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].

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].

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.

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.

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.

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].

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].

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].

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.

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].

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].

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].

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].

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.

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].

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.

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].

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].

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].

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].