes, rural people have to change their lifestyle, and this has placed wastewater setups under burden. Unfortunately, in several instances, this has led to efficient wastewater treatment systems not being put in place. In certain cases, it is difficult and expensive to build centralized wastewater treatment plants after the fact. With the increased use of distributed and non-networked technologies, it is more likely that we will develop a decentralized wastewater treatment infrastructure, which helps to encourage improved system robustness and lower economic and environmental costs [37].

#### *Microbial Electrolysis Cell Mechanism for Wastewater Treatment with Simultaneous Hydrogen Production*

The microbial electrolysis cell is a capable technique for removing organics while simultaneously producing hydrogen gas. When mixed with other elements, hydrogen is a plentiful element on Earth (water, hydrocarbons, etc.). To produce biohydrogen in a pure and regulated form, several industrial procedures are necessary. Among a variety of fuels (gasoline: 47.5 MJ·kg−<sup>1</sup> higher heating value (HHV); 44.5 MJ·kg−<sup>1</sup> lower heating value (LHV)), hydrogen has the greatest thermal efficiency (141.9 MJ·kg−<sup>1</sup> HHV; 119.9 MJ·kg−<sup>1</sup> LHV) and may be preserved for extended periods until being employed in fixed or mobile operations. The benefits of biohydrogen generation from waste matter, such as solid wastes and wastewater, are increased by making the process more sustainable [38]. Hydrogen is an excellent future fuel that can be used to meet the world's energy demands due to its high energy density, ecologically benign combustion profile, and ability to be used at ambient temperature and pressure [39].

MECs provide a different approach for centralized wastewater treatment systems. According to this energy conversion estimate, the chemical energy potential of organic components constituting wastewater's core is roughly 9.3-fold higher than the energy required to treat it [40]. There is a notable increase in employing biomass energy to ge<sup>t</sup> energy from biological anaerobic wastewater treatment because of the simplicity and resilience of this technique [41]. Anaerobic digestion (AD) is a technique that is increasingly being used to generate biogas from wastewater sludge. A process termed acetate digestion provides energy by breaking down acetate to produce CO2 and CH4 [42]. Alternatively, the process is quite sluggish and causes the biogas to produce a significant quantity of CO2, which reduces the energy density. Because biogas contains a high quantity of CO2, it must be stored, and substantial chemical treatment is performed, involving cryogenic separation, to eliminate CO2 until it can be utilized [43]. Compared to the use of biological techniques, MECs provide an alternate method for producing both CH4 and H2.

In MECs, microbes are utilized as biocatalysts to reduce the activation overpotential of a certain redox process, enhancing voltage efficiency and production rate [44]. On the surface of the anode, some microbes can develop a biofilm, which can convert the chemical energy contained in organic molecules into electrical energy. At the cathode, this electrical energy is subsequently used to produce additional useful products, such as H2 and CH4 [45]. To withstand cellular function and growth, certain microorganisms are electrochemically active, which means that they can transfer electrons along with the electrode to keep things running [46]. The chronological development of MEC technology is illustrated in Figure 3.

**Figure 3.** Chronological development of microbial electrolysis cell (MEC) technology [42,47–57].

#### **4. Thermodynamics and Electrochemistry for Hydrogen Production Using MEC**

Because of the thermodynamic limitations, major organic compounds such as volatile acids (e.g., butyrate, acetate, and propionate) and solvents (e.g., ethanol and butanol) cannot be used for fermentative H2 production. However, additional energy is required for overcoming this limitation and for producing hydrogen [58]. In MEC, the required additional energy is provided by the voltage supplied through the power source. However, there is a necessity for higher applied potential than the equilibrium potential (Eeq) of the electrochemical cell (EC) for driving the MEC process. The equation is given as follows:

$$E\_{\rm cell} = E\_{\rm cat} - E\_{\rm anv} \tag{1}$$

where Eeq is the equilibrium potential of the EC, Ecat is the cathodic half-life potential, and Ean is the anodic half-life potential.

Therefore, to find the equilibrium voltage, determining the individual half-life potential using the Nernst equation is required.

Another well-researched electrochemical process is the hydrogen evolution reaction (HER). There are two reasons for this: (i) hydrogen evolution was originally regarded to be one of the simplest electrochemical processes, and (ii) it is a very significant process for society since hydrogen will one day replace fossil fuels as a transportation fuel, because of its simple reaction scheme.

> 2H<sup>+</sup> + 2e− → H2 (Under acidic conditions).

2H2O + 2e− → H2 + 2OH− (Under alkaline conditions).

The equilibrium potential of the hydrogen evolution process is significantly reliant on the pH at the cathode since protons (or hydroxyl ions) are involved, according to

$$\rm E\_{H2,pH}^{\ominus} = -0.059 \times \rm pH. \tag{2}$$

It has been well established that HER proceeds via two successive steps. The initial adsorption of a proton to form adsorbed hydrogen, i.e., the Volmer reaction (H+ + e<sup>−</sup> + Had), is usually considered to be fast; however, there are two possibilities for the subsequent, slower hydrogen evolution process: one is the hemolytic Tafel reaction; the other is the heterolytic Heyrovsky reaction [59].

$$\text{Hg} \\ \text{O} + \text{e}^- + \text{Me} \rightarrow \text{MeH}\_{\text{ads}} + \text{OH}^- \ (\text{volmer electrode} \text{emical discharge step}). \ \ ^\circ$$

MeHads + H2O + e<sup>−</sup> → H2 ↑ +OH− + Me (Heyrovsky electrochemical disorption step).

> MeHads → H2+2Me (Tafel catalytic recombination step).

#### *4.1. Anodic Potential for Hydrogen Production*

In the MEC method, anode respiratory bacteria such as antibiotic-resistant bacteria (ARB) or exoelectrogens residing in anodic biofilms transform organic matter into bicarbonates, electrons, and protons; the electrons are then transported to the cathode via a limited energy supply (−0.300 V vs. standard hydrogen electrode (SHE) formed by ARB), where they interact with protons to produce hydrogen gas. For example, the sodium acetate reaction at the anode is presented below.

$$\mathrm{CH\_3COO^- + 4H\_2O \to 2HCO\_3^- + 9H^+ + 8e^- \dots}$$

In terms of the conceptual anode potential (Ean) for oxidation of acetates under typical biological conditions (pH = 7, T = 298.15 K, [CH3COO−] = 0.0169 mol·L−<sup>1</sup> (1 <sup>g</sup>·L−1), and [HCO3−] = 0.005 mol·L−1), the Nernst equation can be approximated to

$$\mathrm{E\_{an}} = \mathrm{E\_{an}}^{\circ 0} - \frac{\mathrm{RT}}{8\mathrm{F}} \ln \frac{\left[\mathrm{CH\_3COO^-}\right]}{\left[\mathrm{HCO\_3}^-\right]\left[\mathrm{H^+}\right]^9} \tag{3}$$

$$= 0.187 - \frac{8.31 \times 298.15}{8 \times (9.65 \times 10^4)} \ln{\frac{[0.0169]}{[0.005]^2 [10^{-7}]}} = -0.300 \text{ V} \tag{4}$$

where Ean<sup>0</sup> is the standard electrode potential for acetate oxidation (0.187 V), R is the universal gas constant (8.31 J·mol−1·K−1), T is the absolute temperature (K), and F is Faraday's constant (9.65 × 10<sup>4</sup> <sup>C</sup>·mol−1) [60].
