1.1 Introduction

Petroleum is one of the most important reserves used as a fundamental raw material for various industries. It has been in a predominant position in the world energy consumption structure since the 1970s. Petroleum‐derived products, such as plastics, synthetic fiber, and synthetic rubber, are widely utilized in the agricultural, chemical, and pharmaceutical sectors, and have already become necessities in our daily life. Most industries, like the chemical industries and transportation, are heavily dependent upon petroleum and other fossil resources.

However, this fossil‐based economy is facing two serious problems. On one hand, fossil resources are not renewable – they have limited availability on our planet and are running out at a rapid rate. On the other hand, the industrial utilization of fossil resources has already caused many environmental problems, such as greenhouse effects, and air, water, and soil pollution. The serious energy and environmental crisis has aroused global concerns and reflections. The world needs to find substitutes for fossil resources to change the current energy‐intensive and environmentally unfriendly economic model. Therefore, a low‐consumption and high‐value‐added sustainable circular economy system needs to be established. Such a circular economy can replace the linear economy model of “make‐use‐dispose” with a “circular” model, in which the value of resources and products is maintained in the system for a long period. The efficient use of waste or side streams from production processes is another important aspect of a circular economy.

While traditional energy resources, such as petroleum and natural gas, are non‐renewable and will be depleted in the near future, the substitution of emerging resources (energy and materials) and awareness of environmental protection outweigh seeking only economic profits and has become a significant worldwide issue. Besides the utilization of solar energy and wind energy, biomass energy, as an alternative form of energy derived from solar energy, has attracted an increasing amount of attention. Data from the US Energy Information Administration show that the percentage of biomass energy in total energy consumption has increased rapidly in recent years. For instance, in 2016, biomass energy contributed 5.8% to the source of US energy consumption. Biomass energy is a renewable (the only renewable carbon resource), clean (little pollution, low carbon emissions), and abundant resource (Field et al. 2008). It will greatly ease the energy and environment burden if biomass energy could be widely accepted and utilized in industries or in our daily life, and replace fossil resources as the preference in energy consumption. In fact, a global industrial revolution has already begun as the foundations of economic development change from hydrocarbon to carbohydrate, i.e., the transition from a petrol‐based economy to a bio‐based economy, which is a significant trend for sustainable development (Bozell and Petersen 2010) (Figure 1.1).

With the rapid development of industries and improvement in living standards, the generation of waste is also increasing, which has already caused many environmental and social problems (Sharholy et al. 2008). Landfill and incineration are the most commonly used methods of waste management at present. However, they are not ideal solutions owing to the damage to the environment and human health. In particular, organic wastes, characterized by putrescibility, low heat value and high organic matter, could become a great threat to public health, because of the emission of toxic gases (e.g., oxole, dioxine) and the transmission of pathogenic micro‐organisms when improperly treated by conventional methods (Polprasert and Koottatep 2017). But “wastes” can be regarded as valuable resources due to the functional components are present within them (Ong et al. 2018). It allows for their transformation into high‐value products rather than being discarded as useless and unwanted. Overall, this provides solutions to both efficient waste management and provision of feedstock for industrially important products, which are fundamental solutions for sustainable development (Figure 1.2).

Together with increasing demand in both substance and spirit, the world today is facing many problems related to food security, energy consumption, and environmental protection. The development of biomass‐based industries could be one of the great efforts made in order to change this situation. Biorefinery, aimed at sustainable development through utilisation of renewable (and/or waste) resources and integration of high‐efficiency technologies, can play an increasingly important role in achieving a green, circular, and sustainable economy.

In this book, we will take an overview of the development of biochemical processes for the utilization of wastes as a bioresource (Chapters 2 and 3), process integration for waste‐based biorefinery (Chapters 4–7), and closed loop recirculation of waste‐based biorefinery in a bio‐based economy (Chapters 8–10) (Figure 1.3).

1.2 Development of (Bio)Chemical Process for Utilization of Waste as a Bioresource

Wastes are often defined as substances that are no longer useful to the holder. The rapid industrial and economic development of recent years has seen a huge amount of waste generated from human activities, which has caused many environmental and social problems. In fact, most of the wastes we are talking about have the potential for further processing and utilization. The main reasons hindering the effective recycling of the wastes are improper handling strategies and inadequate technologies in related industries. Sustainable waste management should be carried out for both environmental and economic benefits.

A waste stream is the flow of a specific waste, referring to the lifecycle from its source to recovery, recycling, or disposal. In general, waste streams are mainly divided into two groups: material‐related streams (e.g., metals, plastics, bio‐waste) and product‐related streams (e.g., e‐waste, construction waste) (Bourguignon 2015). When talking about the biorefinery concept in waste valorisation, it aims to utilize waste as a bioresource, so we are more interested in the organic (or biodegradable) parts among the whole waste stream. Basically, the suitable sources for waste biorefineries are municipal/domestic waste, agricultural waste, industrial waste, forestry waste, and animal waste (Table 1.1).

