Open AccessArticle

Biomethane Production from Untreated and Treated Brewery’s Spent Grain: Feasibility of Anaerobic Digestion After Pretreatments According to Biogas Yield and Energy Efficiency

Jessica Di Mario

Alberto Maria Gambelli

and

Giovanni Gigliotti

Civil and Environmental Engineering Department, University of Perugia, Via G. Duranti 93, 06125 Perugia, Italy

Author to whom correspondence should be addressed.

Agronomy 2024, 14(12), 2980; https://doi.org/10.3390/agronomy14122980 (registering DOI)

Submission received: 6 November 2024 / Revised: 5 December 2024 / Accepted: 11 December 2024 / Published: 14 December 2024

(This article belongs to the Special Issue Biogas and Biomethane Production from Pretreated Waste Biomasses)

Download

Browse Figures

Figure 1
(A) Batch bioreactor used in this study for biogas production. The biogas generated in the first bottle flows into a second vessel, the “gasometer”, which contains water. The quantity of water flowed in the last vessel allowed us to measure the amount of biogas produced (volumetric method). (B) Inclusion of an alkaline trap, composed of 5 M NaOH and thymolphthalein as a pH indicator, to assess biomethane production. The CO₂ in the biogas was separated according to the reaction outlined in the scheme. "> Figure 2
Daily biogas production for the different biomasses tested. "> Figure 3
(a) Total biogas yield for unit of VS for all biomasses. (b) Average daily biogas yield for the different samples (total Nm3·gVS−1/days of production). "> Figure 4
Comparison between the quantity of biogas produced and the related amount of biomethane contained in it. "> Figure 5
Cumulative energy produced (empty dots) and energy spent (filled dots) for the various samples. "> Figure 6
Efficiency measured for the optimal production period (until reaching the maximum difference between energy produced and energy spent). "> Figure 7
Energy produced (in blue) and energy spent (in red) during the optimal production period and energy produced before the energy spent equaled the energy produced (green). ">

Versions Notes

Abstract

The increasing global energy demand, coupled with the urgent need to reduce CO₂ emissions, has intensified the search for renewable energy sources. Biogas, produced from agro-industrial biomass, presents a viable solution. In beer production, brewery’s spent grain (BSG), the largest by-product by volume, offers potential for bioenergy recovery. This study applied a biorefinery approach to BSG, extracting protein hydrolysates (PH) through mild alkaline hydrolysis and nanostructured lignin (LN) via the Ionic Liquid Method. The objective was to assess biogas production from the residual biorefinery biomass and evaluate the co-digestion of BSG with Olive Mill Wastewater (OMWW) and Olive Pomace (OP), by-products of the olive oil industry. Biogas was produced in lab-scale batch reactors and the quantity of biogas produced was measured via the volumetric method. Conversely, the amount of biomethane obtained was evaluated by introducing, in the production chain, an alkaline trap. Biogas yields were the highest for untreated BSG (1075.6 mL), co-digested BSG with OMWW (1130.1 mL), and BSG residue after PH extraction (814.9 mL). The concentration of biomethane obtained in the various samples ranged from 54.5 vol % (OMWW + BSG) to 76.59 vol % (BSG). An energy balance analysis considering both the theoretical energy consumed by a semi-continuous anaerobic digestion bioreactor and the energy produced as bio-CH₄ revealed that BSG after PH extraction was the most energy-efficient treatment, producing a net energy gain of 5.36 kJ. For the scope, the energy consumption was calculated by considering a PEIO index equal to 33% of the energy produced during the day, showing the highest biogas production. In contrast, the co-digested BSG with OMWW yielded the lowest net energy gain of 1.96 kJ. This comprehensive analysis highlights the energy efficiency of different treatments, identifying which process should be improved.

Keywords:

biogas; by-products; renewable energy; green chemistry; biomethane

1. Introduction

The continuous growth of the global population is driving an increasing demand for energy. However, non-renewable energy sources are insufficient to meet this increasing demand, highlighting the urgent need to transition to renewable energy alternatives. Furthermore, the extensive reliance on fossil fuels is a primary contributor to ongoing environmental challenges, with carbon dioxide emissions from various combustion processes being the leading cause of global warming [1,2].

Consequently, the development and adoption of renewable energy sources have become critical elements of global energy policies aimed at reducing dependence on fossil fuels. In this context, biogas has emerged as a promising clean energy source for both household and industrial applications, offering a practical solution to address the global energy crisis [3,4].

The process that leads to biogas production is anaerobic digestion (AD), an efficient, economical, and environmentally friendly technology in which organic matter is degraded by microorganisms, in the absence of oxygen, through a series of biochemical reactions [5,6].

Microorganisms involved in the anaerobic digestion process can utilize biomass from food and agricultural industries, as well as municipal waste, as feedstock for biogas production. Within the food industry, various by-products can serve as valuable substrates for microbial growth. One such by-product is brewery’s spent grain (BSG), a significant waste material from the beer production chain [7,8,9]. BSG is the primary solid by-product of the beer manufacturing process, accounting for approximately 86% of the total solid by-products generated by the brewing industry [10]. This biomass contains a significant fibrous component composed of lignin, cellulose, and hemicellulose, as well as various phenolic compounds, with ferulic acid and p-coumaric acid being the most prominent. Additionally, it is rich in vitamins, including folic acid, choline, thiamine, and pantothenic acid [11,12,13].

Currently, 70% of BSG is utilized as animal feed, primarily due to its contents of arabinoxylans and β-glucans, which stimulate the activity of beneficial gut bacteria in animals [14]. Additionally, its high protein and fiber contents have attracted attention from the food industry, where it is being explored as an ingredient to enhance the nutritional value of human food products [15,16].

The application of BSG in anaerobic digestion presents significant benefits for both the bioenergy and economic sectors. Nevertheless, only 10% of the total BSG produced is currently used for biogas production. Thus, there is a growing need to increase this percentage and fully harness BSG’s potential as a substrate for biogas generation [17,18].

Pereira Lins et al. [19] evaluated the biogas and methane production potential of BSG, reporting yields up to 580 NL kg⁻¹VS and 330 NL kg⁻¹VS, respectively, under mesophilic anaerobic digestion conditions (37 ± 2 °C).

To enhance anaerobic degradation and thereby increase biogas yield, pretreatments are often necessary to alter the complex structure of raw biomass [20]. In this context, Bochmann et al. [21] investigated the effects of thermal pretreatment on BSG, applying temperatures ranging from 100 °C to 200 °C. The optimal biogas yield was achieved at 140 °C, with pretreated samples producing a total methane volume of 467.6 Nm³ CH₄ kg⁻¹VS, compared with 409.8 Nm³ CH₄ kg⁻¹VS for untreated biomass. Buller L. S. et al. [22] explored the use of ultrasound as a pretreatment for BSG prior to anaerobic digestion, resulting in a significantly higher methane yield of 107.28 L CH₄ kg⁻¹VS, compared with 72 L CH₄ kg⁻¹VS from untreated BSG. Although this method improves biogas quality, it is energetically expensive.

However, biogas production can serve as the final step in a biorefinery approach, wherein waste biomass is first utilized to extract value-added molecules before the residual biomass is employed for AD. The extraction of these molecules can be considered a pretreatment process, as it helps to remove compounds that hinder anaerobic degradation. This results in a simpler biomass structure that is more amenable to degradation by anaerobic microorganisms, ultimately enhancing biogas yield [23]. Lignin, for example, is nearly non-degradable under anaerobic conditions due to the need for oxygen of extracellular enzymes, responsible for its depolymerization [24,25]. The presence of lignin, therefore, inhibits the process of biomass hydrolysis by interfering with anaerobic digestion, leading to a lower biogas and biomethane yield [26,27]. On the other hand, lignin has the potential to be highly valued and used in a variety of sectors, including pharmaceutical, electrochemical, and polymer industries [28]. Del Buono et al. applied nanostructured lignin as a seed treatment in maize, achieving positive outcomes in the plant’s physical, chemical, and biochemical properties [29]. In BSG, the average lignin content is 11.41% [30], and various methods may be used for its extraction, including the Ionic Liquid Method, and microwave-assisted and deep eutectic solvent extraction [31].

