Cascade Processing of Agricultural, Forest, and Marine Waste Biomass for Sustainable Production of Food, Feed, Biopolymers, and Bioenergy

Abstract

An increasing global population, rising energy demands, and the shift toward a circular bioeconomy are driving the need for more resource-efficient waste management. The increase in the world population—now exceeding 8 billion as of 2024—results in an increased need for alternative proteins, both human and feed grade proteins, as well as for biopolymers and bioenergy. As such, agricultural, forest, and marine waste biomass represent a valuable feedstock for production of food and feed ingredients, biopolymers, and bioenergy. However, the lack of integrated and efficient valorization strategies for these diverse biomass sources remains a major challenge. This literature review aims to give a systematic approach on the recent research status of agricultural, forest, and marine waste biomass valorization, focusing on cascade processing (a sequential combination of processes such as pretreatment, extraction, and conversion methods). Potential products will be identified that create the most economic value over multiple lifetimes, to maximize resource efficiency. It highlights the challenges associated with cascade processing of waste biomass and proposes technological synergies for waste biomass valorization. Moreover, this review will provide a comprehensive understanding of the potential of waste biomass valorization in the context of sustainable and circular bioeconomy.

1. Introduction

Today’s society heavily relies on fossil-based resources and linear economic models, where the main focus is on maximizing output and supply, with detrimental consequences to the environment. With the global population projected to exceed 9.1 billion by 2050, the pressure on natural resources and food systems continues to intensify [1]. The linear economic models do not provide a driving force towards sustainable growth as they contribute to over 90 billion tonnes of resource extraction annually, with only 7.2% of materials being cycled back into the economy [2]. Therefore a transition from a linear, fossil-based economy to a circular, bio-based economy that focuses on resource efficiency, recycling, and minimizing waste is necessary [3]. However, this transition faces several challenges, including (1) ensuring stable access to biomass and residual raw materials, (2) addressing price volatility and quality fluctuations, and (3) navigating the complex web of legislation and regulations governing biomass utilization [4]. Efficient utilization of the available bioresources is crucial for the successful implementation of circular bioeconomy [5]. Residual raw materials such as agricultural, forest, marine waste biomass, and industrial biproducts are highly available and renewable resources, often being considered as having low market value [6]. The residuals/waste biomass contain valuable components/nutrients and energy, which can be further utilized in new and innovative ways. The advantages are several, such as reduced exploitation of natural resources and land area, reduced amount of waste, and reduced emission of greenhouse gases. The increased exploitation of residuals also offers opportunities in value creation for pre-existing resources, as well as the possibility of new innovations, technology development, and new business opportunities [7,8,9]. The economic potential of biomass-derived products is also significant, with the global biopolymers market projected to reach USD 38.7 billion by 2030, and the biomass-based energy sector expected to exceed USD 149 billion by 2025 [10].

Worldwide, agricultural, forest, and aquaculture create large quantities of waste biomass, much of which are not fully utilized today, e.g., 140 billion metric tons of biomass wastes are generated annually from agriculture [11].

Table 1. Waste generated vs. waste valorized [12].

Sector	Waste Generated (Million Metric Tons/Year)	Waste Valorized (Estimated %)
Agriculture	140,000	<10%
Forestry	522,000	~5%
Aquaculture	80	<15%

provides a comparative overview of waste generated and valorized across these key sectors, highlighting the significant gap and the need for improved utilization strategies.

The global forest waste biomass, which includes both above-ground and below-ground biomass annually, is estimated to be around 522 billion metric tons [13], while aquaculture production generates significant waste, with estimates indicating around 80 million metric tons of waste per year [14]. Due to their chemical composition, which predominantly comprises cellulose, hemicellulose, starch, and lignin—agricultural, forest, and marine biomass (such as seaweed) are considered to possess substantial potential for applications in the food and feed industry, as well as in the chemical industry and energy sector [15]. However, the chemical recalcitrance and the variation in these waste biomass presents a challenge for the valorization process, leading to a necessity for different processing steps, such as mechanical, thermochemical, and biological treatment [16]. It is crucial to combine these processing steps, where the biomass can be utilized in a cascading processing approach without affecting its current use. Cascading processing aims to maximize the value of circular economy by maximizing resource utilization and efficiency through the efficient conversion of biomass into multiple products [17]. This implies that biomass should be used for high value-added products and not exclusively for energy production. Smit et al. [18] found that combining biomass pre-extraction and fractionation in an integrated cascading process can both maximize biomass utilization and increase feedstock flexibility for biorefineries. The increased competitiveness for bio-based resources will boost interest in using agricultural, forest, and marine waste biomass as an alternative source of food and feed, biopolymers, and bioenergy.

This review focuses on the vast potential of agricultural, forest, and marine waste biomass in driving sustainable production of food, feed, biopolymers, and bioenergy. By exploring the various types of waste biomass and their cascading processing methods, this review aims to present an overview of the diverse valorization pathways and highlight the challenges and opportunities associated with their utilization. The objective is to provide a comprehensive understanding of how these renewable resources can be effectively utilized to support a circular bioeconomy, reduce environmental impact, and pave the way for innovative technological advancements and new business opportunities.

Alignment with the SDGs

The UN’s Sustainable Development Goals (SDGs) include several targets that emphasize the importance of utilizing waste biomass for sustainable development. Target 2.3 aims to double agricultural productivity by converting waste biomass into higher value-added products and materials, while Target 2.4 focuses on ensuring sustainable food production through the production of protein and high-value ingredients for food and feed. Target 9.5 seeks to enhance research and upgrade industrial technologies by developing new sustainable value chains. Similarly, Target 9.B supports domestic technology development and industrial diversification by fostering innovative technological ecosystems in the agricultural, forest, and aquaculture industries. Target 12.4 emphasizes the responsible management of chemicals and waste, and Target 12.5 aims to substantially reduce waste generation by advancing the management of organic waste and developing new technologies for resource reuse and value creation from industrial waste streams. Additionally, Target 12.8 promotes a universal understanding of sustainable lifestyles. Finally, Target 13 focuses on climate action by enabling technologies for the biochemical conversion of industrial wastes into sustainable products that can substitute fossil-based products, thereby contributing to the reduction of CO₂ emissions [19].

