Second- and third-generation biorefineries enable the sustainable management of biomasses within the framework of circular economy principles. This approach aims to minimize waste biomass while generating high-value molecules and bio-energy, such as biogas. Biogas production is achieved via anaerobic digestion, a process where microorganisms metabolize organic compounds in the absence of oxygen to primarily produce CO₂ and CH₄. The efficiency of this process is closely linked to the composition of the biomass and, sometimes, characteristics of the initial matrix can impede the process. To address these challenges, various pretreatments are employed to enhance digestion efficiency and mitigate issues associated with biomass complexity. However, the implementation of pretreatments can be energy-intensive and costly. The extraction of valuable molecules from biomass for various applications can represent a form of pretreatment. This extraction process selectively removes recalcitrant molecules such as lignin and cellulose, which can hinder biodegradation, thereby adding new value to the biomass. These extracted molecules not only contribute to improved anaerobic digestion efficiency but also offer potential economic benefits by serving as valuable inputs across diverse industrial sectors. This article presents a detailed state of the art of the most widespread biomass pretreatments and specifies when biomass is pretreated to improve the biogas yield and, in contrast, when it is treated to extract high-added-value products. Finally, in order to define if the same treatment can be simultaneously applied for both goals, an experimental section was dedicated to the production of biogas from untreated olive mill wastewater and the same biomass after being freeze-dried and after the extraction of polyphenols and flavonoids. The use of pretreated biomass effectively improved the biogas production yield: the untreated olive mill wastewater led to the production of 147 mL of biogas, while after freeze-drying and after polyphenols/flavonoids extraction, the production was, respectively, equal to 169 mL and 268 mL of biogas.

Brewery’s spent grain (BSG) consists of the largest by-product by volume in the beer production sector and offers potential for both bio-composite material production, high-added-value molecular extraction and bioenergy recovery. Aiming at exploring the ideal biorefinery approach for this agro-industrial residual, the present study experimentally investigated several methodologies to enhance the reuse of BSG and proposed a scheme of biorefinery focused on it. According to it, BSGs were firstly tested to produce high-added-value byproducts, such as protein hydrolysates and for the extraction of lignin via ionic liquids-based methods. The residuals were then used for biogas/biomethane production via anaerobic codigestion. The different matrices were rearranged in varying mixtures, aiming at ensuring high availability of nutrients for methanogens, thus achieving higher energy production than what achievable with untreated BSG. For the scope, further agro-industrial wastes were considered. The resulted digestate was finally composted. Untreated BSGs were also directly tested as fillers for bio-composite material production (in a mixture with PHB). Different concentrations were tested and the mechanical properties of each sample were compared with those of pure PHB. Disintegration tests were finally carried out to measure the improved biodegradability of the produced bio-composite material.

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.

Olive cultivation is one of the most widespread agro-industrial activities in the Mediterranean area. However, required pretreatments often affect the anaerobic digestion process, promoting or inhibiting the overall yield. Therefore, the efficiency of Anaerobic Digestion (AD) processes cannot be established in advance but needs to be experimentally validated for each biomass-pretreatment combination. Following the present purpose, these biomasses were firstly treated: the olive pomace (OP) with a procedure based on the use of an ionic liquid (IL) composed of triethylamine and sulfuric acid [Et₃N][HSO₄] to remove hemicellulose and lignin and recover the insolubilized OP, while olive mill wastewater (OW) was processed via freeze-drying. The resulting materials, the pulp from olive pomace (POP) and freeze-dried OW (FDOW), were then digested using lab-scale anaerobic reactors. The biogas production was then compared with the quantity obtained by digesting the same untreated biomasses (OW and OP). The FDOW showed the highest biogas production due to the freeze-drying treatment that led to some morphological and structural surface modifications of OW (respectively, 658 mL vs. 79 mL/g for the two matrices), prompting microorganism activity. Conversely, the method based on the use of IL significantly reduced the nitrogen content of POP, thus resulting in the lowest biogas production, which ceased by the second day. To address this issue, we co-digested POP with the brewery’s spent grain, a biomass rich in nitrogen. This step enhanced the biogas yield of POP, resulting in an extended anaerobic digestion period and the production of 466 mL/g. Additionally, we tested FDOW in co-digestion with BSG to evaluate improvements in production. The codigestion of the two matrices increased the biogas yield of FDOW from 944 to 1131 mL/g.

