Exploring glass recycling: Trends, technologies, and future trajectories
Article information
Abstract
Glass is a solid-like, chemically stable, and transparent material that is commonly used in many applications, including packaging, construction, consumer products, and automobile industries. In light of increasing global concerns over plastic pollution, there is a renewed focus on using glass alternatives to plastic materials. Challenges persist due to the weight, fragility, significant energy use, and resource consumption in the manufacturing process, despite its infinite recyclability and minimal environmental impacts. This study explored the overview of glass recycling, including the properties of glass, the current status of global recycling efforts, collection and recycling methods, challenges, and future directions. We created material flows of glass bottles by analyzing the recycling status and systems of selected countries with higher glass recycling rates and by using the most current data available. We also examined the current technologies used in collecting and recycling glass, and the future of glass recycling. Introducing renewable energy, a deposit return scheme (DRS), extended producer responsibility (EPR) system, and digitalization technologies at the recycling stage are expected to effectively enhance sustainable production and consumption of glass towards a circular economy.
Abstract
Graphical Abstract
1. Introduction
Current global production of glass is approximately 130 million tons per year, according to previous studies in 2018 and 2021 [1,2]. Such production consists of container glass (48%), flat glass (42%), tableware (5%), and other glass products (5%) [2]. Approximately 40 million tons of glass products were estimated to be produced in the EU [3], while an annual production from the US was found to be 12.3 million tons [4]. Globally, various types of glass products are manufactured, including soda-lime glass, borosilicate glass, lead glass, fused silica glass, barium crown glass, and photochromic glass, depending on their applications (Table 1).
Types of glass, components and applications.
Soda-lime glass is the most common type of glass, comprising about 90% of the glass manufacturing industry [5]. It is primarily used for container glass (e.g., bottles and jars) and flat glass (e.g., windows for buildings and vehicles) [6]. In contrast, borosilicate glass is extensively used in cookware and laboratory equipment due to its high chemical resistance [7]. The diverse composition of these glasses, suited to their specific applications and differing physicochemical properties, poses technical challenges in manufacturing new products from mixed recycled glass of unknown types. Additionally, recycling efforts encounter limitation such as restricted markets for recycled glass, lack of public awareness, and technical issues in glass separation [8].
Despite these challenges, considering the energy and material use in glass manufacturing and the reduced space in landfills, glass recycling is critical. For instance, the specific energy required for melting crystalline raw materials is 2.68 GJ/ton of soda-lime glass [9], which accounts for 75–85% of the total energy consumption in soda-lime glass production [10]. However, it is reported that using recycled glass in the melting process can significantly reduce energy consumption and CO2 emissions due to the lower temperature required to melt the cullet compared to virgin materials [11]. Furthermore, glass recycling can conserve significant amounts of natural resources such as sand, soda-lime and feldspar for every ton recycled [12]. Moreover, disposing of waste glass in landfills can severely impact the available landfill space. Cline et al. estimated the landfill space impact factors for 14 waste components, with waste glass being among the materials with the highest impact factor due to its low compactibility [13]. Therefore, glass recycling is crucial not only from a waste management perspective but also for future sustainable material use.
This paper aims to provide a comprehensive review of the current status of glass recycling and the technologies used for glass separation and recycling around the world. Challenges, suggestions, and future directions for improving glass recycling are discussed and summarized, based on up-to-date statistics and a critical review of selected countries (e.g., United States, European countries, Japan, and Korea). Based on the results of this research, we intend to explore the possibility of using glass as an alternative to plastic, which has recently become an environmental issue of concern, and to propose ways to activate the use of glass from institutional, technological, and civic participation perspectives. The novelty of this paper is that it provides a comparative analysis for policy measures and recent statistics of glass recycling in a global trend. This study also highlights the key factors for successful glass recycling in selected countries. Current challenges to enhance glass recycling are identified, and future directions towards a glass resource circulation are suggested.
2. Types and Applications of Glass
Glass is traditionally regarded as a silica-rich solid material formed by cooling it from a melt [14]. As glass manufacturing methods, raw materials, and applications of glass have become increasingly varied and complex, the approaches to understanding glass have evolved. Shelby defined a glass as “an amorphous solid completely lacking in long-range, periodic atomic structure and exhibiting a region of glass transforming behavior” [14]. Any material, whether organic, inorganic, or metallic, formed by any technique that exhibits glass transformation behavior is considered glass. Glass transformation behavior refers to the structural changes of a super-cooled liquid as its temperature decreases below the melting temperature. As temperature changes, the molecular structure of a melt is continuously transformed exhibiting changes in viscosity until the melt becomes a ‘frozen liquid’, which is a glass.
Viscosity is one of the most important properties of glass-forming liquids, significantly impacting glass manufacturing [15]. Essentially, a melt becomes less viscous with the addition of impurities. For example, vitreous silica, primarily consisting of pure silica, shows the highest viscosity among glass products. The viscosity of a pure silica melt can be substantially altered by adding alkali oxide, which introduces non-bridging oxygen and reduces connectivity [16]. Structural changes in the melt not only influence glass transforming behavior but also affect other physicochemical properties such as chemical durability and fragility. Lower viscosity in glass can also reduce processing temperatures. Pure silica softens at approximately 1,200°C, while the glass transformation temperature of vitreous silica ranges from 1,060°C to 1,200°C, with a melting point of 1,670°C to 1,755°C. In contrast, soda-lime glass has a glass transformation temperature ranging from 550°C to 580°C, with a melting point of approximately 1,500°C [14].