2. Methodology for Data Collection, Selection, and Review

In this review, the data collection was divided into three main actions. Firstly, several databases were utilized, including Web of Science, PubMed, Elsevier, and Academic Google. Secondly, the databases were extensively searched focusing on four relevant topics: (1) agricultural, forest, and marine waste biomass; (2) cascade processing of waste biomass; (3) technological innovations; and (4) challenges and research gaps. Based on the extensive literature search (500+ papers), a number of studies were read and classified considering their relevance to the subject of the present review. In total, 170 relevant papers were finally selected and reviewed. The study selection process is illustrated in the PRISMA flow diagram in Figure 1.

Literature articles and reviews on cascade processing of biomass have been previously published and many of them focus on resource efficiency and circular economy [17,20,21,22,23]. It is important to emphasize that the present work focuses on the importance of each step in the cascade processing and the possible products from each step are identified. Moreover, synergies, technological innovations and challenges and research gaps between agricultural, forest, and marine biomass in a cascade processing are also highlighted in this work. As a result of the literature review, five products categories arising from cascade processing have been identified and illustrated in Figure 2: (1) food and feed ingredients, (2) extractives and tannins, (3) single cell protein production, (4) biopolymers, and (5) bioenergy.

3. Waste Biomass

3.1. Agricultural Waste

Agricultural waste streams described in this literature review includes common crop wastes, such as stover, straw, husks, stalks, and brewery spent grain. Crop residue production is estimated to be around 2800 Mtonnes/year globally [24]. The total amount of estimated EU crop waste in 2030 is estimated to achieve 139 Mtonnes/year [25]. More specifically, 700 Mtonnes of rice straw, 500 Mtonnes of wheat straw, 100 Mtonnes of rice husks, and 1 billion tonnes of corn stovers are produced yearly [26,27]. About 20 kg of brewery spent grain (BSG) is generated from every 100 L of beer produced. BSG represents up to 85% (w/w) of a brewery’s total waste, and annually, around 40 Mtonnes BSG are produced worldwide [28,29]. These agricultural waste biomasses represent rich sources of carbohydrates, hemicellulose, lignin, protein, silica, minerals, and ash. Due to high fibre content, agricultural crop residues are often used as animal feed directly, which minimize the transport and refining costs [30]. Other relevant applications where the fibrous material from the agricultural waste residues is used are, e.g., as a sustainable alternative in paper and board, fibreboard and packaging, as well as for bioenergy production. Valorisation into bioenergy production is further described in Section 4.

3.2. Forest Waste

Waste biomass from the forestry sector is generated throughout the entire value chain, including logging, conversion, and final handling. After logging operations, residues like branches and tops are produced. The current amount available of forestry residues was estimated to be 40 Mtonnes/year [25]. For environmental reasons, it is recommended to leave 30% of these residues in the felling area as the nutrient is returned to the soil to support future forest growth [31]. The withdrawn logging residues are primarily used as a source of bioenergy, particularly in chip heating units. During wood conversion, where saw timber and pulp are the primary products, several residuals are generated, like bark, sawdust, wood chips, ash, and sludge. Sawdust and woodchips can be utilized in their raw form as bedding in barns and paddocks [32]. Additionally, they serve as an energy source, particularly in chip heating units. Traditionally, bark is used in gardens, to protect sensitive plants and prevent weed growth, and it can also be utilized for energy purposes. Bark contains up to 50% on a dry basis of extractives and lignin, which can be utilized as a renewable source of chemicals. Feng et al. [33] summarized the valorization of bark to chemicals and materials in a literature review.

Utilizing biological residuals, such as waste wood, is essential for advancing climate-friendly renewable energy solutions. Additionally, these residues can be used in the production of feed through the production of single-cell proteins. Regardless of the application, the residuals need energy-intensive pretreatment, like drying and mechanical processing. The production of food and feed, as well as bioenergy is further described in Section 4 “Cascade Processing of waste biomass”.

3.3. Marine Waste Biomass

Macroalgae, commonly known as seaweeds, are categorized as brown, green, or red based on the colour of their thallus [34] and hold significant potential for biorefinery and cascade processing due to their unique composition and abundance. Seaweeds typically consist of 80–90% water, with their dry weight comprising approximately 50% carbohydrates, 1–3% lipids, and 7–38% minerals [35]. The protein content of seaweeds varies significantly, ranging from 10 to 47%, and they contain high proportions of essential amino acids. Due to their nutrient richness [36], seaweeds are highly suitable for human consumption. Moreover, their favourable amino acid profile makes them a promising protein source [37]. This is particularly true for red and green algae, which have a higher protein content compared to most terrestrial crops. Despite the relatively low lipid content, the lipid fraction in seaweed contains bioactive components that have demonstrated beneficial effects on human health [38]. The environmental conditions and temperature in which seaweed grows influence its chemical composition, growth rate, and size [39,40]. Due to their rich nutritional profile, seaweeds serve as an ideal feedstock for producing a wide range of valuable products in a biorefinery, including biofuels, bioactive ingredients, chemicals, and other high-value products such as bioplastics and bioethanol [41]. For cascade processing of seaweed, the initial extraction of carbohydrates can be followed by the extraction of proteins and lipids, with each stage producing valuable products. The remaining biomass can be used for applications such as animal feed, fertilizers, or even as a source of renewable energy. Seaweeds are efficient at carbon sequestration during growth, helping to mitigate climate change [42]. Additionally, the production of biofuels and bio-based products from seaweed can reduce reliance on non-renewable resources and promote sustainable development. The economic potential of seaweed biorefineries is significant, with the potential to generate high-value products and create new market opportunities [42].

