Characterization and Sustainable Valorization of Brewers’ Spent Grain for Metal Ion and Organic Substance Removal

Abstract

Brewers’ spent grain (BSG) is the dominant solid side stream from wort separation, generating about 20 kg wet BSG per 100 L of beer and contributing hundreds of millions of tons annually worldwide, and thus a strategic feedstock for circular solutions in the brewing sector. This study situates BSG within that sustainability context and assesses its performance for removing metal ions and organic contaminants. A critical literature review with selected techniques (SEM, NIR/MIR, TGA) has been combined. SEM reveals a rough, fibrous–lamellar microtexture with high pore density, large pore-area fractions, submicron median equivalent diameters, and elevated edge density, consistent with accessible surface and mass-transfer pathways. Compiled adsorption evidence shows that raw and engineered BSG effectively capture diverse cations, including Cu(II), Cr(III/VI), Pb(II), Mn(II), U(VI) and selected rare-earth elements (REEs), demonstrable reusability, and fixed-bed breakthrough on the order of tens to hundreds of hours. Preservation options (drying, cooling/freezing, thermal inactivation, oxygen control) that enable safe storage and logistics for deployment have also been outlined. Overall, BSG emerges as a reliable, scalable biosorbent, with SEM-derived descriptors providing practical tools for performance prediction, while spectroscopic and thermal methods support material monitoring and process integration within a brewery’s circular economy.

1. Introduction

Industrial discharges containing toxic metal ions continue to threaten aquatic ecosystems and human health worldwide, prompting increasingly stringent regulatory limits and a search for cost-effective, sustainable treatment technologies [1]. Within this context, low-value lignocellulosic residues are attractive precursors for sorbents, because they combine abundant functional groups with favorable life-cycle footprints compared with many mineral or synthetic media [2,3]. Breweries generate one of the world’s largest food-sector side streams, brewers’ spent grain (BSG), which is a husk- and pericarp-rich residue produced during wort separation. BSG typically constitutes the majority of a brewery’s solid by-products [4,5] and has been repeatedly highlighted as a strategic feedstock for circular bioeconomy solutions owing to its volume, composition (cellulose/hemicellulose/lignin with proteins and phenolics), and year-round availability [1,2].

Over roughly two decades, the literature progressed from proof-of-concept batch tests on raw BSG to engineered, selective sorbents and continuous-flow demonstrations. Early studies reported uptake of Cu(II), Cd(II), Pb(II), Ni(II), and others by unmodified BSG, with equilibria frequently described by Langmuir and Freundlich isotherms and pseudo-second-order kinetics [3,6]. Subsequent strategies—citric-acid esterification, nitro-oxidation, ion-imprinting, and thermochemical conversion to biochars/activated carbons—improved capacity, selectivity, and in some cases regeneration, yielding materials that rival commercial carbons (e.g., for Cr(VI) via reduction–adsorption under acidic conditions) [7,8,9,10,11,12]. Parallel streams upcycle fiber-rich BSG fractions to cellulose adsorbents effective for Pb and Mn in real waters, aligning with food-sector circularity [13], while studies on organics (dyes, pharmaceuticals) demonstrate π–π interactions, hydrogen bonding, and electrostatics on BSG-derived surfaces [14].

Despite this progress, key issues remain. Feedstock variability (malt bill, mashing regime, lauter technology) induces compositional/microstructural differences, contributing to scatter in capacity and kinetics; reviews call for standardized reporting and pretreatment protocols [15,16]. Matrix effects in realistic waters (competing Ca²⁺/Mg²⁺, ionic strength) often depress performance versus single-solute tests, motivating application-relevant matrices [3,6]. Modification trade-offs are nontrivial: oxidation, amination, or high-temperature activation elevate performance but add cost and footprint; TEA/LCA are needed to ensure net sustainability [7,8,9]. Scale-up questions—bed design, breakthrough behavior, robust regeneration— are addressed by a smaller but growing set of fixed-bed studies and remain critical for translation beyond batch [12].

Within this context, a structured review of BSG-based metal sorbents with quantitative microscopy to bridge microstructure and performance is integrated. A reproducible SEM image-analysis pipeline (automated scale detection; pore segmentation and size statistics; edge/texture descriptors) to characterize surface features relevant to site density and mass-transfer accessibility is applied, and adsorption evidence across raw, modified, and carbonized BSG in light of those descriptors is synthesized. By linking texture properties to sorption behavior, practical predictors to guide material selection and low-energy pretreatments consistent with circularity goals are identified.

Practical implications include: (a) reporting standardized morphological metrics alongside sorption characteristics; (b) designing studies under real-matrix conditions (hardness, salinity); (c) integrating LCA/TEA with modification selection; (d) transitioning from batch to continuous-flow tests with breakthrough and regeneration; and (e) reporting $q_{max}$ together with specific preservation energy per tonne of BSG and simple TEA sensitivities (energy price, transport radius). These practices should improve reproducibility, lower implementation costs, and shorten the path to industrial deployment.

The aim of this work is to (i) situate BSG-based metal adsorption in a sustainability, process, and economic context; (ii) critically review performance and mechanisms across literature sources; and (iii) quantify SEM-derived texture parameters that rationalize adsorption trends.

2. Materials and Methods

This work followed a reproducible systematic-review workflow based on the PSALSAR framework (Protocol Search Appraisal Synthesis Analysis Report) as described by Mengist et al. [17,18] and reported. Where meta-analysis was not feasible, synthesis without meta-analysis (SWiM) reporting items were applied.

• Protocol

The review question, eligibility criteria, outcomes of interest, and analysis plan a priori were specified.

2.

Search (literature review)

ScienceDirect, Scopus, Wiley Online Library, and Taylor & Francis, combining controlled terms and free text related to brewers’ spent grain (BSG), adsorption/valorization, and microscopy were queried.

3.

Appraisal

After deduplication, records were screened by title/abstract and then full text against predefined inclusion/exclusion criteria (study design, analytes, matrices, comparators). Study quality and risk of bias were assessed using fit-for-purpose checklists.

4.

Synthesis

Data were extracted to structured tables; when studies were too heterogeneous for meta-analysis, structured narrative synthesis following SWiM items (grouping logic, standardized metrics, and certainty of evidence) was performed.

5.

Analysis

Adsorption isotherms, kinetics and mapped microstructural descriptors from SEM to performance where reported or computable were summarized.

6.

Report

Data collected from the literature were reported in the form of an internal report and the final version was presented in the manuscript.

SEM images were acquired in secondary-electron mode; samples were sputter-coated with a thin Au/Pd layer; accelerating voltage and working distance were kept constant across images. The SEM image analysis included the following steps:

(1)

Image preprocessing

• Conversion to grayscale and gentle CLAHE contrast normalization (Contrast Limited Adaptive Histogram Equalization, clip limit = 0.02) to equalize the lighting without excessively darkening the edges.
• Further steps were performed on an 8-bit image (0–255).

(2)

Scale calibration (µm/px)

• Automatic detection of the scale bar length in the bottom left corner:

  -

  cutting out the ROI (bottom ~25% of the height, left ~60% of the width),

  -

  brightness thresholding in the ROI (≥90th percentile), bright pixel mask,

  -

  morphological opening (disk = 1) to remove small disturbances,

  -

  in each line of the mask, the longest horizontal segment of “True” is searched for; its length in pixels is taken as the strip length.

• Conversion factor µm/px = 2 µm / strip length px.
• The following parameters are determined from the converter: FOV (Field of View) width µm, FOV height µm, FOV area µm².

(3)

Pore segmentation (dark according to SEM)

• A slight Gaussian blur (σ ≈ 1.0) for high-frequency noise reduction.
• Global Otsu thresholding on a blurred image.
• Pores operationally defined as pixels below the threshold (darker areas/cavities).
• Morphological opening (disk = 1) with the removal of very small objects (min size = 20 px) to filter out noise.
• Component labeling and object area determination (in px²).
• For each object, the equivalent diameter (the diameter of a circle with the same area) was calculated according to Equation (1):

$$d_{eq}=2\sqrt{{\displaystyle \frac{A}{\pi }}}$$

(1)

where A—object area.

Then, pixels are converted to micrometers by dividing by micrometers per pixel.

• Pore density scaled to 1000 µm² was calculated according to Equation (2):

$$density={\displaystyle \frac{pore count}{FOV area {\mu m}^2}}\times {10}^3$$

(2)

(4)

Texture and edges

• Edge density—the fraction of pixels classified as edges by the Canny detector (σ = 1.5; other parameters are the library defaults). Interpretively: a synthetic image roughness/edginess indicator.

(5)

Visual quality control

• For each sample, an overlay with pore boundaries (outlined by mask dilation and XOR) and a page with the detected scale bar (highlighted in green in the SEM images) were generated to verify the accuracy of the calibration and segmentation.

(6)

Assumptions and limitations of the analysis method

• Pores are an operational definition (dark areas on SEM); depending on the contrast/SEM settings, they may correspond to depressions, cavities, or topographic shadows.
• Equivalent diameter assumes a circular equivalent; for elongated objects, it provides a conventional measure (good for comparisons, not for shape metrology).
• Edge density also depends on imaging parameters (sharpness/saturation); it is worth comparing images with similar settings.
• Calibration assumes the scale bar is a uniform, horizontal bar in the bottom left corner; unusual positioning/graphics may require manual adjustment.

These SEM procedures constitute the analysis step within PSALSAR framework.

3. The Development of the Brewing Industry: From Fermentation to Global Production

Brewing is among the oldest food processes, shaping technology, economy, and culture from the Neolithic to modern times. Early evidence from Jiahu (7000–9000 BCE) indicates fermented beverages of rice, honey, and fruits [19]. In Mesopotamia and ancient Egypt, beer had dietary and economic roles, as a medium of exchange, export, and taxed commodity [20]. In Greece and Rome it was associated with lower social strata, while monastic breweries of medieval Northern Europe professionalized production; German monks introduced hops, extending shelf life and enabling broader trade [21,22,23].

The 19th century catalyzed industrialization through pasteurization, cooling, controlled fermentation, steam power, and rail logistics, delivering consistent quality and mass distribution [23,24]. The 20th century brought mature “beer engineering”: stainless steel and glass-lined equipment, and widespread adoption of cylindrical-conical fermenters (CCTs) reduced contamination, improved safety, and compressed production cycles [24]. Advances in yeast genetics and biochemistry enabled selection for aroma, alcohol tolerance, and low off-flavor formation; aseptic propagation and cryopreservation supported robust operations [25,26]. Since the late 20th/early 21st century, sustainability and circularity have become central, with best-practice water use below ~3 hl/hl, heat and CO₂ recovery, valorization of spent grains/yeast, and advanced wastewater treatment [1]. Data analytics integrated quality monitoring with supply chains and sales, shortening time-to-market; current innovation directions include de-alcoholization with aroma retention, preservative-free stability, and sensory profiling via hop/yeast biotransformation [27,28,29].