In addition, BSG is characterized by high protein content, ranging from 26% to 30% [32], primarily consisting of hordeins and glutelins [33]. This protein fraction can be recovered by using various extraction techniques and utilized as a value-added product [34]. Moreover, protein extraction can enhance the anaerobic digestion (AD) process by reducing the significant amounts of ammonia released during protein hydrolysis, which inhibits the methanogenesis step and negatively impacts the percentage of bio-CH₄ produced [35].

Given that the proteins in BSG are not highly soluble, several studies have focused on enzymatic treatments to produce protein hydrolysates with enhanced functional properties. The bioactive properties of BSG protein hydrolysate, including antioxidant and anti-inflammatory activities, indicate its potential applications in the pharmaceutical industry. Additionally, it can be utilized in the food industry for its emulsifying and foaming characteristics [32].

In a third-generation biorefinery, various valuable molecules are produced; ultimately, the anaerobic digestion process yields biogas and digestate, a solid–liquid residue that can serve as biofertilizer capable of replacing chemical fertilizers in sustainable agriculture [36]. This is attributed to its rich contents of organic matter (C), macronutrients (N, P, K), and micronutrients. Specifically, the liquid fraction can be utilized as a biofertilizer, while the solid component acts as a soil amendment. This is due to the substantial reduction in easily degradable organic matter during anaerobic digestion, resulting in a digestate that is rich in recalcitrant compounds [37,38]. However, considering the huge quantities of BSG-based digestate that could be produced worldwide, future destinations must also be planned for this agro-industrial residual. Within this context, Karimi and colleagues proposed to advantageously use digestate (in their study, from municipal solid waste) to produce activated carbon [39].

Therefore, the aim of this study is to apply a biorefinery approach to BSG by extracting nanostructured lignin and protein hydrolysates. Subsequently, we will assess the biogas yield of the residual biomass and compare it with the biogas potential of raw BSG.

To also evaluate the impact of co-digestion with other agro-industrial biomasses, we co-digested BSG with by-products from the olive mill industry, Olive Mill Wastewater (OMWW) and Olive Pomace (OP). Additionally, we evaluated the energetic cost of the entire biorefinery process to effectively assess its overall feasibility by comparing the energy required for the entire biorefinery with the energy produced in the form of biogas.

2. Materials and Methods

2.1. Materials, Procedures, and Analyses

The digestate employed in this study was characterized, and the results are shown in Table 1.

The Brewery’s spent grain (BSG) used in the experiment came from a local brewery company located in Perugia. The characterization of the BSG is reported in Table 2.

BSG was digested anaerobically but also used to extract protein hydrolysates (PHs) and to isolate lignin and hemicellulose. These extraction processes were carried out in parallel starting from the same raw biomass (BSG). After extraction, both residual biomasses were used to evaluate their potential biogas production through anaerobic digestion.

Humidity Content and Volatile Solid Determination

Humidity content was measured following the official protocol [40]. Two grams of each biomass was put in a crucible and dried in an oven at 105 °C until constant weight. The test was carried out in triplicate. The moisture percentage of the sample was calculated as the difference between the initial wet weight and the dried weight.

To determine the volatile solid content, the protocol used by [41] was used. Two grams of dried sample for each biomass was weighed in ceramic crucibles. The test was performed in triplicate. The samples were placed in a muffle with an increasing temperature up to 550 °C. Samples were kept at 550 °C for 24 h. Crucibles were then placed in a desiccator to reach room temperature before weighing the sample. The volatile solid content was estimated as the difference between the initial and incinerated weights.

2.2. Production of BSG Pulp

The Ionic Liquid (IL) Method was employed for the extraction of lignin and hemicellulose, using a solution of triethylamine and sulfuric acid [Et₃NH][HSO₄], as described by [42]. After solubilizing these components with EtOH, the remaining biomass, primarily composed of cellulose, was recovered by vacuum filtration, and it is referred to as Pulp (BSGp in the text).

2.3. Extraction of Protein Hydrolysates from BSG

The protocol for obtaining PHs involved hydrolyzing the initial biomass under alkaline conditions at mild temperature (<100 °C), followed by the isolation of PHs. During this hydrolysis process, proteins were broken down into smaller peptides and amino acids. The isolation of PH from the initial biomass determined high contents of organic compounds, carbohydrates, lipids, nitrogenous compounds, and fibers in the residual biomass, referred to as BSGph in the text.

2.4. Anaerobic Bioreactors

Anaerobic digestion was carried out in batch experiments by using 50 mL bioreactors at mesophilic temperature, i.e., 37 °C. To measure the volume of biogas produced, we used the volumetric methods, and the biomethane ratio was analyzed with an alkaline trap (0.5 M NaOH, with thymolphthalein as a pH indicator). Therefore, biomethane was measured every day, and the overall quantity measured allowed us to express it as a percentage of the total quantity of biogas produced. Each bioreactor consisted of 75% of dry weight (relative weight of 1.8 g) of the inoculum and the remaining 25% (relative weight of 0.6 g) of the tested dry biomass. The control consisted only of the inoculum. The total volume of each batch reactor was 75 mL. The treatments in the experiment were the following:

-: Control—inoculum only;
-: Treatment 1 (BSG): 1.8 g dw (dry weight) of inoculum (¾) and 0.6 g dw of BSG (¼);
-: Treatment 2 (BSGph): 1.8 g dw of inoculum (¾) and 0.6 g dw of BSG-PH (¼);
-: Treatment 3 (BSGp): 1.8 g dw of inoculum (¾) and 0.6 g dw of BSGPulp (¼);
-: Treatment 4 (OMWW + BSG): 1.8 g dw of inoculum (¾), and the ¼ was composed of 0.3 g (dw) of BSG + 0.3 g (dw) of OMWW;
-: Treatment 5 (OP + BSG): 1.8 g dw of inoculum (¾), and the ¼ was composed of 0.3 g (dw) of BSG + 0.3 g (dw) of OP.

A schematization of the experimental apparatus is provided in Figure 1.

2.5. Statistical Analysis

All analyses in this study were conducted in triplicate, and the values presented represent the means of these replicates. The significance of differences between mean values was determined by using a one-way ANOVA test, with a significance threshold set to a p-value < 0.05.

3. Results and Discussion

3.1. Biogas Production

Table 3 presents the daily biogas yield of all the treatments. Among the treatments, BSG and BSG + OMWW demonstrated the highest biogas production, 1075.6 mL and 1130.4 mL, respectively. The main difference between these two treatments was the production period, which, in the presence of OMWW (BSG + OMWW), was strongly reduced, passing from 62 days in BGS alone to 32 days.

This process improvement may be attributed to the degradation of OMWW, which is enhanced in the presence of additional macronutrients. In this case, BSG serves as a source of macronutrients such as nitrogen and carbon, contributing to the overall enhancement of the process [43]. The residual biomass after IL treatment (BSGp) generated slightly lower biogas yields than OMWW + BSG and BSG, with a shorter production period of 21 days. The shorter production period may be attributed to the removal of lignin, which normally can hinder anaerobic digestion (AD) steps, leading to an extended overall process [44]. In contrast, the poorest results were observed for BSG after enzymatic hydrolysis (BSGph) and for the OPP + BSG mixture, where the production volumes were 382.3 mL and 452.7 mL, respectively—both significantly lower compared with the other treatments. The low biogas production from BSGph could be attributed to a significant reduction in nitrogen content, caused by protein extraction, which leads to an imbalance in the C/N ratio essential for anaerobic digestion process [45].