4. Cascade Processing of Waste Biomass

Cascade processing is a stepwise biomass valorization strategy that enables the sequential recovery of multiple products from a single feedstock, enhancing both economic and environmental outcomes. Existing biorefinery processes aim to efficiently utilize and convert lignocellulosic biomass to multiple products, such as biochemicals, biomaterials, and bioenergy. Consequently, this requires the development of sequential pretreatment strategies to enhance the removal of hemicellulose and lignin and increase the availability of cellulose. As such, cascade processing integrates multiple extraction and pretreatment steps where the synergistic effects of each stage can be efficiently monitored. A cascade process was developed by Castilla-Archilla et al. [43] for the fractionation of brewery spent grain, where the proteins and fibres were recovered in the solid fraction while carbohydrates were released in the liquid fraction. Pardilho et al. [44] published an exploration study on cascade processing of macroalgae waste biomass, while Baghel [45] proposed a multistage cascade bioprocessing model for macroalgae biomass.

4.1. Principles and Importance of Cascade Processing

Cascade processing of woody biomass has been increasingly discussed as a key principle upon which to base efficient utilization of biomass, especially in the European Union [46]. Cascade processing is a systematic approach aimed at maximizing resource efficiency and minimizing waste by utilizing waste biomass in a hierarchical manner to produce value-added products [47,48]. The core principle of cascade processing is to sequentially extract and utilize various components of biomass, ensuring that each fraction is used to its highest potential before moving on to the next. This method not only enhances the overall value derived from biomass but also contributes to sustainable resource management and environmental conservation. In general, the cascade processing of waste biomass involves the following key steps [48]: (1) initial pretreatment, (2) extraction of high-value components, (3) chemical and biological pretreatment and fractionation, and (4) energy production and recovery. A schematic diagram is shown in Figure 3. To complement the conceptual schematic (Figure 3), a simplified illustration of hypothetical mass and energy flows between cascade processing steps is provided in Figure 4. This figure is based on typical values reported in the literature [49] and highlights the sequential reduction in biomass mass and associated energy inputs across pretreatment, extraction, fermentation, and energy recovery stages.

4.1.1. Initial Pretreatment

Agricultural and marine waste biomass, due to the high dietary fibre content, can be used as animal feed directly, while forest waste biomass requires mechanical pretreatment. In some cases when waste biomass is used as animal bedding, drying is requested. This subsection is illustrated in Figure 5.

Mechanical pretreatment involves processes such as cutting, shearing, and high-pressure homogenization, which reduce particle size and increase surface area [50]. Mechanical pretreatment is often the first step in the biomass conversion process and can be combined with other methods to enhance efficiency [51,52,53]. This step is crucial for making the biomass components accessible for further processing such as centrifugation, filtration, solvent extraction, drying, and chromatography are well known from the literature [54]. For example, beta-glucans are soluble fibres extracted from the cell walls of oats and barley. In the literature, several potential health benefits are given, like reduction in heart disease, reduction in glycaemic index, and lowering of cholesterol. Beta-glucans are also used in foods and cosmetics due to their texturizing properties [55].

4.1.2. Extraction of High-Value Components

The second step involves the extraction of high-value components such as proteins, lipids (also called extractives), xylooligosaccharides, and phenolic compounds, as illustrated in Figure 6.

Proteins

According to the Food and Agriculture Organization of the United Nations (FAO), the world’s population is projected to reach 9.7 billion by 2050 which will inevitably lead to an increase in protein needs. Likewise, the UN predicts that demand for protein will have increased by more than 50% by 2050 compared to 2020 levels [56]. Conventional protein sources found on the market today, such as soybean products, have negative environmental impact, contributing to almost 20% of tropical deforestation [57]. As a result, the development of alternative proteins is a necessity to contribute to the protein deficiency on the market. Lignocellulose biomass waste, including agricultural, forest, and marine residues due to their composition and availability represents low-cost carbon source for protein production, such as microbial proteins called single-cell proteins. Single-cell proteins (SCPs) are derived from several species of microorganisms, such as fungi, yeast, microalgae, and bacteria. An advantage of producing single-cell protein from lignocellulosic biomass as a carbon source is the rapid production due to the growth rate of microorganisms; its production does not depend on the seasonal and climate variations and it can be produced throughout the year, reducing greenhouse gas emissions [58]. The production of SCPs from lignocellulosic biomass involves the following steps: (1) the preparation of the carbon substrate, (2) choice of microorganism, (3) fermentation process, and (4) the separation of microbial biomass and its processing. The following aspects should be considered when choosing the carbon substrate for SCP production: accessibility, cost of the substrate, and of the pre-treatment method, transportation cost, and the concentration of protein in the final microbial biomass [59]. The selection of microorganisms for SCPs is very important as this will decide the foam formation, oxygen requirement, growth rate, tolerance to pH and temperature, genetic stability, and productivity/yield of specific low-cost substrates during the fermentation process [60].

Market applications. Due to their high protein content, amino acid profile, as well as carbohydrate, lipids, vitamins, and minerals content, SCPs can be utilized as protein-rich supplements or ingredients for both human and animal feed nutrition.

Commercial examples: Pekilo process was developed in the 1960s by the finish pulp and paper industry, where carbohydrates from sulphite liquor are used as a carbon substrate for filamentous fungus Paecilomyces variotii in a fermentation process to produce protein with an annual capacity of 10,000 tones [61,62]. Fusarium venenatum is one of the most commercially well-known fungal single-cell protein (SCP) species and is utilized to produce a meat alternative, QuornTM. It was successfully launched in 1985 and is currently one of the most well-known SCP products [60]. While traditional Quorn™ production utilizes refined glucose as the carbon source, the underlying bioprocess, fungal fermentation, can be adapted to valorize agricultural and food waste streams. While the Pekilo process and Quorn™ production are notable commercial examples, they represent partial valorization pathways. A more comprehensive example of cascading waste utilization can be seen in integrated biorefineries, such as the pilot-scale AgriProtein model, which utilizes organic waste (e.g., food and agricultural residues) to rear black soldier fly larvae. These larvae are processed into high-protein animal feed, while the residual biomass is further converted into biofertilizers and biogas [63]. This multi-output system exemplifies a cascading approach where waste is sequentially transformed into multiple value-added products, maximizing resource efficiency and minimizing environmental impact. Although such models are still emerging, they demonstrate the potential for the real-world application of cascading biomass valorization strategies.

