Environmental Sustainability of Brewers’ Spent Grains Composting: Effect of Turning Strategies and Mixtures Composition on Greenhouse Gas Emissions

Abstract

The global production of brewers’ spent grains (BSG) is 37 million tons yearly. Composting represents an eco-friendly method to manage and valorize organic by-products in a circular economy model. This project aims to compare two BSG bin-composting mixtures (BSG and wheat straw with pig slurry solid fraction, MIX1, or sheep manure, MIX2) and approaches (manual turning, MT, and static composting, ST). The two mixtures’ physicochemical characteristics and greenhouse gas (GHG) emissions were assessed during the process. The evolution of physicochemical properties is reported in detail. Headspace samples of GHG emissions were collected and analyzed with gas chromatography coupled with specific detectors. Carbon dioxide (CO₂) emissions were 34.3 ± 0.03 and 31.0 ± 0.06 g C kg⁻¹ fresh matter (FM) for MIX1-MT and MIX2-MT, and 28.8 ± 0.01 and 31.2 ± 0.02 g Ckg⁻¹ FM for MIX1-ST and MIX2-ST. Methane emissions were negligible (all conditions < 0.086 ± 0.00 mg C kg⁻¹ FM). Nitrous oxide (N₂O) emissions from composting are affected by the substrate, bulking material, pile dimension, and manure. Particularly, the total emissions of N₂O, estimated as CO₂ equivalents, were 45.8 ± 0.2 and 63.0 ± 0.4 g CO₂ eq kg⁻¹ FM for MIX1-MT and MIX1-ST, respectively. In both composting approaches, MIX2 showed a low CO₂ equivalent (1.8 ± 0.02 and 9.9 ± 0.05 g CO₂ eq kg⁻¹ FM for MT and ST), likely due to incomplete decomposition. The bin-composting process represents a solution for recycling and reusing organic waste and livestock manure in small to medium-sized breweries. The solid fraction of the pig slurry resulted in the most suitable manure.

1. Introduction

During the late 1980s, there was a swift rise in beer’s market share within the global alcohol market, coinciding with the surge of craft brewing in the United States (US), subsequently spreading to other parts of the world [1]. Craft beers are produced by small/medium and independent breweries, typically in quantities of several thousand hectoliters (with the maximum volume allowed determined by national laws and regulations). This definition starkly contrasts with the production of “mainstream” beers, brewed in millions of hectoliters by industries like Anheuser-Busch and MillerCoors in the US [2]. According to the latest Barth report 2023–2024, worldwide beer production has been estimated at 1884 million hectoliters, while 625, 574, and 511 million hectoliters were produced in the American, Asian, and European continents, respectively [3]. At the same time, the brewing industry can generate significant quantities of by-products, including spent hops, yeast, and spent grain, with the latter constituting 85% of the total brewing industry solid by-products [4]. Specifically, for every 100 L of beer produced, 20 kg of brewers’ spent grain (BSG) are generated [5,6]. Therefore, the worldwide annual production of BSG was calculated at approximately 37 million tons [7], among which 12.5, 11.5, and 10.2 million tons were generated in the American, Asian, and European continents, respectively [3]. Many authors [8,9,10] have stated that the disposal of BSG poses a significant challenge for the brewing industry, primarily due to the large volumes produced, its low market value, and the high moisture content that favors microbial fermentations that adversely impact storage. To reduce landfill disposal and move toward a circular economy, considerable research has been undertaken to investigate various methods for recycling BSG. Currently, BSG is commonly recycled in the animal feeding sector [11], often resulting in low or even zero economic benefits for breweries [12]. BSG has also been explored for various alternative uses, including producing value-added compounds like xylitol [13] and lactic acid [14], cultivating microorganisms [15], or serving as a raw material for extracting sugars, proteins, acids, and antioxidants [16]. Moreover, Bianco et al. [17] emphasized that BSG can be effectively utilized to accelerate waste decomposition due to the growth of fungal and bacterial species naturally associated with BSG.

Composting involves the controlled decomposition of organic matter (OM) through regulated temperature, moisture, and aeration, serving as a recycling technique [18]. This process leads to the production of a stabilized final product that is free of phytotoxicity and pathogens and rich in humic compounds [19,20]. Furthermore, several authors have affirmed the effectiveness of composted OM as a valuable asset for enhancing [21,22] and conditioning soil [23]. This offers a feasible substitute for costly chemical fertilizers, ultimately decreasing the overall expenses linked with crop production [24,25]. However, composting has the disadvantage of generating and releasing greenhouse gases (GHGs) [26]. During the process, oxygen (O₂) is consumed, and carbon dioxide (CO₂) is released, along with methane (CH₄), nitrous oxide (N₂O), and other volatile substances, such as ammonia (NH₃) [27]. Furthermore, aiming to obtain an organic fertilizer, Assandri et al. [9] explored the potential of recycling BSG through the composting process in bins. More in detail, they proposed two different composting mixtures utilizing the solid fraction of pig slurry and sheep manure as the starting material, coupled with wheat straw as a bulking agent [28].

Despite the extensive literature on composting, there is a lack of studies explicitly focusing on the environmental sustainability of BSG composting. Additionally, the impact of the turning operation on GHG emissions during BSG composting remains largely unexplored. Therefore, the aim of this study was to investigate the BSG composting mixtures proposed by Assandri et al. [29] and compare two different bin-composting strategies (turned and not turned) in terms of physicochemical characteristics and GHG (CO₂, CH₄, and N₂O) emissions. Moreover, this research evaluated if the bin-composting approach is a suitable method to optimize the by-products produced by breweries while obtaining compost for organic fertilization, resulting in sustainable waste management within the circular economy framework.

