1 Introduction

In Europe, grasslands span over 17% of the total land area, fulfilling essential roles in biodiversity conservation, safeguarding water resources, preventing soil erosion, and acting as substantial carbon reservoirs [1,2,3,4]. Ireland’s oceanic climate, with abundant precipitation throughout the year, provides naturally favorable growing conditions for grass. As a result, grasslands in Ireland account for 80% of its agricultural land [5] with an average yield of 15 Mt of DM per hectare [6] compared to 11 Mt of DM in the rest of Europe [7]. However, grass is a promising biomass that remains underutilized [4, 8]. Developing technologies for valorizing grass can keep grasslands from being transformed into “higher yielding” intensive croplands, which have a higher environmental impact than grasslands [1, 2, 4].

The processing of grass can also help achieve the aim of the Renewable Energy Directive EU 2001/2018 (REDII) to transition into the use of energy derived from renewable sources. Anaerobic digestion (AD) is presently one of the most beneficial technologies for bioenergy production [9]. This has resulted in a 51% increase in AD plants in 2 years, from 2018 to 2020, treating organic waste streams due to incentives offered by many EU countries [10]. According to RePowerEU objectives, new targets have been set to reach 35 bcm per year of biogas and biomethane in the EU by 2030, which would substitute 20% of natural gas imports [11].

Even though AD has been established as an industrial process for decades, it is a complex biochemical process prone to process instability, such as low biodegradability and clogging when using fiber-rich materials like energy crops and grasses [12]. The addition of pretreatments before AD was reported to be effective in increasing the digestion efficiency of such feedstocks [13, 14]. Positive effects of the reduction in the size of the substrate on biogas production have been observed, as shorter grass lengths result in quicker degradation and higher methane yield [12]. Moreover, ensiling as a storage method has also been observed to increase biogas yields [15] and can thus also be considered an efficient pretreatment. The ensiling process produces organic acids, which act as preservatives that hinder the unwanted growth of microorganisms while hydrolyzing the lignocellulosic structure of grass, making it more prone to break down further during AD [15]. Pretreatment, therefore, appears as a crucial step in improving the efficiency of the AD of grass.

In addition to energy, the United Nations has emphasized the need to transform current agricultural practices into sustainable agricultural production systems to limit global temperature rise by 2 °C [16]. Traditional agricultural practices follow linearity, where nutrients are imported and discarded in a way that can cause soil pollution and other environmental impacts due to leaching, volatilization, denitrification, erosion, and runoff [17]. Nutrient recovery technologies can assist in closing these nutrient loops, returning them to the fields. Digestate, a by-product of the AD process, contains non-digested resilient organics, water, and macro- and micro-nutrients that can be used as fertilizers [18]. Moreover, digestate provides organic matter to the soil improving its biological activity and reducing nutrient leaching [19, 20]. Digestate from the AD of grass should possess similar qualities and its potential to be used as a nutrient source is yet to be explored as compared to digestate from animal by-products.

The biorefining of grass extends beyond just transforming it into energy and bio-based fertilizers; it also involves the production of high-value products. These can include leaf protein concentrate for nutritional applications [21], fibers extracted for biomaterials [22], and bio-based chemicals obtained through fermentation processes [23]. This diverse valorization approach significantly enhances the utility and economic value of grass in a biorefinery context. In that sense, lactic acid (LA, 2-hydroxy propanoic acid) is one of the main targeted products in grass biorefineries, as it is commonly used in various industries, such as food, cosmetic, and pharmaceutical industries [24]. The global demand for LA is expected to reach 1.96 million tonnes in 2025 [25]. LA is witnessing a rising demand, particularly for its crucial role in manufacturing PLA (polylactic acid), a bio-based alternative to conventional plastics [26]. One of the major obstacles in PLA production is its elevated cost, primarily attributed to the high expenses associated with producing LA. The potential viability of grass as a source for LA production holds a critical role in reducing production costs. Moreover, using low-cost, non-food biomass to produce LA for the food industry avoids the land-use debate common in the bioeconomy [27, 28].

