*3.5. Hydrogen Production*

Hydrogen generation occurs once hydrolysis has been completed and the acidic stages of acidogenesis and acetogenesis begin, which will consequently become CH<sup>4</sup> through acetoclastic methanogenesis. Hydrogen once generated begins to disappear when it is transformed into methane using hydrogenotrophic methanogenesis. It is therefore a good indicator of process development and hydrolysis speed, although its use is not widespread as it is difficult to detect and measure against other indicators such as pH. This research will use it for this purpose, providing added value and novelty to the study.

It is to be expected that hydrogen will form during the first few days and the larger its production and the faster the production peak is reached, the faster and deeper the hydrolysis process will be. As soon as hydrogen is generated, that is, as soon as hydrolysis is complete, the production of acidic elements by acidic elements through acidogenesis and acetogenesis begins, so it is accompanied by a decrease in pH. When the peak is reached it is understood that hydrolysis has completed, and when it begins to disappear hydrogen it is inferred that hydrogenotrophic methanogenesis begins. Methanogenesis should also be acetoclastic, that is, it must be formed from acetic acid and acidic elements. If methanogenesis occurs correctly, hydrogen reduction (by its transformation into methane) is accompanied by an increase of pH (by the transformation of acidic elements into methane). Any other development with hydrogen is identified with a process failure, inhibition or stop, for example if hydrogen reduction is slowed and also coincides with a pH that is maintained at low levels, it is understood that there has been a build-up of acidic elements that has led to inhibition of methanogenesis, and the process has been inhibited without methane production through acetoclastic methanogenesis.

Figure 4 represents the average of gross hydrogen generation curves obtained in each BMP test, along with the error bars. The ANOVA analysis demonstrates by the level of significance that in all cases curves can be resembling the middle curve and studying the process through it.

There is a first hydrogen peak on day 3. This means that hydrolysis ends on day 1, and as hydrogen grows the acidic phases begin. The hydrogen generation for this peak is 0.690 (±105.727%) NmL of hydrogen measured under normal conditions, per 100 g of digested CrM residue (Table 3). The variability of the data is very high, which is logical since production occurs in less than a day and many averages are performed after this period, on days 1–3. In any case, the presence of a maximum hydrogen on day 3 is clear in any test curve. From this day the amount of hydrogen begins to decrease, at a certain rate, the reduction stops between days 4 and 5, and resumes until the 6th day. This change in slope in the reduction of H<sup>2</sup> may be due to some kind of slowdown, which should be studied and determined with subsequent analyses, such as pH evolution, although it is likely to be due to an accumulation of VFA, since this substrate is prone to release them as stated in Section 3.1.

Thereis a second peak ofH<sup>2</sup> generation on day 9 althoughlower value 0.330 (±134.450%) NmL for every 100 g of CrM residue introduced into the reactor). The appearance of this second peak may be due; well begins the digestion of particulate organic matter, which has released the components after hydrolyzing the outer membranes of the waste particles; or inhibition of methanogenesis, which subsequently resumes. Both options may be valid given the characteristics of the CrM residue studied in Section 3.1:


first, that is, The OM directly solubilized and accessible to microorganisms, and once hydrolysis has developed, the OM or COD that is encapsulated in the substrate begins to be digested.

In terms of specific production, the curves are proportional to those already described, there is a first generation peak on day 3 worth 0.027 (±39.903%) NmL per gram if vs. of CrM residue introduced, and a second peak on day 9 worth 0.033 (±349.718%) NmL for each gram of vs. of CrM waste introduced into the reactor.
