According to the United Nations, waste is made up of “materials that are not prime products (that is products produced for the market) for which the generator has no further use in terms of his/her own purposes of production, transformation or consumption, and of which he/she wants to dispose. Wastes may be generated during the extraction of raw materials, the processing of raw materials into intermediate and final products, the consumption of final products, and other human activities”.¹  As the worldwide population, urbanization and economic development evolve and consumption patterns change, solid waste generation is mounting at an alarming rate, with each individual producing an average of 0.74 kg of waste per day (range 0.11–4.54 kg).²  In 2016, circa 2.01 × 10⁹ tons of waste were generated globally, and this value is expected to reach 3.40 × 10⁹ tons by 2050.²  Waste represents a threat to both the environment and human health due to its significant contribution to greenhouse gas emissions and pollution, negatively affecting climate change and public health.^3–5  When incorrectly disposed and/or poorly managed, waste may contaminate the ecosystems, promote diseases, and hamper economic development by limiting socio-economic activities.^2,4  It may also have important societal and legal impacts, with opportunities for illegal activities at various stages of waste management.⁶ 

According to the United Nations Environment Programme (UNEP), waste management is a basic human need, also regarded as a basic human right.⁷  As a way to mitigate the environmental, societal, and health burden of waste generation while boosting resource efficiency, governments are forced to take action.⁴  In line with this, several strategies for waste management have been adopted, including waste reduction/avoidance or waste valorisation.^3,8  For that purpose, the implementation of effective waste prevention and management actions along with a transition from a “take-make-use-dispose” to a circular economy, in which waste is considered a valuable resource, is critical.^3,8–10  In this chapter, waste generation and composition are described and the evolution of waste management systems is discussed in light of the major challenges and innovations in the field.

Waste has been produced since the beginning of civilization; and humans have been developing strategies to deal with the different types of waste ever since (see Figure 1.1). The first evidence of waste management dates back to 3000 BC in Knossos, Crete, where waste was placed in pits in the ground and covered with soil.^11,12  At that time, waste management strategies were already accompanied by recycling activities. During the bronze age (2000 BC), the Greeks and other Europeans started to melt down old and broken metal items¹¹  to produce new items, in what can be considered as the start of metal recycling. At about the same time, i.e., 2000 BC, in China, waste composting and bronze recycling were already part of the population's daily routine.¹³ 