Xylooligosaccharides

Xylooligosaccharides are non-digestible oligosaccharides, extracted from lignocellulosic fraction (especially agricultural wastes) composed of xylose units with prebiotic properties. Prebiotics are widely used as non-digestible food supplements mainly due to their different health benefits for various systemic disorders such as gastrointestinal, cardiovascular, neurological, inflammatory, oncological, and endocrine systems [64].

Extractives and Phenolic Compounds

Lipids, waxes, fat, phenolic compounds, and alkaloids are compounds belonging to this collective group termed extractives. The quantity and quality of extractives depend on factors such as genetic and geographical origin, climate, soil, and age [65]. Extractives have a wide range of applications, including medical and biotechnological uses, pharmaceuticals, and food preservation [66]. Lignans, also known as phytoestrogens, are found in plants, especially in seeds, grains, and legumes. Their structure is similar to estrogen, thereof their name phytoestrogens. Lignans are known for their antioxidant and anti-inflammatory properties [67]. Various phenolic compounds can be extracted from agricultural waste, e.g., cereal brans. They have been stated to have antioxidant effects, providing resistance against radical deterioration, cancer, and cardiovascular diseases.

Tannins, natural polyphenolic compounds found in various plants, have diverse industrial applications. In the leather industry, they are essential for tanning hides, transforming them into durable leather [68]. In the food and beverage sector, tannins contribute to the astringency and flavour of products like wine and tea. Their antioxidant, antiseptic, and anti-inflammatory properties make them valuable in pharmaceuticals and nutraceuticals [68]. Additionally, tannins are used in wood adhesives as a sustainable alternative to synthetic options [69], and in cosmetics for their beneficial effects on skin health. They also serve as metal chelators, binding to metal ions to mitigate their harmful effects. These versatile applications underscore the importance of tannins across multiple industries. Globally, the tannin market was valued at approximately USD 2.47 billion in 2022 and is expected to grow at a compound annual growth rate (CAGR) of 6.6% from 2023 to 2030 [68]. This growth reflects the increasing demand for tannins in various industries.

4.1.3. Chemical and/or Biological Pretreatment and Fractionation

Agricultural, forest, and marine waste biomass require pretreatment and fractionation to efficiently convert the oligomers (cellulose and hemicellulose sugars) to monomer sugars and produce biopolymers; see Figure 7.

Pretreatment and Fractionation

Acid pretreatment involves using dilute acids to hydrolyze hemicellulose, making cellulose more accessible for enzymatic hydrolysis [70,71,72]. Alkaline pretreatment targets lignin removal, improving the digestibility of cellulose and hemicellulose [73,74]. Deep eutectic solvents (DESs), a greener alternative to ionic liquids, have shown promise in lignin removal and enhancing enzymatic hydrolysis [75,76,77]. Organosolv pretreatment uses organic solvents such as ethanol, methanol, acetone, or organic acids to fractionate biomass into cellulose, hemicellulose, and lignin, facilitating their subsequent conversion [78,79,80,81,82]. Ionic liquid pretreatment effectively dissolves lignocellulose, although it faces challenges related to cost and environmental impact [77,83]. Oxidative methods such as ozonolysis and wet oxidation, are effective in breaking down lignin and enhancing biomass digestibility [83]. Ammonia fibre explosion (AFEX), SO₂, and hydrogen peroxide treatments are also used to disrupt the lignocellulosic structure, improving enzyme accessibility [83]. Enzymatic pretreatment utilizes specific enzymes to break down the cellulose and hemicellulose in biomass, making sugars more accessible for further processing [84]. This enhances the efficiency of downstream conversion processes, such as fermentation and biofuel production [85]. Emerging technologies, including ionizing and non-ionizing radiation, pulsed electric fields, ultrasound, and high pressure, are being explored as sustainable green pretreatment solutions for large-scale lignocellulosic biomass utilization. These methods offer promising alternatives for environmentally friendly processing [86].

Biopolymers

Cellulose, hemicellulose, starch, chitin, and alginate can be used in packaging, coatings, flexible films with superior properties, and even biomedical materials, making them crucial feedstocks for sustainable material development [87,88]. For instance, alginate is derived from marine biomass like brown seaweed and is composed of mannuronic and guluronic acid units. It is widely used in biopolymer production to make flexible films, particularly for food packaging, owing to its gel-forming and moisture-retention properties [87]. Starch is a naturally occurring polysaccharide found primarily in plant storage organs like tubers and seeds (e.g., potatoes, maize, and wheat). It is composed of amylose and amylopectin and serves as a major energy reserve in plants. In biopolymer production, starch is often used to manufacture biodegradable films, packaging materials, and compostable bags due to its good film-forming ability and availability from renewable sources. Cellulose, the most abundant organic polymer on Earth, is a structural component of plant cell walls, particularly in wood, cotton, and agricultural residues. It is a linear polysaccharide made up of β-1,4-linked glucose units. Cellulose can be processed into nanocellulose or cellulose derivatives (e.g., cellulose acetate) and used in packaging, coatings, and even biomedical materials, making it a crucial feedstock for sustainable material development [87,88]. Finally, chitin is a naturally occurring polysaccharide found in the exoskeletons of arthropods (e.g., crabs, shrimp, and insects) and the cell walls of fungi. It consists of N-acetylglucosamine units and is second only to cellulose in natural abundance. Chitin and its derivative chitosan are used in wound dressings, water purification, packaging materials, and biodegradable films due to their antimicrobial properties and biocompatibility [89,90].