Switching LA production from chemical synthesis to microbial synthesis allows for the use of renewable biomass, low-temperature fermentation, and the production of optically pure LA by selecting the appropriate strain [25]. The synthesis of bio-based LA from biomass encompasses a sequential biochemical conversion methodology. Initially, complex polysaccharides within the biomass are depolymerized into simpler monosaccharides through acid hydrolysis, followed by enzymatic hydrolysis to enhance the fermentability of the resultant sugars. These monosaccharides are then subjected to microbial fermentation, utilizing specialized bacteria, which metabolize the sugars to yield LA as the primary metabolic product. The use of renewable biomasses such as molasses [29], milk whey [30], date juice [31], starchy materials [32], and lignocellulosic biomass [33] to produce LA using lactic acid bacteria has already been carried out whereas studies producing LA by fermenting grass are scarce.

This study introduces a biorefinery scheme for converting grass into valuable products such as LA, biogas, and bio-based fertilizers, emphasizing the use of a combination of pretreatment techniques to enhance the value chain of grass biomass. Pre-treated grass streams were evaluated for their biogas potential, and the economic value of derived products—electricity, heat, and nutrients—was assessed to determine the effectiveness of different pretreatment methods. An alternative valorization pathway was explored by fermenting milled and ensiled grass to produce LA, to analyze the potential revenue generated from this process in comparison to biogas production. This analysis provides insights into optimizing grass-based biorefineries, suggesting that production strategies may require reconsideration to maximize value generation from grass feedstocks.

2 Material and methods

2.1 Feedstock characteristics

This study used meadow grass from UCD Lyons Farms, Newcastle, Co. Dublin, Ireland, that comprised a mixture of ryegrass, white clover, red clover, Yorkshire fog, creeping buttercup, and chick weed. The grass was harvested in June 2021, vacuum-packed, frozen, and transported to Denmark for experimentation. Following the NREL protocol [34], the compositional analysis of grass resulted in a cellulose content of 29.4 ± 0.4%, hemicellulose content of 14.3 ± 3.0%, and total lignin content of 27% divided into 12.3% acid-insoluble lignin and 14.7% acid-soluble lignin.

2.2 Experimental setup and pretreatments

Figure 1 illustrates the experimental workflow and main target products from this study.

Fig. 1
figure 1

The overview of the different grass pretreatments carried out in this study. The milled (M), ensiled (E), milled and ensiled (EM), and solid fraction after enzymatic hydrolysis (EMEh) are the streams that were analyzed for BMP. Only the liquid fraction after the enzymatic hydrolysis was studied for its potential to produce lactic acid (LA) using two different bacteria

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A physical pretreatment was done to reduce the grass size to 10 mm using a cutting mill (Retsch SM2000). In the ensiling process, 100 g of grass sample (U = untreated or M = milled) was added to 500-mL anaerobic bottles. Each bottle was flushed with nitrogen for 30 min to create anaerobic conditions, sealed, and stored at 35 °C for 3 weeks. At the end of 3 weeks, the pH of the grass was measured to ensure an efficient ensiling (4.2–4.5). To measure the total solid content (TS) and total volatile solids (VS), the procedure mentioned in Standard Methods [35] was followed. An oven was used for TS (UF110, Memmert, Germany) and a muffle furnace for VS determination (LE 24/11, Nabertherm, Germany). The TS and VS of the grass fractions after pretreatment are given in Table 1. After ensiling, the U stream was named “E” and the M stream was named “EM.”

Table 1 List of grass streams and their total solids (TS) and volatile solids (VS) used in this study. Results are presented as mean values of triplicates and standard deviations were lower than 0.1% for all the analyzed conditions
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2.3 Biochemical methane potential (BMP) determination

BMP experiments on the grass streams obtained after the above-mentioned pretreatments (Section 2.2) were conducted to assess their impact on biogas production and methane yields. The design of the BMP experiments was carried out following the guidelines from Angelidaki et al. [36]. The inoculum was obtained from the Hashøj biogas plant in Denmark, where food, industrial, and animal waste are co-digested at mesophilic conditions.