2. Materials and Methods

The trial was carried out at the Institute of Sciences and Technologies for Sustainable Energy and Mobility (STEMS)—Italian National Research Council (CNR)—in Turin, Italy (44°57′ N, 7°36′ E, 245 m above sea level). BSG was obtained from a middle-sized brewery in Piedmont. The BSG composting mixtures—MIX1 and MIX2—proposed by Assandri et al. [29] have been investigated. The PSF was obtained from a fattening pig farm in Piedmont (Italy) at the end of a separation process (with a screw press Chior, mod. COM300/600), and the SM was collected in a farm in Piedmont (Italy) where sheep are raised in paddocks with a slow change in the litter (about 4 times per year). The mixtures were obtained by blending BSG and wheat straw (WS) with the solid fraction of pig slurry (PSF) for MIX1 and sheep manure (SM) for MIX2. Specifically, MIX1 comprised 48% BSG, 15% WS, and 37% PSF, while MIX2 consisted of 42% BSG, 18% WS, and 40% SM. All the above percentages refer to the weight of fresh matter.

Table 1. Physicochemical characteristics of the initial mixtures. Mean values and standard deviations of three replicates.

Compost	Moisture *	pH	TOC	OM	TKN	NH4+-N	NO3−-N	C/N
	%		g kg−1	%	%	mg kg−1	mg kg−1
MIX1	73.2 ± 1.07 a	8.3 ± 0.06 b	515 ± 0.23 a	88.6 ± 0.29 a	2.8 ± 0.11 a	1091 ± 5.03 b	74 ± 1.66 b	18.2 ± 0.78 a
MIX2	73.0 ± 1.52 a	8.7 ± 0.12 a	488 ± 2.06 a	85.8 ± 0.58 b	2.6 ± 0.09 b	1623 ± 4.07 a	103 ± 6.24 a	19.1 ± 0.15 a

TOC: total organic carbon; OM: organic matter; TKN: total Kjeldahl nitrogen; NH₄⁺-N: ammoniacal nitrogen; NO₃⁻-N: nitrate nitrogen; C/N: TOC to TKN ratio. * Calculated based on fresh weight. ^a,b Different letters represent significant differences between physicochemical characteristics between mixtures. Significant differences were determined by ANOVA and the Bonferroni post hoc test (p = 0.05).

shows the average physicochemical characteristics of the initial mixtures. These physicochemical characteristics influence the composting process. In the literature, the optimum moisture range values for composting have quite a variability, with several authors agreeing on a 60–65% water content [30]. The starting moisture levels of the mixtures were a bit elevated for optimal conditions, possibly leading to anaerobic conditions. The pH values of this study were within the suitable ranges for good microbial activity (pH 6.7–9.0) [31]. The C/N ratio initial levels of both mixtures were approximately in the optimum range for composting (ratio between 20 and 30), as highlighted by Assandri et al. [9]. In general, these initial parameters are more suitable for composting.

The composting process occurred within identical 300 L polyethylene bins (with a base diameter of 600 mm, top diameter of 700 mm, and height of 880 mm), as described and designed by Assandri et al. [29] in their previous study. During the experimental period, two different composting strategies were compared: turning composting, with the manual turning of the mass (MT), and static composting, without turning operations (ST). For the manual turning of the compost, in the MT strategy, the bin containing the pile of compost was turned upside down, and then the compost was turned with a shovel before being reintroduced into the bin. For each condition, three replicates were tested, resulting in a total of twelve bins (three bins for MIX1-ST, three for MIX1-MT, three for MIX2-ST, and three for MIX2-MT). Throughout the entire composting duration, the MT strategy bins underwent seven turns, with a turn on days 5, 12, 17, 21, 26, 33, and 46. During the composting trial, the temperature of each bin was consistently monitored using a Type K thermocouple probe connected to a multichannel acquisition system (Testo 176 T4, Milan, Italy). The probe was positioned in the center of the composting mass to avoid influences from the surrounding environment, and the temperature was recorded once per hour in each bin. The average daily temperature for each condition (n = 3) was then calculated. The ambient air temperature was measured in two different places in the area where the experimental system was stored. The device type was the same as the one used for the bins, a type K thermocouple probe connected to a multichannel acquisition system (Testo 176 T4).

2.1. Gaseous Emission Measurements

The greenhouse gas emissions (CO₂, CH₄, and N₂O) were measured throughout the composting process (104 days), including the maturation phase. The sampling frequency was adjusted according to the composting trend, gradually decreasing as gaseous fluxes diminished. More detailed measurements were conducted three times a week during the first four weeks and then once or twice a week for the remaining period. All gas sample replicates (n = 3 for each condition) were collected simultaneously, following the approach employed by Peyron et al. [32] and Pittelkow et al. [33]. Throughout each turning operation, gaseous emission rates from the MT strategy were assessed before turning. The emissions were also measured after the turning but are not reported in this paper as they are negligible. Emissions were measured using a non-steady-state closed chamber technique adapted for this study from Livingston and Hutchinson [34]. Each lid was equipped with a pressure vent valve to maintain pressure equilibrium designed according to Hutchinson and Mosier [35] and diametrically opposite to the gas sampling port (Figure 1). The gases’ concentrations built up under the lid of composting bins. Before measuring the GHG emissions, the air of the composting units was exchanged to start the sampling as close as possible to ambient concentrations [36]. Air-tight 30 mL propylene syringes (Sigma-Aldrich, Milan, Italy) were used to collect the inside headspace gas samples from the sampling port (Figure 2) to evaluate gas fluxes. At every sampling event, three gas samples were collected from each bin at 0, 10, and 20 min after the lids were closed and sealed. Before sampling, all holes, including the drainage one, were closed (Figure 2).