Pursuing the so-defined biorefinery approach, residual biomass, such as agro-industrial wastes, should first be exploited for the extraction and production of high-value-added products and then processed for energy valorisation through anaerobic digestion (AD). However, the treatments applied to achieve the first goal could impact biogas yield. This problem can be solved by co-digesting the treated biomass with others. In this study, Brewery’ Spent Grain (by itself, a good biogas producer) was treated with an ionic liquid (IL) composed of triethylamine and sulfuric acid [TEA][HSO₄] for lignin removal. The residual biomass (pulp, BSGp) was then used for biogas production. The tests revealed a marked reduction in the total quantity of biomethane (per unit of volatile solid—VS). In detail, 6.82 × 10⁻⁴ Nm³CH₄/gVS of biomethane was produced with BSGp, against 1.31 × 10⁻³ Nm³CH₄/gVS with BSG. The lack of organic nitrogen after the IL-based treatment prevented biogas production, resulting in a shorter production period. To compensate for the nitrogen deficiency and restore the optimal C/N ratio, BSGp was mixed with Lemna minor (LM), an aquatic weed with a high nitrogen content. By itself, LM cannot be considered a good biogas producer as proven in this study. However, the co-digestion of LM with BSGp extended the production period and kept the daily production close to that registered in test made with the sole BSGp, thus achieving a total biomethane production equal to 1.83 × 10⁻³ Nm³CH₄/gVS, even higher than the one registered with untreated BSG.

This study focuses on biogas production within lab-scale semi-batch bioreactors using agro-industrial wastes and dry biomass of an invasive aquatic species. In particular, the primary objective is to increase the yield of anaerobic digestion processes, with a specific focus on reducing CO₂ emissions associated with the degradation of biomass, by co-digesting different raw biomasses and agro-industrial wastes. In detail, the experiments concerned the pulp of Brewery’s Spent Grain (BSGp), consisting of the residual of Brewery’s Spent Grain after fiber deconstruction with ionic liquids–based treatment, and Lemna minor L. (LM). The two biomasses were studied separately and then co-digested. Co-digestion was carried out using a 1:1 (VS basis) mixture of Lemna minor and Brewery’s Spent Grain pulp. Due to the lack of organic nitrogen, BSGp showed low biogas production if compared with untreated BSG (1.14 × 10⁻³ vs. 1.71 × 10⁻³ Nm³/gVS). Differently, LM has a high nitrogen content and, when digested alone, produced 9.79 × 10⁻⁴ Nm³/gVS. The co-digestion tests allowed us to reach the highest performance: 2.94 × 10⁻³ Nm³/gVS. In terms of bioenergy production, the two biomasses showed high synergy when used in co-digestion. The amount of energy produced was calculated using a lower heating value (LHV) of CH₄ equal to 52 MJ. The results showed that co-digestion yielded 64.9 ± 0.6 MJ/kgVS, followed by BSG (43.3 ± 5.3 MJ/kgVS), BSGp (25.6 ± 0.3 MJ/kgVS), and LM (19.3 ± 1.0 MJ/kgVS). In addition, in terms of CO₂ avoided, the following results were achieved: 0.38–0.40 gCO₂/gVS with BSGp, 0.73–0.8 gCO₂/gVS with LM. Conversely, co-digestion tests allowed for the avoidance of 1.68–1.91 gCO₂/gVS. In conclusion, co-digesting BSGp with Lemna minor yields more methane and less CO₂ per unit processed, providing an effective way to convert readily available waste and biomass into bioenergy.