BMP tests were conducted in triplicates at an organic load of 2 gVS/L and a working volume of 200 mL with a headspace of 340 mL. After adding the samples, the bottles were flushed with nitrogen for 4 min to maintain anaerobic conditions. The bottles were then sealed shut with rubber corks and aluminum caps, stored in an incubator at 37 °C, and shaken manually every other day to ensure homogenization. The pressure in the bottles was monitored twice a week using a manometer (Delta Ohm HD2124.2), and the increase in pressure was calculated over weeks. The experiment was stopped when a steady pressure was observed in the bottles. The produced gas in the bottles was analyzed for nitrogen, carbon dioxide, and methane concentrations using gas chromatography, as detailed in Section 2.6.

2.4 Enzymatic hydrolysis

Acid pretreatment was performed on the EM stream in an autoclave at 121 °C for 40 min at 18% solid loading using 1% sulfuric acid solution. Enzymatic hydrolysis was performed with the acid-treated grass using a cellulase enzyme blend (SAE0020, Novozyme Corp, Denmark). A 2-L bioreactor with an enzyme loading of 10 FPU/g glucan was used at 200 rpm, 50 °C, and pH of 4.8 was controlled automatically using 8M NaOH for 24 h. Samples of 5 mL were taken during this process and frozen until needed.

2.5 Fermentation

After enzymatic hydrolysis, the hydrolysate was separated from the solid residue using a centrifuge at 10,000 rpm for 10 min. The hydrolysate (LEMEh) was used for fermentation, while the solid fraction (EMEh) was analyzed for its biochemical methane potential (BMP). Lactobacillus delbrueckii and Pediococcus acidilactici were selected as lactic acid–producing bacteria for fermentation and were grown in a de Man–Rogosa–Sharpe (MRS) medium [37] until reaching their exponential growth phase, achieved 16 h after the inoculation. The fermentation was carried out in duplicates in 1-L anaerobic bottles fitted with a heating jacket set at 37 °C and placed on a magnetic stirrer set at 60 rpm. On reaching the desired temperature, 10% (v/v) inoculum was added to the fermenter. The pH of the broth was set to 8 using 10 M NaOH after inoculation, and no pH control was done afterwards, as mentioned in Zhang et al. [38]. A sample of 5 mL was taken during this process and the samples were frozen until needed. The pH of the broth became acidic with the production of LA; this indicated the advancement of fermentation, which was concluded once a constant pH was observed (24 h after inoculation).

2.6 Analytical methods

The total nitrogen (TN) of the samples was analyzed with a CN analyzer (Skalar Analytical BV, Breda, The Netherlands). The determination of phosphorus (P) and potassium (K) in the grass digestates was done by digesting 0.1 g of sample in 5 mL 90% nitric acid in a microwave (Ultra Wave 1 and 2, Milestone Srl, Italy). The digested sample was filtered using a filter paper, diluted as per requirement, and analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES; Thermo Scientific™ iCAP™ 7400, Thermo Fisher Scientific, Waltham, MA, USA).

Intermediate sampling was carried out during fermentation and frozen until necessary. At the end of the experiment, the samples were filtered and diluted, ready to be analyzed using the high-performance liquid chromatography (HPLC; Shimadzu Nexera XR equipped with an Aminex® HPX-87H Ion Exclusion Column) as mentioned in [39].

The produced biogas analysis was conducted by injecting 0.2 mL gas from the headspace of the bottle into a gas chromatography equipment (GC; Thermo Fisher Trace 1310 Gas Chromatography) as mentioned in [36].

The BMP obtained during the experiments was calculated using the following Eq. 1:

$${\text{BMP}}_{\text{Grass}}\frac{\text{mL} {\text{CH}}_{4}}{{g}_{FM}}=\frac{{\% {\text{CH}}_{4}}_{GC}\times {V}_{\text{headspace}}\times ( 1+\left( {P}_{\text{sample}}- {P}_{\text{blank}}\right) \times t}{{g}_{FM}}$$
(1)

Here, %CH4 is the content of methane in the biogas, Vheadspace is 340 mL, Psample is the pressure of sample in bar and Pblank is the pressure of blank in bar, t is 0.88 (273 / (273 + 37)), and FM is the amount of grass added in FM in grams. Equation 1 was used to calculate the methane produced in mL/gFM, which was converted into m3/gFM.