Thirty mL air samples were collected and injected into twelve mL evacuated vials closed with butyl rubber septa (Exetainer^®®® vial from Labco Limited, Lampeter, UK) [37]. A fully automated gas chromatograph (Agilent 7890A) with a Gerstel Maestro MPS2 autosampler was used to determine the gas concentration in the collected samples. After the injection (injector temperature 70 °C), the sample is split into two lines for gas detection; line 1 is equipped with two packed columns (Supelco Sigma Aldrich Porapack Q; Milan, Italy, and Porapak QS, Milan, Italy) kept at 80 °C and with a thermal conductivity detector (TCD) for CO₂ detection and a flame ionization detector (FID) for CH₄ quantification, placed in-line. On this line, He is used as carrier gas at 30 mL/min flow. Operating temperatures are 200 °C for the TCD and 250 °C for FID. Line 2 is equipped with two packed columns (Sigma Aldrich Porapack Q) and with an electron capture detector (ECD) for N₂O quantification; on this line, a 5% argon-CH₄ mix is used both as carrier and makeup gas (30 mL/min). The ECD operating temperature is 350 °C. All detectors are from Agilent Technologies (Agilent Technologies, Waldbronn, Germany). Each line is preceded by a 500 μL loop for sample volume determination; the system allows sample edge and tail cutting by a two-valve system to limit time analysis at nearly six minutes. Minimum detectable concentrations for each gas are as follows: 110 ppb for CH₄, 16.5 ppm for CO₂, 10 ppb for N₂O. Fluxes were calculated based on the increasing concentration in the chamber headspace over time. The concentration increase was assessed using either a linear or nonlinear method based on the observed emission pattern. The calculation method was based on the recommendations of Hutchinson and Mosier [35] and Livingston and Hutchinson [34] and with atmospheric pressure and temperature corrections [38].

The global warming potential (GWP) was calculated to assess the potential future impacts of emissions from different gases upon the climate system in a comparative way [27]. GWP is a relative measure, reflecting a greenhouse gas’s capability to trap heat in the atmosphere. It compares the heat retention of a specific mass of the gas in question to that of a comparable mass of CO₂ [39]. In this study, the GWP estimation was calculated according to the Intergovernmental Panel on Climate Change (IPCC) [40]. The global heating potential over a 100-year timespan of a GHG is calculated by the ratio of the greenhouse effect of one kilogram of the considered GHG with that of one kilogram of CO₂. The GWP of CH₄ is 27 and the GWP of N₂O is 273.

2.2. Physicochemical Analyses

For the analyses of the physicochemical properties, five sub-samples (100 g) were collected from each bin and mixed to form a composite sample of 500 g. Each condition was tested three times (once for each bin), yielding three replicates of the composite samples per condition per sampling. The total set of composite samples collected for the physicochemical analyses were five for each condition: one on day zero, one on day 104, and three intermediate samples set at days 13, 46, and 74 to explore the progression of the mixtures during the composting process. The moisture content was determined using a gravimetric method, which involved measuring the weight loss after drying the samples at 105 °C in a ventilated oven until a constant weight was achieved for 24 h [21]. The pH of water extracts, prepared at the ratio 1:10 (w/v), was measured using a Hanna HI 9026 portable pH meter (Hanna Instruments, Woonsocket, RI, USA) fitted with a glass electrode and combined with a thermal automatic compensation system [41]. Samples for the total organic carbon (TOC) analysis were prepared by treating the dried samples with sulfuric acid (H₂SO₄; Sigma-Aldrich, Milan, Italy) to eliminate any inorganic carbon (C). The samples were then analyzed by a C analyzer (Carlo Erba Instruments, Rodano, Milan, Italy) [42]. Total Kjeldahl nitrogen (TKN), total ammoniacal nitrogen (NH₄⁺-N), and nitrate nitrogen (NO₃⁻-N) were determined using the Kjeldahl standard method. The OM-percentage loss (OM loss %) by mineralization was calculated through the initial (X_i) and final (X_f) ash concentrations according to the following Equation (1) [43]. The ashes’ content was obtained by incineration of the dry matter at 500 °C for ca. 2 h [44].

OM loss (%) = 100 − 100 [X_i (100 − X_f)]/[X_f (100 − X_i)]

(1)

2.3. Statistical Analyses

The data of physicochemical parameters and differences in cumulative gas emissions were collected as three replicates per condition and subjected to an analysis of variance (ANOVA). Before the ANOVA analysis, the data underwent assessment for normal distribution using the Shapiro–Wilk test. When required, appropriate transformations, such as log transformation, were applied to enhance the distribution of residuals and ensure the homogeneity of variance. Differences between means were subsequently determined using the Bonferroni post hoc test (α = 0.05). The statistical analyses were conducted using R statistical software version 4.4.0 (24 April 2024 ucrt). The results are reported as mean value ± standard deviation (SD).

