• Biorefineries are emerging new facilities to use organic waste as feedstock.

In response to the classical linear economic procedures, which have eventually led to an emergence related to critical environmental issues such as global warming and climate change, new strategies are being developed to break this linearity in favour of circular approaches by reducing the dependence on fossil fuels and introducing new ways of economic development. The objective of these novel paradigm is to implement sustainable production processes and development, according to the Sustainable Development Goals (SDG) promoted by the United Nations (United Nations, 2017). Some of these strategies include Circular Economy as a trend specifically designed to increase process sustainability.

In this framework, biorefineries are installations for the design of products and processes that reduce or eliminate the generation of substances that are hazardous for humans and/or the environment, replacing them by biomaterials (US Environmental Protection Agency, 2024). Biorefineries’ performance must be analysed across the life cycle of any obtained bioproduct (design, manufacture, use and disposal) to be really sustainable; therefore, Life Cycle Assessment (LCA) is an essential tool for their evaluation in terms of environmental impact. On the other hand, Circular Economy draws from concepts derived from Industrial Ecology, Industrial Symbiosis and Smart cities. Circular Economy is defined by the European Commission as: “A model of production and consumption, which involves sharing, leasing, reusing, repairing, refurbishing and recycling existing materials and products as long as possible”. In this way, the life cycle of products is extended (European Union, 2015), although other definitions can be found (Ellen MacArthur Foundation, 2024). Nevertheless, in all cases, Circular Economy is focused on the recovery of what is now considered waste to increase sustainability, developing a novel economy based on the paradigm of materials circularity within the production circle, avoiding the increasing generation of waste.

An important point when considering the environmental sustainability of new configurations of biorefineries is their analysis using powerful tools as Life Cycle Assessment (LCA). Although its application to complex biorefineries is still in an incipient point, several examples can be already found in literature. For instance, Gaffey et al. (2024) presents an excellent review of 59 preliminary LCA performed in different biorefineries. Since some of them are quite simple, it is evident that the amount of data necessary to perform rigorous analysis in more complex installations is still not available. Being some of these complex biorefineries in the designing stage, to obtain reliable and consistent conclusions about the environmental benefits of them will be one of the main lacks to cover in future research (Martínez-Arce et al., 2024).

The use of renewable materials and biodegradable products, the energy efficiency and the use of renewable sources of energy, coupled with pollution prevention, are some of the principles that Circular Economy and biomaterials have in common. In this context, the concept of biorefinery appears as a new installation including several processes to maximise energy and biomaterials recovery. A biorefinery is defined as “an industrial facility (or a network of facilities) that utilizes biomass as raw material for the sustainable production of a wide range of marketable products and energy” (Cherubini, 2010). More specifically, if energy and bioproducts are simultaneously produced, biorefineries achieve a high level of sustainability (Molina-Peñate et al., 2022). Additionally, it is noteworthy to note that biorefineries also present an excellent opportunity to recycle waste and by-products from industrial processes, which gives biorefineries the possibility of using locally available materials, being this point another important advantage. This review highlights the importance of two technologies for organic waste-based biorefineries: on one hand, the consolidation of anaerobic digestion (AD) as a provider of renewable energy and, on the other hand, the emerging role of solid-state fermentation (SSF) as a novel strategy to obtain a wide variety of bioproducts of added-value from organic waste and side products. Besides, in second term, AD is as a producer of digestate, an organic material that can be further processed in the biorefinery. The exponential presence that AD (European Biogas Association, 2022) will have in the overall picture of renewable energy, as a source of locally available raw materials, will result in an increasing amount of digestate, being an excellent starting material for SSF in the production of new biomaterials. Therefore, the success in the complementarity of both biotechnologies is a critical factor for the development of a future generation of biorefineries, which maximize the production of renewable energy and bioproducts substituting its chemically-produced twins.

Biorefineries use organic waste and biomass as input material. The heterogeneity of these materials according to its seasonal (quality and quantity) and geographical variations and source diversity, makes impossible to have a unique classification of biorefineries. This fact, jointly with the continuous evolution of biorefinery definition, has resulted in a classification that is periodically reviewed. The International Energy Agency proposes four categories to classify biorefineries (Annevelink et al., 2022):

1) Related to feedstock: it can be classified as primary biomass and secondary biomass. Primary biomass corresponds to feedstock from agriculture, marine, forestry and nature management. Secondary biomass corresponds to the residues produced from the use of primary biomass. Table 1 presents the main subcategories included in primary and secondary biomass, according to this classification. It is important to clarify that biorefineries can work on these two different types of biomass, being primary biomass raw materials whereas secondary biomass are biomass-derived waste (Lago et al., 2019; Anvari et al., 2024). Considering these different types, it is also worthwhile to mention that the technologies used in the corresponding biorefineries should be adapted for a proper performance.