2.7 Revenue calculation

The possible revenues generated from the produced biogas were calculated as shown below.

The electricity production using 1 tonne of FM was calculated using Eq. 2:

$$\text{Electricity}\frac{\text{kWh}}{tFM}= {\text{BMP}}_{\text{m}3}\times {\text{Electricity value}}_{\text{Methane}}\times {\text{Conversion efficiency}}_{\text{elctricity}}$$
(2)

where BMP m3 stands for the biochemical methane potential of the grass stream in m3, the electricity value of methane used was 10 kWh/m3 [40], and the conversion efficiency was considered to be 41% [41].

The heat produced using 1 tonne of FM was calculated using Eq. 3:

$$\text{Heat}\frac{\text{kWh}}{tFM}={\text{BMP}}_{\text{m}3}\times {\text{Thermal Efficiency}}_{\text{Methane}}\times {\text{Energy Value}}_{\text{Methane}}\times \text{Conversion factor}$$
(3)

where BMPm3 stands for the biochemical methane potential of the grass stream in m3, the thermal efficiency of methane is 44%, the energy value of methane used was 36 MJ/m3, and the conversion factor for MJ to kWh used was 0.28.

2.8 Statistical analysis

All measurements were performed in triplicates, and mean data were presented along with standard deviation. Analysis of methane yield changes caused by different pretreatment technologies was done using Minitab Statistical Software version 21 (Minitab Ltd., Coventry, UK). Analysis of variance (ANOVA) followed by Tukey’s method and 95% confidence interval was used to determine significant differences in experimental results.

3 Results and discussion

3.1 Effect of pretreatments on biogas production from grass

Ensiled (E), milled (M), milled + ensiled (EM), and untreated (U) grass from the same origin were used in this study to understand the effects of the tested treatments on biogas production (Figure 2).

Fig. 2
figure 2

BMP of pre-treated grass samples expressed per tonne FM and VS and their methane content in the biogas is presented in the figure. U untreated, M milled, E ensiled, EM milled and ensiled. Means of BMP and methane concentration of the pre-treated grass that do not share a letter (A, B, C or x, y, z or a, b, c) are significantly different between each category (/tFM, /tVS, or %)

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The methane yields achieved from the AD of U, E, M, and EM samples were 289, 332, 324, and 343 m3 CH4/tVS, respectively. All pretreatment methods tested enhanced the methane yield significantly when compared to the untreated grass, while no significant differences were observed between the pretreatments (p > 0.05).

Ensiling the grass resulted in a BMP that was similar to M and EM grass when expressed as m3 CH4/tFM (p > 0.05). Interestingly, the two grass fractions subjected to ensiling (E and EM) showed a significant difference when expressed in m3 CH4/tFM (p < 0.05). Ensiling caused a reduction of the TS content of the initial grass (see Table 1), so a higher amount of FM was required to maintain the required VS content as outlined in the experimental design of the BMP test. Consequently, this led to a comparatively lower yield per ton of fresh matter (tFM). A similar loss of TS content during ensiling has been observed in switchgrass [42] and in other substrates such as maize, hemp, beets, and beet tops [43]. The process of ensiling results in the generation of organic acids, causing the grass to become acidic. This acidity, unlike in AD, could hamper the activity of methane-producing microbes [44], ultimately leading to a reduction in the methane yield on a larger scale.

The purpose of milling as a pretreatment is to reduce the particle size and to increase the access of hydrolytic enzymes to the cellulose and the hemicellulosic content of the grass. The M stream followed the EM stream with a high methane yield per tonne of FM which is possibly due to the increased surface area of the milled grass fibers, improving the access of microorganisms and enzymes to the substrate. The methane content of all biogas samples fell within the range of 68 to 77%. Milling of grass in M and EM also led to a higher methane content by 3% and 9% compared to U in biogas, respectively. The enhancement in methane yield resulting from size reduction has been supported in the case of sisal fiber particles, as observed by Mshandete et al. [45]. Additionally, Kaur, M. [46] observed that reducing the particle size of rice straw and bagasse led to an augmentation in both biogas yield and digestibility. In addition to increased methane yield compared to U, particle size reduction by mechanical pretreatments helps in handling, transportation, and storage space of biomass [47].