Additionally, the co-digestion of BSG with OP (OP + BSG) resulted in a longer production period compared with the other treated and co-digested BSG samples. The reduced biogas production observed in the co-digested sample OP + BSG can be attributed to the inhibitory effects of phenolic compounds present in Olive Pomace, which may negatively impact the microbial communities responsible for the final stages of AD. This inhibition reduces both biomethane yield and overall biogas output. In contrast, the prolonged and enhanced biogas production from OP co-digested with BSG, compared with the other treatments, may be explained by the composition of raw BSG. Rich in proteins and exhibiting high biogas potential, raw BSG contributes to increased biogas yields. However, when BSG is subjected to certain extraction processes, lignin removal, and protein hydrolysates extraction, its composition is altered, resulting in reduced biogas production.

For the co-digestion with OMWW, the high concentration of phenolic compounds in OMWW, higher than in OP, caused a significant inhibitory effect on biogas production, strongly reducing the production.

The evaluation of daily production (Figure 2) for each biomass showed a general trend across all samples: an initial phase of high productivity followed by a gradual decline over time. BSG exhibited relatively steady production, maintaining a range of 40–60 mL for the first 16 days before production began to decrease. In contrast, OMWW + BSG had more variable production. The highest peak occurred on the first day, with 213.9 mL produced, marking the most productive day. A second significant peak appeared on day 14, with a daily output of 92 mL.

Figure 3a shows the cumulative biogas production per unit of VS. This evaluation confirms that both OMWW + BSG and BSG are the most productive matrices, highlighting how the addition of OMWW improves the anaerobic digestion (AD) of raw BSG, reducing the processing time and increasing biogas yield.

The graph below (Figure 3b) displays the average daily biogas production for each treatment calculated by dividing the total yield by the production days. In this case, BSGp (the residue after IL treatment) shows the highest average daily production, followed by OMWW + BSG. The lower daily production of BSG was due to its extended production period of 62 days, compared with the other matrices, which completed the process earlier. Both graphs in Figure 3 confirm that BSGph and OPP + BSG are not the most productive treatments.

In addition to the quantitative evaluation of biogas production, a qualitative assessment was conducted to measure the CH₄ concentration in the biogas mix. By using a 0.5 M NaOH solution, the CO₂ in the biogas was dissolved in the solution, and the remaining gas volume was considered pure bio-CH₄. In Table 4 and schematically in Figure 4, the values of the CH₄ concentration for all the treatments are reported. The biogas produced from BSG had the highest CH₄ content, with 76.59%, equivalent to 823.8 mL in volume. Following BSG, the BSGph and OP + BSG treatments showed bio-CH₄ concentrations of 71.69% and 66.05%, respectively. However, due to their lower biogas production potential, the actual volumes of CH₄ produced were quite small, 274.1 mL for BSGph and 299 mL for BSG + OPP. The bio-CH₄ produced from the residue after treatment with IL (BSGp) showed a significant reduction in methane concentration, decreasing by more than 15% compared with untreated BSG. The methane content dropped from 76.59% in BSG to 60% in BSGp. The lowest result was observed in the co-digested OMWW + BSG. Although this treatment achieved the highest biogas yield, the methane quantity was lower, with only 54.5% CH₄. In terms of volume, OMWW + BSG still ranked second, but the overall quality of biogas was the poorest. The reduction in methane concentration may be attributed to the toxic effects of phenolic compounds on acetoclastic methanogenesis, the final step in methane production during the anaerobic digestion process [46].

3.2. Energy Evaluations

Considering that biogas production requires energy to operate the plant, it is essential to evaluate the total energy consumed throughout the process and compare it to the energy produced, primarily in the form of biomethane. The energy demand in anaerobic digestion includes different steps, from the possible initial pretreatment of the biomass to its transport, the keeping of plant optimal conditions, the biogas upgrade process, and the handling of final products, such as digestate. Therefore, the process can only be considered efficient when the energy output exceeds the energy input. In this section, we analyze the energy balance of the treated sample, and the energy produced by each biomass was calculated by considering that the LHV (Lower Heating Value) of CH₄ is 52 MJ/kg (Table 5).

The energy consumption in the process was estimated, as it was not possible to precisely calculate the energy required for steps such as feedstock transportation and biomass pretreatment. Additionally, the lab-scale batch reactor used in this study could not be directly compared to a full-scale plant, further complicating exact energy assessments. Therefore, the energy required was calculated as a percentage of the energy produced from biogas, using the Primary Energy Input Output (PEIO) ratio set to 33% based on the average literature data [47,48,49,50] (Table 6).

The production period considered for calculating energy consumption did not encompass the entire production duration of our analysis. Since we used a batch reactor, the biogas production trend has an initial exponential phase followed by a gradual decline. However, as our energy calculations are based on a semi-continuous plant model, the energy consumption was estimated as a percentage of the production during the initial phase.

Table 6 shows that BSG produced the highest total energy, generating 20.62 kJ over the entire production period, but ranked third in energy consumption, at 0.38 kJ per day. In contrast, OMWW + BSG produced the second highest energy output, but due to exceptionally high biogas production on the first day (Figure 2), it became the most energy-consuming treatment, with 0.96 kJ per day. The two residual biomasses after the extraction of valuable molecules gave different results. The extraction of lignin (BSGp) allowed for a reasonable energy yield of 12.24 kJ, with an energy requirement of 0.41 kJ per day. In contrast, the residue after protein hydrolysis extraction (BSGph) significantly reduced the energy potential, yielding only 6.86 kJ, the second lowest after OPp + BSG, which also had low energy production (7.48 kJ) despite having the lowest daily energy cost of 0.12 kJ/day.

To emphasize the importance of maintaining a positive balance between the energy produced and the energy consumed, Figure 5 and Table 7 illustrate that reaching the end of the anaerobic digestion process is not advantageous, as the energy expended exceeds the energy generated. Furthermore, when the energy spent equals the energy produced, this situation is also unfavorable in terms of energy balance. In this case, there is no surplus energy available for other applications; instead, we only generated enough energy to meet the operational energy requirements. Figure 5 and Table 7 also demonstrate that for OMWW + BSG, the trend is distinctly negative, with the energy consumed nearly matching the energy produced almost immediately, at the beginning of the process. In contrast, OP + BSG and BSG showed the longest production period before the energy produced reached the energy spent.

In Table 7, we can observe that BSG yielded the best results, achieving the highest (Ep-Es)_max of 7.08 kJ in 17 days, compared with the other treatments. Similar results were obtained for BSGp, which had the second highest (Ep-Es)_max of 5.36 kJ, reached after 19 days of anaerobic digestion. These values indicate that beyond these points, biomethane production becomes inefficient, as the energy spent exceeds the energy produced. Therefore, the addition of OMWW to BSG cannot be considered an efficient co-digestion treatment, as its (Ep-Es)_max was only 1.96 kJ, reached after just 1 day.

Figure 6 presents the efficiency calculated for the optimal production period for each treatment. Efficiency was determined by dividing the energy produced during the optimal period (when the difference between energy produced and energy spent is at its maximum) by the total energy produced in that period. An efficiency value of 1 represents the most efficient scenario. In this graph, OMWW + BSG emerges as the most efficient treatment, with a value of 0.67. However, this result requires further clarification, as the optimal period for this treatment lasts only one day, making the energy production during this time very small compared with its total energy output.