By 500 BC, in ancient Greece, the first waste law was introduced, requiring garbage to be dumped at least one mile away from the city (Athens). During the Roman Empire, a publicly organized waste management system was implemented.^14,15  At that time, waste collection systems were mostly intended to protect the health of the inhabitants and to improve the esthetic appearance of the cities,¹²  with waste being dumped in areas away from the cities. During the Middle Age, waste collection systems were not improved and the disposal of garbage and human and animal excreta directly into the streets was a common practice. Consequently, in England, in the late XIII century, a law enforcing all householders to keep the front of their house clear from garbage was adopted, and waste collectors “Rakers” were employed across London to rake and remove trash from the streets.¹¹  With the bubonic plague (XIV century), the impact of waste in the dissemination of disease highlighted the importance of maintaining the streets clean from human and animal excreta and other types of waste. This was later reinforced by neo-Hippocratic medicine, which associated the excess of mortality and disease in the cities with the unhygienic environment, prompting the implementation of new policies and waste management techniques in Europe.¹⁵  From the 1770s to the 1860s, almost all urban waste was used in agriculture or in industry. Human and animal excreta as well as food residues produced in cities were used as fertilizers in agriculture, for example. Plant rags were used to produce paper and bones and other animal parts were used to produce grease, glue, gelatine, and candles, among many other products.¹⁵  In those times, waste was considered as a valuable resource and a source of economic profit. Cities, more populated than the countryside, generated higher amounts of waste and thus, during the XIX century, they were the main source of raw materials to be used in agriculture and industry.⁷  The waste from industry was also considered as a resource; cotton waste from Manchester's textile industry, for example, was used for papermaking.¹⁵  Therefore, the first wave of industrialization was accompanied by a growth in the recycling sector¹⁶  and a circular model of development was in place. However, as the industrial development increased, the demand for raw materials also increased and the generation of waste failed to reach this demand. Furthermore, the development of synthetic alternatives to fertilizers and the development of the coal and later the petroleum industry provided low-cost materials, turning waste into a disposable item.¹⁵  Since their recycling was no longer profitable, they ended up being dumped into the environment, and strategies to deal with this increasing amount of unnecessary waste were developed such as incineration (without energy recovery) and landfilling. The culture of take-make-use-dispose started to flourish and not even the war period with the scarcity of raw materials and the inevitable return to reuse/recycling prevented the establishment of this new consumption paradigm based on a linear development model. By the fifties, “Throwaway Living” as coined by Life Magazine in its August 1955 issue¹⁷  was the norm. The huge amount of waste produced was either incinerated or deposited in landfills, preferably away from the cities, with this latter solution being more common in the USA where the available land was more abundant than in Europe or Japan. Yet, with the exponential growth of the cities, these facilities were no longer “far of sight”. The undesirable proximity of suburban communities with waste facilities potentiated by the NIMBY syndrome (Not In My BackYard) lead to some concerns in the affected populations. With increasing evidence of the deleterious effects of waste on the environment and human health along with the environmental crisis of the 1960s and 1970s, waste started to be regarded as a global problem and legislative measures were adopted.¹⁵  The Organisation for Economic Co-operation and Development (OECD) universally adopted the “Polluter pays” principle in 1972 and in Europe, for example, the Waste directive was adopted in 1975 (Council Directive 75/442/EEC of 15 July 1975 on waste), while in the United States the Solid Waste Disposal Act was adopted in 1965 and the Resource Conservation and Recovery Act in 1976. In 1989, the Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and their Disposal was adopted. Despite these measures, waste production continued to increase (see Figure 1.2).¹⁸ 

In Turkey, for instance, an increase from 10 000 × 10³ tons in 1975; to 31 352 × 10³ in 2005 and to 34 533 × 10³ in 2018 occurred.¹⁸  In the United States, a similar increasing tendency was observed (116 212 × 10³ tons in 1975; 230 180 × 10³ in 2005 and 265 225 × 10³ in 2018). On the other hand, countries like Japan witnessed in recent years a decrease in municipal waste production (from 42 094 × 10³ tons in 1985 (no data available for 1975), to 52 720 × 10³ in 2005 and then back to 42 716 × 10³ in 2018). This decrease over the last decade was a consequence of the substantial investments in waste management and recycling made by the Japanese government.¹²  Similarly to Japan, the European Union also adopted legislative and financial incentives toward waste reduction and waste valorisation with the goal of reaching a zero-waste society (see Box 1.1 and Section 1.6). Nevertheless, the results on EU waste reduction over the last decade have been disappointing, with a slight increase in the average amount of waste generated per capita in the EU-27, from 4881 tons per capita in 2008 to 5234 tons per capita in 2018 (see Figure 1.3).¹⁹ 

These results highlight the difficulties associated with implementation of efficient waste management practices, which are deeply associated with waste composition itself and the disposal and treatment routes.^20,21  Generally, waste management struggles with the diversified, complex, and variable nature of waste, making the choice of the ideal collection, disposal, and treatment routes a difficult task.^20–22  In the following sections, the composition of waste is described and challenges associated with its management are discussed.

Global municipal solid waste composition is depicted in Figure 1.4 and, according to the World Bank, can be categorized within five types:²  (i) food and green, which entails food, yard, and green waste; (ii) dry recyclables, which correspond to plastic, paper and cardboard, metal, and glass; (iii) rubber and leather; (iv) wood; and (v) others that can arise from sources other than households. Although waste composition highly depends on a countries’ income level, food and green waste is the most prominent, accounting for 44% of global waste composition. Dry recyclables follow, representing 38% of global waste composition.