3. Results and Discussion

3.1. Composting Trial

3.1.1. Temperature Trends

The temperature patterns are shown in Figure 3a (MIX1) and Figure 3b (MIX2). The temperature trend reflects microbial activity and the progression of the composting process [31,45] and classifies the composting phases [46]. Several authors [47,48,49,50,51] agree that temperatures above 55 °C for 3 to 5 days are required to achieve proper compost sanitization, while temperatures between 35 and 40 °C create optimal conditions for microbial diversification [52]. At the beginning of the composting process, the thermophilic phase was achieved under all conditions 6–7 h after the bins were closed. This rapid increase in temperature indicates that brewery by-products, when combined with livestock manure, supply readily available nutrients to the microorganisms involved in the composting process [18]. Considering MIX1, temperatures remained above 55 °C for 6 and 7 days for the MT and ST strategies, respectively. In contrast, for MIX2, the number of days was reduced to 3 and 4 days for the ST and MT strategies, respectively. The temperatures for MIX1 dropped below 40 °C on the 21st day in both strategies. For MIX2, the temperatures went below 40 °C on the 10th and 12th days for the MT and ST strategies, respectively. As suggested by Tang et al. [53], values below 30 °C indicate the end of the mesophilic phase. In our study, this phase ended on the 29th and 36th days for MIX1 in MT and ST strategies, respectively. Meanwhile, for MIX2, the mesophilic phase ended 31st for MT and 28th for ST. In general, the MT strategy increases oxygen levels within the mixtures, stimulating microbial activity and causing temperatures to rise again [54]. This was also observed in our study, where temperatures increased up to and including the penultimate turning (33rd day). Subsequently, temperature trends aligned with the ambient temperature pattern. After the final turning operation (day 46), temperatures in both mixtures remained stable, indicating the end of the active process phase.

3.1.2. Physicochemical Properties

The evolution of the physicochemical properties during the composting process is reported in

Table 2. Physicochemical properties evolution during the composting process. Mean values and standard deviations (SD) of 3 replicates.

Treatment	Day	Moisture * (%)	pH	TOC (g kg−1)	OM (%)	OM Loss ** (%)	TKN (%)	NH4+-N (mg kg−1)	NO3-N (mg kg−1)	C/N
MIX1-MT	13	73.6 ± 1.57 a	8.1 ± 0.15 b	497 ± 1.7 ab	85.8 ± 0.30 ab	22.2 ± 0.55 ab	3.0 ± 0.02 a	706 ± 9.90 c	75 ± 1.09 b	16.6 ± 0.04 bc
	46	70.5 ± 0.87 a	6.4 ± 0.00 c	465 ± 0.4 a	80.2 ± 0.08 c	47.6 ± 1.56 a	3.5 ± 0.06 a	471 ± 2.91 c	100 ± 1.11 c	13.2 ± 0.21 d
	74	67.4 ± 0.29 a	6.1 ± 0.00 b	457 ± 3.9 a	78.8 ± 0.65 a	52.0 ± 0.64 a	3.8 ± 0.11 a	380 ± 6.81 b	460 ± 9.15 c	12.2 ± 0.44 b
	104	40.6 ± 0.63 b	6.2 ± 0.06 c	424 ± 1.3 b	76.9 ± 0.67 ab	56.9 ± 1.73 a	3.7 ± 0.10 a	187 ± 3.92 a	490 ± 3.05 bc	11.6 ± 0.29 c
MIX2-MT	13	75.6 ± 2.04 a	8.7 ± 0.00 a	488 ± 4.0 b	84.1 ± 0.68 b	12.1 ± 1.10 ab	2.7 ± 0.15 b	728 ± 2.08 b	108 ± 6.24 a	18.1 ± 0.96 b
	46	45.2 ± 2.52 b	9.0 ± 0.06 a	471 ± 1.3 a	81.1 ± 0.22 b	28.7 ± 4.39 c	3.0 ± 0.06 c	684 ± 3.76 b	155 ± 1.51 ab	15.5 ± 0.29 b
	74	37.0 ± 1.37 b	8.8 ± 0.44 a	445 ± 4.2 a	76.8 ± 0.73 a	45.3 ± 0.46 a	3.2 ± 0.03 b	363 ± 6.73 b	503 ± 1.97 c	14.0 ± 0.15 a
	104	18.4 ± 0.73 d	9.4 ± 0.00 a	425 ± 9.5 b	73.6 ± 1.61 b	53.9 ± 1.64 ab	3.1 ± 0.13 b	281 ± 7.70 b	560 ± 1.73 a	13.9 ± 0.28 b
MIX1-ST	13	74.8 ± 0.57 a	8.5 ± 0.12 a	509 ± 3.7 a	88.0 ± 0.81 a	4.7 ± 9.22 b	3.1 ± 0.06 a	834 ± 1.40 ab	79 ± 8.54 b	16.4 ± 0.41 c
	46	71.2 ± 0.04 a	6.5 ± 0.06 c	475 ± 0.9 a	82.0 ± 0.08 a	41.2 ± 1.47 ab	3.3 ± 0.03 b	820 ± 6.76 a	114 ± 1.73 c	14.3 ± 0.11 c
	74	68.6 ± 3.14 a	6.1 ± 0.06 b	456 ± 11.3 a	79.1 ± 1.79 a	50.8 ± 6.75 a	3.7 ± 0.10 a	328 ± 6.73 c	572 ± 1.21 b	12.5 ± 0.67 ab
	104	46.0 ± 1.76 a	6.0 ± 0.12 c	449 ± 0.3 a	77.4 ± 0.05 a	55.7 ± 1.37 a	3.5 ± 0.17 a	247 ± 3.00 a	836 ± 9.64 c	12.9 ± 0.64 bc
MIX2-ST	13	58.4 ± 3.97 b	8.7 ± 0.12 a	486 ± 8.8 b	81.1 ± 1.30 c	28.3 ± 9.41 a	2.6 ± 0.11 b	1418 ± 3.00 a	105 ± 5.29 ab	18.6 ± 0.50 a
	46	55.2 ± 1.37 ab	8.8 ± 0.10 b	470 ± 7.6 a	79.2 ± 0.36 d	37.0 ± 1.65 b	2.8 ± 0.06 d	587 ± 1.02 ab	179 ± 9.17 a	16.9 ± 0.61 a
	74	43.2 ± 7.68 b	8.4 ± 0.32 a	459 ± 2.1 a	78.0 ± 8.96 a	37.0 ± 2.29 b	3.4 ± 0.22 ab	490 ± 9.41 a	627 ± 1.11 a	13.4 ± 0.80 ab
	104	26.6 ± 0.86 c	9.0 ± 0.00 b	451 ± 11.5 a	77.7 ± 1.98 a	41.7 ± 9.41 b	2.9 ± 0.04 b	408 ± 5.03 a	931 ± 9.85 ab	15.5 ± 0.21 a