The efficiency of BSG, at 0.52, was still higher than that of the two residual biomasses, BSGp and BSGph, which had efficiencies of 0.38 and 0.36, respectively, making BSGph the least efficient process.

In Figure 7, the graphs display the total energy produced and consumed during the optimal period, as well as the energy produced when Ep equaled Es. We can observe that for all biomasses, except BSG, the energy produced in the calculated optimal period was the same as the energy produced when Ep equaled Es. However, for BSG, there was a noticeable difference between the two values, indicating that biogas potential remained even after the identified optimal period. Nevertheless, if we aim to optimize the process in terms of energy efficiency, the best approach is to stop anaerobic digestion when Ep equals Es, even if biogas production potential remains. This is because, after this point, the net available energy decreases day by day.

The evaluation of the energy balance during anaerobic digestion is crucial when considering the scale-up of the process to a real plant. For the process to be efficient, the energy produced must always exceed the energy consumed. In this study, this type of analysis helps identify which treatments could be viable for a real plant and which require improvements in the anaerobic digestion process.

Based on the energy analysis, BSGp is a good compromise for a biorefinery approach [36]. It is the residual biomass obtained after extracting nanostructural lignin, and it reaches a value of (Ep-Es)_max of 5.36 kJ in 19 days. This performance contrasts with the other residual biomass from the BSG biorefinery, BSGph, which shows a much lower (Ep-Es)_max of 2.2 kJ and the lowest efficiency of 0.36. The untreated BSG shows a positive energy balance with the highest (Ep-Es)_max and better process efficiency. However, its full biogas production potential cannot be fully exploited without reducing efficiency. This makes BSGp a better candidate for maintaining both energy production and efficiency, particularly as it is residual biomass post-extraction of a valuable molecule and fits on a biorefinery approach. On the other hand, OMWW + BSG turns out to be the least effective treatment in terms of energy efficiency. With a high daily energy cost of 0.96 kJ/day, a very short optimal production period of only 1 day, and a low energy yield of just 1.96 kJ during this period, it is not a suitable candidate for further development.

4. Conclusions

This work focuses on the recovery of brewers’ spent grain (BSG), the primary solid by-product of beer production, through a third-generation biorefinery approach. This method enables the extraction of valuable molecules with diverse applications, enhancing their economic significance. In a 3G biorefinery, the final step usually involves biogas production from residual biomass. In this study, we assessed the biogas production potential of both biorefinery products and untreated BSG. The valuable compounds obtained included protein hydrolysates and nanostructured lignin, both produced through green methodologies. Additionally, we tested two other treatments co-digesting the BSG with by-products from the olive oil production chain. While the biorefinery approach aligns well with the principles of a circular economy by enhancing waste management sustainability, it is crucial to assess the energy efficiency of the process for real-scale applications. This study focused on energy balance analysis and the evaluation of anaerobic digestion efficiency for each tested biomass. The results confirmed the importance of such evaluations before scaling up to pilot or industrial plant, as not all samples proved to be energy-efficient. In detail, we determined the following:

(1): BSGp proved to be the most efficient biorefinery residue, with a value of (Ep-Es)_max of 5.36 kJ. BSG also demonstrated efficiency, showing the highest (Ep-Es)_max among the samples, even if to keep this by-product efficient from the energetic point of view, it is not convenient to produce all the potential energy.
(2): In contrast, BSGph yielded the lowest (Ep-Es)_max, with the lowest efficiency value of 0.36. This assessment is valuable for refining the process, as it helps identify weak points in the biorefinery system and provides opportunities for optimization.
(3): The co-digested samples, OPp + BSG and OMWW + BSG, did not perform better than BSGp and BSG. In fact, the co-digestion of BSG with OMWW yielded the worst results in terms of energy production during the optimal period identified in the analysis.

This confirms that energy evaluation is a critical decision-making tool for designing an efficient biorefinery process, as it allows for the assessment of pretreatments, methodologies, and potential co-digestion mixtures for improved efficiency.

Author Contributions

Conceptualization, J.D.M. and G.G.; methodology, J.D.M. and A.M.G.; validation, G.G.; formal analysis, A.M.G.; investigation, J.D.M.; resources, G.G.; data curation, A.M.G.; writing—original draft preparation, J.D.M. and A.M.G.; supervision, G.G.; project administration, G.G. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the European Union—NextGenerationEU—as part of the National Innovation Ecosystem (grant ECS00000041—VITALITY (to Prof. Giovanni Gigliotti and Prof. Carla Emiliani)) promoted by Ministero dell’Università e della Ricerca (MUR). We thank University of Perugia and MUR for their support within the VITALITY project.

Data Availability Statement

Data are already available within the manuscript. Further data are available upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