In most cases, it is not efficient to convert the waste directly into products. Hence, before they are ready as substrates for biorefineries, certain pretreatments or modifications are required to make them accessible to the following downstream reactions, which may improve the conversion efficiency. Pretreatment methods are mainly classified into mechanical, physical, chemical and biological types, and each method has its advantages and disadvantages depending on the characteristics of the waste or specific need of the subsequent conversion (or production) process. Thus, it is an important consideration for the biorefinery management to decide the best pretreatment option. A brief summary of pretreatment methods is provided below.

1.3 Process Integration for Waste‐Based Biorefinery

The concept of “biorefinery” was first put forward in Science in 1982 (Bungay 1982). Derived from petroleum refinery, biorefinery aims to produce a variety of valuable chemicals, materials, and energy by maximizing the conversion efficiency and minimizing cost and waste generation. What makes it different from petroleum‐based biorefinery is the combination of facilities using biomass as feedstocks instead of fossil resources. Biorefinery overturns the stereotype of producing one product from biomass, as found in traditional chemical industries. Instead, it makes the most of every component of substrates and converts them into various products in order to achieve full resource utilization and value maximization. Biomass conversion is the core procedure of the biorefinery. Physical, chemical, and biological methods play important roles in this conversion process, such as mixing, separation, pyrolysis, modification, fermentation, and enzyme catalysis.

Based on the complexity of feedstock, combination of technologies and variety of outcomes, biorefineries are divided into three types: (I) single feedstock, single process and single major product; (II) single feedstock, multiple processes, and multiple major products; (III) multiple feedstocks, multiple processes, and multiple major products (Fernando et al. 2006). Biorefineries are capital‐intensive projects, and it would be very difficult, and also costly, to increase the productivity or conversion efficiency when the designer put too much weight on just one conversion technology. Obviously, type III biorefineries are the most advanced and optimal, because they can efficiently produce multiple outcomes to meet market demands and achieve profitability. Also, they are capable of using different substrates when required after structural adjustment, which makes them flexible in industrial practice. However, there is still a long way to go for the commercial existence of such integrated biorefineries. It is not easy to achieve the combination of multiple inputs and considerable outputs on a large scale, and in one platform. Nevertheless, there are extensive works under way in many places on the design and feasibility of such facilities (Clark and Deswarte 2008).

Along with increasing population and urbanization, waste generation as well as its accompanying problems has become a global concern. Waste biorefineries are able to offer satisfying waste management solutions given the current circumstances. Although there is no standard classification for existing waste biorefineries, in most cases we divide waste biorefineries into different types according to the origin of feedstocks. For example, there are biorefineries based on food waste, agricultural waste, industrial waste, and wastewater.

1.3.1 Food Waste Biorefinery

Food waste is the food mass left before or after human consumption throughout the supply chain, including those arising from production, handling, storage and disposal. It is among the most widely generated biowastes in the world, amounting to about 1.3 billion tonnes globally every year (Paritosh et al. 2017). Food waste can be roughly divided into two groups: avoidable food waste and unavoidable food waste. Avoidable food waste is edible before being discarded, and we can usually decrease this waste by changing consumption strategies; while unavoidable food waste mainly originates from food (processing) industries and needs to be properly disposed of or recycled.

The organic fraction is the dominant component of food waste – carbohydrates, proteins and lipids that can be converted into simple compounds like glucose, amino acids and fatty acids through further conversion processes. Anaerobic fermentation is the most promising process for food waste valorisation to produce biogas and volatile fatty acids (Dahiya et al. 2018). Metabolite‐rich effluents are generated during these specific processes. They have great potential to improve the conversion efficiency through integrating with other bio‐processes to obtain additional products such as biopolymers, bioelectricity, and animal feed (Figure 1.4).

1.4 Closed Loop Recirculation in a Bio‐based Economy

Landfill and incineration have been widely accepted as waste management strategies for a long time. However, drawbacks of these traditional waste management methods have already emerged. Landfilling is a serious waste of land resources, and without treatment, the buried wastes could result in serious problems in the future, such as heavy metal pollution, groundwater pollution, and fatal bacterial/viral diseases. Similarly, the disadvantages of incineration include public health problems and secondary environmental pollution (e.g., air pollution and acid rain).