From fermentation to global production, the industrial scale of brewing creates a concentrated, compositionally consistent lignocellulosic by-product brewers’ spent grain (BSG). BSG is generated at multi-million-ton annual scales, making it a candidate for circular strategies (from stabilization to valorization). In this manuscript historical and technological drivers to practice are linked: the characterization of BSG using various techniques (SEM-based surface microtexture, MIR/NIR, and TGA) that are directly informative for adsorption design was performed, and utilization pathways relevant to removal of metal ions and organic substances was operationalized.

Structurally, BSG inherits a layered, fibrous wall texture from the barley husk/pericarp and detailed SEM-based descriptions are reported in Section 10.

4. Economic Approach

The modern brewing industry operates in conditions of high competition, globalization, and increasing diversity in consumer preferences. From an economic perspective, there is a strong market concentration, a few of the largest companies, such as AB InBev, Heineken, Carlsberg, and Molson Coors, account for the majority of global beer production, which indicates a classic tendency toward oligopolization in the consumer goods sector [27]. Capital consolidation allows corporations to achieve economies of scale, both in terms of raw material purchases and logistics, which contributes to reducing unit costs and strengthening their market position [24].

Simultaneously, the craft and premium beer segments are growing in importance, playing a key role in the process of product differentiation and value creation. Economists point out that the development of the craft market is an example of niche competition within an oligopolistic global structure, responding to consumer demand for quality, locality, and innovation [28,30]. Craft breweries thus contribute to the growth of innovation dynamics in the sector, also forcing global corporations to invest in new styles, flavor variations, and non-alcoholic beers [29].

The modern economic approach in brewing also includes sustainability issues. Effective management of water and energy resources, as well as recycling by-products, are becoming not only an environmental requirement but also an element of competitive advantage. Analyzes indicate that companies investing in sustainable strategies generate long-term cost savings, improve brand reputation, and attract investors more easily [1].

Within this market structure, the variables that matter for this study are those that govern BSG supply: volumes, composition variability, and collection/logistics pathways. Consolidation concentrates steady, high-throughput streams, while craft growth increases geographic dispersion and seasonal variability; both trends intersect with circular-economy imperatives (heat/CO₂ recovery, wastewater treatment, and by-product utilization) that increasingly prioritize spent-grain valorization [1,24,27,28,29,30]. In the following sections characterization that informs adsorbent design and sustainable valorization routes relevant to removing metal ions and organic contaminants from water are focused on.

An additional dimension is the globalization of beer trade. International corporations utilize exchange rate differences, access to cheaper raw material markets, and expansion into emerging markets to diversify their revenue portfolio and minimize macroeconomic risk. At the same time, the ongoing digitalization and the use of data analytics support the process of supply chain optimization, demand forecasting, and product innovation [24,29].

Among the global corporations in the brewing industry, one can distinguish Anheuser-Busch InBev (AB InBev), which remains the world’s largest beer producer, with an annual production exceeding 490 million hectoliters and a portfolio of over 600 brands present in nearly 150 countries [31]. Among them, the Corona brand gained particular significance, being recognized as the most valuable global beer brand in 2025 according to the Kantar BrandZ ranking, primarily owing to its non-alcoholic variant, Corona Cero, and extensive sponsorship activities [32].

The next largest corporation in terms of scale is Heineken N.V., producing over 240 million hectoliters annually and present in over 190 markets. Heineken, the flagship brand, is one of the most recognized premium beers in the world and is among the top beer brands in terms of value [33]. Third place goes to China Resources Snow Breweries, whose production exceeds 100 million hectoliters annually and which dominates the Chinese largest beer market [34]. In fourth place is the Carlsberg Group, with an annual production of over 100 million hectoliters and brands such as Carlsberg, Tuborg, Kronenbourg 1664, and Grimbergen [35]. Among the global leaders, Molson Coors Beverage Company, formed by the merger of Molson and Coors and the acquisition of Miller Brewing, should also be mentioned. Her portfolio includes brands like Coors Light, Molson Canadian, and Miller Lite, which play a significant role in the North American market [36]. In Asia, Tsingtao Brewery is significant, with its flagship brand Tsingtao being one of the most consumed Chinese beers worldwide [34].

Constellation Brands, the largest beer importer in the USA, plays a significant role in the American market. This company’s portfolio includes the Modelo brand, which became the best-selling beer in the American market in 2023, demonstrating the growing importance of premium brands and the influence of Mexican traditions on the global beer landscape [37].

Due to the increased activity of multinational beer companies purchasing existing breweries, building new facilities in emerging markets, and licensing production of their brands outside of their home countries, the pace of globalization for beer has significantly accelerated in recent years. The demand for these products has increased in developing nations as living standards and incomes have changed. With revenues of almost 59.4 billion USD in 2024, Anheuser-Busch InBev was the biggest beer company in the world, as shown in Figure 1. With $32.8 billion in sales, Heineken Holding took the second place [38].

In terms of beer production volume in 2023, China was the top producer, as shown in Figure 2. China brewed about 359.08 million hectoliters of beer that year. With 193.03 million hectoliters of beer produced that year, the United States came in second [39].

In Figure 3 the statistic depicts the global beer production trend from 1998 to 2024. Beer output increased from 1.3 billion hectoliters in 1998 to around 1.88 billion hectoliters in 2024. The primary constituents of beer, which is a popular beverage worldwide, are water, malt, hops, and yeast [40,41].

5. The Brewing Process

The brewing process consists of unit operations that convert cereal raw materials into a sensorially stable beverage: mashing (enzymatic hydrolysis of starch and proteins at elevated temperature), wort separation from the solid phase, wort boiling with the addition of hops, hot wort clarification and cooling, aeration and fermentation carried out by brewing yeast, followed by conditioning, maturation, optional filtration, colloidal stabilization, and packaging. These findings are consistent with the microbiological perspective of the process (the role of malt and yeast enzymes, fermentation management, and diacetyl reduction) and with process engineering analyzes in brewing (the selection of mashing, wort boiling, cooling, and packaging technologies in the context of energy balances) [17,42].

Operationally, brewery sustainability converges on resource efficiency (water/energy) and by-product valorization. Because BSG is the dominant solid residue, it is the most direct target for circular strategies that create local value chains and lower treatment loads. In this manuscript, sustainability is made actionable by (i) characterizing BSG with image-based and spectroscopic techniques that correlate with adsorption-relevant surface features, and (ii) mapping valorization pathways into adsorbents suited for the removal of metal ions and organic substances—consistent with current literature and brewery practice [43,44,45,46,47,48,49,50].

In malting (outside the scope of the brewhouse itself, but crucial for later stages), the grain is soaked to a moisture content of approximately 42–48%, which initiates germination and the activation of the enzymatic system. This is followed by drying (kilning), and the malt is stabilized while preserving the activity of amylases and proteases. In the brewhouse, the malt (and any unmalted raw materials) is milled and mixed with water, undergoing mashing in temperature profiles chosen for extraction purposes (conversion of starch to fermentable sugars, protein modification). After the conversion is complete, the wort is separated (in a lauter tun or mash filter), which removes the solid phase in the form of brewers’ spent grains (BSG) [4].

Boiling ensures wort sterilization, isomerization of α-acids, coagulation and precipitation of proteins (the so-called hot break), and evaporation of undesirable precursors of off-flavors (e.g., DMS). Whirlpooling/trub separation after boiling and rapid cooling limit oxidation, while aeration prepares the wort for fermentation. The mechanisms of these operations and their significance for microbiology and beer stability are described in the literature [13].

In fermentation, yeast converts sugars into ethanol and CO₂, simultaneously synthesizing selected higher alcohols and esters. Inoculation parameters, temperature, and oxygen management determine the aromatic profile. After primary fermentation, the beer matures (diacetyl reduction, cold clarification), and can then be filtered, stabilized, and packaged. An overview indicates that the selection of technology and conditions at these stages shape both the quality and energy consumption of the process [13,40].

After boiling with hops and clarification (whirlpool), the wort is rapidly cooled in a plate heat exchanger (PHE) and then directed to the fermentor. The hygiene and energy consumption of this section depend on a properly designed Clean-in-Place (CIP) procedure, particularly the pre-rinsing stage, which aims to wet and initially loosen deposits before cleaning with alkali or acid. Modeling and experimental results with PHEs contaminated with protein deposits indicate that the cleaning effect depends mainly on temperature and flow velocity, while time is of secondary importance and primarily improves the effect at low flow rates. Pre-rinsing efficacy in PHEs depends mainly on temperature and velocity. Temperature ~45 °C and the highest feasible plate-gap velocity gave the best cleanliness, while overly high temperatures can fix protein deposits; extending time helps chiefly at low velocities [42].

BSG is the main solid byproduct of brewing, which is produced during wort separation (after mashing) as an insoluble residue of husks and seed coat fragments with a small portion of the endosperm. According to the literature, BSG accounts for approximately 85% of all solid by-products in a brewery, which quantitatively corresponds to about 20 kg of wet BSG per 100 L of brewed beer (this value refers to the mass ratio to the malt input) [41,43].

Although BSG is formed earlier (during wort filtration), CIP PHE does not affect its generation. However, optimizing the pre-rinse directly supports the subsequent steps in the brewing process: stable wort cooling, microbiological safety during transfer to the fermentor, and lower water/energy and chemical consumption throughout the entire CIP cycle. This understanding of integrating cleaning activities with the stages of the process completes the sustainability loop in the brewery without interfering with the BSG production stage itself [42].

BSG is a lignocellulosic material and, in terms of its composition, contains cellulose, non-cellulosic polysaccharides (mainly arabinoxylans), and lignin, as well as a significant proportion of proteins and phenolic compounds. The moisture content of fresh BSG after wort filtration is approximately 70–80% (w/w). This characteristic, along with the heterogeneity resulting from the malt-raw material recipes and mashing conditions, determines the processability and valorization directions [41,43].

BSG contains a stable microflora (mainly aerobic thermophilic bacteria) and has a limited shelf life due to its high moisture content, which implies the need for rapid collection, preservation (e.g., drying, acidification, bioconversion). At the same time, comparative studies published in the literature from many breweries confirm the variability of BSG’s chemical and microbiological composition, depending on the raw materials and technology used [44].

Technological choices in the brewhouse (mashing and boiling profiles, separation, cooling, and stabilization systems) impact both the energy balance and the nature of by-products (including BSG), reflecting the growing emphasis on energy efficiency and the circular economy in brewing. Overview reports indicate that the largest breweries consume an average of approximately 39 kWh per hectoliter, with thermal energy being dominant. Therefore, energy integration and the utilization of BSG (for food, feed, and bioenergy purposes) are important directions for modernization [40,45].

