## 1 Introduction
However, if butyric acid is among the products, the reaction's yield in hydrogen production will be equal to 2 moles of H\({}_{2}\), as shown in the development of Equation (2) [7]. \[\text{C}_{6}\text{H}_{12}\text{O}_{6}+2\text{H}_{2}\text{O}\to 2\text{CH}_{3} \text{COOH}+4\text{H}_{2}+2\text{CO}_{2}\text{ }\Delta\text{Go}=-206\text{ kJ mol}^{-1} \tag{1}\]  
\[\text{C}_{6}\text{H}_{12}\text{O}_{6}+2\text{H}_{2}\text{O}\to 2\text{CH}_{3} \text{COO-}+2\text{HCOO-}+4\text{H}^{+}+2\text{H}_{2}\text{ }\Delta\text{Go}=-209.1\text{ kJ mol}^{-1} \tag{2}\]  
\[2\text{HCOOH}\to 2\text{CO}_{2}+2\text{H}_{2}\text{ }\Delta\text{Go}=-6\text{ kJ mol}^{-1} \tag{3}\]  
In real conditions, the yield will be less than 4 moles because besides the mixed production of butyric and acetic acid, it is likely that hydrogen-consuming bacteria such as methanogenic organisms, sulfate-reducing bacteria (SRB), nitrate-reducing bacteria (NRB), propionate producers, iron-reducing bacteria, and lactic acid bacteria are present within the microbial consortium [10]. The recovery of biohydrogen through dark fermentation using biomass as a substrate has gained significance in recent years. Specifically in the field of biofuels, there is a variety of research focused on biohydrogen production from a wide range of substrates, from microalgae cultivation to agro-industrial waste. In addition to substrates, microbial consortia and process variables such as temperature, pH, organic loading, and the substrate-inoculum ratio have also been studied to ensure the best possible biohydrogen production [11]. Regarding microorganisms participating as inocula in the dark fermentation process, both pure and mixed cultures have been used. With pure cultures, bioaugmentation tests have been conducted with strains of bacteria like _Clostridium pasteuranium_, _Clostridium acetobutylcium_, and _Lactobacillus bulgaris_. It was found that in bioaugmented cultures, the metabolic process was more stable than in mixed cultures, but hydrogen production was limited due to trophic competition between the strains and the original consortium [12]. On the other hand, when a mixed culture is subjected to a mixed micro-oxidative environment, the hydrogen yield remains similar to pure culture conditions, but after 9 days, the process becomes unstable, possibly due to prolonged stress on the culture [13]. pH has been considered an important parameter for achieving higher biohydrogen production, with the process's by-products depending on it and its variation during the process. For a pH lower than 5, production is primarily of lactic acid. If the pH value falls within the range of 5 to 5.5, the main products are biohydrogen and butyric acid. Finally, if the pH value exceeds 6, the main products are biohydrogen and sulfate [14]. The optimal pH for production is 5.5 since it results in the lowest production of volatile fatty acids. Low pH values inhibit enzymatic and hydrogenase activities, which consume hydrogen [15; 16]. Waste materials as substrates for biohydrogen production have been utilized in different studies. Food waste has shown performance results in hydrogen production of 49.9 mL H\({}_{2}\)F/g-VS [17], whereas OFMSW at a temperature of 37 \({}^{\circ}\)C yielded a production of 41.7 (\(\pm\)2.3) ml H\({}_{2}\)/g-VS, attributed to the presence of organic compounds such as carbohydrates. These carbohydrates, with short chains, are easily assimilated by fermentative bacteria. The biogas obtained consists of H\({}_{2}\) and CH\({}_{4}\), representing 28% and 72% of the total energy recovery from the OFMSW [18]. Organic loading is considered a relevant variable since the overall efficiency of the fermentation process depends on it. If the organic loading is too low, there will be poor biohydrogen production, while if it is too high, the process will be inhibited [19]. Organic loading plays an essential role in defining the metabolic pathways the fermentation process will take.