References

Khalil, M.; Berawi, M.A.; Heryanto, R.; Rizalie, A. Waste to energy technology: The potential of sustainable biogas production from animal waste in Indonesia. Renew. Sustain. Energy Rev. 2019, 105, 323–331. [Google Scholar] [CrossRef]
Gambelli, A.M.; Rossi, F. Re-definition of the region suitable for CO₂/CH₄ replacement into hydrates as a function of the thermodynamic difference between CO₂ hydrate formation and dissociation. Process Saf. Environ. Prot. 2023, 169, 132–141. [Google Scholar] [CrossRef]
Atelge, M.R.; Krisa, D.; Kumar, G. Biogas Production from Organic Waste: Recent Progress and Perspectives. Waste Biomass Valor 2020, 11, 1019–1040. [Google Scholar] [CrossRef]
Kasinath, A.; Fudala-Ksiazek, S.; Szopinska, M.; Bylinski, H.; Artichowicz, W. Remiszewska-Skwarek, A.; Luczkiewicz, A. Biomass in biogas production: Pretreatment and codigestion. Renew. Sustain. Energy Rev. 2021, 150, 111509. [Google Scholar] [CrossRef]
Kabeyi, M.J.B.; Olanrewaju, O.A. Biogas production and applications in the sustainable energy transition. J. Energy 2022, 1, 8750221. [Google Scholar] [CrossRef]
Adekunle, K.F.; Okolie, J.A. A review of biochemical process of anaerobic digestion. Adv. Biosci. Biotechnol. 2015, 6, 205–212. [Google Scholar] [CrossRef]
Achinas, S.; Achinas, V.; Euverink, G.J.W. A technological overview of biogas production from biowaste. Engineering 2017, 3, 299–307. [Google Scholar] [CrossRef]
Rafiee, A.; Khalilpour, K.R.; Prest, J.; Skryabin, I. Biogas as an energy vector. Biomass Bioenergy 2021, 144, 105935. [Google Scholar] [CrossRef]
Deng, S.; Liu, L.; Li, X.Y.; Xue, W.; Liang, J.; Yu, Z.; Lin, L. Rapid granulation of aerobic sludge for treatment of brewery wastewater: Aeration strategy and nitrogen removal mechanism. J. Environ. Chem. Eng. 2024, 115108. [Google Scholar] [CrossRef]
González-García, S.; Morales, P.C.; Gullón, B. Estimating the environmental impacts of a brewery waste–based biorefinery: Bio-ethanol and xylooligosaccharides joint production case study. Ind. Crops Prod. 2018, 123, 331–340. [Google Scholar] [CrossRef]
Lisci, S.; Tronci, S.; Grosso, M.; Karring, H.; Hajrizaj, R.; Errico, M. Brewer’s spent grain: Its value as renewable biomass and its possible applications. Chem. Eng. Trans. 2022, 92, 259–264. [Google Scholar]
Ikram, S.; Huang, L.; Zhang, H.; Wang, J.; Yin, M. Composition and nutrient value proposition of brewers spent grain. J. Food Sci. 2017, 82, 2232–2242. [Google Scholar] [CrossRef]
Chetrariu, A.; Dabija, A. Brewer’s spent grains: Possibilities of valorization, a review. Appl. Sci. 2020, 10, 5619. [Google Scholar] [CrossRef]
Lao, E.J.; Dimoso, N.; Raymond, J.; Mbega, E.R. The prebiotic potential of brewers’ spent grain on livestock’s health: A review. Trop. Anim. Health Prod. 2020, 52, 461–472. [Google Scholar] [CrossRef] [PubMed]
Naibaho, J.; Korzeniowska, M. Brewers’ spent grain in food systems: Processing and final products quality as a function of fiber modification treatment. J. Food Sci. 2021, 86, 1532–1551. [Google Scholar] [CrossRef] [PubMed]
Lynch, K.M.; Steffen, E.J.; Arendt, E.K. Brewers’ spent grain: A review with an emphasis on food and health. J. Inst. Brew. 2016, 122, 553–568. [Google Scholar] [CrossRef]
Carlini, M.; Monarca, D.; Castellucci, S.; Mennuni, A.; Casini, L.; Selli, S. Beer spent grains biomass for biogas production: Characterization and anaerobic digestion-oriented pre-treatments. Energy Rep. 2021, 7, 921–929. [Google Scholar] [CrossRef]
Colussi, I.; Cortesi, A.; Gallo, V.; Vitanza, R. Biomethanization of Brewer’s spent grain evaluated by application of the Anaerobic Digestion Model No. 1. Environ. Prog. Sustain. Energy 2016, 35, 1055–1060. [Google Scholar] [CrossRef]
Pereira Lins, L.; Gotardo Martinez, D.; Furtado, A.C.; Janine Carvalho Padilha, J. Biomethane generation and CO2 recovery through biogas production using brewers’ spent Grains. Biocatal. Agric. Biotechnol. 2023, 48, 102597. [Google Scholar]
Mankar, A.R.; Pandey, A.; Modak, A.; Pant, K.K. Pretreatment of Lignocellulosic Biomass: A Review on Recent Advances. Bioresour. Technol. 2021, 334, 125235. [Google Scholar] [CrossRef]
Bochmann, G.; Drosg, B.; Fuchsa, W. Anaerobic Digestion of Thermal Pretreated Brewers’ Spent Grains. Environ. Prog. Sustain. Energy 2015, 34, 1092–1096. [Google Scholar] [CrossRef]
Buller, L.S.; Sganzerla, W.G.; Lima, M.N.; Muenchow, K.E.; Timko, M.T.; Carneiro, T.F. Ultrasonic pretreatment of brewers’ spent grains for anaerobic digestion: Biogas production for a sustainable industrial development. J. Clean. Prod. 2022, 355, 131802. [Google Scholar] [CrossRef]
Wainaina, S.; Awasthi, M.K.; Sarsaiya, S.; Chen, H.; Singh, E.; Kumar, A.; Ravindran, B.; Awasthi, S.K.; Liu, T.; Duan, Y.; et al. Resource Recovery and Circular Economy from Organic Solid Waste Using Aerobic and Anaerobic Digestion Technologies. Bioresour. Technol. 2020, 301, 122778. [Google Scholar] [CrossRef] [PubMed]
Hernández-Beltrán, J.U.; Hernández-De Lira, I.O.; Cruz-Santos, M.M.; Saucedo-Luevanos, A.; Hernández-Terán, F.; Balagurusamy, N. Insight into pretreatment methods of lignocellulosic biomass to increase biogas yield: Current state, challenges, and opportunities. Appl. Sci. 2019, 9, 3721. [Google Scholar] [CrossRef]
Fan, Y.V.; Kleměs, J.J.; Perry, S.; Lee, C.T. Anaerobic digestion of lignocellulosic waste: Environmental impact and economic assessment. J. Environ. Manag. 2019, 231, 352–363. [Google Scholar] [CrossRef] [PubMed]
Ju, X.H.; Engelhard, M.; Zhang, X. An advanced understanding of the specific effects of xylan and surface lignin contents on enzymatic hydrolysis of lignocellulosic biomass. Bioresour. Technol. 2013, 132, 137–145. [Google Scholar] [CrossRef] [PubMed]
Mustafa, A.M.; Poulsen, T.G.; Sheng, K. Fungal pretreatment of rice straw with Pleurotus ostreatus and Trichoderma reesei to enhance methane production under solid-state anaerobic digestion. Appl. Energy 2016, 180, 661–671. [Google Scholar] [CrossRef]
Yu, O.; Kim, K.H. Lignin to materials: A focused review on recent novel lignin applications. Appl. Sci. 2020, 10, 4626. [Google Scholar] [CrossRef]
Del Buono, D.; Luzi, F.; Puglia, D. Lignin Nanoparticles: A Promising Tool to Improve Maize Physiological, Biochemical, and Chemical Traits. Nanomaterials 2021, 11, 846. [Google Scholar] [CrossRef]
Zeko-Pivač, A.; Tišma, M.; Žnidaršič-Plazl, P.; Kulisic, B.; Sakellaris, G.; Hao, J.; Planinić, M. The potential of brewer’s spent grain in the circular bioeconomy: State of the art and future perspectives. Front. Bioeng. Biotechnol. 2022, 10, 870744. [Google Scholar] [CrossRef] [PubMed]
Xu, J.; Kong, Y.; Du, B.; Wang, X.; Zhou, J. Exploration of mechanisms of lignin extraction by different methods. Environ. Prog. Sustain. Energy 2022, 41, e13785. [Google Scholar] [CrossRef]
Wen, C.; Zhang, J.; Duan, Y.; Zhang, H.; Ma, H. A mini-review on Brewer’s spent grain protein: Isolation, physicochemical properties, application of protein, and functional properties of hydrolysates. J. Food Sci. 2019, 84, 3330–3340. [Google Scholar] [CrossRef] [PubMed]
Celus, I.; Brijs, K.; Delcour, J.A. The effects of malting and mashing on barley protein extractability. J. Cereal Sci. 2006, 44, 203–211. [Google Scholar] [CrossRef]
Connolly, A.; Cermeño, M.; Crowley, D.; O’Callaghan, Y.; O’Brien, N.M.; Fitz Gerald, R.J. Characterization of the in vitro bioactive properties of alkaline and enzyme extracted brewers’ spent grain protein hydrolysates. Food Res. Int. 2019, 121, 524–532. [Google Scholar] [CrossRef] [PubMed]
Chen, Y.; Cheng, J.J.; Creamer, K.S. Inhibition of anaerobic digestion process: A review. Bioresour. Technol. 2008, 99, 4044–4064. [Google Scholar] [CrossRef]
Bhatia, L.; Bachheti, R.K.; Garlapati, V.K.; Chandel, A.K. Third-generation biorefineries: A sustainable platform for food, clean energy, and nutraceuticals production. Biomass Conv. Bioref. 2022, 12, 4215–4230. [Google Scholar] [CrossRef]
Wang, W.; Lee, D.J. Valorization of anaerobic digestion digestate: A prospect review. Bioresour. Technol. 2021, 323, 124626. [Google Scholar] [CrossRef]
Koszel, M.; Lorencowicz, E. Agricultural use of biogas digestate as a replacement fertilizer. Agric. Agric. Sci. Procedia 2015, 7, 119–124. [Google Scholar] [CrossRef]
Karimi, M.; Shirzad, M. Sustainable industrial process design for derived CO₂ adsorbent from municipal solid wastes: Scale-up, techno-economic and parametric assessment. Sust. Mat. Technol. 2024, 41, e01091. [Google Scholar] [CrossRef]
AOAC Official Methods of Analysis of AOAC International. Helrich, K. (Ed.) Association of Official Analytical Chemists: Rockville, MD, USA, 1997; Volume 80, Available online: https://law.resource.org/pub/us/cfr/ibr/002/aoac.methods.1.1990.pdf (accessed on 5 November 2024).
Montegiove, N.; Gambelli, A.M.; Calzoni, E.; Bertoldi, A.; Puglia, D.; Zadra, C.; Emiliani, C.; Gigliotti, G. Biogas production with residual deriving from olive mill wastewater and olive pomace wastes: Quantification of produced energy, spent energy, and process efficiency. Agronomy 2024, 14, 531. [Google Scholar] [CrossRef]
Cequier, E.; Aguilera, J.; Balcells, M.; Canela-Garayoa, R. Extraction and characterization of lignon from olive pomace: A comparison study among ionic liquid, sulfuric acid, and alkaline treatments. Biomass Convers. Biorefinery 2019, 9, 241–252. [Google Scholar] [CrossRef]
Misi, S.N.; Forster, C.F. Batch co-digestion of multi-component agro-wastes. Bioresour. Technol. 2001, 80, 19–28. [Google Scholar] [CrossRef]
Li, P.; Liu, D.; Pei, Z.; Zhao, L.; Shi, F.; Yao, Z.; Li, W.; Sun, Y.; Wang, S.; Yu, Q.; et al. Evaluation of lignin inhibition in anaerobic digestion from the perspective of reducing the hydrolysis rate of holocellulose. Bioresour. Technol. 2021, 333, 125204. [Google Scholar] [CrossRef] [PubMed]
Fricke, K.; Santen, H.; Wallmann, R.; Hüttner, A.; Dichtl, N. Operating problems in anaerobic digestion plants resulting from nitrogen in MSW. Waste Manag. 2007, 27, 30–43. [Google Scholar] [CrossRef]
Borja, R.; Alba, J.; Banks, C.J. Impact of the main phenolic compounds of olive mill wastewater (OMW) on the kinetics of acetoclastic methanogenesis. Process Biochem. 1997, 32, 121–133. [Google Scholar] [CrossRef]
Pöschl, M.; Ward, S.; Owende, P. Evaluation of energy efficiency of various biogas production and utilization pathways. Appl. Energy 2010, 87, 3305–3321. [Google Scholar] [CrossRef]
Berglund, M.; Börjesson, P. Assessment of energy performance in the life-cycle of biogas production. Biomass Bioenergy 2006, 30, 254–266. [Google Scholar] [CrossRef]
Montegiove, N.; Gambelli, A.M.; Calzoni, E.; Bertoldi, A.; Emiliani, C.; Gigliotti, G. Olive pomace protein hydrolysate waste valorization through biogas production: Evaluation of energy produced and process efficiency. Chem. Eng. Trans. 2024, 109, 319–324. [Google Scholar]
Di Mario, J.; Montegiove, N.; Gambelli, A.M.; Brienza, M.; Zadra, C.; Gigliotti, G. Waste biomass pretreatments for biogas yield optimization and for the extraction of valuable high-added-value products: Possible combinations of the two processes towards a biorefinery purpose. Biomass 2024, 4, 865–885. [Google Scholar] [CrossRef]