6. Towards Sustainable Brewing

Brewing sustainability increasingly integrates environmental performance with circular management of by-products (Figure 4). Core measures include lowering the carbon footprint through renewable energy and optimized logistics/vehicle fleets [43,44,45,46] and deploying automation/AI/IoT for real-time control of water, energy, and raw-material use, which reduces waste and operating costs [47,48]. Within this framework, BSG utilization is a key circular lever, linking process efficiency with resource recovery and valorization pathways developed in the next section.

Beer production is highly energy-intensive, involving steps such as mashing, wort boiling, cooling, and packaging. Many studies have shown that implementing waste heat recovery technologies, using heat pumps, and employing high-efficiency cooling systems can reduce energy consumption by up to 30% [51,52]. Another significant direction is the recovery of carbon dioxide produced during fermentation and its reuse in beer carbonation, which not only reduces emissions but also lowers production costs [53,54].

An important issue is water management and wastewater reuse. Brewery wastewater, if untreated, can be a significant environmental pollutant. Water remains a key resource in beer production, and the average water consumption rate in modern breweries has decreased from 5 to 7 hl of water/hl of beer to below 3 hl/hl, and in best practices, even to 2.5 hl/hl [1,55]. Membrane technology, sequencing batch reactors (SBRs), and nitrification-denitrification systems enable the treatment of brewery wastewater to a level suitable for reuse in technological processes or agriculture. This is particularly important in areas where water scarcity is a concern, and it helps to reduce the environmental footprint of brewing [56,57].

A significant potential lies in the utilization of by-products, spent grain, spent hops, and yeast, which can serve as valuable raw materials for feed production, biogas, and protein supplements [4]. Many breweries are implementing the “zero waste brewery” concept, which aims for full integration of waste management within a circular model. They are adopting regenerative agricultural practices that improve soil health and reduce dependence on synthetic fertilizers and pesticides [58,59,60]. Furthermore, sourcing ingredients locally reduces transportation emissions and supports regional economies, contributing to a more resilient supply chain [43].

Another aspect is packaging, which is a major source of brewery waste. Breweries increasingly adopt lower-impact options, that is, less plastic, recyclable/biodegradable materials, and refillable bottles/kegs, reducing environmental footprints while meeting consumer demand for eco-friendly packaging [44].

It is essential for breweries to recognize the importance of reducing their carbon footprint. Measures include transitioning to renewable energy sources, such as wind and solar, to power production processes. In addition, breweries are focusing on reducing the carbon footprint associated with their distribution networks by optimizing delivery routes and using electric or low-emission vehicles [43,45]. Some companies are even offsetting their emissions through environmental projects, like tree planting or renewable energy investments [46].

Furthermore, technological innovations are playing a crucial role in promoting sustainability in the brewing industry. For instance, the adoption of automation, artificial intelligence (AI), and the Internet of Things (IoT) allows for real-time monitoring of resources like water, energy, and raw materials, optimizing their usage. These technologies help breweries reduce waste, increase efficiency, and lower operational costs [47,48].

Another aspect is that sustainability is not only about environmental and economic factors but also involves social responsibility. Many breweries are increasingly focused on community engagement, supporting local causes, providing fair wages and working conditions, and improving the quality of life for employees [49]. Additionally, the brewing industry is taking steps to reduce alcohol-related harm through awareness campaigns and by offering low- or non-alcoholic alternatives [50].

These circular strategies, by focusing on BSG stabilization and utilization pathways directly relevant to removing metal ions and organic substances, are discussed in the next sections.

7. Utilization of Brewers’ Spent Grain (BSG): Current Applications and Future Potential

Brewers’ spent grain (BSG) is one of the main by-products generated during beer production. It is composed mainly of insoluble cellulose fibers, proteins, lignin, and carbohydrates, making it a valuable source of raw materials for use in various industrial sectors. According to estimates, BSG constitutes the majority of solid waste generated by breweries, with a global mass of hundreds of millions of tons annually [4]. These wastes pose a significant challenge for the brewing industry, but they also offer a wide range of utilization possibilities, contributing to sustainable development in brewing. This chapter will present the current and potential applications of BSG in various industries, such as biotechnology, food processing, energy production, composite materials, and bioplastics. This section operationalizes the utilization pathways introduced in Section 3.

7.1. Biorefining Utilization of BSG

Due to its high cellulose content, BSG is a raw material with great potential for biofuel production. It can be used for bioethanol production through fermentation processes, utilizing cellulolytic enzymes to break down cellulose into fermentable sugars. Research has shown that enzymatic hydrolysis processes can effectively convert BSG into bioethanol, which can be used both as a fuel and as an additive to fossil fuels [61]. Additionally, BSG can be used in biogas production due to its high fiber content, which is broken down by microorganisms in an anaerobic process. Biogas can be used to produce electricity or heat [62].

BSG can also be used in the production of other biochemicals, such as organic acids (e.g., acetic acid, lactic acid) and various enzymes, which can be used in the food or cosmetic industries. BSG fermentation allows for the acquisition of bioactivity, which increases its value as a raw material in the biotechnology industry [63,64,65,66]. The possibility of producing biochemicals from BSG can help increase the efficiency of utilizing this material within a circular economy.

7.2. Application of BSG in the Food Industry

BSG is a valuable source of dietary fiber and protein that can be used in the production of functional foods. BSG flour, which is produced by appropriate drying and milling of the hammer mill, can be used in the production of bread, cookies, bars, and other baked goods. The fiber content in such products improves their health properties, aiding digestion and preventing heart disease. BSG also has a high protein content, making it an ideal ingredient for protein products such as protein powders and dietary supplements. The use of BSG in the production of functional foods is gaining importance, especially in the context of the growing demand for health and dietary products [67,68].

BSG can also be used in fermentation processes where microorganisms, such as lactic acid bacteria, can transform BSG into bioactive substances like organic acids, amino acids, and peptides, which have antioxidant, anti-inflammatory, and antimicrobial properties. These bioactive compounds are used in the production of dietary supplements, functional foods, and cosmetics [65,66,69].

7.3. Using BSG for Energy Production

One of the most popular ways to utilize BSG for energy production is to convert it into biogas through anaerobic digestion. Studies have shown that BSG, both in its raw form and after pre-processing, can be effectively used for biogas production. For example, in research conducted by Di Mario et al. (2024), it was found that raw BSG and its mixture with olive mill wastewater (OMWW) yielded the highest biogas production, reaching 1075.6 mL and 1130.1 mL, respectively, under laboratory conditions. Additionally, the use of pre-biorefining processes, such as protein hydrolysate extraction, can improve biogas production efficiency. For example, the residues after protein extraction from BSG showed better energy efficiency, achieving a positive energy balance of 5.36 kJ [70].

Pyrolysis of brewers’ spent grain (BSG) is a promising method for converting this waste into a valuable product—biochar. Biochar, a solid product of pyrolysis, is used in various fields such as energy, agriculture, and environmental remediation. BSG pyrolysis involves the thermal decomposition of biomass under anaerobic conditions, resulting in the formation of three main products: biochar, bio-oil, and syngas. Process parameters such as pyrolysis temperature, reaction time, and heating rate have a significant impact on the composition and properties of these products. Research conducted by Zabaleta et al. (2024) showed that pyrolysis of BSG at temperatures of 673 K, 773 K, and 873 K resulted in biochar with a high carbon content (over 62%) and a high calorific value (over 23 MJ/kg). These results suggest that BSG can be effectively used to produce biochar, which can serve as a biofuel or a soil amendment material. The properties of biochar obtained from BSG depend on the pyrolysis parameters. It was found that biochar obtained at 673 K had the best energy properties, making it a suitable candidate for use as a biofuel. In contrast, biochar obtained at higher temperatures (773 K and 873 K) showed a beneficial effect on lentil seed germination, suggesting its potential application in agriculture as a soil amendment [71]. Other studies, such as the work by Świechowski et al. (2023), also confirm the positive effects of using BSG biochar in improving soil quality and increasing crop yields [59].

Biochar obtained from BSG can be used in various fields, such as energy, agriculture, or environmental remediation. Because of its high calorific value, biochar can be used as a biofuel in industrial boilers or in cogeneration processes. As a soil amendment, biochar can increase water retention, improve soil structure, and provide nutrients for plants. Due to its adsorption properties, biochar can be used to remove pollutants from groundwater or wastewater. An example is the use of biochar from BSG in heavy metal water purification, where it has been shown to be highly effective in removing pollutants such as lead and cadmium [72,73,74].

Another method of recovering energy from BSG is to burn it or co-burn it with other fuels, such as lignite. Research conducted by Vasileiadou (2024) has shown that BSG can be effectively used as a solid fuel, and co-firing it with lignite improves fuel quality, increasing the energy efficiency of the combustion process. Additionally, burning BSG in industrial furnaces can contribute to reducing greenhouse gas emissions, provided the process parameters and flue gas treatment technology are appropriately selected [75].

BSG can also be used in cogeneration systems, which allow for the simultaneous production of electricity and heat. The use of BSG in such systems allows for the effective utilization of brewery waste and the improvement of energy efficiency in industrial plants. An example is the application of BDI BioGas technology, which allows for the integration of BSG fermentation processes with existing brewery infrastructure, enabling sustainable and economical energy recovery from waste [76,77]. An example of a practical application is the BrewDog company in the UK. BrewDog has invested in a biogas plant that processes brewing waste, which powers its beer production and delivery fleet. This project aims to reduce CO₂ emissions by 7500 tons annually [78]. Meanwhile, the oldest brewery in the USA, Yuengling Brewery, has installed a 400 kW biogas cogeneration system, which covers approximately 20% of the plant’s electricity demand, contributing to significant cost savings [79]. Simultaneously, the installation of cogeneration systems at two Scottish & Newcastle breweries in the UK has allowed for the use of BSG and local wood shavings to produce steam and electricity, covering approximately 60% of their electricity needs and 100% of their heat needs [80].

In conclusion, the use of brewers’ spent grain (BSG) for energy production is a promising strategy toward the sustainability of the brewing industry. Owing to various processing methods such as anaerobic fermentation, pyrolysis, combustion, or integration with cogeneration systems, BSG can be effectively utilized for the production of biogas, biochar, electricity, and heat. Further research and development of BSG processing technology are essential to fully utilize its potential for energy production.

