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Article

A Cluster Analysis of Cooperative Recycling Behaviors for Post-Consumer Plastic Waste in Urban Areas: A Case Study on Sendai, Kawasaki, and Kyoto City in Japan

by
Zhuojiao Yu
1,*,
Xiaoyue Liu
1,*,
Jeongsoo Yu
1,
Mohammad Sujauddin
2 and
Gaku Manago
1
1
Graduate School of International Cultural Studies, Tohoku University, Sendai 980–8576, Japan
2
Department of Environmental Science and Management, North South University, Bashundhara, Dhaka 1229, Bangladesh
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(17), 7939; https://doi.org/10.3390/su17177939
Submission received: 9 August 2025 / Revised: 28 August 2025 / Accepted: 1 September 2025 / Published: 3 September 2025
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Abstract

Post-consumer plastic waste poses increasing challenges in urban areas, where recycling heavily relies on consumer cooperation. In Japan, two recycling routes for post-consumer plastic waste from households exist, the municipal recycling route and the retailer recycling route, with the latter requiring more voluntary effort. This study aims to explore the diversity of consumers’ cooperative behaviors in Japan’s post-consumer plastic waste recycling system, with a focus on the retailer route. We conducted an online survey with 758 respondents from Sendai, Kawasaki, and Kyoto in urban Japan, using a structured questionnaire based on the knowledge–attitude–practice (KAP) framework. K-means clustering was conducted to identify behaviorally distinct consumer groups. Three clusters were revealed: Fully Engaged Consumers, Knowledge-Driven Consumers, and Passively Engaged Consumers. These groups exhibited distinct differences in cooperative recycling behaviors and socio-demographic characteristics. Our findings demonstrate the heterogeneity of consumer cooperation and underscore the importance of targeted strategies. By focusing on the retailer recycling route and consumer segmentation, this study addresses key gaps in Japan’s research on urban plastic waste. The results provide a theoretical and empirical foundation for differentiated policy-making, ultimately supporting the transition to a more sustainable and circular economy in post-consumer plastic waste recycling in urban Japan.

1. Introduction

The plastic problem is a major challenge worldwide. Plastics are widely used across industries due to their affordability, durability, and versatility [1,2]. However, large-scale production and disposal have led to severe environmental and health concerns [3,4]. In 2023, global plastic production reached approximately 413 million tons, but only 10% was recycled [5]. The COVID-19 pandemic has further exacerbated the issue, driving a sharp increase in single-use plastics and post-consumer plastic waste [6,7,8].
In the EU, approximately 61% of post-consumer plastic waste collected in 2020 originated from packaging [9], underscoring the urgent need to enhance the recycling system targeting post-consumer plastics. Many Asian countries face similar challenges. In Indonesia, inadequate infrastructure and fragmented governance limit effective recycling, especially in urban areas [10]. In China, despite announcing a ban on imported plastic waste in late 2017 and implementing it in early 2018 [11], it continues to face challenges related to informal recycling and inadequate sorting systems [12]. Even countries with advanced waste management systems, such as South Korea and Japan, have experienced setbacks, particularly under China’s ban on imports [13]. Japan ranks second in the world for both per capita plastic consumption and plastic waste exports [14]. Following China’s ban on plastic imports, Japan’s plastic waste exports to China and Southeast Asia decreased from 1.36 million tons to 0.91 million tons [1]. This presented Japan with a significant challenge in managing the large amount of plastic waste domestically.
To manage post-consumer plastic waste from households, Japan has two primary recycling routes: municipal recycling, regulated under the Containers and Packaging Recycling Law (CPRL), and retailer recycling, which relies on voluntary consumer use of recycling boxes placed in supermarkets and other retailer outlets. In 2023, Japan generated approximately 7.69 million tons of plastic waste, of which about 25% was material recycled, 9% was chemically recycled, 54% was thermally recycled, 8% was incinerated (without power or heat generation), and 4% was landfilled [15]. While Japan’s effective utilization rate of plastic waste reached 89% in 2023, one of the highest in the world [15,16], over half of it was thermal recycling. Thermal recycling plays a crucial role in the circular economy framework, particularly for plastics that can no longer be recycled due to the deterioration of their physical and chemical properties. This approach ensures energy recovery and prevents landfilling, but its heavy reliance is inconsistent with international trends toward sustainable development and circular economy principles. Notably, according to Article 3 (17) of the Waste Framework Directive (Directive 2008/98/EC) in the EU, thermal recycling is classified as “energy recovery” and is therefore excluded from the recycling rate [17]. Hence, to align with European and international standards, Japan needs to further increase the share of material and chemical recycling while gradually reducing its dependence on thermal recycling. Since both material and chemical recycling require relatively high-quality input, improvements in recycling technologies must be accompanied by active consumer cooperation, such as better sorting and cleaning of plastic waste, to enhance the downstream recyclability of materials.
In response to these challenges, Japan enacted the Act on Promotion of Resource Circulation for Plastic (the New Plastic Law) in 2022, aiming to strengthen Extended Producer Responsibility (EPR) and share responsibilities among producers, retailers, municipalities, and consumers to improve plastic recycling and advance a sustainable and circular economy for plastic waste [5]. The law requires producers to enhance product design and take greater responsibility for recycling, promote material and chemical recycling alongside thermal recycling for plastic waste. These assumptions are broadly consistent with the EU’s Circular Economy Action Plan (CEAP), which prioritizes the circulation of resources and recovery of high-quality materials [18]. However, unlike the EU, where energy recovery is not considered recycling [17], Japan has historically relied more heavily on thermal recycling [19]. This new law marks Japan’s shift in policy toward convergence with international standards by increasing the share of material and chemical recycling. However, technical and institutional reforms alone are insufficient. Post-consumer plastic recycling relies heavily on individual behavior, such as sorting and washing plastic activities that vary across socio-demographic groups and regions. In urban areas, where plastic waste is concentrated and lifestyles are diverse, encouraging consumer participation presents challenges. Understanding consumer behavior is, therefore, crucial for developing effective and inclusive recycling policies.
This study aims to explore the diversity of consumer cooperative behaviors in the household post-consumer plastic waste recycling system in urban Japan, with a particular focus on the retailer recycling route. We conducted an online survey in three major Japanese cities: Sendai (Miyagi Prefecture, Tohoku region), Kawasaki (Kanagawa Prefecture, Kanto region), and Kyoto (Kyoto Prefecture, Kansai region), using a structured questionnaire based on the knowledge–attitude–practice (KAP) framework. By applying K-means cluster analysis, we identified distinct consumer groups and analyzed their plastic-related KAP and socio-demographic characteristics. This study aims to address the existing gaps in research on the cooperative behavior of the retailer recycling system and the lack of segmentation in Japanese consumers’ post-consumer plastic waste recycling behaviors. Ultimately, this study provides a theoretical and empirical foundation for differentiated policy-making to promote a more sustainable plastic waste recycling system in urban Japan.