The combination of pretreatments (EM) resulted in the highest methane yield if expressed in m3 CH4/tFM (p < 0.05) (Figure 2). EM had similar TS content to the untreated control (Table 1), indicating that the higher BMP observed for EM can be attributed to an actual increase in digestibility resulting from the combined treatments. This is further confirmed by the rapid increase in pressure observed in the EM bottles during the BMP experiments as compared to the other samples (data not shown). The BMP was observed to increase by 14% and 12% for E and M grass, respectively. However, when grass underwent combined ensiling and milling (EM), the BMP enhancement reached 17%. This increase in BMP for the EM treatment compared to individual E and M treatments could be attributed to a synergistic effect. The reduction in particle size from milling likely improved the ensiling process, which in turn further augmented the BMP, demonstrating the beneficial impact of integrated preprocessing techniques [48]. Enhanced biodegradability of ensiled grass upon mechanical pretreatment was also observed by Tsapekos et al. [48].

Every 1 m3 increase in methane potential due to additional pretreatment steps per tonne FM of grass increases the electricity production by 4.1 kWh. Therefore, milling the grass fraction increased the electricity production by 25 kWh/tFM, whereas milling and ensiling resulted in an increase of 44 kWh/tFM. To better understand the feasibility of the milling pretreatment, considering that ensiling had no energy cost, 1 ha of grassland is taken as an example. The grass yield for Ireland is 10.2 tDM/ha/year [49]; therefore, the example land would produce 44 tonnes of fresh grass annually, assuming similar TS content as the grass in this study. In a study conducted by the Idaho National Laboratory and Vermeer Manufacturing Company [50], the energy consumed to mill 1 tonne DM of switch grass using a hammer mill was ~7kWh. Assuming the same amount of energy would be consumed to mill grass from this research, 71.4 kWh/year would be required to mill 1 ha of grass and in return gain a net additional 1011 kWh/ha/year of electricity from M and 1913 kWh/ha/year from EM. Therefore, the application of pretreatment could prove advantageous as the resultant increase in electricity generation surpasses the corresponding consumption.

3.2 Nutrients in the digestate of grass streams

The three main macro-nutrients necessary for plant growth, N, P and K (NPK), were found in all the grass digestates, albeit in different proportions, as shown in Figure 3. The composition of digestate generated by each grass stream represented in N:P: K (kg/tDM) was 3:11:45, 3:10:31, 7:8:42, and 6:10:31 for U, M, E, and EM, respectively.

Fig. 3
figure 3

The nitrogen:phosphorus:potassium (N:P:K) compositions (kg/tDM) in the different grass digestates are presented in the figure. U untreated, M milled, E ensiled, EM milled and ensiled

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The nutrients in the digestate can help to reduce the farmers’ dependency and expenses on chemical fertilizer and to make farming more sustainable. To understand the fertilizer potential of these digestates, the nutrients (NPK) required to grow a hectare of potatoes were used as a baseline for comparison. According to Czekała et al. [51], 70–90% of the feedstock weight is conserved in digestate, which implies that 1 tonne of grass will produce 0.8 tonnes of digestate. According to Prince Edward Island Analytics Laboratories [52], the NPK requirements of potato plants are 185–135–135 (kg/ha). Considering EM as the best-performing pathway, 31 tonnes of digestate (DM basis) will fulfill the nutrient requirements of 1 ha of a potato field. Since the examined digestate is a by-product of mono-digested grass, no legal limitations for digestate use on land regarding its N content are applicable with respect to Nitrates Directive 91/676/EEC. Thus, the whole digestate fraction produced can be applied to the fields. As mentioned previously, the FM of grass/ha/year is considered to be approximately 44 tonnes. Therefore, digestate produced with the grass harvested from 0.9 ha of grassland could potentially contain the nutrient required to grow 1 ha of potatoes, enabling farmers to substitute chemical fertilizers with digestate and transition toward sustainable farming. Assuming the farmer used urea, triple superphosphate, and potash as sources of NPK with concentrations of 46-0-0, 0-20-0, and 0-0-50 and priced at €265/Mt, €360/Mt, and €303/Mt, respectively [53], they would save 431€/ha in chemical fertilizers by using digestate. According to Hendriks et al. [54], potato fields supplied with bio-based nutrient sources show similar yields as chemical fertilizers. Nonetheless, a further study should be carried out to understand the nutrient uptake of plants when grass digestates are used as a nutrient source and confirm the assumptions of this study.