8. Methods for BSG Preservation and Storage

The preservation of Brewers’ Spent Grains (BSG) is essential for extending its shelf life and enhancing its potential applications. This by-product, with its high moisture content, is prone to microbial spoilage, making preservation techniques crucial. Several methods, both chemical and physical, have been investigated for effective storage and stabilization of BSG to ensure its safety and usability in food, feed, and other value-added applications.

8.1. Chemical Methods

An effective method for chemically stabilizing BSG is to lower the pH using organic acids (mainly formic and propionic acids) and additives based on them (e.g., sodium formate or calcium propionate). Acidification limits the growth of yeasts, molds, and proteolytic bacteria, reduces dry matter losses, and inhibits protein breakdown (decrease in NH₃-N). In a study on wet brewer’s grains, sodium formate (3 g/kg fresh weight) was more effective than calcium propionate (3 g/kg) in improving fermentation quality and the microbial profile during 20 days of storage (higher lactic acid content, lower pH, reduction of Clostridium) [81].

Mixtures of formic (FA) and propionic acids (PA) combine a rapid, strong pH reduction (FA role) with the long-lasting fungistatic effect of propionates (PA role). Consequently, they inhibit yeast and mold growth, limit the activity of butyric acid and proteolytic bacteria, and reduce dry matter (DM) losses and ammonia (NH₃-N) accumulation. Literature highlights that for wet brewers’ grains (WBG), higher doses of the mixture are particularly effective in preventing microbial contamination and nutritional degradation during short-term storage [82].

It was shown that a formic-propionic acid mixture at a level of 0.75% fresh weight provides significant protection for WBG against contamination and component degradation compared to lower doses; at the same time, doses of 0.50–0.75% of formic acid alone and approximately 0.40% of propionic acid alone also improve stability, but the highest short-term effectiveness was achieved with the higher dose of the formic-propionic acid mixture. This position is based on a synthesis of results from practical and controlled experiments, including field and pilot-scale studies [82].

Many WBG experiments test commercial propionic acid-based preparations (often multi-component, sometimes also containing other acids), and their findings are useful for planning the supply of FA + PA mixtures. Specifically, even mixing with the mass usually provides a more consistent improvement in the fermentation profile and lower DM losses than surface application, especially in the first 14 days. After 28 days, both methods are able to reduce losses compared to the control. In practice, this means that if we are using FA + PA mixtures, it is worth striving for the most homogeneous distribution of the agent within the material possible [83].

In studies on WBG preservation, it has been shown that acid additives (including propionic acid preparations) lower the pH, modify the volatile fatty acid profile, and reduce total DM losses compared to a control without additives. For propionic acid preparations themselves, significant benefits have also been demonstrated under summer conditions and differences between application strategies; from the perspective of FA + PA mixtures, this suggests that the formic acid component can further accelerate acidification and enhance the antifungal effect of PA, which confirms the logic of using mixtures in WBG with very high humidity [83,84,85].

As a cheaper preservative, salt (NaCl) is sometimes used, which acts osmotically and can inhibit some microorganisms, but in studies on WBG, its effectiveness was lower than acidic additives, especially during longer storage. Therefore, salt is considered more as an auxiliary additive rather than a substitute for acidification. On the other hand, bioconservation (e.g., inoculation with lactic acid or propionic acid bacteria) also lowers the pH and generates organic acids in situ. Fermentation of the liquid fraction of BSG to propionic and acetic acid increased antimicrobial activity and stability, but requires process control [82,86].

In conclusion, it should be emphasized that the short shelf life of BSG is a bottleneck in logistics, and acidification remains a cornerstone of stabilization practices alongside physical methods (drying, pressing, ensiling). Chemical methods are often combined with others (e.g., rapid draining and immediate acidification), which improves microbiological safety and reduces nutrient losses before further use (as feed or in biotechnology).

8.2. Physical Methods

Dehydration (drying) is the most commonly used method for storing and preserving BSG due to its high efficiency, long-term storage capability, and wide range of potential further uses. BSG contains about 70–80% water, making it susceptible to spoilage and microbial growth. Dehydration reduces the moisture content to below 10%, which prevents the growth of bacteria and mold, allows for long-term storage and transport, reduces the volume of the material, and enables further processing and use in various industries. One of the traditional methods is drum drying (rotary), which involves mechanically removing water from the BSG and then drying it in a drum oven. This process is energy-intensive, but effective in removing moisture and stabilizing the product. A modern technique is microwave vacuum drying (VMD), which uses microwaves in a vacuum to rapidly remove water. It is characterized by a shorter drying time and better retention of BSG’s nutritional value. An innovative method that uses superconducting water vapor to effectively remove moisture. It allows for a reduction in energy consumption and an improvement in process efficiency. Furthermore, the steam generated from the BSG can be reused in other processes. Another method is convective drying, during which air of a specific temperature and velocity flows through the BSG, removing moisture. Studies have shown that a temperature of 50 °C and an air velocity of 1.6 m/s are optimal for effective BSG drying [87,88,89].

In summary, the benefits of BSG dewatering include long-term storage without spoilage risk, versatile use as animal feed, a food ingredient for humans, a raw material for biofuel production or packaging material, as well as reducing waste in breweries. As a consequence of various drying methods, breweries can effectively manage this valuable raw material, contributing to sustainable development.

One technique used is lowering the temperature, which slows down the metabolism of microorganisms and endogenous enzymes. It was noted that during storage of wet BSG at 4 °C (over 16 days), the number of aerobic bacteria remained below 10⁶ CFU/g; no microbial activity was observed in frozen samples. At the same time, both 4 °C and 20 °C promote sugar utilization over time, which has been attributed to enzyme activity (e.g., xylanases, esterases, cellulases). It can be concluded that cooling/freezing extends the microbiological safety of wet material but does not completely stop biochemical changes. Therefore, as a standalone method, it is most useful for short logistics chains [82,90].

Autoclaving (at approximately 120 °C for 1 h and frozen) effectively sterilizes BSG and allows for long-term, safe storage of the material for research purposes or technologies requiring a sterile medium. However, this is an energy-intensive process and not always practical on an industrial scale. Milder alternatives are being considered, such as heating/pasteurization and newer volumetric heating techniques (e.g., microwave heating, as well as ohmic heating in fermentation substrate applications), which allow for reducing the thermal load while maintaining the effect of inactivating the microflora [82,89,91].

Changing the gas conditions is a physical method of modulating the microflora during storage. Controlled industrial studies have shown that aerobic storage of BSG promotes fungal invasion, including potential aflatoxin producers (Aspergillus flavus), while anaerobic storage limits mold growth but can lead to the proliferation of lactic acid bacteria and Bacillus. From this, it can be inferred that oxygen reduction techniques (e.g., vacuum or modified atmospheres) can reduce the risk of fungal contamination, but the bacterial microflora should be monitored simultaneously, especially during longer storage times [92].

In conclusion, it should be stated that in a technological context, drying (ranging from hot air, superheated steam, to VMD/lyophilization) is an effective and practical physical technique for BSG stabilization, as it reduces water activity and simplifies logistics. Drying ensures stable, long-range logistics but raises energy demand; cooling/freezing suits short chains and should be weighed against biochemical drift and local energy mix. For short supply chains, cooling/freezing effectively extends the microbiological safety of wet BSG, although it does not completely stop enzymatic changes. Thermal inactivation (including autoclaving) provides the highest level of microbiological safety at the expense of energy and possible functional changes. Oxygen control, on the other hand, can be a useful supplement, but it requires conscious management of the microflora profile.

8.3. Ensiling Methods

One method of preservation through ensiling is mixing BSG with dry, starch-rich components (e.g., dry cornmeal) to increase DM and provide sugars for lactic acid bacteria. In wet brewers’ grains (WBG) studies, the addition of dry, ground corn improved the fermentation profile, reduced DM losses, and increased aerobic stability compared to BSG ensiled without additives. This effect was confirmed in both controlled trials and experiments with varying proportions of grain supplement. Co-ensiling BSG with fibrous materials (e.g., corn stalks) can also improve fermentation quality, especially when combined with microbiome modification through biological additives [85,93].

The use of bacterial inoculants accelerates acidification, increases the proportion of lactic acid, and limits early deviations in the volatile acid profile. Experiences with wet brewery grains have shown that the addition of lactic acid bacteria (including strains from the genus Lactobacillus) accelerated the pH drop and improved fermentation, and the combination of bacterial inoculants with the addition of carbohydrates (e.g., spent grains/molasses) had a synergistic effect. Another studies showed that in mixed systems (WBG with fibrous raw materials), beneficial effects were observed after simultaneous addition of cellulase and or laccase and bacterial inoculants; the enzymes increased the supply of sugars for the bacteria and favored the dominance of desired strains, which translated into lower pH and better stability [94,95].

In other works, the addition of calcium oxide was tested in BSG. In controlled doses, CaO reduced fermentation losses and improved the nutritional value and aerobic stability of BSG silages, most likely by modifying the cell wall structure and fermentation profile. However, due to its alkaline nature, careful dosage selection and pH monitoring are required to avoid inhibiting the desired contribution of lactic acid fermentation [96].

In subsequent studies on WBG, the silage’s behavior during the aerobic phase after the silo was opened was also evaluated. The selection of additives (biological/chemical) and prior DM level determine the aerobic stability and surface heating rate. In comparative studies, the fermentation profile, DM losses, and aerobic stability were significantly better with an appropriate change in DM level (e.g., by adding dry corn) and the use of preservatives [85,97].

In conclusion, the appropriate approach to ensiling BSG is: (i) increasing the dry matter content and the supply of rapidly fermentable substrate by co-ensiling with dry cereal components, (ii) using bacterial inoculants (possibly with the addition of enzymes in mixed systems) to accelerate acidification and improve stability, and (iii) targeted use of propionic acid preservatives to limit yeast/mold and losses, considering newer options (e.g., CaO) under strictly controlled conditions. Maintaining consistent anaerobic conditions and appropriate DM is critical for limiting clostridial fermentation and nutrient losses.

9. Structure of the Barley Grain

Barley grain is defined as a dry, non-dehiscent fruit (caryopsis) in which the pericarp is fused with the seed coat, forming a functional unit [98]. At the base of the outer covering (husk), specifically the lemma and palea, adhering to the pericarp, lies a special cementing layer produced by the pericarp epidermis. The presence and quality of this layer are crucial for the proper maturation of the grain, especially in the malting industry [99].

The glume (husk) consists of the lemma (dorsal side) and the palea (ventral side). Both of these structures have four layers: an outer epidermis, sclerenchyma, parenchyma, and an inner epidermis. The inner epidermises are directly adjacent to the cementing layer, forming a permanent bond with the caryopsis. In the lemma layer, greater thickness and mechanical resistance are observed in varieties less susceptible to damage during harvesting (grain skinning), which confirms the relationship between husk structure and the quality of the malting raw material [100]. The emphasis on the anatomy of the barley grain stems from the fact that the dominant fractions in BSG (husk, pericarp, aleurone layer) are a direct derivative of the caryopsis structure. This is what gives the observed fibrous-lamellar morphology and edge/pore density of the BSG its characteristics, which correlate with the availability of sorption sites and mass transport kinetics.