2. Literature Review

Post-consumer plastic waste, including containers, packaging, and mixed plastic products discarded by households, poses significant recycling challenges due to its contamination, volume, and complexity [8]. As mentioned, there are two recycling routes for post-consumer plastic waste from households. Previous studies have shown that plastic waste collected from supermarkets is cleaner and well-sorted, making it more suitable for material recycling [20,21]. A more recent study has highlighted the importance of precise material identification in post-consumer food containers and packaging to enhance recycling efficiency [22]. While other studies emphasized the role of voluntary collection in advancing Japan’s circular economy [23,24], it is also important to note that material and chemical recycling require higher-quality plastic waste as input. Therefore, beyond technological improvements such as advanced sorting and identification, consumers’ active cooperation through better separation and cleaning of plastics remains essential for expanding these recycling routes and reducing reliance on thermal recycling.
Consumers play a central role in the post-consumer plastic waste recycling system, particularly through source separation and cooperation, which impact both recycling rates and downstream processing efficiency [25,26,27]. Numerous studies have demonstrated that socio-demographic characteristics, environmental awareness, environmental attitudes, social pressure, and social norms collectively influence consumer recycling behavior [28,29,30,31,32,33]. A more recent study in Japan has highlighted the role of socio-demographic factors in shaping household plastic waste generation and recycling rates in the municipality recycling route [34], suggesting that these factors also influence consumers’ cooperation in post-consumer plastic recycling. However, few have explicitly focused on plastics, and even fewer address Japan’s retailer recycling context [2].
The KAP framework is widely used in the study of consumer behavior. It showed that increased knowledge and positive environmental attitudes often lead to higher participation in recycling [30,35]. Studies on youth and students show that while awareness is generally high, actual practices are often limited due to motivational and structural constraints [36,37,38]. Social media and public engagement also shape these behaviors [25,39]. In the context of post-consumer plastic waste, studies have found that inconvenience and complexity hinder practice, despite positive attitudes [3,40]. While KAP offers valuable insight, most related studies focus on general or e-waste and overlook segmentation of plastic-specific cooperative behavior.
To uncover behavioral diversity, K-means clustering has been widely applied in environmental research. It helps identify consumer segments based on knowledge, attitudes, practices, and demographics. Applications include the use of plastic bags [41], health and green fast food restaurant choices [42], and household food waste [43,44]. In Japan, some studies used cluster analysis to analyze low-carbon and food recovery behaviors [45,46]. Broader applications include pro-environmental behaviors [47], carbon capability [48], and sustainable consumption [49]. Nevertheless, few studies apply K-means to examine the cooperative recycling behaviors of post-consumer plastic waste in Japan.
In summary, the existing research on behavioral heterogeneity in Japan’s voluntary retailer recycling system is limited. This study fills this gap by integrating the KAP framework and K-means clustering to identify and characterize consumer segments in the household post-consumer plastic waste recycling system in urban Japan. The findings aim to inform more targeted and effective recycling strategies.