3.3 Lactic acid production from ensiled and milled grass

In addition to its potential for energy and nutrient recovery, the grass was evaluated as a source of fermentable sugars specifically for LA production, utilizing exclusively the EM grass fraction. The EM grass was first treated with diluted acid followed by enzymatic hydrolysis to produce a sugar-rich broth. L. delbrueckii and P. acidilactici were chosen as lactic acid–producing bacteria since they are homo-fermentative and robust enough to grow in a wide pH range of 4.2–8.5 and different temperatures of 20–53 °C [55]. L. delbrueckii and P. acidilactici yielded 240 kg LA/tFM and 230 kg LA/tFM, respectively. A conversion efficiency of 0.7 g/gsugar was achieved, which is comparable to that of the same bacteria used by Zhang et al. [38] to produce LA from organic household waste.

In the process of LA fermentation, the solid residue from the enzymatic hydrolysis (EMEh) was separated and tested for its BMP. This EMEh stream had a low methane potential of 118 m3 CH4/tVS (corresponding to 4 m3 CH4/tFM) because most of the cellulose was hydrolyzed into soluble sugars, resulting in a solid phase rich in lignin, which is biologically recalcitrant. Therefore, the viability of employing this lignin-rich stream for AD is limited. Instead, it could be valorized for lignin recovery, as mentioned in a study by Dutta et al. [56] who generated streams abundant in glucan and lignin from grass, hardwood, and softwood. Similarly, Chopda et al. [57] produced fermentable glucan along with a lignin-rich stream with comprehensive applications from oat husks. Implementing such strategies would enable the biorefinery to diversify its product range, consequently boosting revenue.

3.4 Revenue generated from products

Figure 4 presents the electricity and heat production in kWh from the biogas of anaerobically digested U, E, M, and EM grass fractions. An increased BMP of the EM grass resulted in a 16% and 10% increase in electricity and heat as compared to U grass. In recent years, the cost of electricity has shown significant variability, ranging from 0.12 to 0.2 €/kWh, while gas prices have fluctuated between 0.03 and 0.06 €/kWh. Given this volatility, farmers must adopt self-sustaining measures to mitigate the risk of unforeseen increases in operational costs. In January 2022, the commercial electricity rate in both Ireland and the Euro area stood at 0.2 €/kWh, while the corresponding gas prices were 0.06 €/kWh in both regions.

Fig. 4
figure 4

Average electricity (kWh) and heat (kWh) produced per tonne of fresh matter calculated based on the biomethane potential of the pre-treated grass are presented in the figure. U untreated, M milled, E ensiled, EM milled and ensiled. Means of kWh electricity and heat of pretreatments that do not share a letter (a, b, and c in between electricity and A, B, and C between heat) are significantly different between each category (electricity/heat)

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The revenue generated from each pathway is shown in Figure 5 for comparison. If the generated electricity and heat from EM are used on a farm, a total of 87 €/tFM in operational cost could be saved by the farmer. Furthermore, the electricity required to operate an AD plant digesting farm waste was calculated as 6 kWh/tFM [58]. Therefore, assuming the electricity for operating a grass-fed AD plant to be similar to that of a farm waste-fed AD, the economic benefits seem to outweigh the additional operations and costs associated with biogas production from all streams of grass. This would help make the grass biorefinery more self-sufficient and generate additional revenue from selling electricity.