By applying changes in physicochemical properties and glass transformation behavior resulting from the addition of chemical compositions, various types of commercial glass products have been developed to facilitate the glass-making process in less aggressive conditions and meet special needs. For example, soda-lime glass is manufactured by adding soda ash (Na2CO3, Sodium Carbonate) to reduce the processing temperature, but it can lead to a reduction in chemical durability. So lime (CaO, Calcium Oxide) is added to reinforce those reduced chemical properties (Fig. 1) [17]. Although soda-lime glass may have relatively low chemical durability and thermal shock resistance, it is most widely used for commercial purposes such as glass packaging due to its low transformation temperature.
The process of soda-lime glass manufacturing.
Vitreous silica is a single-component glass commercially available in a wide range of purities. It is used in a wide variety of applications, from “low tech” tubing and container applications to “high tech” applications, such as lens elements in microlithography steppers and optical waveguides. Vitreous silica exhibits superior chemical resistance, low thermal expansion, and excellent optical transmission from the vacuum ultraviolet to the near-infrared. Thermal history and trace contamination can have significant effects on its properties, including electrical conductivity and absorption characteristics. The high viscosity of vitreous silica, even at its high melting point, limits its use in traditional glass-forming techniques. Its manufacture usually involves specialized methods, such as flame hydrolysis or high temperature sintering in helium or vacuum [18].
Borosilicate glass is manufactured by adding boric oxide (B2O3) to strengthen thermal shock resistance, chemical durability, and electrical resistivity compared to those of soda-lime glass. In addition to boric oxide (B2O3), barium oxide (BaO), sodium oxide (Na2O), or aluminum oxide (Al2O3) can be added depending on the commercial application [14].
3. Current Trends of Glass Recycling
The global glass recycling rate is estimated to be only 21% of the total glass produced, with container glass achieving the highest recycling rate of approximately 32%. In contrast, the recycling rate for flat glass is significantly lower, at about 11% [19]. In 2022, North America and Europe account for 29% and 27% of the global market share of glass packaging, respectively, [20] and have well-equipped recycling programs and infrastructure relative to other regions. Therefore, we analyze and compare the current status of the United States, a major market in North America, as well as the EU.
3.1. The United States
In the United States, approximately 7.5 million tons of glass are disposed of in landfills each year as municipal solid waste (MSW) [21]. The total glass generation for all products was 12.3 million tons in 2018, accounting for 4.2% of municipal solid waste (MSW) streams. Of these, 9.8 million tons were container glass, representing 3.3% of MSW generation [22]. Container glass constitutes over 90% of discarded glass by weight, primarily because more than 60% of produced glass is container glass and it has a much shorter life cycle compared to other types of glass. According to the US EPA, in 2018, only 39.6% of beer and soft drink glass bottles, 39.8% of wine and liquor bottles, and 15% of food and other glass jars were recycled [23].
In 2018, about 3.1 million tons of container glass were recycled, amounting to 31.3% of the total waste container glass generation. Furthermore, 1.3 million tons (13.3%) were combusted with energy recovery, and the remaining 5.45 million tons (55.4%) were landfilled [22]. In 2019, around 1.8 million tons were recycled into container glass, and 1.5 million tons were used for other purposes such as fiberglass or aggregates [24].
Data from 1960 to 2018 exhibit that the total amounts of waste glass have been decreasing overall since the 1980s. The recycling amount in the US peaked around 2010 and has either stabilized, plateaued, or slightly declined [22] (Fig. 2(a)). But the recovery and recycling rate of container glass varies significantly across the US. In general, several states (e.g., Oregon, California, and New York) with bottle refund programs reported higher average recycling rates of 63%, compared to 24% in other states without such programs [25]. Regionally, the Eastern and Western regions showed high recycling rates, while the Central and Southern regions resulted in lower recycling rates of less than 20%, as shown in the Fig. 2(b)[26].
(a) Glass waste management in the US between 1960 and 2018, (b) Glass recycling rates by state in the US (2018).
3.2. The European Union
In 2021, 14.8 million tons of glass packaging were placed on the market in Europe and 11.9 million tons of glass packaging waste were collected [27], largely 98% originating from municipal waste, with only 2% from pre-consumer sources [28]. Most countries collect waste bottles along with household waste, but exceptionally with Finland collecting 84% through the DRS and Ireland collecting 79% through direct acquisition from consumers [28].
Since 2017, the collection rate has been increasing, and as of 2021, the EU’s average collection rate for glass bottles has reached up to 80.1%. The Close-the-Glass-Loop, a European multi-stakeholder partnership, aims to achieve a collection target of 90% from post-consumer container glass by 2030, ensuring that it is recycled back into the container glass production loop (Fig. 3(a)). Collection rates vary significantly among the EU countries, ranging from 36% to 114%. Across Europe, Northern and Western Europe have recycling rates of over 80%, while Eastern and Southern Europe have below-average recycling rates. (Fig. 3(b)) [29]. Out of the EU countries, 13 countries have implemented the DRS, covering a total population of 144.2 million people. Under this system, their average return rate is found to be 90%, with deposit values ranging from €0.07 to €0.25. The systems in most countries are centralized and operated on a return-to-retail basis [30].