Lignin, the second most abundant biopolymer on Earth after cellulose, constitutes 15–40% of lignocellulosic biomass and presents enormous potential for high-value polymer development. It is currently an underutilized by-product of the pulp and paper industry, particularly in countries like Sweden where kraft pulping is a major industrial activity [91,92]. Traditionally incinerated for energy recovery, lignin’s rich aromatic and functional structure allows for the development of renewable polymers and chemicals, presenting an opportunity to upgrade waste streams into performance materials. It can be used to create biocomposites for automotive parts [93], construction materials, and packaging [94]. Additionally, lignin can be converted into powdered activated carbon (PAC) for mercury sequestration in power plant flue gas, harnessing its similarity to lignite coal. It also has potential as a source of biofuels, contributing to sustainable energy efforts [95]. Furthermore, lignin can be transformed into various chemicals, including vanillin, phenols, and other aromatic compounds, offering diverse industrial applications [96]. The global lignin market size was estimated at USD 1.08 billion in 2023 and is projected to grow at a compound annual growth rate (CAGR) of 4.5% from 2024 to 2030 [97]. In terms of volume, the annual production of lignin is substantial, with estimates suggesting that over 50 million tons of lignin are produced worldwide each year. This production is driven by its increasing demand in various applications as described above [98]. Applications ranging from automotive components and construction materials to carbon fibres and packaging, underscoring lignin’s role in building a resilient bioeconomy while addressing waste management [91]. However, several challenges such as variability in lignin feedstocks, poor miscibility with most thermoplastics, and high brittleness need to be addressed through advanced process engineering, plasticization, and compatibilizer use [95]. These biopolymers need to be deconstructed to monomer form if the bioplastics shall be synthesized by polymerization (e.g., PLA and PHA).

Bioplastics

PLA production: PLA is typically produced from lactic acid (LA), which in turn is derived from carbohydrate-rich biomass such as corn, sugarcane, or food waste via bacterial fermentation [99]. While chemical synthesis of LA is possible, it is often limited by low yields of the desired L-LA isomer and higher costs. PLA’s versatility and biodegradability make it suitable for high-barrier applications in packaging, substituting conventional plastics like PS and PP [100].

PHA production: PHAs, on the other hand, are microbial polyesters produced intracellularly by various bacteria under nutrient-limited conditions with excess carbon sources. The type of PHA produced depends heavily on the carbon source provided. These may include gases (e.g., methane and CO₂), alcohols (e.g., glycerol and ethanol), sugars (e.g., glucose and xylose), organic acids (e.g., acetic, butyric, and valeric acid), and alkanes [101]. Within a cascade framework, such carbon-rich substrates can be derived from agricultural residues, food waste, or the volatile fatty acids produced during anaerobic digestion, thereby ensuring the integration and circularity of biomass valorization [102,103,104].

4.1.4. Energy Production and Recovery

The final step in the cascade processing chain is energy production and recovery. The residual biomass, which has minimal value for further material extraction, can be used for energy production through processes such as fermentation, combustion, and anaerobic digestion, as illustrated in Figure 8.

Annually in Europe approximately 950 million tons of biomass are produced, from which approximately 300 million tons of fuel equivalent to petroleum fuel can be produced. This data means that biomass waste can provide around 65% of Europe’s total oil consumption [105]. Waste biomass can be converted to bioenergy via thermochemical and biological conversion processes. The thermal conversion of biomass is a well-established method for generating energy and chemicals from renewable sources, significantly contributing to the reduction in fossil CO₂ emissions. For example, utilizing biological residuals, including wood (both primary and secondary woody biomass), agricultural byproducts, marine, and food waste, is essential for promoting climate-friendly renewable energy solutions [30,106,107,108]. In 2022, renewable energy accounted for 22% of the gross final energy consumption of the EU. Bioenergy represented approximately 60% of all renewable energy sources, with almost 70% of EU’s domestic biomass energy purposes being derived from wood (solid biofuels). Additionally, biogas, liquid biofuels, and renewable waste also contributed to the bioenergy mix [109].

Biogas

Biogas is a reliable renewable energy source, as its production is climate-independent, and biogas can be stored and used on request. This is advantageous compared to solar radiation and wind energy [110]. The global capacity of biogas production is increasing yearly. In 2023, 21,400 MW of biogas was produced [111], and Europe, Northern America, and China account for more than 80% of the global biogas production. Biogas is produced by the breakdown of organic matter, such as food waste, agricultural, marine and animal waste, the organic fraction of municipal waste, and industrial sludge (e.g., sludge from wood processing industry and aquaculture) through a process called anaerobic digestion. This process occurs in the absence of oxygen and involves different groups of microorganisms (especially methanogenic archaea) breaking down the organic material to produce a mixture of gases, primarily methane (CH₄; 55–70%) and carbon dioxide (CO₂; 30–45%). To a lesser extent, components like hydrogen sulphide (H₂S; 1–4%), nitrogen (N₂; 0–1%), oxygen (O₂; 0–1%), water vapour (0–1%), and trace elements are present [110]. The biogas yield and the concentration of the constituents will vary, based on the biogas source and the digestion conditions. Co-digestion of different substrates improves the biogas production potential, providing a better nutrient balance for the microorganisms involved in the degradation [110,112]. For example, fish sludge, rich in fats, has been verified as a promising co-substrate to increase the biogas yield [113]. For the fish farming industry in Norway, it is estimated that around 52,000 tons of nitrogen and 7000 tons of phosphorous are lost to the sea every year ([114], and references therein). Therefore, fish sludge is not only an energy source, but also a nutrient source. However, technology for collection and treatment before further use is immature, and investments must be made before applications for use in the production of biogas and soil fertilizer are enabled.