The caryopsis includes three main structures: the pericarp, the testa (seed coat), and the nucellar cell layer. The pericarp forms the outer covering of the caryopsis, acting as a protective and functional layer [98].

The endosperm is divided into two main zones: the surrounding living aleurone layer and the quantitatively dominant starch endosperm zone (the main storage site for reserve substances). The starchy endosperm accounts for about 75% of the grain’s weight and consists of dead cells filled with starch granules embedded in a protein matrix. Their walls contain polysaccharides: in a proportion of approximately 75% (1,3;1,4)-β-glucan and 20% arabinoxylan. Within this zone, the cells have different shapes: irregular on the sides; prismatic in the region between the ventral groove and the dorsal side; and smallest and most uniform in the sub-aleurone (closest to the aleurone) [101,102,103].

The aleurone layer, unlike the starchy endosperm, is composed of living cells (2–4 rows). They have thick, solid walls (mainly arabinoxylan) and are rich in proteins, lipids, vitamins, and minerals. They play a key role during germination: in response to gibberellin produced by the embryo, they release hydrolytic enzymes (e.g., α-amylase) that degrade the starch and proteins of the starchy endosperm, thus enabling the embryo to develop [100].

The embryo is the smallest part of the grain (~2–3%), but it is crucial for the plant’s further growth. It consists of an embryonic axis (with coleoptile, plumule, apical meristem, radicle, coleorhiza) and the scutellum—a reduced seed leaf (cotyledon). The scutellum facilitates the secretion of hormones and enzymes into the endosperm and the transport of nutrients to the embryo during germination [104,105]. A graphical illustration of a barley grain, along with a description, is presented in Figure 5.

10. Chemical Composition and Surface Microstructure Properties of BSG

Based on numerous studies, dried Brewers’ Spent Grain (BSG) is primarily a lignocellulosic residue from wort production, containing 70–85% water and easily fermentable components when fresh. In terms of dry matter, the main fractions are cellulose, hemicellulose, and lignin, which together account for approximately 50–70% of the dry matter, protein 15–24%, and smaller amounts of lipids (2.5–10.6%) and ash (2.3–7.9%) [106,107]. The chemical compositions of BSG based on literature reports are presented in

Table 1. Composition of BSG based on the literature.

	Components [% Dry Weight]
Ref.	Cellulose	Hemicellulose	Lignin	Proteins	Ashes	Extractives	Others	Carbohydrates	Crude Fiber	Moisture	Lipids	Acid Detergent Fiber
[107]	49.4		8.8	14.5	4.9	–	-	–	–	8.3	9.2	–
[108]	25.4	21.8	11.9	24	2.4	–	21.8	–	–	–	10.6	–
[109]	23–25	30–35	7–8	19–23	4–4.5	–	–	–	–	–	–	–
[110]	16.8	28.4	27.8	15.3	4.6	5.8	–	–	–	–	–	–
[111]	16.8	28.4	27.8	–	4.6	–	22.4	–	–	–	–	–
[112]	–	–	–	6.4	2.3	–	–	–	–	–	2.5	23.3
[113]	–	–	–	2.2	7.9	–	–	79.9	3.3	6.4	–	–
[114]	–	22–29	13–17	20–24	–	–	2.7–8.9	–	–	–	–	–
[115]	51		20.1	17.6	–	–	–	–	–	–	5.4	–
[116]	–	–	16	31	4.0	–	1.7–2.0	–	–	–	3–6	–
[117]	21.9	29.6	21.7	24.6	1.2	–	–	–	–	–	–	–
[118]	25.3	41.9	16.9	–	4.6	–	–	–	–	–	–	–
[119]	0.3	22.5	–	26.7	3.3	–	1	–	–	–	–	–
[120]	12	40	11.5	14.2	3.3	-	2.0	-	-	-	13	-
[121]	31–33		20–22	15–17			11–13.5				6–8
[122]	26.0	22.2	–	22.1	1.1	–	–	–	–	–	–	–
[123]	21.7	19.2	19.4	24.7	4.2	–	–	–	–	–	–	–

.

Traditional methods for determining the chemical composition of BSG are complex and time-consuming, so it seems better to use other techniques that are equally precise, repeatable, faster, and economical. Among spectrophotometric techniques, near-infrared (NIR) spectroscopy appears promising for determining chemical composition. NIR is used to characterize chemical composition by analyzing spectra in the wavelength range of 700 to 2500 nm to provide qualitative and quantitative information about complex samples. The results of studies using this method to detect cellulose, hemicellulose, and lignin in BSG samples have been published in the literature [124,125,126]. Other studies on BSG have also used mid-infrared spectroscopy (MIR). The possibility of rapid, non-invasive, and chemometrically validated assessment of components such as protein, carbohydrates, and lipids, as well as monitoring fractionation/structural changes during the process, has been demonstrated [124,127,128,129]. Thermogravimetric analysis (TGA) quantifies moisture and composition of lignocellulosic materials by tracking mass change with temperature [130]. For BSG, TGA/DTG was used to (i) assess drying kinetics; (ii) characterize pyrolysis (often with EGA) and volatiles; (iii) evaluate combustion/co-combustion and support proximate analysis; and (iv) examine the thermal behavior of fractions (e.g., lignin) and derived materials (hydrochars/biochars) [75,131,132,133]. Raman microscopy can map cellulose, hemicellulose, and lignin at submicron scales, though peak assignment may be challenged by baseline complexity [134,135,136,137].

Figure 6 presents SEM images of BSG focused on 2D surface microtexture/topography with operational, image-based descriptors rather than bulk morphology or specific surface area. Particle surface is rough, irregular, and fragmented, with grooves/striations, microcracks, translucent voids indicating local porosity, and frayed fibrous edges–features typical of barley husk/pericarp that dominate BSG after mashing; abrasion primarily removes endosperm, while husk, pericarp, aleurone, and testa largely persist, imparting a layered surface microtexture [138]. More porous zones between lamellae likely mark separation of the hemicellulose–lignin matrix from cellulose micro/nanofiber bundles; SEM/AFM studies similarly report secondary fiber exposure after protein/lignin washing, increasing roughness and delamination [139].

Reviews consistently describe BSG as having a husk-derived, fibrous-layered surface microtexture [124,138]. In Figure 6, rough surfaces, microcracks, lamellar layers, and microfiber bundles characteristic of seed-coat and aleurone fractions are evident; pretreatments (steam, steam explosion, and DES) further delaminate and open the structure, enhancing surface accessibility [140,141,142]. The flake-like agglomerates and irregular, angular particles with local cavities accord with SEM observations of BSG in composites, including variable matrix adhesion [143]. Colloid milling typically yields particles < 10 µm, while the 2 µm scale in Figure 6 emphasizes wall microtextures (roughness, micropores) rather than whole-particle size [144]. Studies of 22 BSG variants link BSG type to structural parameters–and thus to SEM appearance and filler behavior (e.g., brittleness, polymer compatibility) [145]; application work (snack breads, dye adsorption) further shows that BSG’s rough microstructure affects food texture and the availability of active adsorption sites [14,146]. The calculated BSG parameters based on the analysis of SEM images are presented in

Table 2. BSG parameters based on the analysis of SEM images A and C (Figure 6).

Parameters	BSG SEM Image A	BSG SEM Image B
Pores (µm/µm2):
Pore count	194	220
Pore density [pore count/1000 µm2]	154.18	227.22
Pore area share [14,146] (fraction of FOV)	0.500	0.608
Diameters (equivalent) [µm]	p50 = 0.171; p90 = 0.546; max = 26.251	p50 = 0.187; p90 = 0.708; max = 15.334
Pore average surface area [µm2]	3.240	2.674
Pore Shape (dimensionless):
Extension (AR = major/minor)	p50 = 1.88; p90 = 3.17	p50 = 1.98; p90 = 3.27
Circularity = 4πA/P2	p50 = 0.799; p10 = 0.353	p50 = 0.723; p10 = 0.282
Roundness = 4A/(π·major2)	p50 = 0.471	p50 = 0.441
Eccentricity	p50 = 0.846	p50 = 0.863
Solidity	p50 = 0.892	p50 = 0.880
Texture (GLCM, Dimensionless):
Contrast	424.570	442.706
Homogeneity	0.156	0.142
Energy	0.014	0.013
Correlation	0.957	0.944
Entropy	17.235	17.052
Edge Density (Canny)	0.077	0.093

.

Explanation of the parameters used in Table 2:

• Pore count/pore density/pore area share—how many pores are in the field of view, how many per 1000 µm², and what percentage of the field is occupied by pores (dimensionless).
• Equivalent diameter (p50/p90/max)—circular diameter of an object with the same area; p50 = median, p90 = 90th percentile (upper tail), max = largest detected (the maximum of the distribution of d_eq in µm).
• Average pore area—average area of a single pore (µm²).
• AR (major/minor)—elongation (1 = circle; >1 = ellipse/slit).
• Circularity (4πA/P²)—roundness (1 = perfect circle; lower values = more jagged).
• Roundness (4A/(π·major²))—another measure of roundness (sensitive to the major axis).
• Eccentricity—elliptical eccentricity (0 = circle, close to 1 = elongated shape).
• Solidity—compactness (A/k. surrounding); lower = more indentations/irregularities.
• GLCM: contrast/homogeneity/energy/correlation/entropy—dimensionless textural descriptors calculated from the co-occurrence matrix; they indicate tone differentiation, homogeneity, orderliness, linear dependence of tones, and randomness of the texture, respectively.
• Edge density (Canny)—the proportion of edge pixels in the image (dimensionless).

Based on the analysis of SEM images, it can be concluded that the microstructure of the BSG sample is characterized by significant porosity and a complex surface texture. Both analyzed images showed a high pore density (around 150–230 pores per 1000 µm²) and a significant pore surface area fraction (approximately 0.50–0.61 (dimensionless, fraction of the imaged field of view)). Size parameters based on equivalent diameter show a strongly skewed distribution: the median p50 falls within the range of 0.17–0.19 µm, while p90 reaches 0.55–0.71 µm, with occasional macropores (maxima up to several tens of µm). This distribution suggests that the matrix dominates the population of submicron pores, with few but geometrically significant large-scale defects.