(a) Average glass collection rates of the EU between 2017 and 2021, (b) Glass collection rates by country in the EU (2021).
4. Case Studies
In this chapter, the status of waste glass bottle recycling and material flow in countries with high recycling rates in Asia as well as Europe and the United States are discussed and analyzed in more detail as case studies. Among many Asian countries, Japan and Korea have accumulated national-level data over a long period of time and have high recycling rates and well-equipped systems. Thus, further studies were conducted on these two countries. In the US, there are a number of states with high recycling rates in the Western and Northeastern regions. California and Oregon state programs are selected as best management practices due to their high recycling rates and regional or population scale comparable to that of other countries in Europe and Asia. In Europe, Northern and Western European countries have relatively higher recycling rates. Among these countries, four countries (Belgium, Germany, the United Kingdom, and France) with populations and GDP levels comparable to those in other regions, were selected as case studies [27].
4.1. The United States
In analyzing glass recycling in the U.S., this study shows the cases of Oregon and California, which have some of the highest glass bottle recycling rates in the country. These states have similar areas and populations to the other countries being compared.
4.1.1. Oregon
In 1971, the Oregon Legislature enacted the ‘Bottle Bill’, the nation’s first deposit-return scheme, aiming at reducing litter from single-use beverage containers. This initiative led to Oregon residents returning over 2 billion beverage containers in 2022, achieving 88.5% redemption rate, one of the highest in the country [31]. Glass bottles comprised 8% of the total recovered containers of all materials [32], with a recycling rate of 73% [25]. Approximately 5 million tons of glass materials were processed in 2022, sourced from 26 full-service Bottle Drop Redemption Centers, 94 bag drop locations, and a network of reverse vending machines and hand-count at stores. There were one million Bottle Drop accounts in Oregon, which is a quarter of the State’s 4 million residents. The Bottle Bill was recently updated to increase the redemption value to 10 cents and to cover a broader range of container types. It was originally designed to reduce litter and protect the environment and Oregon’s Bottle Bill has also become a significant part of the community’s social fabric. Residents have the option to donate their deposits to various charities by purchasing blue bags (designed for donations) instead of green bags used for refunds. In addition to ensuring high-quality recycling for glass bottles, some of the bottles in Oregon’s system are part of the Oregon Beverage Recycling Cooperative (OBRC)’s Refillable Bottle program. The program is the only statewide refillable bottle program in the United States. This program allows bottles to be washed and reused more than 25 times before recycling [33].
4.1.2. California
California has implemented the Beverage Container Recycling Program (BCRP). This program includes a redeemable fee for certain containers. When consumers in California purchase beverages, they pay a California Redemption Value (CRV) fee. They receive CRV refunds upon redeeming containers at a recycling center or retailers [34]. CRV-eligible beverages are packaged in glass, aluminum, plastic and bi-metal materials. The refunds amount to 5 cents for containers less than 24 ounces and 10 cents for containers 24 ounces or larger. In California, out of 2.8 billion glass bottles sold, 1.8 billion bottles were recovered, resulting in a recycling rate of about 64% [35]. The state offers four collection routes: buy-back centers, curbside programs, drop-off programs, and community service programs. Among these, buy-back centers account for 85% of the collection rate with 1,247 sites, while curbside programs account for 12% with 635 sites. California uniquely requires glass products to contain a minimum percentage of cullet, with 35% cullet content in container glass and 30% in insulation glass [36]. However, the BCRP did not cover all types of wine and spirit bottles, which constitute a third of beverage containers. To improve upon these aspects, the program coverage was expanded to include wine and distilled spirit coolers with over 7% alcohol by volume, 100% fruit juices of 46 ounces or more, and vegetable juices over 16 ounces, effective from January 1, 2024.
4.2. The European Union
4.2.1. Belgium
Among the EU, Belgium showed the highest rate of glass bottle recycling in 2021, with a recycling rate of 114% [27]. This rate exceeds 100% because more glass bottles and jars were collected than those sold on the market within the country that year [37]. In 2022, the recycled quantity of glass bottles per inhabitant was 30.1kg [38]. Despite the absence of a deposit return scheme, Belgium has been notably successful in recycling for years, thanks to its highly efficient and consumer-centric recycling scheme overseen by the Producer Responsibility Organization (PRO), Fost Plus [39]. Additionally, different types of waste are taxed based on the difficulty of processing and disposal, with higher taxes imposed on single-use packaging that could otherwise be part of a reuse system. Currently, this policy applies only to glass beverage bottles [40]. This approach encourages people to be more discerning about what they discard. Plastic bottles, metal packaging, and drink cartons (PMD waste) can be collected together in a ‘blue bag’ for a fee. However, glass bottles, paper, and cardboard are collected at free of charge [41]. Glass is recyclable, and colored glass is required to be separated from clear glass at designated bottle banks or recycling parks [42]. With the efforts of the government, local authorities consistently remind people of reducing waste and donating items to charity or a second-hand shop before disposing of them. Separation and disposal of household waste are compulsory, facilitating easier recycling. Belgium’s local governments recognize the importance of a deposit scheme in reducing litter. A proposal for such a scheme has been made by beverage companies, their representative federations, and Fost Plus. At the same time, a study is exploring various deposit schemes, including the digital options, with the ultimate goal of implementing a smart, digital deposit system across the country by 2025 [38].