Biogas is generated in anaerobic digesters, which can be small-scale for household use or large-scale for industrial applications. It can also be collected from landfills. Biogas has a range of applications, such as the generation of electricity and heat, combustion systems, fuel for vehicles (when upgraded to biomethane), and feedstock to produce biobased chemicals. When purified, it can replace natural gas for cooking and heating [110,115,116]. For improved synergy, cost savings, and practicality, it may be beneficial to co-locate anaerobic digestion biogas plants with biorefineries. Industrial symbiosis like this can facilitate the production of biomethane for transportation fuel, as well as biobased chemicals and other biobased materials [116].

Using biogas has several environmental benefits. It helps reduce greenhouse gas emissions by capturing methane that would otherwise be released into the atmosphere. It also helps manage waste effectively and reduce odours and pathogens associated with traditional waste disposal methods.

A side stream from the biogas production is the digestate residue which is rich in nitrogen, phosphor, and potassium and can be used as a fertilizer in agriculture [117]. A drawback that must be considered when utilizing the digestate is the possible content of heavy metals, pathogens, and other pollutants [118]. Several reviews on biogas as a renewable energy source have been published in the recent years, e.g., [110,115,116,119,120].

Bioethanol

Bioethanol production from lignocellulosic biomass has become an economical and environmentally friendly substitute to fossil fuels [121]. Today’s bioethanol production mainly comes from first-generation bioethanol, with only small amounts of second-generation bioethanol available on the market [122]. The typical bioethanol production process using lignocellulosic biomass consists of pretreatment, hydrolysis, fermentation, and separation. However, the utilization of lignocellulosic biomass waste still represents some challenges, such as high cost of pretreatment, the recalcitrance nature of the biomass, the formation of inhibitors, and the fermentation process. Ghazali and Mustafa [123] summarized the advantages and disadvantages of first-, second-, and third-generation bioethanol production. Lignocellulosic-based biorefineries are researching ways to improve the effectiveness and expandability of the conversion procedures by using inventive pretreatment approaches, enhanced enzyme compositions, and refined fermentation technologies [121]. Borregaard’s Bali^TM produces 20 million litres bioethanol annually. The available commercial bioethanol plants are summarized by Raj et al. [124].

Ash

A byproduct of burning biomass, contains valuable nutrients such as calcium, potassium, magnesium, and phosphorus. However, it also includes potentially harmful heavy metals, like lead, zinc, and cadmium, which require careful handling. Global ash production from biomass incineration is estimated to be 170 Mt/yr [125], whereas Tosti and colleagues report a higher worldwide production of 480 Mt/yr [126]. Production varies based on feedstock type and biomass utilization degree [125]. Given its composition, ash can be used as a soil conditioner and fertilizer. However, factors such as plant nutrient availability, alkaline effects, and heavy metal concentrations [127], as well as separation pathways [128] must be considered. Regulatory requirements for the ash utilization differ between countries [126]. Ashes from industrial-scale wood incineration that cannot be reused result in significant landfill waste, which is space-demanding, and hinders renewability. Developing technical solutions for material flow separation and closing material cycles, will give a value creation for both plant nutrients and heavy metals. Eichenmüller and colleagues have used wet chemical extraction to separate the different substances [128]. Additionally, ash can be used as a raw material in the construction industry, such as in road construction [129] or as an alternative for cement in concrete production [130], contributing to CO₂ emission reductions.

4.2. Advantages of Cascade Processing

Cascade processing offers several advantages compared to traditional single-product approaches [131] such as (1) resource Efficiency: by extracting multiple high-value components from biomass, cascade processing ensures that resources are used to their fullest potential, reducing the need for additional raw materials; (2) waste reduction and lower greenhouse gas emissions: this approach minimizes waste generation and lowers greenhouse gas emissions by efficiently utilizing all parts of the biomass, thereby contributing to a more sustainable and eco-friendly production process; and (3) increased value creation: cascade processing enhances the economic viability of biomass utilization by producing a range of value-added products, thereby increasing the overall profitability and market potential. By following these steps, cascade processing not only maximizes the utilization of biomass but also aligns with the principles of a circular economy, promoting sustainability and reducing environmental impact. This approach enhances economic feasibility and closes material and energy loops, critical to the success of industrial biorefineries.

4.3. Integration of Energy Recovery in Cascade Systems

To achieve seamless integration in cascade processing, both upstream and downstream operations must be optimized in a coordinated and circular manner. Beyond optimizing parameters such as pH, temperature, and feedstock supply, a key opportunity lies in reintegrating the energy produced from residual biomass back into the cascade system [132].

For example, heat generated from combustion or biogas production can be reused for drying biomass, maintaining fermentation temperatures, or powering distillation units. Similarly, electricity generated from biogas or syngas can support mechanical pretreatment operations or drive membrane separation systems. This internal reuse of energy not only reduces external energy demand but also enhances the overall energy efficiency and sustainability of the biorefinery. Such integration transforms energy recovery from a terminal step into a functional input that supports earlier stages of the cascade, creating a self-sustaining loop. This approach aligns with industrial symbiosis principles and strengthens the economic and environmental performance of cascade biorefineries. The Borregaard biorefinery in Norway demonstrates the feasibility of such integrated systems, where energy and material flows are tightly coupled to maximize circularity and minimize waste [133].

5. Technological Innovations

5.1. Innovative Bioreactor Designs for Multi-Product Recovery

Innovative bioreactor designs are crucial for achieving efficient multi-product recovery in bioprocesses, enabling the simultaneous production and extraction of multiple value-added products. Advanced bioreactor configurations incorporate strategies to enhance process flexibility, substrate utilization, and microbial performance. These designs include multi-functional bioreactors capable of handling diverse feedstocks and resilient microbial strains, ensuring stable and high-yield production systems [134]. One of the most promising approaches involves integrated bioreactor systems that allow in situ product recovery (ISPR), minimizing product inhibition and enhancing overall productivity. For example, extractive fermentation using membrane bioreactors or biphasic systems facilitates continuous separation of desired compounds, preventing toxic accumulation and improving process efficiency [135]. Such systems are particularly advantageous in biofuel and biochemical production, where volatile compounds can be selectively extracted to drive equilibrium towards higher yields [136]. Photobioreactors (PBRs) are gaining attention for their effectiveness in producing biochemicals, biopolymers, and biofuels, particularly in microalgae-based systems. These reactors optimize light penetration, nutrient supply, and CO₂ utilization, promoting enhanced biomass productivity [137]. For instance, the LGEM photobioreactor system is designed to optimize light distribution and nutrient delivery, ensuring high production yields while minimizing operational costs [138]. Industrial-scale PBRs have demonstrated volumetric productivities of up to 1.5 kg/m³/day, with closed systems offering improved control and reduced contamination risks [139].