Fig. 5
figure 5

Overview of the grass biorefinery. The outputs and the revenue generated from different grass streams are illustrated. The cost of lactic acid per kg (€ 1.2/kg), for kWh of electricity (€ 0.2/kWh), and heat (€ 0.06/kWh) is subject to change. These do not take into account the cost of operations. U untreated, M milled, E ensiled, EM milled and ensiled, EMEh solid fraction separated after enzymatic hydrolysis, VS volatile solids, FM fresh matter, LA lactic acid, CHP combined heat and power

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The gross revenue generated by producing LA using L. delbrueckii with a selling price of 1.83 €/kg [59] and a yield of 240 kg LA/tFM is 434 €/tFM. In addition to LA, an additional 26 €/tFM can be generated by the solid fraction if it were anaerobically digested.

Assigning each element (NPK) with its singular nutrient fertilizer cost and multiplying it with its concentration as found in each digestate would result in unrealistic revenue from digestate.

The composition of NPK within the digestate exhibits variation [60]. Consequently, when applying the digestate based on its N concentration, the ability to regulate the quantities of P and K that accompany it becomes challenging. While the limitation on applying digestate N is absent in the case of an animal by-product-free digestate, the evaluation of grass digestates revolves around their quality assessment rather than their monetary yield. The value of digestate was expressed in terms of FM of grass required to provide sufficient nutrients per hectare of potato field as discussed in Section 3.2. In the event of monetization of digestate, considering the digestate’s substantial K content, its utilization as a K-fertilizer would yield values of 27, 19, 25, and 19 €/tDM for the U, E, M, and EM digestate streams, respectively. Similarly, should the digestate be used as a P-rich fertilizer, it would generate values of 20, 18, 14, and 18 €/tDM for the corresponding digestate streams.

The pathways that generated the highest revenue were those following the fermentation pathway. There is an increase of 389% in revenue if the EM is not anaerobically digested but instead fermented. For the LA pathways to be economically feasible, additional factors such as the scale of production, the concentration of LA in the broth, the use of chemicals, and the efficient downstream processing of LA would play an important role. Large-scale LA fermentation also entails significant capital and operational costs, as mentioned in a previous study by Manandhar and Shah [32]. Moreover, the cost of raw materials [61], transport, and storage should be also accounted for, while policies regarding agricultural and renewable energy might provide subsidies that could also influence the feasibility of the proposed process [62]. A deeper study of the complete process that takes into account the operational cost of a large-scale setup should be carried out. A proposed study should explore the feasibility of integrating a biorefinery, involving AD and/or fermentation processes, into an existing agricultural operation. This integration should aim to utilize on-farm biomass for energy production and nutrient recovery, potentially reducing operational costs and enhancing sustainability. Additionally, the study should also examine the establishment of a standalone grass biorefinery that processes excess grass from various sources, such as roadsides and grasslands. This could prevent the conversion of permanent grasslands into intensive farmlands and would facilitate a transition toward a bio-based economy. A comprehensive techno-economic analysis, encompassing technical, economic, and social dimensions, is also crucial to assess the viability and impact of such biorefineries.

4 Conclusion

The high availability of grass presents numerous untapped valorization opportunities that have yet to be better developed and that could help to reach the EU Circular Economy goals (including renewable energy). In this study, the combination of milling and ensiling demonstrated the most favorable outcomes, yielding the highest methane potential and content compared to each separate pretreatment method. The grass-derived digestate shows the potential to be used as a source of nutrients in terms of its NPK concentrations. As an alternative high-value route, the enzymatic hydrolysis followed by LA fermentation was tested for the EM stream, resulting in a high conversion efficiency. Although further research is required to evaluate the feasibility of downstream LA processing, its production should be considered due to the high value of this molecule when compared to energy and fertilizers. The establishment of a grass biorefinery could substantially increase farmers’ revenue and contribute to energy and nutrient self-sufficiency. While this study confirms the technical viability of these processes, scaling up and a thorough techno-economic analysis are essential to determine their broader applicability and economic impact. Developing a business model tailored to farmers’ needs will be crucial for facilitating the adoption of this technology.