* Calculated based on fresh weight. ** Calculated according to Equation (1) ^a–d Different letters in the same column represent significant differences between days of analysis. Significant differences were determined by ANOVA and Bonferroni post hoc test (p = 0.05).

. The data in Table 2 demonstrate a gradual decrease in moisture content over time, which is ascribed to metabolic heat generation [55]. Despite both composting strategies employing sealed bins throughout the experiment, from the 13th day onward, MIX1 displayed significantly higher moisture levels than MIX2, with statistical significance (p < 0.05). This difference is likely due to the two investigated manures’ different physical compositions and particle sizes.

Due to the different raw materials used, MIX1 exhibited an initial pH value of 8.3 ± 0.06, while MIX2 had an initial pH value of 8.7 ± 0.12 (Table 1). As composting progressed, the pH of MIX1 decreased rapidly after 46 days in both experimental conditions, stabilizing at final pH values of 6.2 ± 0.06 and 6.0 ± 0.12 for the MT and ST strategies, respectively (Table 2). On the other hand, MIX2 retained alkaline pH values throughout both composting approaches until the end of the process. The differences in pH can also be correlated with the animals’ diet: herbivores (sheep) or omnivores (pigs). There are not only differences in the chemical contents of the diets (higher protein amounts for omnivores and higher cellulose substances for herbivores) but also a distinction in animals’ gut microbiomes, resulting in different nitrogen metabolic processes and the bacterial and fungal composition of the compost [56,57,58]. Consistent with the results mentioned above, this study showed a decrease in pH for MIX1 (pigs) and an increase for MIX2 (sheep).

According to Equation (1), OM mineralization led to a decrease in OM concentration by 56.9% ± 1.73 and 53.9% ± 1.64 for MIX1 and MIX2 in the MT strategy and 55.7% ± 1.37 and 41.7% ± 9.41 for MIX1 and MIX2 in the ST strategy. The statistical analysis revealed significant differences (p < 0.05) between MIX1 and MIX2, suggesting variations in their composting behavior, while the strategies approach only influenced MIX2 significantly (Table 2). According to Santos et al. [43], the higher OM loss observed in MIX1 might be attributed to increased microbial activity during the initial phases of the process. This may arise from the presence of readily degradable materials originating from the solid fraction of pig slurry. On the contrary, in MIX2, the lower loss of OM can be ascribed to the larger amount of recalcitrant compounds, especially those rich in lignin, provided by the wheat straw [20,59]. The mineralization of OM was also verified through the decrease in TOC concentrations in both composting strategies and for both mixtures. The MT strategy exhibited an average reduction of 17.7% and 12.9% for MIX1 and MIX2, respectively. Meanwhile, under the ST strategy, the reduction amounted to 12.8% for MIX1 and 7.6% for MIX2. Similar values were reported in similar composting conditions by Guo et al. [54].

The minor TOC reduction observed in MIX2 may be attributed to its higher proportion of wheat straw, which possesses greater lignification and resistance to degradation. In the MT strategy, the higher reduction in TOC content observed is attributable to the turning system [43,60,61,62].

The rise in TKN concentrations during composting results from OM oxidation [39,63,64]. At the outset of the composting trial, there were no significant differences (p > 0.05) between strategies. As shown in Table 2, after a 104-day composting period, the TKN concentration in MIX1 was significantly (p < 0.05) higher than in MIX2 (3.7% ± 0.10 compared to 3.1% ± 0.13 for MT and 3.5% ± 0.17 compared to 2.9% ± 0.04 for ST).

The NH₄⁺-N concentration decreased during the composting process in all treatments. Simultaneously, the content of NO₃⁻-N increased, mainly due to the nitrification process promoted by nitrifying bacteria such as Nitrosomonas spp. and Nitrobacter spp. The highest concentrations of NH₄⁺-N were observed in static composting (ST). The statistical analyses showed a significant difference (p < 0.05) between MT and ST. This variance might be explained by the loss of volatilized NH₃ during the turning operations [65]. As expected, NO₃⁻-N levels without turning operations were significantly higher (p < 0.05) than those obtained in the MT strategy at the end of the composting process. These results are in accordance with the trends of these two mineral-N forms in other studies [54,66,67].

