Biorefinery Based on Waste Biomass

In a broad sense, all materials with a biological origin can be called biomass, which includes those materials that have been obtained from the direct conversion of solar energy, such as plants and crops, their residues, the materials obtained from their industrial transformation as well as subproducts and residues and the organic fraction of the municipal solid wastes.

Millions of tons of waste biomass are produced every year, either as agricultural or forestry residues or as a result of agroindustrial transformations. As a general rule, these residual materials lack practical applications and represent both an economic cost and an environmental concern, as they need to be adequately handled.

A biorefinery is broadly defined as a facility where biomass is converted into a range of products including bioenergy and renewable (bio) chemicals. If waste biomass, e.g., crop residues, waste from industrial transformation processes, or municipal solid waste, are the feedstock for such an installation, then the benefits can be multiplied when the environmental effects of using waste materials as well as the relevant economic and social aspects are considered. The reduction of dependency on fossil fuels, a net contribution to climate change mitigation, along with higher industrial development and the creation of more jobs are just a few of the aspects that should be considered.

With the above idea in mind, and while also the Sustainable Development Goals into account, particularly those related to affordable and clean energy, decent work and economic growth, and climate action, the use of waste biomass can be seen as a certain opportunity that can be used to take advantage of a renewable, underutilized, and low-cost source of energy and products under the biorefinery scheme, enabling progress towards the circular economy.

The present Special Issue represents a humble contribution to achieve that opportunity through the study of the particular cases that are represented by the six articles included here.

Wastes derived from olive tree cultivation and the olive oil production process is the biomass of choice for three of these articles. Even if olive trees are typical in Mediterranean countries, the healthy properties that are attributed to olive oil consumption has resulted in many countries around the world becoming oil producers. The biomass that is associated with this industry represents a huge amount of the biomass that is produced every year, and in most cases, there is a lack of practical cases where this biomass is handled appropriately and adequately. In this SI, two of the main residues that result from olive oil production are used as starting materials for advanced bioconversion processes.

The work by Domenech et al. [1] is focused on the production of sugars from olive stones, the mean by-product obtained after olive oil extraction and that is currently used as a solid biofuel. In this case, a novel conversion process based on reactive alkaline extrusion is proposed, and the best operational conditions for sugar release are optimized. Considering that up to one million tons of olive stones are separated in the process every year, the relevance of proposing alternative uses is evident.

If olive stones are the main solid by-product of the olive oil production process, mill wastewater constitutes the most common liquid residue. Mill wastewater presents a high organic load and phenolic compound content, making it a real problem from an environmental viewpoint. In the article by Dias et al. [2], Candida tropicalis was selected from a number of microorganisms as being the species that is likely to be the most capable of reducing the negative effects of this wastewater. The capacity of the selected yeast to simultaneously produce added-value compounds in addition to the detoxification of the olive mill wastewater makes this an attractive option for treating these residues.

The works that were commented upon above deal with biorefinery applications that are based on a particular waste biomass that can be obtained in the olive oil sector. The third article that appears in this SI on this topic is aimed at optimizing the location of the biorefinery itself, where the above and other conversion schemes can be put in practice. For that, Cardoza et al. [3] have analyzed the region of Andalucía, which is where more than 70% of the olive waste that is generated in Spain is concentrated. The work uses geographical information system tools to quantify the amount of residue available in the study area, and other relevant elements, such as the environmental fragility, were overlapped to produce a map to determine where the best areas for biorefineries based on olive-derived biomass would be.

Lignin comprises one of the main fractions of biomass composition. It is also a source of renewable compounds that have a broad spectrum of applications in different sectors, such as in the cosmetic, pharmaceutical, and food industries. The work by Alves-Ferreira and coworkers [4] was focused on the lignin obtained from Cistus ladanifer, one of the most important natural shrubs in the Mediterranean basin that occupies nearly two million hectares in the Iberian Peninsula. The main goal of the process was to determine the operational conditions for the delignification of this feedstock to produce a cellulose-rich solid and a liquid in which the phenolic compounds derived from the lignin fraction can be obtained.

The valorization of lignin is also the objective of the interesting article by Pinto et al. [5]. In this article, the side streams of the pulp and paper industry are the starting point for the production of biobased polyols and were tested for their ability to synthesize rigid polyurethane foams. Additionally, the authors demonstrated that both materials can be combined using a chemical production method (vanillin and syringaldehyde), providing a clear example of biorefinery development.

In the work by Carrasco et al. [6], the raw material that is used is a major crop in Bolivia, i.e., quinoa; more precisely, the study is focused on the stalks that are left on the fields after the quinoa has been harvested, which currently have no application. However, their chemical composition includes a relevant content of cellulose and hemicellulose that can be hydrolysed for the production of fermentable sugars. The work then addresses the hydrothermal pretreatment of the raw material to improve the conversion yields following the enzymatic hydrolysis of the pretreated materials.

Globally, relevant aspects of biorefineries that are based on waste biomass have been covered in the selected articles. The Invited Editors are confident that they can contribute to increasing interest in the study of additional raw materials, technologies, and products based on waste biomasses.

The Editors want to thank all of the authors who have contributed to this Special Issue. Additionally, we want to acknowledge the editorial staff at MDPI for their help during the entire process that has allowed this SI to be published.