Figure 1. (A) Batch bioreactor used in this study for biogas production. The biogas generated in the first bottle flows into a second vessel, the “gasometer”, which contains water. The quantity of water flowed in the last vessel allowed us to measure the amount of biogas produced (volumetric method). (B) Inclusion of an alkaline trap, composed of 5 M NaOH and thymolphthalein as a pH indicator, to assess biomethane production. The CO₂ in the biogas was separated according to the reaction outlined in the scheme.

Figure 2. Daily biogas production for the different biomasses tested.

Figure 3. (a) Total biogas yield for unit of VS for all biomasses. (b) Average daily biogas yield for the different samples (total Nm³·gVS⁻¹/days of production).

Figure 4. Comparison between the quantity of biogas produced and the related amount of biomethane contained in it.

Figure 5. Cumulative energy produced (empty dots) and energy spent (filled dots) for the various samples.

Figure 6. Efficiency measured for the optimal production period (until reaching the maximum difference between energy produced and energy spent).

Figure 7. Energy produced (in blue) and energy spent (in red) during the optimal production period and energy produced before the energy spent equaled the energy produced (green).

Table 1. Characterization of inoculum used to prepare various samples (DM = dry matter). WEOC = Water-Extractable Organic Carbon; WEN = Water-Extractable Nitrogen.

Parameter	Digestate
Moisture (%)	88.08
Total Volatile Solids (%)	73.34
pH	8.08
TOC (% of DM)	53.05
TKN (% of DM)	5.47
Total P (g kg⁻¹ DM)	3.11
Total K (g kg⁻¹ DM)	75.62
WEOC (g kg⁻¹ DM)	110.51
WEN (g kg⁻¹ DM)	67.86

Table 2. Characterization of BSG (DM = dry matter). WEOC = Water-Extractable Organic Carbon; WEN = Water-Extractable Nitrogen.

Parameter	BSG
Moisture (%)	78.30
Total Volatile Solids (%)	97.50
pH	5.06
Protein (% of DM)	24.30
TOC (% of DM)	26.66
TKN (% of DM)	3.90
Total P (g kg⁻¹ DM)	1.02
Lipid (% of DM)	4.27
Cellulose (% of DM)	10.48
Hemicellulose (% of DM)	33.24
Lignin (% of DM)	3.99

Table 3. Cumulative biogas production for each matrix tested in the study (means of tests performed and corresponding deviations). Biogas yields are reported in Nm³biogas/gVS. The maximum cumulative biogas yield produced and the duration of AD for any biomass are highlighted (the corresponding cell is colored as the own biomass). BSG = brewery’s spent grain; BSGP = BSG residue after IL treatment; BSGPH = BSG residue after alkaline hydrolysis; BSG + OMWW = co-digestion of BSG and Olive Mill Wastewater; BSG + OPP = co-digestion of BSG with Olive Pomace.