As presented in Table 2, across all examined mixtures, there was a reduction in the C/N ratio with an increase in composting time. For MIX1, this ratio decreased from 18.2 ± 0.78 to 11.6 ± 0.29 and 12.9 ± 0.64 in MT and ST strategies, respectively. Similarly, MIX2 decreased from 19.1 ± 0.15 to 13.9 ± 0.28 and 15.5 ± 0.21 under MT and ST conditions, with significant differences (p < 0.05) between both mixtures and strategies. This reduction in the C/N ratio primarily resulted from the conversion of organic carbon into CO₂ and CH₄ [68], coupled with an increase in total nitrogen content [69,70].

As described in previous works [9,29,71,72], BSG-derived compost is a rich nutrient source that can be used as a soil fertilizer, offering a sustainable alternative to chemical fertilizers. However, immature compost can negatively affect plant growth and seed germination, limiting its agricultural use [73,74,75,76,77]. To assess compost maturity, chemical indicators such as nitrification (NH₄⁺-N/NO₃⁻-N) [78] and the C/N ratio [79] are commonly used. For these parameters, values indicating good compost maturity are below 1.0 and 20, respectively [79]. In our study, at the end of the composting process (104 days), both values gradually decreased in all treatments and dropped below 0.50 and 15.5, respectively, confirming good compost maturation (see Table 2).

3.2. Greenhouse Gas Emissions

3.2.1. CO₂ Emissions

CO₂ emissions are representative of microbial activities [80]. As shown in Figure 4, in all treatments, CO₂ emissions rapidly increased within the first few days due to the rapid degradation of OM under high temperatures. More in detail, maximum emissions were observed of 2973 mg kg⁻¹ day⁻¹ in MIX2-ST on the 3rd day, 2366 mg kg⁻¹ day⁻¹ in MIX1-MT on the 14th day, 2171 mg kg⁻¹ day⁻¹ in MIX2-MT on the 3rd day, and 1937 mg kg⁻¹ day⁻¹ in MIX1-ST on the 3rd day. After the first peak (day 3rd), during the thermophilic phase, the CO₂ emissions declined due to the decreased availability of easily degradable OM [81]. However, after two weeks (day 14th), corresponding to the final part of the mesophilic phase and/or the beginning of the maturation phase, a second CO₂ peak was observed under all conditions. This could be due to the presence of newly easily available degradable OM for mesophilic microorganisms [53]. Considering the two composting strategies investigated, the second peak of CO₂ emissions was higher than the first in the MT system (for both MIXs), highlighting the positive effect of the turning operations. Conversely, in the ST strategy, the second peak was lower than the first, indicating a progressive decline in microbial activity, a more stable OM, and a reduction in O₂ availability due to the absence of turning operations [82]. Finally, the maturation phase is characterized by a lower CO₂ emission, with mean values well below 60 mg C kg⁻¹ d⁻¹.

3.2.2. CH₄ Emissions

There are several aspects that influence the production of CH₄, such as the composting approach, the technology system used (e.g., static or dynamic process, open or closed systems) [83,84], and the waste type and characteristics [85]. In this context, Szanto et al. [86] observed lower N₂O and CH₄ emissions in turned piles than in static systems. Instead, Ermolaev et al. [87] found that the emissions of CH₄ and N₂O were low independently of the level of ventilation. CH₄ is produced in anaerobic conditions by methanogenic bacteria. These conditions can be generated in anaerobic microsites that occur inside the piles with intense microbial activity and O₂ depletion and/or the compaction resulting from an excessive pile height [88]. Therefore, higher emissions of CH₄ may indicate inadequate aeration during composting and a different free-air porosity of raw materials [89]. In this investigation, CH₄ fluxes were predominantly negligible, with the highest measured value being 0.01 mg kg⁻¹ for MIX2-MT noted at the beginning of the composting process, at 10 and 14 days (Figure 5). Negative CH₄ fluxes (−0.002 mg kg⁻¹) were documented in certain instances, suggesting potential methanotrophic bacteria activity. Fukumoto et al. [88] noted that controlling the composting process with smaller piles could assist in reducing CH₄ emissions. The peak showed by MIX2–MT at the beginning of the composting process (day zero) can be attributed to the substantial trampling that the sheep manure underwent, leading to compaction and the formation of anaerobic conditions within the mixture. Moreover, the SM remained in the enclosure for a while before being collected; therefore, the composting process was likely already ongoing. The following peaks of MIX2 could be linked to the nature of sheep’s excrement, which is capable of preserving methanogenic microorganisms in comparison to pigs’ manure. These microorganisms are known to favor CH₄ production under anaerobic conditions in manure digesters [90]. Vice versa, negative values of emission fluxes, though negligible, can indicate methanotrophic activity, where bacteria use CH₄ as a carbon source, reducing emissions [76]. As expected, the CH₄ emission dynamics were also influenced by the turning operations. In particular, the CH₄ emission rate promptly declined after turning due to the improved aeration of the composting mixtures. However, during the period between turnings, the intense aerobic activity led to a reduction in oxygen concentration, creating conditions conducive to anaerobic processes and, consequently, CH₄ production [81]. Fukumoto et al. [88] demonstrated that the anaerobic section within the compost piles diminished and eventually vanished as the composting material matured.