As a result of this classification, lignocellulosic materials are the main feedstock, sugars are the dominant platform, and bioethanol is the main targeted product, although this situation is quickly evolving in accordance to bioproducts that are more interesting, especially those coming from SSF (Oiza et al., 2022).

One of the main objectives of modern biorefineries is to have a zero-waste process. Achieving this goal has large benefits in technical, environmental and economic terms (Sachdeva et al., 2022). In this framework, composting is a suitable technology as it permits a wide variety of raw materials to end up in an organic amendment. It is important to note that, sometimes, it is forgotten that composting was one of the first approaches to circular bioeconomy and this role remains intact (European Compost Network, 2024). Recently, composting has gain more interest when complemented with AD and SSF. Both processes result in organic exhaust material that is neither stable nor mature for some soil applications or for most of the horticultural uses. In this case, composting seems the most adequate strategy, especially for SSF exhaust matrices and the solid fraction of digestate.

In case of AD digested materials, composting has been largely studied to transform digestate into compost with enough stability (Colón et al., 2017). Besides, other advantages of composting have been highlighted to obtain an improved organic amendment with a better environmental performance: nutrient mineralization and humification, decrease of ammonia emissions and, obviously, a considerable shortening in the composting process and a decrease of aeration requirements (Chen et al., 2023; Czekała et al., 2023).

In the case of SSF, its complementarity with AD is beyond energy or compost production. However, the research on the use of exhaust solid waste after obtaining the product of interest is much scarcer. Anyway, two main trends can be observed in recent literature. On one hand, if the bioproduct has application to soil, SSF and composting simultaneously occur and the resulting product is stable enough, as in the case of some biopesticides and biostimulants (Ballardo et al., 2017; Ghoreishi et al., 2023). On the other hand, when a product is extracted from the fermented solid, composting can be a proper technology to obtain a compost-like product, to finally reach a zero waste process. In other cases, digestate can act as raw material to produce hydrolytic enzymes or biosurfactants from organic waste through SSF (Marín et al., 2018; Cerda et al., 2019). Some of these bioproducts can also act as additives to improve AD biogas yields, which is a novel concept of biorefinery that maximises the energy production without using external materials such as biochar or nutrients (Romero-Güiza et al., 2016; Parra-Orobio et al., 2023). For instance, in the case of biosurfactants obtained from municipal solid waste through SSF, the strategy is highly beneficial, as some of these bioproducts are reported to enhance the production of biogas in AD, as the case of sophorolipids (Xu et al., 2019; Oiza et al., 2024). In the case of certain enzymes, they are also reported to act as enzymatic cocktails to shorten the hydrolytic limiting step of AD when treating complex substrates as the source-selected organic fraction of municipal solid waste (Martínez-Valdez et al., 2017).

Biorefineries are called to be advanced substitutes for waste and wastewater treatment plants, in a context where Circular Economy paradigms may be implemented to the detriment of linear strategies, which result in poor recovery of energy and material resources and a high generation of waste to be disposed of. In this framework, modern biorefineries should implement novel biotechnologies, such as SSF, which is in line with the basic principle of circularity: transform waste into raw and profitable materials. SSF needs research and reliable strategies of scaling-up to be a reality in the biorefineries of the next future, to take profit of its huge potential. However, the role of traditional technologies as AD or composting must not be diminished. Particularly, AD will be the main process to provide renewable energy to these facilities, while reducing their carbon footprint. Composting, as a way to produce a stable organic amendment for soil application, must be considered complementary to AD and SSF, with a high relevance in achieving zero-waste processes. In any case, it is evident that these new configurations of biorefineries should be environmentally evaluated with specific tools such as LCA. In summary, the exponential increase of AD (and inherently digestate) will result in a considerable change in the waste management overall picture, which will place AD, SSF and its complementarity in a key position regarding the success of future organic waste-based biorefineries, which should be optimized in terms of renewable energy generation and the variety of bioproducts that may be substitutes of their chemical alternatives.