Additionally and beyond municipal solid waste, there is the need to deal with other types of waste: (i) industrial waste; (ii) agricultural waste; (iii) construction and demolition waste; (iv) hazardous waste; (v) medical waste; and (vi) e-waste.²  The global generation of this “special waste”, in particular industrial waste (12.73 kg of waste per capita per day), outpaces by far that of municipal solid waste (0.74 kg of waste per capita per day)²  – cf. Figure 1.5. Due to its nature, most “special waste” may pose a significant risk of environmental contamination, so that their management requires additional caution and it is generally treated in specialized facilities and subjected to specific regulations (e.g. Basel Convention).

After collection, the disposal and treatment of waste is commonly performed via at least one of the following techniques, depicted in Figure 1.6: (i) composting; (ii) incineration; (iii) landfilling (controlled, sanitary, or unspecified); (iv) open dumping; (v) recycling; and (vi) others (typically open burning of waste).²  Waste is mostly sent to landfills (37%) or illegally discharged in open dumps (33%), whereas a smaller fraction is used for materials or energy recovery (13.5% by recycling, 5.5% by composting, and 11% by incineration) – cf. Figure 1.6.

This scenario, where landfills, open dumping, and incineration are the preferred routes, clearly compromises the sustainability of the entire system and more environmentally sound solutions must be adopted.²³  The modern vision is that waste prevention should be prioritized, followed by material reuse and recycling. When a “new life” for waste is not possible, recovery should be considered, with disposal always being the least preferred option. This prioritization of waste, translated into the Waste Hierarchy (WH), was proposed in 1975 by the European Commission (Council Directive 1975/442/EEC) and is still the basis of the EU waste policy – the Waste Framework Directive – WFD; 2008/98/EC – cf. Box 1.1. Yet, despite being introduced for the first time in 1975, this ranking of waste management practices is still not completely implemented (not even in Europe and other developed nations). Figure 1.7 depicts the evolution of the waste management practices highlighting the waste hierarchy as currently adopted under the Waste Framework Directive (WFD; 2008/98/EC) by the European Commission and other Intergovernmental Agencies.^7,12,14,24 

The development and innovation in waste management have been determined by several factors, including: (i) public health; (ii) environment; (iii) resource value of waste; and (iv) climate change. These factors are key drivers in the Integrated Sustainable Waste Management Framework proposed by the United Nations.²⁵  The first driver, public health, aims to promote healthy living conditions, particularly through an efficient waste collection service, whereas with the second driver the protection of the environment is envisioned.^7,25  This environmental protection should occur throughout the waste chain, particularly during treatment and disposal. The third driver, the resource value of waste, aims to ‘close the loop’ by preventing waste, promoting reuse, recycling, and recovery of both materials and nutrients.^7,25  Climate change, the most recent key driver of sustainable waste management, stresses the need to optimize waste management systems, with strong emphasis on reducing and diverting biodegradable waste from landfill in order to reduce greenhouse gas (GHG) emissions, including methane and other short lived climate pollutants (SLCPs).^7,25,26  According to the IPCC report,²⁷  in 2010, solid waste management accounted for around 3% of global greenhouse gas (GHG) emissions, with most of that attributable to methane emissions from landfill sites. Changes in the current waste management scenario including landfill mitigation and diversion, production of energy from waste, recycling, and other types of improved solid waste management would result in 10 to 15% reduction in global GHG emissions. This decline in GHG emissions could be further reduced to 15 to 20%, if strategies for waste prevention are adopted.⁷  These figures clearly stress the need to develop new strategies for more efficient waste management systems.