4.2.2. Germany
In Germany, the Packaging Ordinance in 1991 was implemented to manage packaging waste and the Packaging Act (VerpackG) was enacted in 2019. The law regulates the two systems, which consist of EPR and DRS. The EPR, dual system, obliges manufacturers and distributors to take back used packaging from consumers free of charge and forward it for recycling. Additionally, compulsory membership in a dual system is required for all obligated companies. A dual system is an approved private-sector company, and there are a total of eleven dual systems in Germany [43,44]. On the other hand, Germany operates a DRS, Pfand system, for single-use beverage containers. This system was extended to all single-use drink bottles and drink cans from 1 January 2022 and covers all milk product packaging from 1 January 2024 [44,45]. In 2019, the total domestic production of glass bottles in Germany was 4.2 million tons, and the total amount of recycling was 3.1 million tons. Approximately 2.4 million tons of them were recycled into glass bottles, while the remaining fraction was used for other glass products or aggregates [24].
4.2.3. The UK
‘The Producer Responsibility Obligations (Packaging Waste) Regulations’ were introduced in 1997 with the aim of reducing the amount of packaging waste ending up in landfills. Under this existing system, packaging producers with an annual turnover of more than £2 million and more than 50,000 tons of packaging production per year must demonstrate that they have met their statutory recycling obligations each year. This can be achieved through the purchasing of evidence notes, called packaging waste recovery notes (PRNs) and packaging waste export recovery notes (PERNs), which are issued by accredited recycling processors or exporters for each ton of packaging waste recycled. PRNs and PERNs can be purchased directly by packaging producers or, more commonly, by compliance schemes acting on behalf of their producer members. This is a market-based system, with the price of PRNs fluctuating based on supply and demand [46].
In late 2018, the UK Government pledged to introduce a DRS to boost drinks packaging recycling rates. The COVID-19 and cost-of-living crises prompted retailers to call for even further delays to the DRS, in addition to the extension to 2024 that was confirmed in spring 2021 [47]. In the UK, container glasses are most commonly collected door-to-door co-mingled with other materials (55%); however, they can also be collected door-to-door in a separate stream (32%) or via bring banks (10%) and at household recycling centers (3%) [24]. Although it is below average compared to other European countries, the UK has improved its ranking on the Global Waste Index from the 20th position in 2019 to 18th in 2022. Such improvement is also reflected in the percentage of glass bottles recycled in the UK, which has increased from 72.5% in 2019 to 76.5% in 2022 [48, 49]. In 2019, the total domestic production of glass bottles in the UK was 2.4 million tons, and the total amount of recycling is 1.8 million tons. 0.9 million tons of them are recycled into glass bottles and the rest are used as other glass products or aggregates. Unlike other countries, the UK has the characteristic of importing 1.2 million tons of glass bottles, accounting for 30% of domestic demand [24].
4.2.4. France
France is a frontrunner in the EU regarding the implementation of EPR schemes. The EPR system was introduced into French legislation in 1975, regulated through Article L. 541-10 of the environmental code, and has been implemented since the 1980s. A new law on the circular economy was adopted in February 2020 [50]. Glass and other four types of packaging materials are managed through the EPR scheme, which only covers household packaging. At the moment, there is no DRS for packaging, although the creation of a DRS was discussed for recycling plastic bottles in the context of the adoption of the Circular Economy Law. Glass packaging is collected by door-to-door services or designated collection points in cities and suburbs, as well as in rural areas through designated collection points. In 2019, about 85% of household glass packaging waste was collected at bring banks, with around 200,000 collection points in use. The remaining 15% was collected door-to-door [24]. Glass packaging is generally collected in one mixed-color stream. In 2019, the total domestic production of glass bottles in France was 4.3 million tons, and the total amount of recycling was 1.9 million tons. About 1.8 million tons of them were recycled into glass bottles, while the rest was used for other glass products or aggregates. It has the second highest recycling rate after wood materials. It increased by 2.3% over the past five years [51].
4.3. Asia
4.3.1. Japan
In Japan, packaging containers are recycled under ‘The Containers and Packaging Recycling Act’. Among the packaging containers, four types of materials (i.e., glass bottles, PET bottles, paper packaging materials, and plastic packaging materials) are recycled through the EPR system. Business entities obligated under the EPR system include manufacturers of containers and users of specified containers and packaging. There are three routes to fulfill recycling obligations: self-collection route, own recycling route, and designated organization route. Most commonly, the designated organization route is chosen, where specified business entities pay recycling contract fees to entrust the recycling obligation to the Japan Containers and Packaging Recycling Association (JCPRA). In addition to the responsibility of specified business entities, consumers are obliged to dispose of glass bottles separately, and municipalities are in charge of collecting and separating containers and packaging [52].
A total of approximately 790,000 tons of glass bottles collected by JCPRA or other collectors were recycled, with 630,000 tons being processed into bottles in 2022. The remaining 160,000 tons was used for construction materials or other purposes. Furthermore, 470,000 tons of reusable bottles were recovered by product producers through retailers or wholesalers and were reused after washing [53].
4.3.2. Korea
In Korea, the circular system of glass bottles has been regulated by ‘The Act on the Promotion of Saving and Recycling of Resources’ since 2003. The system can be divided into two major flows: the recycling flow by the EPR program and the reuse flow through the beverage container DRS. Among the reusable or waste bottles collected via the DRS, those that are not of good quality or damaged during transportation are crushed and recycled. Conversely, among the bottles collected through the EPR system, those of good quality are selected for reuse through a screening process, although this quantity is relatively small.
