*3.3. Methane Production*

Figure 2 represents all the mean methane generation curves obtained during BMP tests. According to the ANOVA analysis performed, all curves can be assumed by the average curve, so this one will be used for the study, along with the error bars and the diagram of boxes and whiskers. The shape of the curves and the information obtained from them is similar to that obtained in biogas generation curves. Digestion occurs in the same way and clearly in two phases, and the stabilization of the process and therefore the generation of methane occurs around 10 days. In particular, the generation of methane obtained is 458.550 (±24.838%) Biogas NmL measured under normal conditions, per 100 g of digested CrM residue (Table 3).

**Figure 2.** Gross methane production mean curve from the anaerobic digestion of 100 g of residue CrM and box diagram of its variation.

Although the dispersion between curves is similar to that obtained in the production of biogas, as inferred from the box and whisker diagram depicted in Figures 1 and 2 the descriptive statistics in Table 3 show that this variability between curves is not pronounced, in fact the dispersion between methane generation curves is less than the deviation between biodegradability curves or biogas production. It follows then that the process is quite stable, and the changes between them are not due to failures in the process, but to changes in the proportion of methane in biogas, which is a good indicator of the stability and development of the process and is therefore studied in a section of its own.

As for the specific production of methane, this is determined at 18.024 NmL of methanol per gram of vs. of CrM residue introduced into the reactor.

### *3.4. Methane Content of Biogas Generated*

It can be seen in Figure 3 that the proportion of methane in biogas begins to detect something before the first day, begins to grow until day 5, slows slightly, resumes until moderately stabilized on day 10. The curves converge around 27% of methane. This double growth and observed slope change reconfirms the two-phase digestion phenomenon that occurs, a first phase in which directly accessible organic matter is digested, and secondly particulate or encapsulated organic matter, although this must be corroborated with a somewhat more thorough analysis such as the evolution of H<sup>2</sup> to be done in subsequent sections.

As noted, and as seen in the mean curve of Figure 4, the average methane content in the generated biogas is 27.485 (±16.201%) % of CH4, resulting in an increase of 128.098% compared to the methane content of the biogas generated by the inoculum, so it is inferred again that the effect of adding CrM substrate has been positive.

**Figure 3.** Methane content mean curve from the biogas produced in the anaerobic digestion of 100 g of residue CrM and box diagram of its variation.

**Figure 4.** Gross hydrogen production mean curve from the anaerobic digestion of 100 g of residue CrM.

The methane content is below 60%, which is the limit considered acceptable for a stable and deeply developed process. In this case it is around 30%, so the process is inferred that it is not entirely complete, as indicated in previous paragraphs and is probably due to low degradation levels, which will be demonstrated later with the mathematical analyses and determinations including the level of biodegradation of the substrate. In this case it has already been indicated that there is first digestion, only of the organic matter directly accessible or solubilized, and subsequently the encapsulation, which will logically not be digested in depth or in full (and will be determined by the analysis of the level of degradation of the substrate). Although some of the results of previous research conducted by other authors yield similar values of methane content, this study explains by the nature of the inoculum, a granular UASB substrate, specially designed to digest solubilized organic matter, and that its characteristic of granule grouping makes the hydrolysis of particulate matter complicated, since microorganisms and hydrolytic enzymes, grouped in a granule have fewer freedoms and the ability to adhere to the walls of the particulate substrate and start hydrolyzing it, being easier directly to act with that COD that is solubilized and directly accessible (COD<sup>f</sup> ).
