The brewing industry is characterized by the large production of by-products. Following the fundamentals of a circular economy, several attempts to recycle brewers’ spent grain (BSG) have been investigated. However, little information is available on its use for composting. Considering the main parameters required for optimal development of composting, the objective of the present review was to analyze the literature to determine whether the microbial and physicochemical characteristics of BSG make it suitable for direct composting. As the main factors in the composting process, we considered the BSG moisture content, total carbon, total nitrogen, C/N ratio, and pH. As described in the literature, the BSG moisture content, C/N ratio, and pH range from 70.6% to 81.3%, 7.1 to 26.5, and 3.8 to 6.9, respectively. This C/N ratio range is lower than the composting target range (20–30). Instead, the mean moisture content in the literature is higher than the 60% to 65% recommended for composting. Optimum pH for aerobic stabilization of compost ranges from 5.5 to 7.5, while the BSG pH in the literature is typically more acidic. Therefore, BSG is not suitable for direct composting. Addition of lignocellulosic bulking agents improves the reduction of moisture content during composting, while also optimizing the substrate properties, such as C/N ratio, air spaces, and pH, to positively affect the composting process. Moreover, livestock manure should be included as a starting material to promote the composting process. In this context, two hypothetical initial mixtures of BSG plus a lignocellulosic bulking agent and livestock manure are presented.

The main challenge of this work is to identify a novel approach to reuse and valorize brewers’ spent grain (BSG) to produce a new source of income for the brewers in terms of self-consumption or selling goods. Therefore, this study explored the composting behavior of BSG mixed with different organic materials: wheat straw with pig slurry solid fraction and wheat straw with sheep manure, MIX1 and MIX2, respectively. The composting process was carried out in bins by comparing two different composting strategies: manual turning (MT) and static composting xxx– without turning operations (ST). During the experimental trial, BSG mixtures were chemically analyzed for pH, total Kjeldahl nitrogen, ammoniacal nitrogen, nitrate-nitrogen, total organic carbon, volatile solids, carbon to nitrogen ratio, and moisture content. Furthermore, the final composted materials were evaluated according to the physicochemical and biological limits fixed by the European regulation (2019/1009) and the Italian law (D.Lgs 75/2010). At the end of the composting process, the C/N ratio ranged from 11.6 to 15.5, the humification ratio ranged from 12.4 to 13.8 and the NH₄⁺-N/NO₃⁻-N ratio was lower than 0.5 indicating, in all investigated treatments, a good degree of maturation. However, as evidenced by the high pH values and low Germination Index, the sheep manure, as starting material, proved less effective than the solid fraction of pig slurry, probably due to excessive trampling and slow litter change. Finally, concerning the two composting strategies investigated, the obtained results highlighted that the composting strategy did not affect the final compost quality.

Introduction: Brewers' spent grain (BSG) is the most important by-product of breweries. Currently, the primary method of recycling BSG is through its use in livestock feed production, due to its high fiber content, non-degradable protein, and water-soluble vitamins. Nonetheless, composting, vermicomposting, and biochar production pathways offer promising alternatives for managing this organic byproduct.Methods: BSG-based substrates were evaluated as bio-fertilizers for use during the transplantation of Lactuca sativa L. and without additional fertilization at the end of the crop cycle. Biochar (B), vermicompost (V), compost (CP), and compost enriched with a microbial consortium (CPE), each mixed at 2 and 4% with peat, were compared with two control treatments: 100% peat (C1) and peat with ternary chemical fertilizer (C2). The experiment was designed with six replicates, half of which were inoculated with Sclerotinia sclerotiorum mycelium during transplantation to assess the contribution of different substrates to pathogen resistance.Results: Overall, it was found that the control treatment C2 was the most effective, yielding the greatest plant height at the seedbed stage and the best results across all parameters at harvest. The V 4%, CP 4%, and CPE 4% treatments showed similar results without significant differences compared to the control. At the end of the seedbed phase, plants treated with CP 4% and CPE 4% exhibited the most substantial root development, with statistically significant differences from all other treatments. Moreover, the CPE 4% treatment demonstrated superior performance, resulting in statistically significant differences (p < 0.05) in the chlorophyll content (α and β) and carotenoid parameters when compared to the control treatment C2. The interaction between the treatment and the pathogen was statistically significant only for the carotenoid content in the CPE 4% treatment and for the flavonoid content in most of the treatments.Discussion: The most promising results were obtained at the nursery phase, highlighting the potential use of processed BSG-based substrates as bio-fertilizers. These products provided plants with both a growth substrate and nutrients, effectively contributing to waste recycling and aligning with the principles of a circular economy. Additional studies are required to investigate the potential use of brewer's spent grain as bio-fertilizers in other vegetable crop species.

Brewers’ spent grain (BSG) is primarily recycled as livestock feed due to its high fiber content, undegradable protein, and water-soluble vitamins. However, BSG composting represents a possible alternative to organic waste management. Adding a microbial consortium further enhances the agronomical properties of the compost intended for fertilizing applications. Microbial-based fertilizers (plant growth-promoting microorganisms, PGPM) are a means to mitigate the adverse environmental impacts of excessive or improper chemical fertilizer use, enhance the direct or indirect uptake of nutrients by plants, and add value to food waste. In a short-term pot experiment on iceberg lettuce (Lactuca sativa L.), this study assessed the effects of compost and pelletized compost from brewers’ spent grain, both enriched with a microbial consortium. In a randomized block experiment, this study compared four organic BSG fertilizers to chemical fertilizer (NPK) and an unfertilized control treatment. The investigation indicates that BSG compost and BSG pelleted compost, with and without bio-inoculum, in general, are comparable to mineral fertilizer treatment; lettuce fresh weight was higher in pots amended with bioprocessed BSG, associated with more significant growth of soil LAB, fungi, and actinomycetes. The investigation outcomes support composting as an alternative recycling process for producing PGM for agricultural applications.

Agricultural and livestock wastes are an important resource for the production of renewable fuels such as biomethane, and the effective management of the components of supply chain, such as available biomass feedstock, are critical to the transition to a low-carbon circular economy. Considering that more than half of the emissions (CO₂eq) generated in agriculture come from the use of fossil fuels to power tractors and other agricultural machinery, replacing diesel fuel in tractors with biofuels produced within the agricultural supply chain could contribute to greenhouse gas emissions reduction and to energy self-sufficiency for the local agricultural sectors. This study evaluated, at the regional level (Piedmont—north-west Italy), the potential production of biomethane from local agricultural wastes (crop residues and livestock manure), the potential energy supply to power tractors and the potential CO₂ emission reduction by replacing diesel fuel. Based on mean annual available agricultural wastes over the last seven years (2015–2021) in the Piedmont region, the annual potential biomethane yield of 910 × 10⁶ m³ year⁻¹ was estimated, equivalent to a thermal energy of 30.1 × 10⁹ MJ year⁻¹, which is 3.8 times higher than the energy requirements for the regional tractors’ fleet. The estimated potential CO₂ emission reduction is about 93.8 t of CO₂ year⁻¹, corresponding to 16.8% potential reduction. The study demonstrates the potential of local agriculture to replace diesel fuel with biomethane from waste to meet energy needs and carbon neutrality.

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.