Average diameters (p50) are sub-micron (0.14–0.19 µm), which means that the microtexture of the cell walls is being measured, not the BSG particle sizes themselves. The pore surface area and pore density are highest in BSG SEM image B, which confirms a more open and rough surface. The AR p90 value of 3–4 suggests a significant contribution from elongated/slit pores, while the decrease in circularity (p10) indicates the presence of irregular shapes.

The shape parameters indicate a predominance of pores with moderate anisotropy. The elongation medians (AR = 1.9–2.0) indicate slightly elliptical shapes, with the distribution tail extending to AR ~3.2 (p90), which means the presence of clearly elongated pores. Circularity indices (p50 = 0.72–0.80; p10 = 0.28–0.35) and roundness (p50 = 0.44–0.47) confirm a mixed surface microtexture: from pores close to isotropy to more irregular ones with sharper edges or indentations in the outline. At the same time, the relatively high median solidity values (0.88–0.89) suggest that despite local irregularities, most pores remain compact, with limited perimeter fractalization. The median eccentricities (0.85–0.86) are consistent with the elliptical outline of many voids, which may translate into directionality of local stresses in the matrix and preferential transport pathways.

The Gray-Level Co-occurrence Matrix (GLCM) reveals a texture with high complexity and strong spatial correlation simultaneously. High entropy (17.05–17.24) along with low energy (0.013–0.014) and low homogeneity (0.142–0.156) indicate a richness of tones and a lack of dominance of single gray levels, typical of rough surfaces with a multi-scale architecture. On the other hand, the high correlation (0.94–0.96) suggests that despite this complexity, there are ordered, continuous bands or clusters of pixels with similar intensity, which is often characteristic of pore networks separated by thin walls. The increased contrast (425–443) and edge density detected by the Canny algorithm (0.077–0.093) further confirm the dominance of sharp tonal transitions associated with pore boundaries and wall microedges.

In summary, the results of the SEM analysis indicate that the BSG microstructure is a highly porous system with a predominance of submicron pores, among which larger, rare macropores are sporadically present. The shape of the pores indicates moderate anisotropy and generally compact outlines, while texture analysis suggests high-complexity roughness but with significant spatial correlation. Such a set of characteristics can imply an increased surface area and facilitate transport (e.g., diffusive), potentially reducing mechanical integrity due to the presence of larger defects. Depending on the application (e.g., filtration, catalytic supports, insulating materials), the observed metrics are consistent with a material optimized for permeability and mass transfer, while requiring control of the macropore fraction to limit local stress concentrations and potential crack initiation.

Attempting a technological interpretation, it should be noted that the sub-micron pore diameters (p50 = 0.14–0.19 µm), pore density (up to 227/1000 µm²), and roughness (edge density 0.08–0.09) suggest a large number of available adsorption sites and favorable ion binding kinetics. Elongated and slit-shaped pores (AR p90 ~3–4) favor capillary binding of metal ions and diffusion along the fibers. Microcracks and local delamination increase the wettability and penetration of enzymes or microorganisms, which typically improves the saccharification rate and fermentation bioprocess yield. The surface microtexture is favorable, the pore surface area is 0.55–0.61 (dimensionless, fraction of the imaged field of view). The porous structure facilitates chemical surface modifications through alkalization, oxidation, or the introduction of functional groups (e.g., –COOH, –NH₂), which can increase the sorption capacity and selectivity toward metals or dyes. The rough, fibrous surface and slit-shaped pores improve physical adsorption and matrix-filler adhesion (resulting in higher tensile or abrasion strength). At the same time, a high proportion of pores can increase absorbency; for structural applications, appropriate modifiers should be considered to mitigate excessive absorbency. A sufficiently dense texture provides a good starting point for producing carriers (e.g., enzyme immobilization) and for pyrolysis into high-surface-area biochar, after which an increase in specific surface area and active sites is expected following carbonization and surface activation. The micro- and sub-microstructure of BSG promotes water binding and influences material properties such as viscosity, elasticity, and crumb porosity. This can support fiber functionality (texture, satiety), but it requires further adjustments and modifications. To complement the textural characterization of BSG samples, it is advisable to perform additional measurements such as BET analysis, Hg porosimetry (µm–nm scale), profilometry, AFM (3D roughness), and water holding capacity tests. To maximize adsorption efficiency, additional processes such as mild steam explosion or alkalization can be considered, which typically increase porosity and surface accessibility [10,142,147,148].

These 2D fractions are FOV-/threshold-/magnification-dependent and therefore represent approximate, image-scale descriptors rather than absolute porosity; sub-resolution pores are not captured.

11. Sorption Properties of BSG

BSG is a lignocellulosic brewing by-product rich in polysaccharides (cellulose/hemicelluloses) and lignin, with an abundant number of hydroxyl and carboxyl groups that can participate in complexation/ion exchange with metal cations and in electrostatic/π–π interactions with the organic pollutant fraction. However, raw BSG exhibits moderate sorption capacities; therefore, chemical modifications and/or conversion to biochar/activated carbons are increasingly used to increase the specific surface area and concentrate functional groups [2,149].

11.1. Sorption Processes of Metal Ions

Conducted studies show that unmodified brewer’s spent grains (BSG) are an effective biosorbent for heavy metals in aqueous solutions. For the tested Cu(II) and Cd(II) cations, the optimal conditions are a slightly acidic pH = 4 and a sorbent dose of approximately 10 g/L, with equilibrium being established very quickly (after 5–10 min.) and high removal efficiency (approximately 93–96%) being maintained. Kinetic analysis indicates that the process is best described by a pseudo-second-order model (R² = 0.99 for Cd(II) and 0.986 for Cu(II)), suggesting the involvement of chemisorption/ion exchange on the BSG surface. Isothermal studies showed a better fit to the Freundlich model (while also having favorable parameters from the Langmuir model), which confirms the heterogeneous nature of the surface and the multilayer nature of the binding. The sorption capacities obtained from the Langmuir equations are as follows: for Cu(II) q_max = 68.98 mg/g, and for Cd(II) up to 85.85 mg/g (at higher BSG doses), which makes BSG competitive or better than many low-cost biomaterials reported in the literature [150,151]. Explaining the aforementioned adsorption isotherms, the Langmuir model assumes monolayer adsorption on a finite number of energetically equivalent sites and is given by Equation (3):

$${\mathit{q}}_{\mathit{e}}={\displaystyle \frac{{\mathit{q}}_{\mathit{m}\mathit{a}\mathit{x}}{\mathit{K}}_{\mathit{L}}{\mathit{C}}_{\mathit{e}}}{1+{\mathit{K}}_{\mathit{L}}{\mathit{C}}_{\mathit{e}}}}$$

(3)

where qe [mg/g] is the equilibrium uptake, C_e [mg/L]—the equilibrium concentration, q_max [mg/g]—the monolayer capacity, K_L—the Langmuir affinity constant.

A useful dimensionless separation factor is R_L = 1/(1 + K_LC₀) (favorable if 0 < R_L < 1).

The Freundlich model describes adsorption on heterogeneous surfaces according to Equation (4):

$${\mathit{q}}_{\mathit{e}}={\mathit{K}}_{\mathit{F}}{\mathit{C}}_{\mathit{e}}^{1/\mathit{n}}$$

(4)

where: q_e [mg/g] is amount adsorbed at equilibrium, C_e [mg/L]—equilibrium concentration in the aqueous phase, K_F—Freundlich constant related to adsorption capacity, n—heterogeneity/intensity parameter.

Based on FT-IR, zeta potential, and SEM-EDS analysis, it can be inferred that there is an electrostatic interaction and ion exchange with surface-settling cations (including Ca²⁺, Mg²⁺) on the BSG. After adsorption, shifts in the FT-IR bands associated with O–H, C=O, and carboxyl groups were observed, and the negative zeta potential (around −16/−17 mV at pH 4) favored the attraction of Cu(II) and Cd(II) metal cations [150,151]. Conceptual mechanism of metal uptake on BSG is presented in Figure 7.

Comparative studies indicate that the competition of other cations in the solution limits the sorption of metals by BSG, and the affinity for BSG usually increases in the series: Mn²⁺ ≈ Zn²⁺ < Ni²⁺ < Cd²⁺ < Cu²⁺ < Pb²⁺. In multicomponent solutions, the presence of competing cations (e.g., Ca²⁺) can significantly limit sorption, while optimal conditions are usually determined within a slightly acidic pH range of 4.5–5.5. The mechanism can be linked to reversible, heterophasic ion exchange on the functional groups of the BSG lignocellulosic matrix, which allows for bed regeneration, although its durability can be limited by the gradual blocking or deactivation of some active sites. The pH dependence is clear (surface protonation/deprotonation), and the isotherm data is often better described by the Langmuir model, while the kinetics are better described by the pseudo-second-order model [3,152].

Comparative studies of two types of BSG (with similar chemical composition but different morphology) showed that all major components of BSG participate in metal binding, and the material is effective not only in model solutions but also in real surface waters. The research results yielded adsorption capacities of around 0.2 mmol/g for Fe³⁺ and 0.1 mmol/g for Mn²⁺, Cd²⁺, and Ni²⁺. Differences in microstructure translate into sorption kinetics and capacities, suggesting that the selection of raw material and BSG pretreatment are of significant application importance [153].

In addition to typical batch studies [154,155], there have been reports of BSG working effectively in fixed-bed columns, which confirms the feasibility of integrating BSG into continuous purification systems. The ability to remove metal ions and the influence of salinity in the columns were confirmed as a significant aspect from the perspective of real waters. The growing literature reports on column adsorption emphasize that proper selection of bed height, flow rate, and breakthrough curve modeling allows for the translation of laboratory results into continuous process implementation [12,156].

Pyrolysis/activation of BSG yields porous materials with high surface area and adjustable surface chemistry. For Cr(VI), it has been shown that activated carbons from BSG (including those nitrogenated in situ) outperform or match commercial ACs (Norit, Filtrasorb) under acidic conditions (where the HCrO₄⁻ formation dominates), suggesting the involvement of reduction/adsorption mechanisms. Column reactors and isotherm studies confirm the effectiveness of these materials [10].

For anionic forms of chromium(VI) (HCrO₄⁻/Cr₂O₇²⁻), the effectiveness of BSG-derived materials increases at acidic pH, and some studies indicate the co-occurrence of Cr(VI)→Cr(III) reduction with sorption (the “reduction–adsorption” mechanism), which is confirmed by spectroscopic analyzes and isotopic fractionation. In multi-waste systems (BSG along with sewage sludge), high removal of Cr(VI) was achieved, with surface properties (basicity, heteroatoms) determining the efficiency [157,158].

