*3.2. Biogas Production*

To determine biogas production, the internal pressure generated inside the digesters during the development of BMP tests is measured, and then translated into the volume of gas generated, as described in Section 2.3.

The most convenient measure to express biogas generation is gross production, that is, biogas generated by a certain amount of waste. However, several authors [37,38] recommend expressing it in terms of specific production, that is, in biogas generation for each vs. containing the residue, in this way it can be compared with another substrate more quickly. Therefore, the results are commented based on gross production but are also displayed based on specific production.

As shown in Figure 1, biogas generation is completed on day 10, producing an average of 1604 (±19.980%) NmL of biogas measured under normal conditions, per 100 g of digested

CrM residue (Table 3). Although there are some failed assays, based on the results of the ANOVA analysis developed, all curves can be considered equal and assumed by the mean curve. This is because, after performing the ANOVA analysis, the significance level is, in all curves except those that the process has been failed, 1000, so that the null assumption of average equality can be accepted, and therefore all curves equal to the mean curve can be assumed. This convergence of results was to be expected since the substrate to be treated, as advanced in Section 3.1 is a substrate rich in simple carbohydrates, which gives it the property to generate stable digestions.

**Production [NmL/100 g of residue CrM] Standard Deviation Coefficient of Variation Relative Error** Biogas 1604.22 NmL 525.39 0.32 19.98% Methane 458.55 NmL 189.61 0.41 24.83% Hydrogen 0.69 NmL + 0.33 NmL 0.60 + 0.15 0.87 + 0.48 105.72% + 134.45% **Specific Production [NmL/g of vs. of residue CrM]** Biogas 63.05 NmL 20.65 0.01 96.53% Methane 18.02 NmL 7.45 0.01 96.80% Hydrogen 0.02 NmL + 0.01 NmL 0.02 + 0.02 0.03 + 0.08 92.60% + 5.28% **Content [% vol]** Methane 27.48 7.22 0.26 16.20% Hydrogen 0.28% + 0.03% 0.14 + 0.14 0.43 + 4.37 39.90% + 349.72%

**Table 3.** Numerical results of the BMP tests when digestion residue CrM. Gross and specific production of biogas, methane and hydrogen, methane and hydrogen content of the produced biogas and descriptive statistics.

> Figure 1 shows the average curve of all the 21 curves obtained, which can be assumed as determined by the ANOVA analysis. It can be seen in them that the initial part of the curves, there are two clearly differentiated slope changes. During the first day there is a delay, and on days 1–4 there is a linear generation, which stops until day 8–9 in which generation resumes to stabilize on day 12. Although it will be demonstrated later in the analysis of the results of the following sections, it is an indicator that anaerobic digestion occurs in two phases. Given the characteristic particulate substrate, in the first place the organic matter is digested directly accessible, that is, the solubilized, and subsequently, after hydrolyzing the particulate matter, it begins to digest it. This assumption will be demonstrated later with the joint analysis of all the variables studied, especially with the evolution of hydrogen, as well as evolutions and compositional changes.

> If the generation of biogas is compared with that obtained by the inoculum alone, the effect of adding the CrM substrate has been positive, by increasing the generation of gas by 311.33%, from the 390 mL of biogas produced by the inoculum to the 1604.22 NmL produced in joint digestion and by increasing the process speed, as you can see by comparing the slope of the start of the curves.

> As for the specific production of biogas, the conclusions obtained are analogous to those of gross biogas production, as well as their curves, which are proportional. Specific production is nothing more than the production of biogas expressed, not by amount of gross waste, but for each gram of volatile solid contained in the residue, hence the curves are proportional. Specific production, as shown in Table 3, is estimated at 63,058 NmL of biogas per gram of vs. of CrM waste introduced into the reactor.