In contrast, waste biorefineries have more opportunities in competition with traditional strategies. With the support of rich feedstocks and superior technologies, it realizes sustainable concepts by effective utilization of biomass resources and achieves numerous value‐added outputs. Developing waste biorefineries could contribute to improving environmental conditions through a shift in the consumption model, from fossil fuels to renewable resources, and efficient waste management. Environmental benefits that waste biorefineries bring, such as the abatement of greenhouse gas emissions and increasing energy savings, would have a positive impact on economic development, especially in developing countries.

Undoubtedly, waste biorefinery could be the optimal solution for a cost‐effective and environmentally friendly economy in such a rapid developing era. As a new concept, however, it still faces many challenges compared with traditional industries in a mature, petroleum‐dominant economy.

Abundant availability of feedstock is one of the outstanding advantages for waste biorefinery. But in most cases, these resources are season‐dependent and widely distributed. Collection, transportation and storage would be major issues for the early stages of waste management. In addition, technologies based on biology still need practical examination. They are usually more complicated than chemical technologies, which makes it difficult for bio‐based substrates to achieve high conversion efficiencies and considerable productivity. Furthermore, despite the fact that many bioproducts are cost‐competitive, low yields and low productivities increase their price and limit their popularity. This means that petroleum‐derived products are still in the dominant position in the market, despite a high oil price, thanks to well developed and highly efficient processing technologies. The rapid realization of economic benefits further makes the fossil‐based industries more favorable, while increasing competitive pressures for bio‐based processes and products.

Nevertheless, the increasing demand for environmental protection and sustainable resource utilization, and improvements in efficiency and productivity of bio‐based processes, will soon outweigh the quick yet non‐sustainable benefits of the petroleum‐based processes. This will promote the development of a bio‐based economy that is both environmentally and economically sustainable.

1.5 Conclusions and Future Trends

Waste biorefinery is full of potential to achieve green and sustainable waste management and bring considerable environmental and economic benefits. Environmental benefits include abatement of greenhouse gas emissions, natural resources conservation, and elimination of other detrimental impacts on the environment caused by traditional waste disposal. As for economic benefits, waste biorefinery helps to break away from the fossil fuel‐dependent situation and decrease the utilization of raw materials owing to widely available renewable waste resources. The development of innovative and optimal technologies could contribute to realizing cost‐efficient outputs of energy recovery and value‐added products from bio‐based refineries in a circular economy. This closed‐loop approach addresses sustainable development goals and will lead many economies to move towards an environmentally friendly development model, which is the reflection of a transitional trend to a circular economy.

The circular economy organizes economic activities into a feedback process of “resources – products – renewable resources,” compared with the “make‐use‐dispose” process of the linear‐increase economic model of traditional industries. It is characterized by low exploitation, high utilization, and low emissions, and all the substrates and energy under this concept can be rationally and circularly utilized for the long term. The circular economy minimizes the impact of economic activities on the environment, and combines green production with waste utilization, which in essence is a form of ecological economy. Core principles of this economic model facilitate the recycling and reuse of material though integrated sustainable approaches (Maina et al. 2017).

However, as upcoming green industries, waste biorefinery should be prepared for the opportunities and challenges when competing with traditional industries. The transition from the current petrol‐dominant economy to a bio‐based economy will require a systematic process involving many aspects. From the perspective of industrial chains, waste biorefinery shows its advantages during the adjustment of feedstocks, energy structure, and technology integration. Changing to renewable waste resources relieves the economic, environmental, and social burdens caused by the traditional linear economy. Integration of various processes enhances the efficiency of resource utilization and achieves considerable outcomes. However, there are still many problems, such as underdeveloped technologies, imperfect policies, and commercialization difficulties. Thus, in this significant period of social transformation toward a more sustainable (economic) system, those working in the area of waste biorefinery must be fully aware of its development needs, grasp opportunities, and strive in the competition. Finally, the objective is green and sustainable management, and the key aspect is to find a balance between innovation and regulation.

To allow optimal resource utilization and maximum economic benefit, a priority‐hierarchy model could be established. This model adopts a waste biomass utilization sequence based on the ranking of the value chain. For example, high value‐added industries, such as the chemical industrial chain, have the highest priority in the waste biomass distribution in order to increase the overall substrate utilization efficiency and economic value. Material and biofuel production would be the next. Subsequently, the residual biomass after distribution can be utilized in heating and electricity production. Additionally, “green policies” are needed to support and guide these industries, especially at the beginning of the research stage and/or industrialization. These policies would involve innovative research, green purchase, taxes, and green consumption, and they will contribute to promoting the development of bio‐based industries, raise public awareness of bioproducts, and stimulate green behavior and consumption.

The above aspects of transitioning to a circular economy based on valorisation of resources in a biorefinery scheme are discussed in detail in subsequent chapters of this book.

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