*3.6. Hydrogen Content of Biogas Produced*

Like hydrogen generation, hydrogen content (Figure 5) can be used as a process indicator. Although methane is one of the final gases of AD and is therefore accumulated, H<sup>2</sup> is an intermediate gas that appears and subsequently transforms into a final compound. The hydrogen ratio of the biogas generated increases in the first few days when hydrolysis, acidogenesis and acetogenesis occur, until the peak is reached as soon as the latter ends. When methanogenesis begins, the ratio of H<sup>2</sup> should drop as hydrogen is transformed into methane by hydrogenotrophic route.

**Figure 5.** Hydrogen content mean curve from the biogas produced in the anaerobic digestion of 100 g of residue CrM.

As noted, there are the two peaks described above. The first and most pronounced occurs on day 3 with a hydrogen content of 0.289 (±9.903%) %, and the second occurs on day 9 reaching a hydrogen content of 0.033 (±349.718%) %. The occurrence of these two peaks reconfirms the possibility of two-phase digestion or a slowdown of the process that is subsequently recovered. To determine the reasons why this occurs, and to give a definitive explanation for the development of the process, we go on to study together all the generation curves together with the pH evolution curve, and to analyze them, taking into account the changes in the composition of the mixture at the end of the digestion process.