Such advancements in bioreactor technology improve the sustainability and economic feasibility of industrial biotechnology applications. Furthermore, hybrid bioreactor systems, combining features of stirred-tank reactors (STRs) with continuous cell retention technologies like perfusion bioreactors, are being explored to enhance volumetric productivity [140]. A notable industrial example includes Novozymes’ large-scale STRs used for enzyme production, operating at volumes exceeding 100 m³, with optimized mixing and aeration strategies to support high-density microbial cultures [141]. These systems have been shown to maintain high substrate conversion efficiencies while minimizing metabolic stress through advanced control of environmental conditions. To ensure optimal performance, such bioreactors are increasingly equipped with real-time monitoring and control systems. For instance, a typical setup may include optical dissolved oxygen (DO) sensors, pH probes, and capacitance-based biomass sensors, all integrated into a programmable logic controller (PLC) or distributed control system (DCS). These sensors continuously feed data into the control system, which adjusts parameters such as aeration rate, agitation speed, and nutrient feed in real time. For example, in perfusion systems, turbidostats or dielectric spectroscopy sensors are used to maintain a constant cell density by regulating the harvest and feed flow rates, ensuring steady-state operation and maximizing product yield.

These designs facilitate the efficient conversion of substrates into target bioproducts while maintaining optimal growth conditions for microorganisms. The design of these bioreactors includes features such as improved mixing, aeration, and temperature control, which are crucial for maintaining the health and productivity of the microbial cultures. Additionally, the integration of real-time monitoring and control systems ensures that the bioreactor operates under optimal conditions, further enhancing product yields and quality [142]. By integrating these innovative bioreactor strategies, the industrial biotechnology sector can achieve higher yields, improved substrate utilization, and reduced production costs, ultimately advancing the circular bioeconomy [143].

5.2. Integrated Upstream and Downstream Processing for Enhanced Bioproduct Recovery

Efficient bioprocessing requires the seamless integration of upstream and downstream processing to maximize resource utilization, improve product recovery, and minimize waste generation. Upstream processing involves the preparation and conditioning of raw materials through mechanical, thermochemical, and biological pretreatment methods, optimizing substrate availability for microbial conversion. Common strategies include steam explosion, dilute acid hydrolysis, and enzymatic hydrolysis, each tailored to break down complex biomass structures and enhance fermentable sugar release [144]. The effectiveness of these pretreatments directly influences downstream recovery, as well-processed biomass leads to higher yields and fewer impurities in subsequent purification steps.

Downstream processing, in turn, encompasses product separation, purification, concentration, and formulation, determining the final quality and commercial viability of bioproducts. Depending on the physicochemical properties of the target compound, separation techniques such as centrifugation, filtration, solvent extraction, and chromatography are employed [54]. For biofuels and biopolymers, membrane-based separation technologies and reactive extraction methods enable efficient product isolation with reduced energy consumption [145]. Advanced bioreactor systems, such as integrated membrane bioreactors (MBRs) and ISPR strategies, further streamline the transition from fermentation to purification by selectively removing target molecules while fermentation is ongoing, thereby improving process efficiency and reducing inhibitory effects [146].

6. Challenges and Research Gaps

The utilization of agricultural, forest, and marine waste biomass for sustainable food, feed, biopolymers, and bioenergy production faces several significant challenges and research gaps. To begin with, efficient harvesting and collection methods are required to minimize labour and environmental impact. The drying and preservation processes must address the high moisture content and prevent microbial spoilage [30]. Processing and extracting valuable components such as carbohydrates, proteins, and lipids is complicated by the biomass’s complex composition and high energy demands. Pretreatment remains a major hurdle for economic feasibility, as it can generate inhibitors like furfural and hydroxymethylfurfural, which interfere with downstream processes such as fermentation. Ensuring consistent quality and removing contaminants is essential for product safety. Biorefineries also face difficulties in separation and purification, which contribute significantly to production costs [147]. These inefficiencies result in higher operational expenses and reduced product yields, undermining economic viability [148].

High production and transportation costs, along with economic feasibility, remain key barriers. These are further compounded by regulatory and policy challenges. The biomass sector must navigate a complex and fragmented regulatory landscape that varies across regions and sectors [92], creating obstacles to efficient and widespread biomass utilization. Additional issues include feedstock unavailability, seasonal and regional variability, and storage limitations [149]. Harmonizing regulations and developing supportive policies at both national and international levels are essential for fostering sustainable biomass projects [150]. For example, the European Union has introduced directives such as Directive (EU) 2018/2001, which sets sustainability criteria for forest biomass, and Implementing Regulation (EU) 2022/2448, which provides operational guidance for compliance [151]. The development of standardized frameworks for assessing cascade processing systems is also critical. Such frameworks would enhance decision-making, resource allocation, and overall efficiency. Innovations in AI, public–private partnerships, and supportive policies can help address these challenges and align biomass utilization with circular bioeconomy principles [42,152,153].

Advanced, energy-efficient, and cost-effective separation technologies are a key research priority. Promising innovations include membrane technologies, advanced chromatography, and novel extraction methods [147]. For instance, membrane separation is being explored for its potential to reduce energy use and improve selectivity [154]. However, scaling these technologies from lab to industrial scale presents challenges related to integration, scalability (such as fouling and yield), and consistent performance [155]. Environmental concerns also arise from traditional separation methods, which often rely on solvents and chemicals. Therefore, developing green and sustainable alternatives is essential to minimize the environmental footprint of biorefineries [156].