3.2.3. N₂O Emissions

The complete nitrification/denitrification process typically converts NH₄ into N₂, a non-polluting gas [91]. On the contrary, as shown in Figure 6, the production of N₂O suggests that during BSG composting, there were incomplete nitrification/denitrification processes. More in detail, nitrification is an aerobic process that initially involves the conversion of NH₃ into nitrite (NO₂⁻) by NH₃-oxidizing bacteria like Nitrospira and Nitrosomonas [92]. This is followed by NO₂⁻ oxidation to NO₃⁻ by NO₂⁻-oxidizing bacteria, such as Nitrobacter [93]. On the other hand, denitrification is primarily responsible for N₂O production. It is an anoxic process facilitated by heterotrophic microorganisms, resulting in the reduction of NO₃⁻ to N₂, with N₂O acting as an intermediate product [94,95]. In conclusion, N₂O can be produced under both aerobic and anaerobic conditions [91,96]. As presented in Figure 6, the N₂O emission peaks were observed between day 12 and day 40, with average temperatures decreasing from 40 °C to 30 °C. Several authors [87,88,97,98,99] have explained the reason for this pattern in their studies: the microorganisms responsible for N₂O production are generally mesophilic and do not proliferate at high temperatures; thus, the emissions of this gas tend to increase after the end of the thermophilic phase. There are also authors [91,100,101,102], though, who reported the highest N₂O emissions in the first week of composting, when the temperatures inside the piles were higher. As suggested by Szanto et al. [86], the timeframe for N₂O emission depends on the specific microorganism involved, since there are methanotrophs capable of ammonium oxidation under thermophilic conditions. An alkaline pH, characteristic of MIX2 (Table 2), inhibits nitrification and promotes the transformation of NH₄⁺ into NH₃, which can be lost through volatilization, as reported by Cayuela et al. as well [103]. These factors likely contribute to the reduced N₂O emission by MIX2 in both composting strategies (Table 2). Moreover, the N₂O emissions are also related to the C/N ratio. In fact, the N₂O emissions are higher when the C/N ratio is lower than 17 [103]. Therefore, the C/N ratio of MIX2 (lower than that of MIX1) could contribute to the lower N₂O emissions of MIX2. In turned compost piles, improved aeration reduces N₂O emissions compared to unturned mixtures by preventing the formation of anaerobic zones. Conversely, in unturned piles, anaerobic conditions promote heterotrophic denitrification and NO₂⁻ accumulation, leading to N₂O production [104]. Enhanced oxygen availability in turned piles inhibits nitrifier denitrification and hydroxylamine oxidation, which are key pathways for N₂O emissions under aerobic conditions [105]. Additionally, aeration improves the physical structure of the compost, minimizing anoxic regions and supporting aerobic nitrification [106]. Overall, the N₂O emission trend reported in this study is similar to those described by other authors [81,88,97].

3.2.4. Cumulative Gas Emissions

Data on cumulative GHG emissions are shown in

Table 3. (a) Cumulative emissions of carbon dioxide (CO₂-C), methane (CH₄-C), and nitrous oxide (N₂O-N) at the end of the composting process and (b) emission factors (%). The reported values are means and standard deviations (SD) of the three replicates (n = 3) of each condition.

GHG	Unit	MIX1-MT	MIX2-MT	MIX1-ST	MIX2-ST
(a) Cumulative gas emissions
CO2-C	mg C kg−1 FM	34,275 ± 27.0 a	31,024 ± 62.8 b	28,769 ± 10.9 c	31,219 ± 24.6 b
CH4-C	mg C kg−1 FM	0.014 ± 0.00 c	0.070 ± 0.00 b	0.004 ± 0.00 d	0.086 ± 0.00 a
N2O-N	mg N kg−1 FM	168 ± 8.0 b	7 ± 0.1 d	231 ± 14.6 a	36 ± 1.9 c
(b) Emission factor *
CO2-C	(%)	3.87 ± 0.04 a	3.62 ± 0.03 b	3.25 ± 0.01 c	3.64 ± 0.03 b
CH4-C	(%)	(0.16 ± 0.01 c) 10−7	(0.82 ± 0.02 b) 10−7	(0.05 ± 0.02 d) 10−7	1.00 ± 0.03 a
N2O-N	(%)	0.595 ± 0.035 b	0.026 ± 0.001 d	0.816 ± 0.025 a	0.142 ± 0.003 c

FM: fresh matter. * Calculated as % from initial OM content for CO₂ and CH₄ and from TKN content for N₂O. ^a–d Different letters represent significant differences between conditions for each gas. Significant differences were determined by ANOVA and the Bonferroni post hoc test (p = 0.05).

