*3.1. Feeding Phases*

Based on the outcomes of the experiment, it was divided into phases. The main characteristics of the feeding phases, except for the start-up phase, are reported in Table 5. Indeed, the start-up phase was characterized by high instability of the main process parameters as well as by a rapid variation of admixture feeding rates over time. Thus, the remaining feeding phases were labeled from 1 to 7.


**Table 5.** Main parameters (with their standard deviations) of anaerobic digestion trials on hemp residues.

1 OLR means "organic loading rate"; 2 HRT means "hydraulic retention time"; 3 C means "percentage of new hemp in the admixture"; 4 R means "digestate recirculation ratio".

Feeding phases were classified according to enzymatic treatment, OLR, HRT, C, and R (Table 5). The two C reference values applied in this study are close to the organic loadings used in the industrial plants of anaerobic digestion.

#### *3.2. Management of the Reactor and Process Stability*

Management of the reactor -TS and VSd.b. trends of the material in the feeding hopper, in the reactor sludge, and the digestate tank are reported in Figures 2 and 3.

**Figure 2.** TS (total solids) (**a**) and VSd.b. (volatile solids on a dry basis); (**b**) trends of the material in the feeding hopper and the digestate tank, concerning consecutive sampling.

**Figure 3.** TS (total solids) (**a**) and VSd.b. (volatile solids on a dry basis); (**b**) trends of the reactor sludge, concerning consecutive sampling.

By comparing the TS and VSd.b. trends in the feeding hopper and digestate, it can be seen that from the 70th sampling, these parameters were notably lower in the discharged sludge than in the corresponding feeding mixtures. The 70th sampling corresponds to the beginning of the 5th feeding phase and this occurrence emerged from the observations related to the 6th and 7th phases as well. In the previous feeding phases, a distinction between the TS and VSd.b. evolution in the fed slurry and the corresponding digestate cannot be seen. This result is related to the higher reference value of C (C2), which was better than the other one (C1).

With regard to Figure 3, the reactor sludge did not show relevant differences in terms of TS and VSd.b. by comparing the loading side to the discharging side, mainly due to a certain mixing of the sludge along the longitudinal section of the reactor. TS and VSd.b. seemed to increase from phase "1" to phase "4" and to decrease in the subsequent phases (characterized by the reference value, C2).

Management of process stability: Concerning the main design and operation process parameters of the reactor, the trends of HRT and OLR are shown in Figures 4 and 5.

**Figure 4.** Hydraulic retention time (HRT) trend.

**Figure 5.** Organic loading rate (OLR) trend.

HRT did not show significant variations during the entire experiment (Figure 4) and it was excluded from the statistical analysis.

OLR (Figure 5) was monitored through the daily determination of TS and VSd.b. in the feeding admixture and adjusted by setting the hemp share in the feeding admixture and the loading flow rate (Figure 2).

Regarding the process stability parameters, the trends of FOS/TAC and pH, biogas production, and biogas composition (CH4, CO2, NH3, H2S) are reported in Figures 6–9.

**Figure 6.** FOS/TAC (volatile fatty acids/buffer capacity) and pH trends.

**Figure 7.** Biogas production trends (GPR) (via biogas metering).

**Figure 8.** Biogas composition (CH4 and CO2, [%wt]) detected by the portable gas analyzer during the experimental period.

**Figure 9.** Gas production rate (GPR) trends of biogas and methane.

By considering the entire experimental campaign, pH varied between 7.2 and 8.0, accompanied by higher values during the first phase.

The FOS/TAC ratio increased over time, from about 0.170 to values close to 0.270. These values are very similar to the typical ones of the industrial plants treating lignocellulosic biomass, such as corn silage.

Overall, the FOS/TAC trend is consistent with the increase in the percentage of new hemp in the feeding admixtures (C2 treatment) (see Table 5) from the fifth feeding phase to the seventh one. It can be supposed that the introduction of higher amounts of hemp provoked a shift of the anaerobic digestion microbial dynamics towards the predominance of acidogenic reactions.