There is a pressing need for standardized frameworks for assessing cascade processing systems [157]. Current methods lack uniformity, making comparisons difficult. A unified framework would support better decision-making, facilitate cross-comparison, and promote collaboration among stakeholders. It would also help assess environmental, economic, and social impacts, driving innovation and supporting circular bioeconomy. In summary, overcoming these technical, economic, and regulatory challenges through research and innovation is vital for the efficient and sustainable utilization of biomass in cascade processing. Enhanced separation technologies and standardized frameworks can significantly improve processing efficiency, reduce costs, and increase product quality, thereby advancing the field [157].

Health and Safety Considerations

The processing of agricultural, forest, and marine waste biomass involves several health and safety risks that must be carefully managed to ensure sustainable and responsible biorefinery operations. One of the primary concerns is the exposure to airborne particulates and bioaerosols during biomass handling, drying, and grinding, which can lead to respiratory issues among workers if not properly ventilated or filtered [158]. Additionally, chemical hazards arise from the use of solvents and reagents in pretreatment and extraction processes, some of which may be toxic, flammable, or corrosive [159]. Thermochemical and biochemical conversion steps may also generate hazardous by-products, such as furfural and hydroxymethylfurfural, which are known fermentation inhibitors and potential irritants [160]. Moreover, microbial contamination during fermentation or storage of high-moisture biomass can pose biological risks, especially when producing food or feed ingredients [161]. To mitigate these risks, it is essential to implement robust occupational safety protocols, including personal protective equipment (PPE), proper ventilation, chemical handling procedures, and regular monitoring of emissions and residues. The development of green and solvent-free processing technologies is also critical to reducing environmental and health impacts. As the biomass sector scales up, integrating health and safety assessments into process design and regulatory frameworks will be vital for ensuring long-term viability and public trust in bio-based industries.

7. Future Perspectives

The future of utilizing agricultural, forest, and marine waste biomass through cascade processing offers significant promise for sustainable bioproduction. This multi-step valorization strategy aligns with emerging technological, economic, and ecological trends, enabling the efficient extraction of high-value products while minimizing waste.

7.1. Innovations in Process Optimization

Cascade processing benefits immensely from recent innovations in process optimization technologies, particularly the integration of artificial intelligence (AI) and machine learning (ML). These tools are transforming biomass valorization by enabling predictive analytics, real-time process monitoring, and adaptive control mechanisms tailored to each stage of the cascade. For instance, AI-driven models can optimize sequential fermentation or enzymatic conversion steps, predicting ideal conditions for microbial consortia or enzyme cocktails, thereby enhancing product yields and reducing energy input. Furthermore, AI can assist in dynamic decision-making across the cascade, such as determining the most efficient extraction sequence or reallocating intermediate streams to alternative valorization pathways, ultimately improving overall resource efficiency and overall sustainability [162]. To provide a structured view of AI/ML integration in biomass utilization, we present a roadmap (Figure 9) outlining key developmental stages over the next decade. This framework includes milestones such as data acquisition, model development, pilot testing, industrial integration, and policy alignment. Each stage is associated with relevant KPIs to guide progress and evaluation. The roadmap complements our discussion on emerging technologies and highlights the strategic potential of AI/ML in advancing circular bioeconomy principles [163,164,165].

7.2. Role of Public–Private Partnerships and Funding Opportunities

The successful implementation and scaling of cascade processing approaches require coordinated efforts supported by robust public–private partnerships and targeted funding mechanisms. Collaborative frameworks involving government agencies, private industry, academic institutions, and non-governmental organizations are essential for accelerating the development and commercialization of cascade-based technologies. Strategic funding initiatives, such as the European Union’s Horizon Europe programme and the U.S. Bioenergy Technologies Office, are instrumental in supporting integrated biorefinery concepts that emphasize staged biomass utilization [166]. These partnerships also facilitate technology transfer, market creation, and the development of regulatory frameworks necessary for mainstream adoption of cascade processing.

7.3. Potential to Align with Circular Bioeconomy Principles

Cascade processing is inherently suited to the principles of the circular bioeconomy, offering a system-based approach to sustainable development. By sequentially recovering multiple products, ranging from high-value biochemicals and nutraceuticals to biofuels and soil amendments, cascade strategies exemplify resource maximization and waste minimization. This closed-loop methodology transforms waste biomass into a portfolio of bio-based products, reducing environmental impact and supporting economic diversification. Integrating cascade processing within the circular bioeconomy model reinforces the transition toward a resilient bioeconomy, lowers dependence on fossil resources, and supports climate goals through reduced greenhouse gas emissions [167].

8. Conclusions

This review offers a comprehensive synthesis of current strategies and innovations in the cascade processing of agricultural, forest, and marine waste biomass, contributing meaningfully to the advancement of circular bioeconomy principles. First, it presents a unified framework for multi-sectoral biomass valorization, integrating diverse feedstocks into a coherent cascade model that emphasizes resource efficiency and product diversification. Second, it bridges the gap between laboratory-scale innovation and industrial application by detailing advanced bioreactor designs, integrated upstream and downstream processing, and real-world case studies with capacity and yield data—demonstrating the scalability and economic viability of these technologies. Third, this review introduces a forward-looking roadmap for AI and machine learning integration in biomass processing, outlining key performance indicators and timelines to guide future digital transformation in biorefineries. Despite these contributions, this review is limited by the variability in biomass composition across regions, the scarcity of industrial-scale data, and the evolving nature of regulatory frameworks, which may affect the generalizability of some findings. Looking ahead, future research should focus on developing standardized metrics for evaluating cascade efficiency, scaling intelligent control systems with real-time sensor integration, and fostering public–private partnerships to align technological innovation with regulatory and market frameworks. By addressing these directions, cascade processing can evolve into a cornerstone of sustainable bioproduction, transforming waste into high-value outputs while supporting climate resilience and economic sustainability.