Day of Production	BSG		BSGp		BSGph		BSG + OMWW		OP + BSG
1	Mean	Dev.	Mean	Dev.	Mean	Dev.	Mean	Dev.	Mean	Dev.
2	60.3	±60.3	83.1	±4.0	41.9	41.9	213.9	7.0	22.0	22.0
3	119.2	±65.4	91.0	±4.0	52.4	52.4	253.4	1.8	44.0	44.0
4	152.6	±70.5	134.5	±0	104.7	83.8	270.9	1.8	57.1	48.3
5	215.4	±66.7	170.1	±4.0	115.2	94.3	288.4	1.8	61.5	52.7
6	235.9	±64.1	209.7	±11.9	125.7	104.7	310.4	10.5	74.7	65.9
7	270.5	±65.4	245.3	±7.9	151.9	110.0	349.8	1.8	101.1	57.1
8	317.9	±66.7	280.9	±4.0	157.1	115.2	363.0	7.0	123.1	44.0
9	344.9	±65.4	328.3	±4.0	204.3	120.5	393.7	15.8	136.3	39.6
10	396.2	±67.9	363.9	±7.9	235.7	130.9	428.7	15.8	153.8	39.6
11	444.9	±70.5	411.4	±7.9	256.6	130.9	459.4	24.5	175.8	44.0
12	500.0	±69.2	454.9	±11.9	277.6	130.9	494.5	42.1	197.8	48.3
13	543.6	±71.8	502.4	±11.9	298.5	130.9	529.5	42.1	215.4	48.3
14	583.3	±73.1	545.9	±7.9	324.7	146.6	569.0	50.9	228.6	44.0
15	621.8	±75.6	589.4	±11.9	340.4	151.9	608.5	59.6	241.7	39.6
16	665.4	±78.2	652.7	±27.7	340.4	151.9	700.5	68.4	259.3	39.6
17	689.7	±79.5	696.2	±23.7	356.1	167.6	726.8	85.9	272.5	44.0
18	707.7	±82.1	735.8	±23.7	366.6	167.6	744.3	103.5	294.5	22.0
19	724.4	±83.3	775.3	±15.8	377.1	167.6	788.2	68.4	294.5	22.0
20	743.6	±82.1	810.9	±11.9	382.3	172.8	840.8	15.8	298.9	17.6
21	760.3	±83.3	814.9	±7.9	382.3	172.8	902.2	36.8	316.5	8.8
22	776.9	±84.6	814.9	±7.9	382.3	172.8	919.7	1.8	334.0	0.0
23	798.7	±83.3	814.9	±7.9	382.3	172.8	924.1	10.5	342.8	8.8
24	812.8	±84.6	814.9	±7.9	382.3	172.8	967.9	7.0	347.2	4.4
25	830.8	±82.1	814.9	±7.9	382.3	172.8	976.7	10.5	356.0	4.4
26	850.0	±83.3	814.9	±7.9	382.3	172.8	989.8	1.8	364.8	4.4
27	860.3	±88.5	814.9	±7.9	382.3	172.8	1003.0	7.0	373.6	4.4
28	873.1	±93.6	814.9	±7.9	382.3	172.8	1016.1	1.8	373.6	4.4
29	884.6	±97.4	814.9	±7.9	382.3	172.8	1046.8	10.5	373.6	4.4
30	889.7	±94.9	814.9	±7.9	382.3	172.8	1060.0	15.8	382.4	4.4
31	894.9	±97.4	814.9	±7.9	382.3	172.8	1064.4	7.0	391.2	4.4
32	902.6	±100.0	814.9	±7.9	382.3	172.8	1073.1	7.0	404.4	8.8
33	910.3	±102.6	814.9	±7.9	382.3	172.8	1108.2	7.0	417.5	4.4
34	916.7	±103.8	814.9	±7.9	382.3	172.8	1108.2	7.0	417.5	4.4
35	925.6	±110.3	814.9	±7.9	382.3	172.8	1108.2	7.0	417.5	4.4
36	935.9	±107.7	814.9	±7.9	382.3	172.8	1130.1	1.8	417.5	4.4
37	939.7	±109.0	814.9	±7.9	382.3	172.8	1130.1	1.8	421.9	0
38	947.4	±111.5	814.9	±7.9	382.3	172.8	1130.1	1.8	421.9	0
39	955.1	±111.5	814.9	±7.9	382.3	172.8	1130.1	1.8	421.9	0
40	955.1	±111.5	814.9	±7.9	382.3	172.8	1130.1	1.8	421.9	0
41	961.5	±110.3	814.9	±7.9	382.3	172.8	1130.1	1.8	421.9	0
42	967.9	±109.0	814.9	±7.9	382.3	172.8	1130.1	1.8	421.9	0
43	974.4	±107.7	814.9	±7.9	382.3	172.8	1130.1	1.8	421.9	0
44	978.2	±103.8	814.9	±7.9	382.3	172.8	1130.1	1.8	421.9	0
45	984.6	±105.1	814.9	±7.9	382.3	172.8	1130.1	1.8	443.9	13.2
46	992.3	±107.7	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
47	998.7	±109.0	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
48	1003.8	±111.5	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
49	1009.0	±114.1	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
50	1012.8	±110.3	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
51	1020.5	±115.4	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
52	1030.8	±117.9	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
53	1032.1	±116.7	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
54	1034.6	±116.7	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
55	1038.5	±117.9	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
56	1042.3	±119.2	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
57	1052.6	±124.4	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
58	1064.1	±130.8	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
59	1064.1	±130.8	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
60	1064.1	±130.8	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
61	1071.8	±133.3	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2
62	1075.6	±132.1	814.9	±7.9	382.3	172.8	1130.1	1.8	452.7	13.2

Table 4. Percentage of biomethane contained in biogas mixture for each sample and relative total volume (mL).

Tested Biomass	% CH₄ in Biogas	CH₄ tot [mL]
BSG	76.59	823.8
BSGp	60	488.9
BSGph	71.69	274.1
OMWW + BSG	54.5	615.9
OPp + BSG	66.05	299

Table 5. Cumulative production of biomethane and corresponding quantity of energy produced.