As described in Section 1.2, throughout history, waste has been handled in different ways, from uncontrolled dumping, to advanced recycling and resource recovery, up to the recently envisioned zero-waste system. Some of these strategies are associated with specific innovations and therefore the history of waste management can be translated into different innovation waves as highlighted by Zaman and Lehmann¹³  and summarized in Figure 1.8. The first wave of innovation corresponds to open dumping and regrettably is still used in many low-income countries.^13,25,26  The second wave of innovation comprises the establishment of uncontrolled landfills, and the first evidence of this practice dates back to 3000 BC in Knossos, Crete.^11,13  Waste composting corresponds to the third wave of innovation and the available evidence indicates that domestic waste composting was already used in China as early as 2000 BC (see Section 1.2).^11,15  The fourth wave of innovation corresponds to recycling and controlled landfill with several examples dating back to the Roman empire, where old or damaged metal items were recycled in order to be used again.^11,13,15  During the XVIII century in the first wave of industrialization, recycling gained even more importance¹⁵  as previously discussed (see Section 1.2). The fifth wave of innovation occurred mostly during the XX century and includes the development of new technologies in order to transform waste into energy, including for example, incineration, pyrolysis-gasification, advanced biological treatments, and advanced recycling and resource recovery facilities.^12,13  More recently, the pursuit of a more sustainable society, in which waste can be regarded as a valuable resource, lead to the development/implementation of sustainable resource consumption patterns and the resource recovery from waste with the ultimate goal of reaching a zero-waste system.¹³  Most of these innovations are based on the cradle-to-cradle design system, which can be defined as “the design and production of products of all types in such a way that at the end of their life, they can be truly recycled (upcycled), imitating nature's cycle with everything either recycled or returned to the earth, directly or indirectly through food, as a completely safe, nontoxic, and biodegradable nutrient”.²⁸ 

As previously highlighted, the strategies to solve the waste problem must be defined within an integrated view, considering not only environmental and human health but also socio-economic aspects. Furthermore, because waste is a global issue, waste management should be addressed at the international level. In line with this, the work developed by the United Nations has played a pivotal role toward the development of a global sustainable waste management system. The symbiotic relationship between the seven Global Waste Management Goals established by the United Nations Environment Programme (UNEP) and the International Solid Waste Association (ISWA) and the seventeen Sustainable Development Goals (SDG) described in the United Nations 2030 Agenda for Sustainable Development^4,5,29,30  provide an excellent articulation between the environmental, economic, and health challenges associated with waste. The Sustainable Development Goals were adopted in 2015 by the United Nations as a universal call to action to end poverty, protect the planet, and ensure that by 2030 all people enjoy peace and prosperity. They encompass 17 integrated goals: No poverty (SDG1); Zero hunger (SDG2); Good health and well-being (SDG3); Quality education (SDG4); Gender equality (SDG5); Clean water and sanitation (SDG6); Affordable and clean energy (SDG7); Decent work and economic growth (SDG8); Industry, innovation, and infrastructure (SDG9); Reduced inequities (SDG10); Sustainable cities and communities (SDG11); Responsible consumption and production (SDG12); Climate action (SDG13); Life below water (SDG14); Life on land (SDG15); Pease, justice, and strong institutions (SDG16); Partnerships for the goals (SDG17). These goals recognize that action should be integrated and that the development must balance social, economic, and environmental sustainability.

The Global Waste Management Goals as proposed in 2015 by the UNEP and ISWA⁷  are: (1) ensuring access for all to adequate, safe, and affordable solid waste collection services, by 2020; (2) eliminating uncontrolled dumping and open burning, by 2020; (3) ensuring sustainable and environmentally sound management of all waste, particularly hazardous waste, by 2030; (4) reducing waste generation substantially through prevention and the 3Rs (reduce, reuse, recycle) and thereby creating green jobs, by 2030; (5) halving global per capita food waste (the biggest fraction of waste generated globally) at the retail and consumer levels and reduce food losses in the supply chain, by 2030; (6) creating green jobs through the circular economy and building sustainable livelihoods by integrating the informal sector into mainstream waste and resource management in the poorest cities; and (7) reducing industrial waste generation at source through waste prevention, resource efficiency, and greater adoption of clean and environmentally sound technologies and industrial processes by 2030.^4,7  Overall, these goals directly deal with questions related to SDGs, as depicted in Figure 1.9. They tackle poverty (SDG 1), hunger, food security, nutrition, and sustainable agriculture (SDG 2), health and well-being (SDG 3), water and sanitation (SDG 6), energy (SDG 7), as well as to making cities more resilient and sustainable (SDG 11).^7,29  Moreover, attaining Global Waste Management Goals will contribute to protect and preserve marine resources and terrestrial ecosystems (SDG 14 and 15) as well as to fight against climate change (SDG 13).^7,29  In addition, economic growth and employment (SDG 8), sustainable consumption and production patterns (SDG 12), as well as inclusiveness, safety, resiliency, and sustainability of cities, infrastructures, and industries (SDG 9 and 13) will be nurtured.^26,29 