In 2009, a group of the Korean traditional alcoholic companies signed a voluntary agreement to share a standardized glass bottle, manufacture the bottles in the standard form, and reuse them collectively by DRS. The implementation of this agreement has contributed to the increase in economic and environmental benefits by reducing logistics costs for collection and exchange of used bottles.
As of 2022, the volume of glass bottles produced was 360,000 tons, with a recycling volume of 292,000 tons, indicating a recycling rate of about 81%, through the EPR [54]. By the DRS, the recovery rate has been maintained above 95% since 2016. In 2022, a total of 4.2 billion glass bottles were put on the market and collected by achieving a 96.4% recovery rate [55]. However, the EPR recycling performance of glass bottles has declined from 363,000 tons in 2005 to 292,000 tons in 2022 [54]. The reason for this decline is that when the deposit for bottles was lower, there was a higher incidence of broken bottles, and the reuse rate was low. But now, as the deposit price per bottle has increased, the reuse rate of bottles has accordingly increased, while the volume of bottles recycled has decreased. Another contributing factor is the decrease in bottle production over time. In Korea, as of 2022, the total production of glass bottles is 569,000 tons, with a total of 489,000 tons of glass bottles recycled through EPR, DRS, and all other pathways, indicating a recycling rate of 85.8% [54, 56].
4.4. Comparison of Recycling Statistics and Material Flow of Glass Bottles
The calculation methods and figures vary for each set of surveyed data, making it difficult to directly compare them. But when comparing two regions, the United States and the European Union, it was determined that the US a lower recycling rate than the EU. Even within the EU and within the US, there are significant differences in collection or recycling rates depending on the state and country. The reason for the lower rate in the US is partly due to its large territory and the lack of consumer’s awareness. On the other hand, variations among the EU countries appear to stem from regional economic and institutional disparities.
When comparing recycling rates by country, Belgium shows the highest rate at 114%, followed by Korea and Germany. The detailed recycling statistics for each country and material flows are summarized in Table 2 and Fig. 4. Analyzing the above cases, it was found that many countries or states operate DRS or EPR systems for recycling and reuse of glass bottles. It is interesting to note that Germany and Korea have implemented both systems. Thirteen countries in the EU and ten states in the US operating with the DRS system also showed relatively high recycling rates.
Comparisons of the glass packaging recycling by country.
The material flow of glass bottles: (a) the US (b) Germany (c) the UK (d) France (e) Japan (f) Korea (POM: put on the market, Unit: thousand tons).
In Europe, nearly 8 out of 10 glass bottles entering the market are collected for recycling, with the vast majority through EPR systems. In comparison, DRS cannot be considered a mainstream solution for tackling glass bottles, which accounted for only 3.1% of recycled glass bottles generated in 2017 and was the main collection method in only two of the 27 EU Member States [57]. In terms of resource savings and environmental aspects, DRS has many advantages, but the system may conflict with EPR system and lead to overlapping infrastructure investment costs for producers and potential confusion for consumers. Thus, an efficient operation plan must be prepared so that the two systems can operate in harmony with each other.
5. Technologies in Glass Recycling
With the exception of beverage glass bottles that are reused or directly returned to grocery stores, waste glass at households is either separately collected at designated drop-off bins (single-stream collection) or collected alongside other recyclable waste such as paper, plastic, and scrap metal (multi-stream collection). Collected waste glass is commonly separated by a number of sorting processes at a material recovery facility (MRF) and crushed down to glass cullet with 1–2 cm in size. After removing contaminants such as plastic, metals, ceramics, and other colored cullet through a further separation process, the cullet can be used as secondary raw materials for manufacturing glass or other products (Fig. 5) [17].
Glass waste collection and recycling process.
5.1. Collection and Sorting
The collection and sorting methods for waste glass vary depending on the route of disposal. Waste glass is typically generated from four major sources: 1) the glass manufacturing process, 2) the municipal solid waste (MSW) stream, 3) construction, renovation, and demolition sites, and 4) end-of-life motor vehicle recycling facilities [58]. Firstly, in glass manufacturing processes, waste glass is generally produced during the glass forming stage and post-forming processes, which include fire polishing, annealing, cutting, and inspection. The defective and leftover glasses in these processes are discarded and scrapped as waste glasses. The waste glass is crushed and reused in the form of cullet again [59]. Secondly, the method of collecting and sorting waste glass discharged through MSW varies by region and municipality. Single stream recycling is the most common approach in the US, but the quality of single stream recyclables is low. In contrast, waste glass from a dual stream is clean enough to ensure that a high fraction of the discarded waste glass bypasses the MRFs and is sent directly to cullet suppliers. A dual system or curbside separation is rare in the US, but it has been successfully implemented with a substantial increase in waste glass recycling rates in some cities. For example, Fayetteville in Arkansas has a system in place where waste glass is sorted into bins at the curbside. This produces a clean, high quality waste glass stream. Other locations, such as Kansas City, follow the European model and have a large number of drop-off bins for waste glass [59]. Thirdly, numerous glass debris is often found in construction and demolition (C&D) sites, but the amount of waste glass by weight in C&D debris waste is relatively small, 1.5 million tons in the EU, due to the predominance of other heavy major C&D debris waste such as concrete, drywall, and ferrous metal [60]. Lastly, waste glass separated from end-of-life vehicle bodies is mostly flat glass (windshield, back and door glass). Windshield glass commonly used in vehicles contains polyvinyl butyral (PVB) for reinforcement and transparency. Each windshield contains approximately 1 kg of PVB sheet, which can be a resource that needs to be recycled for the reuse of PVB [61].