Daily biogas production (Figure 7) was characterized by high variability during the experimental campaign. It showed a significant increase of the biogas produced during the experimental phases from "5" to "7" (C2), accompanied by a rapid increase during the fifth phase and a decrease over the sixth one (without enzymes). The last feeding phase showed rising values for most of the days (to reach about 1.1 m<sup>3</sup> biogas per day), corresponding to the coupling of higher C and enzymatic treatment. It can be considered as the most suitable among all the treatments applied. On average, the CH4 content [%wt] in the biogas produced during the entire experimental campaign was 53.8 ± 2.2. CO2 content [%wt] was 45.6 ± 2.4. CH4 concentration showed a slow reduction from the phase "1" to the phase "4". From the phase "5", a gradual increase of its concentration was detected. CH4 and CO2 trends (Figure 8) did not show any peak attributed to organic overload.

Performance parameters: The performance parameters, SGP and GPR, of the anaerobic digestion trials are shown in Figures 9 and 10.

**Figure 10.** Specific gas production (SGP) trends of biogas and methane.

SGP of biogas/methane is the energy yield of an anaerobic digestion system regardless of OLR, thus it plays an important role when assessing the energy yield of the considered process.

Both trends showed an increase starting in the fifth feeding phase. It was more evident during the seventh period.

By comparing these outcomes with those obtained on a laboratory scale [30,32], it can be pointed out that the present work provided higher SGP values both for biogas and methane than the previous literature on anaerobic digestion trials of hemp straw. Overall, the managemen<sup>t</sup> of the reactor and the process stability was enhanced with respect to biogas production, especially in the last part of the experiment, characterized by the combination of higher values of C and the application of enzymes.

SGPs of the present work are lower compared to those obtained in the same pilot plant, using vegetable feedstock characterized by high degradability (lower lignin contents and higher starch content). Due to the lignocellulosic nature of this crop, the specific biogas/methane production of hemp straw was lower than those related to raw potatoes (0.68 Nm<sup>3</sup> of biogas ·kgVS−<sup>1</sup> and 0.37 Nm<sup>3</sup> of methane ·kgVS−1) [53], potato chips (0.81 Nm<sup>3</sup> of biogas ·kgVS−<sup>1</sup> and 0.47 Nm<sup>3</sup> of methane ·kgVS−1) [54], and fruit and vegetable wastes (0.78 Nm<sup>3</sup> of biogas ·kgVS−<sup>1</sup> and 0.43 Nm<sup>3</sup> of methane ·kgVS−1) [55].

Experiments carried out on the same pilot digester using admixtures made of different kinds of feedstock (admixture composition: 30%wt of shredded corn, the remaining part consisting in whey, vegetable water, pomace pitted, and manure to maintain the OLR between 2.5 and 3.5 kgVS·<sup>m</sup>3reactor·d−1) reported SGPbiogas from 0.623 ± 0.212 Nm3·kgVS−<sup>1</sup> to 0.768 ± 0.227 Nm3·kgVS−<sup>1</sup> and SGPmethane from 0.281 ± 0.160 Nm3·kgVS−<sup>1</sup> to 0.438 ± 0.096 Nm3·kgVS−<sup>1</sup> [56]. Also, hemp straw residues' performance in anaerobic digestion was found to be similar to other lignocellulosic crops [57], the specific methane production of which is between 0.17 and 0.39 Nm3·kgVS−1. Results are comparable to those reported by [58] for other agricultural crops, such as oats, flax, and sorghum, but lower than the hemp energy yields considered there.

Thus, hemp straw residues used in the same pilot plant showed lower SGP of biogas and methane probably because of the lignocellulosic composition. The results obtained by the residues considered in this work could be a ffected by the harvesting time, which is the reproductive stage when the lignin content is higher than in the vegetative stage. By considering that the reproductive stage is the core of the supply chain scenario assumed in this study, related to the extraction of oil from seeds, the results already discussed about the potential of biogas production from straw residues sugges<sup>t</sup> the recovery of this low-value by-product to energy conversion. By considering the ultimate analysis, the carbon:nitrogen ratio is useful to define the biomass suitability for biochemical (ratio <30) or thermochemical processes (ratio >30). The ratio of hemp straw residues is 28.9 (Table 3), thus it can be considered for both, but with slightly higher suitability for biochemical conversion.