Recent work shows significant progress in consequence of ecological modifications. Mechanochemical esterification of BSG with citric acid (high-energy solid-state milling) significantly increased the Langmuir capacities: to 65.83 mg/g for Pb²⁺, 24.72 mg/g for Cd²⁺, and 15.11 mg/g for Ni²⁺ (best described by the Langmuir isotherm). Compared to the raw BSG, this resulted in an increase of 232% (Pb), 164% (Cd), and 576% (Ni), respectively. This is an important direction because it avoids the use of organic solvents and remains scalable [12].

Other studies have presented a rapid method for esterifying wood flour with citric acid, conducted in DMF with sodium hypophosphite monohydrate (NaH₂PO₂·H₂O) as a catalyst, which shortened the modification time to 2 h at 140 °C while simultaneously allowing for swelling and modification of the material. FTIR confirmed the formation of ester bonds (new bands at 1726 and 1167 cm⁻¹), and the point of zero charge shifted to pH_p𝒵$𝒞$ = 3.0 (from 5.8 for raw BSG), expanding the pH range favorable for cation sorption. Under conditions of pH = 6, C₀ = 10 mM, dose 2 g/L, 25 °C, the esterified material reached very high sorption capacities after approximately 30 min.: Cu²⁺ 104.13; Pb²⁺ 293.30; Zn²⁺ 232.10; Cd²⁺ 296.61; Ag⁺ 205.80 mg/g, which represents an increase of about 43–94% compared to the raw BSG [7].

Converting BSG to biochar significantly increases the adsorption efficiency of selected ions. For Cr(VI)/Cr(III), the optimized systems (700 °C, ZnCl₂ activation) achieved an equilibrium adsorption capacity of 78 mg/g. In contrast, biochar produced from BSG (at 500, 600 and 700 °C) effectively removed cobalt and strontium ions, which is significant from the perspective of remediating waste streams containing radionuclides. The choice of pyrolysis/activation temperature determines the pore structure and density of surface groups, and thus the selectivity [159,160].

Low-energy hydrothermal treatment of BSG (at 150 °C; Maillard reaction participation) increases the number of carboxyl groups (1.46 mmol/g) and allows for high capacities for UO₂²⁺ (up to 221 mg/g) and selected lanthanide ions to be obtained without additional activation; nitrogen groups also participate in the mechanism. This indicates the attractiveness of BSG as waste biomass for the recovery of La³⁺, Eu³⁺, Yb³⁺, and UO₂²⁺ metals [8].

Nitrogen oxidation of BSG increases the density of acidic groups (up to 1.3 mmol/g) and creates multiply regenerable sorbents dedicated, among other things, to uranyl. Concurrently, BSG materials with surface modification are being developed, which, at very low biosorbent doses, maintain high selectivity and stability toward UO₂²⁺, potentially making BSG suitable for preparing targeted sorbents for specific ionic forms [9,161].

The use of a fiber-rich product to obtain cellulose sorbents enables the effective removal of Pb and Mn from water (including contaminated tap water), with a good fit to the Langmuir isotherm. This approach combines the valorization of BSG in line with the principles of a circular economy and the production of sorbents with repeatable parameters [13]. Brief literature results of metal ion adsorption using BSG are presented in

Table 3. Summarized results of metal ion adsorption studies using BSG.

Ref.	Sorbent BSG	Metals	pH/Matrix	qmax [mg/g]	Best-Fit Isotherm	Kinetics Model	Regeneration	Notes
[12]	Citric-acid mechanochemically esterified BSG	Pb(II), Cd(II), Ni(II)	pH 4.5 (acetate buffer)	Pb 65.83; Cd 24.72; Ni 15.11 (Langmuir)	Langmuir	―	―	Bottle-point isotherm; KL reported
[9]	Nitro-oxidized BSG	U(VI) (uranyl)	pH 4.7; batch; C0 = 900 mg/L	297.3; fast uptake ~1 h	―	Rapid uptake (∼1 h)	≈60% capacity retained after 5 cycles	High –COOH (~1.3 mmol/g); works in simulated seawater
[8]	Mild-hydrothermally treated BSG (ABSG)	U(VI), La(III), Eu(III), Yb(III)	U: pH 4.7; REE: pH 5.7	U 221; La 38; Eu 68; Yb 46	Langmuir	―	―	Greener prep; Maillard-derived activation
[160]	BSG biochar	Co(II), Sr(II)	―	C0 3.30–5.52; Sr 1.46–3.04 (298–318 K)	―	―	Reusability: C0 75.3→36.2%; Sr 93.6→32.7% over 4 cycles	Competitive ions reduce capacity
[162]	Unmodified BSG	Cr(III)	batch + expanded bed column	16.7 (Langmuir)	Langmuir	Pseudo-second-order (initial) + intraparticle diffusion	Column: breakthrough 58 h; saturation 199 h	Alkali pretreatment not beneficial
[6]	Unmodified BSG	Cu(II)	pH 4.2; batch	10.47 (Langmuir)	Langmuir	Pseudo-second-order	―	Early demonstration of BSG as biosorbent
[159]	BSG biochar (ZnCl2-activated, 700 °C/30 min., 12.5%)	Cr(VI)	batch	78.13 (Redlich–Peterson)	Redlich–Peterson	Pseudo-first-order; equilibrium <100 min.	―	Optimized via factorial
[13]	BSG fiber-derived cellulose (TEMPO-oxidized)	Pb(II), Mn(II)	Contaminated tap water	Pb 272.5; Mn 52.9 (Langmuir)	Langmuir	―	―	Fabricated from fiber-rich product
[153]	Unmodified BSG	Fe(III), Mn(II), Cd(II), Ni(II)	Real waters (surface/groundwater)	Fe 11.2; Mn/Cd/Ni 5.5–11.2	―	―	―	Benchmarked vs guidelines
[161]	Surface ion-imprinted BSG (IIP-BSG)	U(VI) (uranyl)	pH 4.6; batch; high ionic strength tolerant	165.7 (Sips)	Sips	Internal mass transfer controlled	≈90% capacity retained after 5 cycles	High selectivity vs. Eu(III) (SU > 80%)
[7]	Citric acid esterified spent grain (ESG); DMF + NaH2PO2·H2O; 140 °C, 2 h	Cu(II), Pb(II), Zn(II), Cd(II), Ag(I)	pH 6; single-ion nitrates; batch; C0 = 10 mM; dose 2 g/L; 25 °C	Cu 104.13; Pb 293.30; Zn 232.10; Cd 296.61; Ag 205.80 (C0 = 10 mM)	―	Equilibrium in ~30 min. (fast)	―	pHPZC: ESG 3.0 vs. RSG 5.8; FTIR ester bands at 1726 and 1167 cm−1; capacities +43–94% vs. RSG
[163]	BSG biochar (BC)	Cr(VI)	pH 2	9.36	Freundlich	Pseudo-second-order	―	Chemisorption; film/external binding dominates
[163]	BSG biochar, KOH-activated (ABCK)	Cr(VI)	pH 2	8.94	Freundlich	Pseudo-second-order	―	Chemisorption
[163]	BSG biochar, H3PO4-activated (ABCP)	Cr(VI)	pH 2	7.1	Freundlich	Pseudo-second-order	―	Chemisorption; intraparticle (pore) diffusion contributes
[163]	Raw BSG	Cr(VI)	pH 2	7.02	Freundlich	―	―	Not modeled in the paper; capacity reference for comparison

and comparison of q_max is shown in Figure 8.

Across the compiled studies (Table 3), modified BSG sorbents span $q_{\mathrm{m}\mathrm{a}\mathrm{x}}$ from 7 to 10 mg/g for Cr(VI) on BSG biochars up to ≈270–300 mg/g for Pb(II)/U(VI) on BSG-derived cellulose and nitro-oxidized/hydrothermally treated BSG under acidic conditions and Langmuir fits. Unmodified BSG typically exhibits lower capacities (e.g., Cu(II) ≈ 10 mg/g; Cr(III) ≈ 17 mg/g). Literature comparisons further indicate that BSG-derived activated carbons can perform competitively with commercial carbons for selected organics.

Literature-reported capacities for widely used biosorbents fall within ranges comparable to those of some BSG materials (

Table 4. Comparative capacities of selected non-BSG biosorbents for metal removal.

Ref.	Biosorbent & Modification	Metal	qmax [mg/g]	Key Conditions	Best-Fit Isotherm/Kinetics
[164]	Chitosan nanoparticles (cross-linked; nano-size)	Pb(II)	≈398	batch; aqueous; pH typically acidic–near-neutral	(reported in study; high monolayer capacity)
[165]	Chitosan–bentonite composites / beads	Pb(II)	42.5–94.6	batch; Pb(II) aq.; varied pH; composite ratios 90/10–50/50	Langmuir & Freundlich both fit well
[166]	Orange peel (powder, uncarbonized; H2O2-treated / raw)	Cd(II)	up to 128.23	pH ≈ 4.5; 318 K (batch)	Langmuir (best); kinetics: PFO in this study
[167]	Modified rice husk (chemical modification)	Cu(II)	≈43.5	25 °C; pH ≈ 7; ${\mathit{C}}_0$ = 400 mg/L (batch)	Langmuir/Freundlich modeling performed
[168]	Banana-peel/alginate magnetic biobeads (nano-Fe3O4)	Cr(VI)	370.4	pH = 2; 25 °C; Langmuir analysis (batch)	Langmuir & Freundlich; kinetics: PSO

): chitosan nanoparticles for Pb(II) report a q_max of 398 mg/g; chitosan–bentonite beads reach 42.5–94.6 mg/g for Pb(II); H₂O₂-treated orange peel yields up to 128.23 mg/g for Cd(II) at pH ≈ 4.5; chemically modified rice husk reaches ≈ 43.5 mg/g for Cu(II); and magnetic banana-peel/alginate beads achieve 370.4 mg/g for Cr(VI) (Langmuir; PSO). These values corroborate that BSG-derived sorbents (Table 3) are competitive within the broader biosorbent landscape.

11.2. Sorption Processes of Organic Substances

BSG, in its unprocessed form and after simple modification (acid, base, fungal pretreatment), effectively removes cationic and anionic dyes, including methylene blue, Congo red, and malachite green, from aqueous solutions. The ionic strength has a significant influence, and the effect of pH is consistent with the dye’s charge (favoring surface deprotonation/protonation). Experiments in a column (flow) system have also been confirmed, which is significant when attempting to increase process efficiency. Additionally, magnetically modified BSG facilitates sorbent separation and maintains high efficiency in dye removal [14,169,170]. In the literature, the adsorption of nitrates after prior modification of BSG (introduction of anion-favorable groups) is also reported, which shows that appropriate functionalization allows for addressing both cations and anions in water [171].