5.2. Recycling
Waste glass recovered through such collection routes is sent to materials recovery facilities (MRF) and transformed into cullet through various processes [17]. The waste glass that meets the necessary standards can be sold to cullet suppliers. A fraction of the glass entering MRFs ends up in landfills. The primary reason most cullet is sold to container manufacturers is that 60% of the waste glass stream originates from container glass. The availability of end-of-life flat glass cullet is limited by stock. The lifespan of flat glass used in the building and automotive sectors is assumed to be 75 years and 13 years, respectively [58]. The lifespan of container glass is typically less than one year, except for the fraction of reused containers. Hence, there is little accumulation of waste container glass stock and end-of-life cullet becomes a new product within the same year. And the glass industry, such as flat glass, does not accept post-consumer cullet due to the higher quality standards and different manufacturing technology required for other glass products [62]. Another reason is the lack of obligation to properly dismantle and sort windows or glazing from buildings during the renovation or demolition process. Instead, end-of-life glass is often crushed together with other building materials and used for pavement aggregates or disposed of in landfills. Consequently, it is estimated that only 5% of end-of-life glass is effectively recycled into new flat glass products [63].
Glass packaging made of soda-lime glass can be successfully manufactured using crushed glass cullet. Utilizing crushed waste glass as a substitute for aggregate in concrete or asphalt mixtures has been evaluated as another means of glass recycling [64]. Despite its excellent physical and mechanical properties, the morphologies of crushed glass and the formation of alkali-silica reaction gels under high alkali conditions may reduce the workability of a concrete-crushed glass mixture [65]. In case of the partial replacement of cement with glass flour, the obtained strength values were lower than those of samples without glass flour; however, the differences do not exceed 10% after a concrete maturation time of 28 days. The results of the partial replacement of natural aggregate with glass cullet showed that the strength values for the 5% and 10% content are slightly lower than the strength of the samples without glass cullet [66]. Nevertheless, among the numerous opinions about the applicability of crushed glass, it is generally found that the compressive strength, flexural strength, and modulus of elasticity of concrete improve with the inclusion of crushed waste glass. The optimum level of fine aggregate replacement has been reported to be around 20% [67,68].
In addition to reusing waste glass as a raw material or for aggregate replacement, the applicability of pulverized glass cullet as an alternative beach fill material was evaluated [69]. The size of pulverized cullet for evaluation was in the range of 0.33 to 0.89 mm, and it was suggested that coarse cullet would be preferable with respect to erosion prevention. The sharpness, workability, and sphericity of pulverized cullet with a mean grain size of 0.49 mm were tested in another study [70], leading to the conclusion that pulverized glass cullet would be compatible with sand, suitable for handling, and have a low potential for cuts. Despite Makowski et al. reporting no significant environmental issues with the application of glass cullet [69], there are concerns about the not-yet-known environmental consequences of spreading glass cullet over beaches [71].
5.3. Potential Benefits of Recycling
There are several advantages to using recycled glass cullet to manufacture new glass containers. Approximately 1 ton of virgin materials can be conserved for every ton of recycled glass used in place of raw materials. This conservation includes about 590 kg of sand, 186 kg of soda ash, 172 kg of limestone, and 73 kg of feldspar [72]. In a similar study, each ton of cullet saves 1.2 tons of raw materials, including 850 kg of sand [73]. Moreover, it reduces energy consumption by 2.5–3.3% for every 10% of cullet added to the melting batch, and substituting 1 ton of cullet for raw materials saves 322 kWh [74]. It takes less energy to melt cullet than virgin raw materials because the chemical reactions required to convert the raw materials into glass have already occurred, and subsequent remelting of cullet only requires the energy to reheat it to a molten state.
Additionally, it can reduce CO2 emissions required to melt the glass by 300 kg per ton of cullet used [73] or 1 ton of glass that goes back to re-melt saves around 185 kg of carbon dioxide (25–50% reduction compared to using virgin raw materials). On average, a 10% increase in cullet in the furnace decreases its CO2 emissions by 5% [75]. Achieving a 50% glass recycling rate reduces greenhouse gas emissions by up to 1.4 million tons annually. The equivalent corresponds to taking 300,000 cars off the road each year [25].
The service life of a glass melting furnace can be increased by up to 30% due to decreased melting temperatures and a less corrosive batch [74]. With a mix of raw materials and cullet, there are fewer chemical reactions, resulting in less chemical attack on the furnace structure. It makes glass forming easier to manage because greater furnace flexibility and higher temperatures can be achieved due to reduced energy usage. The life cycle assessment (LCA) results indicate that producing recycled aggregates from C&D waste, instead of using crushed stone, can reduce net environmental impacts by about 50%. Furthermore, producing 1 ton of recycled fine aggregates from waste glass instead of river sand can save approximately 185 MJ of non-renewable energy consumption and 14 kg of CO2eq. And GHG emissions result in a net environmental impact savings of about 59% [76,77].