	BSG		BSGp		BSGph		OMWW + BSG		Opp + BSG
Day	CH₄ [kg]	Ep [kJ]	CH₄ [kg]	Ep [kJ]	CH₄ [kg]	Ep [kJ]	CH₄ [kg]	Ep [kJ]	CH₄ [kg]	Ep [kJ]
1	2.22 × 10⁻⁵	1.16	2.40 × 10⁻⁵	1.25	1.45 × 10⁻⁵	0.75	5.62 × 10⁻⁵	2.92	6.99 × 10⁻⁶	0.36
2	4.39 × 10⁻⁵	2.28	2.63 × 10⁻⁵	1.37	1.81 × 10⁻⁵	0.94	6.65 × 10⁻⁵	3.46	1.40 × 10⁻⁵	0.73
3	5.62 × 10⁻⁵	2.92	3.88 × 10⁻⁵	2.02	3.61 × 10⁻⁵	1.88	7.11 × 10⁻⁵	3.70	1.82 × 10⁻⁵	0.94
4	7.94 × 10⁻⁵	4.13	4.91 × 10⁻⁵	2.55	3.97 × 10⁻⁵	2.07	7.57 × 10⁻⁵	3.94	1.95 × 10⁻⁵	1.02
5	8.70 × 10⁻⁵	4.52	6.06 × 10⁻⁵	3.15	4.34 × 10⁻⁵	2.26	8.15 × 10⁻⁵	4.24	2.37 × 10⁻⁵	1.23
6	9.97 × 10⁻⁵	5.18	7.08 × 10⁻⁵	3.68	5.24 × 10⁻⁵	2.73	9.18 × 10⁻⁵	4.78	3.21 × 10⁻⁵	1.67
7	1.17 × 10⁻⁴	6.09	8.11 × 10⁻⁵	4.22	5.42 × 10⁻⁵	2.82	9.53 × 10⁻⁵	4.96	3.91 × 10⁻⁵	2.03
8	1.27 × 10⁻⁴	6.61	9.48 × 10⁻⁵	4.93	7.05 × 10⁻⁵	3.67	1.03 × 10⁻⁴	5.37	4.33 × 10⁻⁵	2.25
9	1.46 × 10⁻⁴	7.59	1.05 × 10⁻⁴	5.46	8.13 × 10⁻⁵	4.23	1.13 × 10⁻⁴	5.85	4.89 × 10⁻⁵	2.54
10	1.64 × 10⁻⁴	8.53	1.19 × 10⁻⁴	6.18	8.85 × 10⁻⁵	4.60	1.21 × 10⁻⁴	6.27	5.59 × 10⁻⁵	2.91
11	1.84 × 10⁻⁴	9.58	1.31 × 10⁻⁴	6.83	9.58 × 10⁻⁵	4.98	1.30 × 10⁻⁴	6.75	6.29 × 10⁻⁵	3.27
12	2.00 × 10⁻⁴	10.42	1.45 × 10⁻⁴	7.54	1.03 × 10⁻⁴	5.36	1.39 × 10⁻⁴	7.23	6.85 × 10⁻⁵	3.56
13	2.15 × 10⁻⁴	11.18	1.58 × 10⁻⁴	8.20	1.12 × 10⁻⁴	5.83	1.49 × 10⁻⁴	7.77	7.27 × 10⁻⁵	3.78
14	2.29 × 10⁻⁴	11.92	1.70 × 10⁻⁴	8.85	1.17 × 10⁻⁴	6.11	1.60 × 10⁻⁴	8.31	7.68 × 10⁻⁵	4.00
15	2.45 × 10⁻⁴	12.75	1.88 × 10⁻⁴	9.80	1.17 × 10⁻⁴	6.11	1.84 × 10⁻⁴	9.56	8.24 × 10⁻⁵	4.29
16	2.54 × 10⁻⁴	13.22	2.01 × 10⁻⁴	10.45	1.23 × 10⁻⁴	6.39	1.91 × 10⁻⁴	9.92	8.66 × 10⁻⁵	4.50
17	2.61 × 10⁻⁴	13.56	2.12 × 10⁻⁴	11.05	1.26 × 10⁻⁴	6.58	1.95 × 10⁻⁴	10.16	9.36 × 10⁻⁵	4.87
18	2.67 × 10⁻⁴	13.88	2.24 × 10⁻⁴	11.64	1.30 × 10⁻⁴	6.77	2.07 × 10⁻⁴	10.76	9.36 × 10⁻⁵	4.87
19	2.74 × 10⁻⁴	14.25	2.34 × 10⁻⁴	12.18	1.32 × 10⁻⁴	6.86	2.21 × 10⁻⁴	11.48	9.50 × 10⁻⁵	4.94
20	2.80 × 10⁻⁴	14.57	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.37 × 10⁻⁴	12.32	1.01 × 10⁻⁴	5.23
21	2.86 × 10⁻⁴	14.89	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.41 × 10⁻⁴	12.56	1.06 × 10⁻⁴	5.52
22	2.94 × 10⁻⁴	15.31	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.43 × 10⁻⁴	12.62	1.09 × 10⁻⁴	5.67
23	3.00 × 10⁻⁴	15.58	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.54 × 10⁻⁴	13.21	1.10 × 10⁻⁴	5.74
24	3.06 × 10⁻⁴	15.92	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.56 × 10⁻⁴	13.33	1.13 × 10⁻⁴	5.88
25	3.13 × 10⁻⁴	16.29	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.60 × 10⁻⁴	13.51	1.16 × 10⁻⁴	6.03
26	3.17 × 10⁻⁴	16.49	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.63 × 10⁻⁴	13.69	1.19 × 10⁻⁴	6.18
27	3.22 × 10⁻⁴	16.73	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.67 × 10⁻⁴	13.87	1.19 × 10⁻⁴	6.18
28	3.26 × 10⁻⁴	16.96	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.75 × 10⁻⁴	14.29	1.19 × 10⁻⁴	6.18
29	3.28 × 10⁻⁴	17.05	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.78 × 10⁻⁴	14.47	1.22 × 10⁻⁴	6.32
30	3.30 × 10⁻⁴	17.15	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.79 × 10⁻⁴	14.53	1.24 × 10⁻⁴	6.47
31	3.33 × 10⁻⁴	17.30	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.82 × 10⁻⁴	14.65	1.29 × 10⁻⁴	6.68
32	3.36 × 10⁻⁴	17.45	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.91 × 10⁻⁴	15.13	1.33 × 10⁻⁴	6.90
33	3.38 × 10⁻⁴	17.57	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.91 × 10⁻⁴	15.13	1.33 × 10⁻⁴	6.90
34	3.41 × 10⁻⁴	17.74	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.91 × 10⁻⁴	15.13	1.33 × 10⁻⁴	6.90
35	3.45 × 10⁻⁴	17.94	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.33 × 10⁻⁴	6.90
36	3.46 × 10⁻⁴	18.01	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.34 × 10⁻⁴	6.97
37	3.49 × 10⁻⁴	18.16	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.34 × 10⁻⁴	6.97
38	3.52 × 10⁻⁴	18.31	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.34 × 10⁻⁴	6.97
39	3.52 × 10⁻⁴	18.31	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.34 × 10⁻⁴	6.97
40	3.54 × 10⁻⁴	18.43	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.34 × 10⁻⁴	6.97
41	3.57 × 10⁻⁴	18.55	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.34 × 10⁻⁴	6.97
42	3.59 × 10⁻⁴	18.68	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.34 × 10⁻⁴	6.97
43	3.61 × 10⁻⁴	18.75	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.34 × 10⁻⁴	6.97
44	3.63 × 10⁻⁴	18.87	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.41 × 10⁻⁴	7.34
45	3.66 × 10⁻⁴	19.02	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
46	3.68 × 10⁻⁴	19.14	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
47	3.70 × 10⁻⁴	19.24	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
48	3.72 × 10⁻⁴	19.34	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
49	3.73 × 10⁻⁴	19.41	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
50	3.76 × 10⁻⁴	19.56	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
51	3.80 × 10⁻⁴	19.76	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
52	3.80 × 10⁻⁴	19.78	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
53	3.81 × 10⁻⁴	19.83	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
54	3.83 × 10⁻⁴	19.90	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
55	3.84 × 10⁻⁴	19.98	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
56	3.88 × 10⁻⁴	20.18	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
57	3.92 × 10⁻⁴	20.40	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
58	3.92 × 10⁻⁴	20.40	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
59	3.92 × 10⁻⁴	20.40	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
60	3.95 × 10⁻⁴	20.54	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48
61	3.96 × 10⁻⁴	20.62	2.35 × 10⁻⁴	12.24	1.32 × 10⁻⁴	6.86	2.97 × 10⁻⁴	15.43	1.44 × 10⁻⁴	7.48

Table 6. From left to right: total biogas production period; Ep _TOT: total energy produced; E_S/day: energy spent per day.

Sample	Prod. Days	Ep _TOT [kJ]	Es/day [kJ]
BSG	63	20.62	0.38
BSGp	22	12.24	0.41
BSGph	21	6.86	0.25
OMWW + BSG	37	15.43	0.96
OP + BSG	47	7.48	0.12

Table 7. From left to right: maximum difference between cumulative energy produced and cumulative energy spent; day where the previous parameter was measured; day where the energy consumption equaled the energy production.

Sample	(Ep − Es)_max [kJ]	Days (Ep − Es)_max [kJ]	Days (Ep ≈ Es)
BSG	7.08	17	52
BSGp	5.36	19	30
BSGph	2.2	18	28
OMWW + BSG	1.96	1	4
OPp + BSG	5.22	32	61

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

© 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Share and Cite

MDPI and ACS Style

Di Mario, J.; Gambelli, A.M.; Gigliotti, G. Biomethane Production from Untreated and Treated Brewery’s Spent Grain: Feasibility of Anaerobic Digestion After Pretreatments According to Biogas Yield and Energy Efficiency. Agronomy 2024, 14, 2980. https://doi.org/10.3390/agronomy14122980

AMA Style

Di Mario J, Gambelli AM, Gigliotti G. Biomethane Production from Untreated and Treated Brewery’s Spent Grain: Feasibility of Anaerobic Digestion After Pretreatments According to Biogas Yield and Energy Efficiency. Agronomy. 2024; 14(12):2980. https://doi.org/10.3390/agronomy14122980

Chicago/Turabian Style

Di Mario, Jessica, Alberto Maria Gambelli, and Giovanni Gigliotti. 2024. "Biomethane Production from Untreated and Treated Brewery’s Spent Grain: Feasibility of Anaerobic Digestion After Pretreatments According to Biogas Yield and Energy Efficiency" Agronomy 14, no. 12: 2980. https://doi.org/10.3390/agronomy14122980

APA Style

Di Mario, J., Gambelli, A. M., & Gigliotti, G. (2024). Biomethane Production from Untreated and Treated Brewery’s Spent Grain: Feasibility of Anaerobic Digestion After Pretreatments According to Biogas Yield and Energy Efficiency. Agronomy, 14(12), 2980. https://doi.org/10.3390/agronomy14122980

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Menu