Brewer’s spent grains (BSG) are an available and inexpensive precursor for sorbents after thermal conversion to biochar (pyrolysis) or hydrochar (HTC), followed by activation (e.g., KOH) and/or magnetization (Figure 9). Review and research papers highlight that temperature, conversion time, and activation conditions influence the development of micropores, specific surface area, and surface chemistry, which are crucial for affinity toward organic pollutants. Furthermore, understanding the mechanism of KOH activation for BSG allows for precise design of porous structures, and even the method of raw material grinding influences the morphology of the resulting biochar [147,172].

Activated hydrochar from BSG (AHC-BSG) removes paracetamol more effectively than raw BSG/HC, dominated by π–π interactions and H-bonding. Forming alginate beads enables fixed-bed operation alongside batch tests, improving mechanical/operational properties. Activated biochar/hydrochar variants are also effective for organic pollutants. Hydrochar and activated carbon from BSG effectively remove acetaminophen and the selection of HTC/pyrolysis and activation parameters (e.g., KOH, H₃PO₄) significantly shapes the morphology and sorption capacity [163,173].

Biochars from BSG obtained at 300–700 °C effectively remove the pesticide pymetrozine. The sorption capacity depends strongly on pyrolysis temperature and pH ≈ 4 and equilibrium/kinetics follow the general pattern stated above [174,175].

Iron oxide (Fe₃O₄) composites on activated hydrochar from BSG enable not only the adsorption of 2-chlorophenol but also its rapid degradation in a heterogeneous Fenton system, combining surface retention with catalytic decomposition. The description of the mechanism indicates a mixed character of interactions (π–π, H-bonds, electrostatics) and Sips-type isotherms for heterogeneous surfaces [176].

Co-pyrolysis of BSG with sewage sludge produces biochars with high Mg/P availability and favorable texture, which translates into increased ammonium nitrogen sorption from water. These results provide a basis for designing BSG sorbents aimed at removing ammonium ions and mitigating eutrophication [177,178].

Activated carbons with an increased nitrogen content and developed microporosity were produced from BSG, effectively adsorbing phenol. Their effectiveness and process parameters (pH, dosage, contact time) were compared with commercial carbons, indicating the competitiveness of materials derived from BSG [11]

Magnetically modified BSG-based sorbents enable rapid recovery from the medium, which is an advantage in the operation of the installation. Simultaneously, systems are being developed that combine adsorption on carbon materials with BSG and immobilized horseradish peroxidase (HRP), enabling simultaneous capture and oxidative degradation (e.g., of Orange II). An increase in capacity and a favorable fit to mixed-type isotherms were demonstrated. From a separation practice perspective, magnetic activated carbons produced from BSG (MAC, doped with Fe₃O₄) are promising, as they combine high adsorption capacity with the ability to be easily recovered in a magnetic field [179,180].

In summary, it can be concluded that the effectiveness of (modified) BSG sorbents against organic pollutants is due to the synergistic action of: (i) π–π interactions between the aromatic domains of carbon and the aromatic rings of dyes and phenols, (ii) hydrogen bonding with O-functional groups, (iii) pH/pHpzc-regulated electrostatic interactions, and (iv) tuning the number and type of active sites through chemical activation and magnetization. Applications in column systems have been demonstrated for dyes and pharmaceuticals, confirming the technology’s deployment potential [169]. Literature review results of organic substances adsorption using BSG are shown in

Table 5. Summarized results of organic substances adsorption studies using BSG.

Ref.	Pollutant	Sorbent	Key Conditions	Isotherm Model, qmax [mg/g]	Kinetic Model	Key Findings
[170]	Congo Red (CR, azo dye)	Chemically pretreated BSG (BGPOH—NaOH; BGPH—H2SO4; BGPB—white-rot fungus)	Batch tests; CR 300 mg/L in some tests; ambient T; dosage varied (0.1–2.0 g)	Langmuir; qmax = 149 (BGPOH), 147 (BGPH), 117 (BGPB)	Pseudo-second order (better fit than PFO)	All pretreatments remove CR; BGPOH highest capacity; adsorption spontaneous; lower T favored
[14]	Methylene Blue (cationic dye)	Raw (unmodified) BSG	Batch; pH 7; 20–50 °C; 0.150 g BSG / 50 mL; C0 = 50–500 mg/L	Langmuir; qmax = 80.31	Pseudo-second order	High removal in synthetic wastewater; favorable RL; good reusability (≤5 cycles)
[174]	Pymetrozine (pesticide)	BSG-derived biochar (slow pyrolysis 300–700 °C; best ~400 °C)	Batch; optimal pH ≈ 4; 25–45 °C; 70–80% removed in first 60 min.	Langmuir; qmax = 22.020 (25 °C), 26.032 (35 °C), 31.606 (45 °C)	―	Adsorption endothermic; Langmuir/Freundlich fits with R ≈ 0.995–0.999
[176]	2-Chlorophenol (2-CP)	Fe3O4/activated hydrochar from BSG (FeOHC; FeOHC-C)	Batch; pH 3–6; 25 °C; also acts as a heterogeneous Fenton catalyst	Sips; equilibrium capacity ≈ 24.63 (FeOHC), 18.70 (FeOHC-C)	Elovich	Bifunctional: adsorption + Fenton oxidation; spontaneous/exothermic; good reusability
[14]	Methylene Blue (MB)	Raw BSG (sieved 53–500 µm)	Batch; 298.15 K; pH ≈ 6.8–7; C0 = 15–150 mg/L; dose = 10 g/L; t = 240 min.	Langmuir (qmax,cal = 37.45; KL = 0.025; R2 = 0.929); Freundlich (n = 1.16; R2 = 0.993)	Pseudo-second order (R2 ≈ 0.999)	85–96% removal across C0; faster uptake for smaller particles; uptake with pH to ~8–10
[14]	Congo Red (CR)	Raw BSG (sieved 53–500 µm)	Batch; 298.15 K; pH ≈ 6.8–7; C0 = 15–150 mg/L; dose = 10 g/L; t = 240 min.	Langmuir (qmax,cal = 19.65; KL = 0.114; R2 = 0.953); Freundlich (n = 1.46; R2 = 0.944)	Pseudo-second order (R2≈ 0.998–1.000)	85–96% removal across C0; pH optimum ~7; higher dose removal

.

12. Conclusions

The brewing industry has evolved from its early origins to a globally recognized sector marked by industrialization, technological advancements, and increasing sustainability efforts. While large corporations dominate global production, the craft beer segment continues to grow, fostering innovation and product differentiation. Key stages of the brewing process, such as mashing, boiling, fermentation, and maturation, can be optimized through energy-efficient technologies, leading to enhanced product consistency and reduced environmental impact. Sustainable brewing practices, including energy recovery, water management, and by-product utilization, are integral to the industry’s future, with breweries focusing on minimizing their carbon footprint while improving efficiency.

From a sustainability standpoint, the preservation route and logistics dominate the short-term footprint of BSG valorization. Drying is effective but energy-intensive; high-moisture BSG (≈70–80%) requires substantial dewatering to <10% moisture, whereas newer options (e.g., VMD, superheated steam) can shorten drying time and improve efficiency, especially when process heat is cascaded or steam is reused. Cooling/freezing extends microbiological safety only in short supply chains and does not fully stop biochemical change; thermal sterilization is robust but energy-heavy, so milder volumetric heating is being explored. These choices should be matched to the transport distance, turnaround time, and targeted product.

Economically, breweries increasingly leverage circular strategies that cut disposal costs and create value from BSG. On-site energy recovery and biogas CHP projects reported material CO₂ reductions (e.g., up to thousands of tons per year) and notable electricity/heat coverage, translating into operating cost relief when integrated with existing infrastructure. Where energy prices or carbon intensity are high, coupling hydrothermal/pyrolytic routes with heat integration and selecting low-energy stabilization can materially improve OPEX. Techno-economic screening should therefore compare (i) drying vs. short-haul cold logistics; (ii) raw BSG vs. BC/HC with chemical activation; and (iii) batch polishing vs. fixed-bed columns, accounting for regeneration cycles. Overall, BSG-based sorbents are low-cost and circular-compatible, but scaling should be guided by complementary TEA/LCA that includes preservation energy, transport, activation chemicals, and realistic performance in real waters.

The presented research confirms that post-fermentation brewer’s spent grain (BSG) is an attractive, low-cost, and circular economy-compliant sorbent material for removing metal cations and organic substances from water and wastewater. The SEM analysis and surface texture revealed the presence of submicrometer porous structures and a high density of edges/defects, which can act as easily accessible binding sites and shorten the mass transport path. Calculated parameters such as pore density in the range of 154–227 pores/1000 µm², a significant proportion of pore surface area, and the recorded edge density are portable morphological indicators that can be used to assess materials and predict their sorption behavior.

The literature data collected in the study show that raw BSG is capable of removing a range of metal ions (e.g., Cu(II) and Cr(III)) with isotherms most often described by the Langmuir/Freundlich models and pseudo-second-order kinetics, while chemical modifications (esterification with citric acid, nitro-oxidation, ion imprinting) and thermal transformations to biochar/activated carbons significantly increase capacity, selectivity, and reusability. The examples provided (including nitro-oxidized and imprinted materials showing resistance to ionic strength and performance retention after multiple cycles) indicate that BSG and its derivatives can compete with commercial sorbents, especially with an appropriate selection of pH and other process conditions.

This work also delineates limitations and divergences: batch variability (malt recipe, mashing/filtration) alters composition and microstructure, yielding wide ranges of capacity and kinetics. Matrix effects in real waters, competition between Ca²⁺ and Mg²⁺, higher ionic strength, and low-cost, low-footprint material upgrades often result in lower efficiency compared to single-solute tests; these factors must be vetted by complementary techno-economic and environmental analyses. Process translation further requires studies on column/flow design, breakthrough behavior, and robust regeneration.

Operationally, effective stabilization and logistics are prerequisites to preserve BSG sorption activity and microbiological safety. Drying—ranging from convective to superheated steam, lyophilization, or VMD—provides practical, energy-efficient preservation for storage and distribution; cooling/freezing extends the use of wet BSG in short supply chains; and thermal inactivation plus oxygen control serve as complementary options, all chosen with energy balance and functional changes in mind.

In summary, the aim of this work was to (i) analyze metal adsorption on BSG within a broad context of sustainable development and process practice, (ii) review performance and mechanisms in the literature, and (iii) quantitatively link microstructure to adsorption performance through a standardized SEM image analysis pipeline. Minimally processed BSG, when properly stabilized and applied under appropriate pH/ionic strength, can provide competitive metal-ion removal. Targeted chemical-thermal modifications further enhance capacity and selectivity but require a cost-environmental assessment. However, simple, accessible SEM metrics (e.g., submicron pore statistics, edge density) are useful predictors of performance that can accelerate material analysis and scaling up to continuous operation.