Overall, tackling waste management goals will facilitate early progress against 12 of the 17 SDGs. In order to succeed, it is necessary to adopt an economic model able to maintain the entire utility and value of materials and products and to valorise wastes.^31,32 

In the modern era marked by the massive consumption of short-lived and disposable goods as well as intense industrial activity, the “take-make-use-dispose” economy described in Section 1.2 still prevails.³³  The linear flow of materials, in which current consumption and production patterns rely, contribute to the huge levels of waste generated worldwide. As represented in Figure 1.10, if one can reframe those into circular flows – by developing a circular economy – unused or end-of-life goods could serve as resources and raw materials to be reused, re/upcycled, or remanufactured within the same or even other industries.^10,33–35  In other words, by looking at the entire value chain and life cycle of a given product, waste generation can be alleviated and unavoidable waste valorised, thus sustaining resource efficiency and sustainable development.^32,33,36,37  With this aim, the European Commission proposed the Closing the loop – New circular economy package, highlighting the benefits that industries and consumers can enjoy from both environmental and economic viewpoints: (i) value will be generated from products and materials that no longer serve through recovery of high-value compounds; (ii) the carbon footprint will be mitigated by using recycled materials; (iii) industries will depend less on external resources; and (iv) industrial competitiveness in Europe will be enhanced.^9,31,38  Within this framework, industries are encouraged to develop their products in a more conscious way by prioritizing durability and their ability to be reused and recycled.^9,10,34,38  It should be stressed that assuring that the quality and safety of the recovered and recycled materials is at least equivalent to that of the original ones is critical due to consumer scepticism.³⁹  In addition, by fomenting a zero-waste approach through prevention and valorisation, instead of common approaches (e.g., energy recovery and landfill), a more sustainable management system can be accomplished.^{9,21,32,36,38}  In fact, as recently summarized by the Ellen MacArthur Foundation, the circular economy “offers a solutions framework for economic renewal, innovation, and industrial transformation. Through new forms of value creation that emerge from redesigning production and consumption systems, the circular economy is an innovation agenda that can lead to better growth”.⁸  For instance, circular economy has been suggested as highly promising in the context of food waste^16,40,41  and dry recyclables^42–44  as well as agricultural waste,⁴⁵  medical waste,^46,47  mining waste,⁴⁸  and e-waste.^49,50  The circular economy also contributes to tackling climate change, by: (i) eliminating waste and pollution to reduce GHG emissions across the value chain; (ii) keeping products and materials in use to retain the energy embodied within them; and (iii) regenerating natural systems to sequester carbon in soil and products.