(a). Due to the turning operations, MIX1-MT showed the highest CO₂ emission (34.3 g ± 0.03 C kg⁻¹ FM). Then follows MIX2, likely thanks to the higher presence of a bulking agent (wheat straw) that increases the free-air porosity and air storage useful for microbial activity [91,101], with 31.2 ± 0.02 and 31.0 ± 0.06 g C kg⁻¹ FM for MIX2-ST and MIX2-MT, respectively. The fact that both composting approaches for MIX2 presented comparable CO₂ emissions is probably linked to the characteristics of the SM. Both MT and ST have free-air porosity due to WS (lower emissions than MIX1-ST), but since the SM was collected after being trampled on by the livestock, it was compacted, leading to the formation of lumps in the compost and local anaerobic conditions that were not affected by the turning. Finally, the lowest emissions were recorded for MIX1-ST. The turning operation improved the blending of elements like oxygen, nitrogen, carbon, and water, and significantly increased (p < 0.05) the total CO₂ emissions observed during the 104-day trial in MIX1–MT compared to MIX1-ST. At the same time, no significant differences were found between the MT and ST strategies for MIX2. Cumulative CO₂ emissions (Figure 7a) were similar for all treatments, with the levels of CO₂ emission decreasing as the composting process progressed. Overall, the CO₂ cumulative emissions were 10 times lower than those reported by Williams et al. [102] with windrows and by Santos et al. [39] in vessels with food and green wastes. The curves showed two different phases of C mineralization: the first phase with a steeper slope, indicative of the rapid decomposition of the most easily biodegradable compounds and a high degree of microbial activity, while in the second phase, the CO₂ production stabilizes due to the presence of more resistant compounds, with a consequent reduction in microbial activity [62]. The cumulative emissions of CH₄ and N₂O are strongly linked to the type of manure. The composting bins with sheep’s manure presented a significantly higher production of CH₄ than those with pigs’ slurry (Figure 7b and Table 3a), while the results are reverted for N₂O (Figure 7c and Table 3a). For N₂O, the composting method influences the gas emissions, too. In fact, both mixtures showed a significantly higher cumulative production of N₂O with the ST approach. This result is in accordance with Szanto et al. [86].

3.2.5. Global Warming Potential (GWP)

The CO₂ emitted from composting processes is usually accounted for as neutral concerning global warming [107,108]. While CH₄ and N₂O are strong greenhouse gases, they are 27 and 273 times more potent than CO₂ over a 100-year time horizon, respectively. From the total emission of CH₄ and N₂O, we can estimate a CO₂ equivalent emission factor for the whole composting process of 45.8 ± 0.21 g CO₂ eq kg⁻¹ FM for MIX1-MT and 63.0 ± 0.39 g CO₂ eq kg⁻¹ FM for MIX1-ST (

Table 4. Global warming potentials (GWP, mg CO₂ eq kg⁻¹ FM) of the gasses emitted during the composting process. The reported values are means and standard deviations (SD) of the three replicates (n = 3) of each condition.

GWP	Unit	MIX1-MT	MIX2-MT	MIX1-ST	MIX2-ST
CH4	mg CO2 eq kg−1 FM	0.38 ± 0.0 c	1.90 ± 0.0 b	0.12 ± 0.0 d	2.32 ± 0.1 a
N2O	mg CO2 eq kg−1 FM	45,815 ± 211 b	1784 ± 15.7 d	62,950 ± 386 a	9875 ± 50.5 c
TOT	mg CO2 eq kg−1 FM	45,816 ± 211 b	1786 ± 15.7 d	62,950 ± 386 a	9877 ± 50.5 c

FM: fresh matter. ^a–d Different letters represent significant differences between conditions. Significant differences were determined by ANOVA and the Bonferroni post hoc test (p = 0.05).

). These values fall within the range (20 to 65 kg CO₂ eq Mg⁻¹) reported by Amlinger et al. [97] for an entire composting process of biowaste and green waste. The CO₂ equivalent emissions found for the MIX2 in both strategies were very low, equal to 1.8 ± 0.02 g CO₂ eq kg⁻¹ for MIX2-MT and 9.9 ± 0.05 g CO₂ eq kg⁻¹ FM for MIX2-ST (Table 4). Amlinger et al. [97] reported that values exceeding the range mentioned above (20 to 65 kg CO₂ eq Mg⁻¹) may indicate system mismanagement, such as an unbalanced initial mixture (excessive moisture, high available N sources, low C/N ratio, and air-filled pore space) or insufficient aeration and mechanical mass turning [97]. On the other hand, values below this range (20 to 65 kg CO₂ eq Mg⁻¹) suggest the presence of extreme and atypical conditions, such as the composting duration being too short, incomplete decomposition, or a very high C/N ratio [97]. Thus, the very low CO₂ equivalent emissions recorded for MIX2 with both strategies suggest that sheep manure is less suitable for composting BSG. This result is likely linked to the fact that the manures of pigs and sheep were collected and prepared differently for composting. Pigs’ slurry was collected continuously and underwent a solid/liquid separation process, and only the solid part was used. Sheep’s manure was collected altogether and after being trampled on and compacted by the livestock. This starting composting characteristics of MIX2 probably led to local (within the composting pile) anaerobic conditions.

4. Conclusions

The compost from BSG and sheep or pigs’ manure represents an effective strategy to manage and valorize breweries’ by-products, obtaining a soil amendment with a high nitrogen content. The emissions of CH₄ and N₂O from the composting process are affected by several factors, including the characteristics of the initial substrate, the bulking material, and the manure used. The initial characteristics of the by-product play a significant role in determining the degradation potential and consequent availability of C and N during the process. The proportion of the bulking agent in the mixture and the turning frequency of the pile were key factors determining the O₂ concentration inside the composting pile. A higher proportion of bulking agents and the mechanical turning of the mass promote aerobic conditions, which decrease CH₄ production. Moreover, the dimensions of the piles can also affect the GHG emissions; in fact, the disappearance of the anaerobic portions is indirectly proportional to the amount of mass being composted, meaning that smaller piles can facilitate more efficient decomposition and minimize anaerobic conditions, leading to reduced GHG emissions. Therefore, carrying out an easy-to-implement on-site composting process in small to medium breweries could lead to the valorization of agro-industrial waste (BSG) and, at the same time, minimize the production of greenhouse gases. Thanks to its physicochemical characteristics, the solid fraction of the pig slurry resulted in the most suitable manure for the composting process.