5.4. Reuse
Despite the benefits of recycling from an economic efficiency and environmental standpoint, recycling container glass through processing it into cullet is usually less profitable compared to its reuse [78]. As we have seen earlier, bottle reuse has largely been implemented through DRS. In the reuse process, after the removal of caps, all the returned bottles collected through various routes are washed and disinfected. In this process, wastewater and solid waste composed of discarded caps, labels, and damaged bottles are generated. Then the contents are filled again and distributed to consumers. LCA results show that the impacts of the bottle reuse system are mainly associated with the distribution stage, particularly the transportation of the bottles from the bottling plant to the local distributor [79]. Therefore, these regional characteristics should be considered in reusing waste bottles.
6. Challenges in Glass Recycling
Despite the advantageous properties of glass that enable recycling, the use of cullet is not without disadvantages. The following factors as below may cause technical, economic, and environmental problems, eventually leading to a declining recycling rates [80].
6.1. Heterogeneity
Non-recyclable materials are often mixed with recyclable ones. This presents a challenge and increases costs for MRFs that need additional labor and equipment to separate recyclable materials into metal, paper, plastic, and glass. Metallic impurities, such as bottle caps or foil from wine bottles, can cause significant refractory damage and shorten furnace life. Due to its heterogeneity, the recycling of glass cullet as a raw material is limited for manufacturing other glass products, such as flat glass, borosilicate glass, and glass fiber, which require specific constituents for consumer applications [17]. Moreover, impurities may lead to undesired air emissions and water pollution [81,82]. The inability to recycle waste glass regardless of color, contamination, composition, and types of bottles, remains a scientific, engineering, and financial problem [21].
6.2. Energy Consumption
LCA studies have shown that natural gas and electricity consumption are the most influential factors contributing to environmental impacts in glass manufacturing [77,83,84]. Natural gas is the most impactful during the core phase due to the large quantities used in this process. The high energy intensity in the glass industry continues to be a major issue, affecting both CO2 emissions and costs, which currently account for over 20% of the total costs [85]. Excluding mining and transportation, the largest sources of carbon emissions originate from the glass melting process (85%) and the decomposition of carbonates (15%). The melting of glass is responsible for 95 million tons of anthropogenic carbon [86].
6.3. Transportation and Logistics
To collect clean bottles, some areas have established glass redemption centers; however, consumers must transport the glass to these facilities themselves. Such inconvenience can reduce the consumer’s participation rate and increase transportation energy. Compared to plastic, the heavy weight and brittle properties of glass bottles have inherent problems, which result in increased energy consumption and difficulties in handling during transportation or logistics. The environmental benefits gained from avoiding landfill disposal are offset by the increased transport-related cost and impacts [87].
6.4. Carbon Emissions
Recycled glass aggregates produced by the crushing process reduce carbon emissions by 46.7% compared to natural sand obtained from quarries. However, when the washing process was added to the crushing process to produce recycled aggregates, CO2 emissions increased by 89.9% compared to natural sand extraction. Therefore, it is evident that proper separation of waste glass in the collection stage greatly contributes to CO2 reduction. [83].
7. Future Directions in Glass Recycling
To address the challenges described in the previous chapter, this study discusses future directions and solutions to enhance glass recycling (Table 3).
Challenges and future directions associated with glass recycling.
7.1. Source Separation and Collection
As previously discussed, the emission indicators suggest that higher environmental gains can be achieved by collecting waste glass through separate bins [83]. Studies show that a separate curbside glass recycling bin model is the most environmentally friendly option, which can lower environmental impacts by around 40–60% per ton of asphalt production compared to the mixed curbside recycling bin model [88]. Case studies have proved that a dual-stream collection system may be the most suitable for a city like Erie, Pennsylvania, in the US. A dual stream is also more cost-effective than other solutions [89]. As in the case of Korea, if beverage companies manufacture the glass bottles in the same form and collectively reuse them, they can benefit by reducing the costs of logistics and manufacturing new bottles.
7.2. Sorting Process
The development of glass color sorting and ceramic detection technologies is important. As in other industries, new attempts are being made to solve these problems by introducing artificial intelligence (AI) technology. It is proposed that a hybrid Convolutional Neural Network-Support Vector Machines model (CNN-SVM model) be used for the classification of contaminants in glass recycling. This model employed a CNN as a feature extractor and an SVM as a classifier for the extracted features. This model achieved the highest classification accuracy of 100% among the other models [90]. In addition to the recovery of cullet, there is also research on methods to collect and recycle the residual materials generated during the recycling process of cullet. If these residuals, such as plastics, metals, synthetic materials, and cork, can be recycled alongside glass, it could contribute to reducing waste and improving the economic viability of glass recycling [91].
The OBRC’s patented Smart Count AI aids in both rapidly and accurately counting containers and crediting customers’ Bottle Drop accounts. The Smart Count AI system utilizes image recognition, scanners, cameras, advanced software, and AI to quickly identify, and count deposit-eligible containers received through green bags and blue bags. After the count, the customer's account is immediately credited [32].