Recently, industries across different sectors have started to transition toward a circular economy, and governments have started to develop and implement circular economy roadmaps and strategies. The support for circular economy initiatives is also emerging in the financial sector. Several initiatives have been launched, including a measurement tool for assessing progress toward a circular economy across a company's operations (e.g. Circulytics). With the revolution already under way, it is now essential to define the target goals in order to allow the scale-up of this transition. In line with this, in January 2021, the Ellen MacArthur Foundation established five universal circular economy policy goals, around which governments and businesses can align to achieve their common objectives.⁸  These goals are: (i) stimulate design for the circular economy; (ii) manage resources to preserve value; (iii) make the economics work; (iv) invest in innovation, infrastructure, and skills; (v) collaborate for system change. With the establishment of these goals, a path forward is provided and it is expected that they will foster circular economy innovations to emerge at scale. It should be stressed that the transition toward a circular economy requires the cooperation between different sectors and the involvement of several factors, including for example governments, intergovernmental organizations, financial organizations, private sector, non-governmental organizations, scientific and educational organizations, civil society, and media and social networks.²⁶  Within this integrated collaboration lies the possibility to develop novel and innovative technologies able to transform society. Governments, for example, may have an important role in the implementation and enforcement of chemicals and waste policies, in the adoption of reuse and recycling standards and on the development of strategies to meet WHO guidelines for pollutants.²⁶  Intergovernmental organizations facilitate international cooperation in science-policy interfaces and are pivotal for the advance in UN system-wide efforts including the promotion of synergies among scientific assessments and multilateral environmental agreements through norms, implementation, financing, capacity-building, and technological cooperation.²⁶  Financial organizations provide financing to develop and implement improved waste management, throughout all stages of the process including research and development activities. The private sector plays an important role, being responsible, for example, for (i) reducing waste and resource use and encouraging sharing, reuse, and recycling; (ii) promoting and supporting plastic free/environmentally friendly packaging, which will contribute to waste prevention; (iii) conducting transparent risk assessments of the impact of chemicals on the environment and human health; (iv) increasing the use of green chemistry, investing in waste recycling, and setting high standards for waste disposal.²⁶  Non-governmental organizations (NGOs) by advocating for policies and regulations that promote investment in sustainable development, and by working directly with communities and local municipalities, play a critical role in the implementation of any major societal change as the zero-waste society envisioned. Scientific and educational organizations are responsible for the education of the next generation of citizens and are also responsible for an important fraction of the R&D work that is the basis for the innovations in the waste sector. Media and social networks are responsible for raising awareness and informing the civil society and the other players of the major challenges, achievements, and breakthroughs in the waste management sector.

Generally, the transition from an unsustainable waste management system toward a sustainable one requires that the processes involved are also sustainable. This can only be achievable if the new valorisation processes, as well as the recovered products, are aligned with the twelve Green Chemistry principles.⁵¹  In fact, as recently recognized by the United Nations,⁵²  Green Chemistry is challenging chemistry in order to provide a more sustainable process, while enabling innovation in the chemistry field that will in turn contribute to sustainable development. Green metrics can be employed to assess the real sustainability of chemical processes, in particular the “E(nvironmental)-factor” proposed by Roger Sheldon, which represents the ratio between the mass of waste generated during production and the mass of final product obtained.⁵³  In the chemical industry, however, the “E-factor” reported for fine chemicals (5–50 kg of waste per kg of product) and pharmaceuticals (25– > 100 kg of waste per kg of product) production are very high.⁵³  Still, one cannot forget that fine chemicals are globally produced at a lower weight than e.g. petrochemicals, and some debate may arise. Even though in all cases, the design of chemical processes should envision less wasteful and less toxic reactions, safer products, innocuous solvents and auxiliaries, the use of renewable feedstocks and raw materials, among others.⁵¹ 

Obtaining high-value chemicals from waste by chemical or biological conversion as well as by direct dissolution, extraction, and separation are promising approaches to replace land disposal and incineration.^{22,39,45,54–60}  Such unconventional processes, however, are still not competitive with traditional, well-established approaches.^39,61  First, the economic viability is compromised by the high capital investment, high operational costs, and modest recovery efficiencies.⁶¹  Second, the energy consumption is high due to the extreme conditions of pressure and temperature as well as the multiple reaction/operation steps required.^39,61  Third, unconventional processes struggle with high levels of hazardous waste and high toxicity of the chemical auxiliaries and solvents usually involved.^39,61  To pursue a sustainable waste management approach, the commonly used volatile organic solvents should be replaced by “solvent-free” conditions whenever possible; most processes are, however, solvent demanding and alternative solvents are viable candidates.^53,62  In general, alternative solvents are cleaner, safer, and easier to recycle, while keeping or improving the efficiency of traditional techniques and the quality of the recycled/recovered products.^63,64  Examples of alternative solvents include water, supercritical fluids, fluorous solvents, ionic liquids (ILs), and deep eutectic solvents (DES).⁶³  The application of a particular class of alternative solvents, namely ILs, in the valorisation of diverse categories of waste through recovery of high-value compounds will be overviewed and discussed in this book.