7.3. Flat Glass and Others
Polyvinyl butyral (PVB) post-production waste, collected from the windshields of end-of-life vehicles and post-consumer building laminated glass, is a valuable polymeric material that can be reused [92]. The weight proportion of the LCD glass panel in the TV or tablet was found to have steadily increased up to 85%. A study proposed a novel method for recycling cathode ray tube (CRT) glass as heavy aggregates in order to prepare radiation shielding concrete [93]. Most automobile glass is recycled through the scrapping process of used automobiles [94]. However, for building glass, the proportion is smaller compared to that of all buildings, resulting in less effective recycling. In Korea, the enforcement decree of ‘The Construction Waste Recycling Promotion Act’ requires recyclable materials to be separated before demolishing public buildings of a certain size or larger. Therefore, expanding the scope to include more buildings could contribute to increasing the recycling rate of flat glass.
7.4. Renewable Energy
It is suggested that introducing renewable energy to production and recycling processes could be a way to further achieve sustainability [88]. Due to its flexibility, glass is already found in many renewable systems, such as the tempered soda-lime silica glass used in modern solar panels, glass fibers used in windmill turbines, as well as glass-based hydrogen fuel cell candidates and substrates for future perovskite photovoltaic cells [21]. Clean glass material was recovered from photovoltaic waste by a series of thermal processes and manual separation. Photovoltaic waste glass was recovered in a proportion of 80.83% from the total mass of photovoltaic waste [95]. Additionally, a glass fiber separator for sodium-ion batteries can be recycled using a simple slurry sieving technology [96].
7.5. Porous Glass
The processed porous glass materials were evaluated for potential uses in various applications, including: 1) removing contaminants from wastewater and lake water [97,98] 2) serving as construction materials, such as insulation panels [99], 3) being utilized as electromagnetic shields [100] and 4) supports for enhancing catalytic performance [101]. Despite the great potential for recycling waste glass as porous glass, controlling the pore structure and various factors such as the chemical and physical properties of waste glass, the type and number of surfactants, and forming agents were reported as challenging [102,103].
7.6. Consumer Awareness through Education
Environmental education is a process that allows individuals to explore environmental issues, engage in problem solving, and take action to improve the environment. In this regard, the following points should be emphasized in education and campaigns on recycling waste glass bottles. In order to effectively reuse glass bottles, it is crucial to keep the bottle's mouth undamaged for consumer safety as well as to avoid putting any foreign substances inside before returned to glass manufacturers. Additionally, source separation of glass by type and color may be required for promoting recycling. Environmental education is more than just information about the environment; it should lead people to active participation and networking [104].
7.7. Policy and Legislation
The success of policy implementation depends on a suite of determining factors that exist at different levels of society, including the ability of the central government, local governments, and civic organizations to convince the public of the objectives or rationale of a policy [105], and whether these policies are designed in alignment with the local cultural background [106].
8. Conclusions
This study examined the current efforts and management of glass recycling, including the properties of glass, current global trends, technologies, challenges, and future directions. Reuse and recycling practices of glass are crucial due to the conservation of raw materials, economic benefits, an alternative to plastic pollution, and CO2 reduction. Despite such potentials of glass recycling, there are a number of challenges, including the nature of glass itself, an inefficient collection system with high transportation costs, low economic incentives, and insufficient policy associated with collection and recycling. The weight and fragility of glass may further complicate the collection process, while the mixture with foreign substances and various colors often degrades the quality of the recycled product and makes its recycling process more complex and less economic benefits.
To overcome such challenges, technology that can reduce the weight and increase the strength of glass needs to be developed. A more efficient collection system for a variety of glass, followed by the sorting and screening processes with an advanced AI system may be necessary. Technical measures to reduce energy consumption through renewable energy and the recycling of waste glass from solar or wind power generators are also essential. Moreover, it is expected that using porous glass as a recyclable method can be applied to various fields, regardless of color or material.
When analyzing the present recycling rates among various countries, it is evident that the United States exhibits a lower rate compared to other regions due to territorial and economic obstacles. Some of the states such as Oregon and California have shown higher recycling rates through voluntary participation, citizen education, and the implementation of DRS. Many countries and cities in the EU that have implemented DRS usually exhibit higher levels of recycling rates. Despite the absence of DRS, Belgium has the highest recycling rate in Europe, however, partly attributed to its robust regulations and an enhanced collection system at the national level. A number of European countries and Japan have adopted an EPR system, whereas Germany and Korea have implemented both EPR and DRS in parallel, leading to a notable increase in recycling rates.
In Europe, recycling rates are determined by the regional economic infrastructure, indicating that overall economic growth at a national level is necessary to increase recycling rates. Additional research is needed on the current status and policies on glass recycling in Asia and other regions where accurate statistics of glass recycling and collection are still lacking.
A higher recycling rate of glass can be achieved by selecting a system (e.g., DRS, EPR, or a combination of both) suitable for considering each country's collection and recycling infrastructure, along with detailed policy measures. Some of the measures may include producing glass products with closed-loop system by producers, sorting and collecting recyclable resources such as glass before disassembly of products and demolition of buildings and mandating the use of recycled materials in glass products by regulation. If technical and institutional supports by governments and recycling industry as well as active participation to source separation of glass by consumers are enhanced, glass recycling would contribute to sustainable production and consumption as an alternative to plastic in the future.
Acknowledgments
This study was supported by Chungnam National University.
Notes
Conflict-of-Interest Statement
The authors declare that they have no conflict of interest.
Author Contributions
Y.C.J. (Professor): Conceptualization, Study plan and Supervision; H.D.K. (Professor): Methodology, Data organization, Analysis and Reviewing; C.R.B (Ph.D. Student); Methodology, Data organization, Analysis, Writing and Editing.