3. Method and Materials

3.1. Data Collection

This study aims to explore the cooperative behavior of consumers in the household post-consumer plastic waste recycling system in urban Japan, with a particular focus on consumer cooperative behaviors in the retailer recycling route. The goal is to segment consumers using clustering analysis to identify characteristic patterns among groups with different levels of cooperation. Questionnaire surveys are widely used in environmental behavior research to collect structured information on individuals’ cognition, awareness, and behavior toward waste management [43,44,50,51]. This study used a structured questionnaire to conduct an online survey of residents in three large cities: Sendai, Kawasaki, and Kyoto. These cities are government-designated cities with populations exceeding one million and are located in different parts of Japan (northeastern, central-eastern, and western Japan). Moreover, they differ in their household plastic waste management systems, particularly in the separation and collection rules. Therefore, the selection of these three cities provides a meaningful representation of consumer behavior in urban Japan.
The questionnaire design was based on the KAP theoretical framework, which has been widely used in the study of consumer recycling behavior [3,30,35,40]. It measured consumers’ cooperative behavior toward plastic waste recycling by combining KAP items with additional questions on willingness to pay (WTP) for eco-friendly packaging and designated garbage bags. To ensure contextual relevance, the questionnaire was adjusted to reflect the specific characteristics of Japan’s post-consumer plastic waste recycling system, including motivations for using different recycling routes and recent policy changes such as the New Plastic Law. The questionnaire was initially prepared in Japanese and consisted of two sections. The first section collected the socio-demographic characteristics of respondents, including their city of residence, gender, age, education level, and other relevant details. The second section consists of 11 single-choice and multiple-choice questions, including five categories: (1) Knowledge and attitudes towards general environmental and plastic issues, (2) Knowledge and attitudes towards the plastic-related policies, (3) Attitudes and practices towards pro-environmental behaviors, (4) Attitudes and practices towards plastic recycling behaviors, and (5) Willingness to pay (WTP). The English version of the questionnaire is available in the Supplementary File S1.
The survey was conducted online by a professional research company. A stratified sampling strategy was employed by the city (Sendai, Kawasaki, Kyoto) to ensure balanced and representative coverage of the population structure in the three cities, taking into account gender and age groups. Respondents under the age of 20 were excluded from the survey. In Japan, most social surveys are generally targeted at adults aged 20 and above [52,53,54,55], as individuals under 20 are legally minors and typically do not take the primary responsibility for household waste management. Since this study investigates consumer cooperation in post-consumer plastic waste recycling, the focus was placed on adults who are directly responsible for complying with the CPRL, including the obligations of waste separation and cleaning before disposal. The survey was conducted from 10 April to 17 April 2025, and a total of 758 valid questionnaires were collected after data cleaning. All participants gave informed consent for the anonymous data to be used for academic research. The study protocol, including the questionnaire design and survey procedure, was reviewed and approved by the Research Ethics Committee of the Graduate School of International Cultural Studies, Tohoku University.

3.2. Data Analysis

To reveal potential consumer characteristics based on post-consumer plastic recycling behavior, we used K-means cluster analysis. This multivariate exploratory technique groups individuals based on similarities between selected variables, thereby revealing different consumer groups [56]. The specific steps of the analysis are as follows: (i) 16 variables were selected and categorized into five types, including recycling behaviors, knowledge and attitudes, pro-environmental practices, willingness to pay, and socio-demographic characteristics as shown in Table 1; (ii) All variables were standardized using the Z-score standardization method to eliminate scale bias; (iii) Euclidean distance was selected as the similarity measure; (iv) K-means clustering algorithm was performed, and randomization was repeated 25 times to ensure the stability of the solution; (v) The optimal number of clusters was determined to be 3 using the elbow method [57]; and (vi) The average silhouette coefficient was calculated to assess the clustering quality and validity [58]. The analysis was performed using R (version 4.5.0), and visualizations were created using the “factoextra” and “ggplot2” packages.
As shown in Table 1, the study selected 16 variables across five categories to identify heterogeneous consumer groups. The recycling behavior indicator distinguishes between the municipal recycling route (MR), which requires purchasing designated garbage bags, and the retailer recycling route (RR), which allows consumers to bring their waste to recycling boxes without needing to purchase designated garbage bags. These two variables directly reflect consumers’ actual cooperation with existing systems, which previous research did not consider. Previous studies have typically asked respondents questions such as whether they believe plastic pollution is a serious issue, whether they can properly sort their waste, and whether they are aware of the dangers associated with plastic incineration [3,35]. However, recent studies have shown that public awareness and attitudes toward plastic pollution have significantly improved [2,3]. Therefore, this study adjusted the knowledge and attitudes indicator to reflect respondents’ awareness and attitudes toward Japan’s recently introduced New Plastics Law (NPL) and Japan’s three current recycling methods (PRM), tailored to Japan’s context and characteristics.
Pro-environmental practices indicators, including frequency of washing plastic waste (FWP), practices of reducing single-use plastics (RSP), and attitudes and practices towards environmental protection activities (AEPA). FWP was included because, under CPRL, consumers are responsible for properly cleaning plastic waste before disposing of it separately for recycling. However, different municipalities have varying standards for cleaning plastic waste. For example, Kyoto City has strict cleaning requirements for plastic waste, which mandate the removal of stains and oil residues. Sendai City, on the other hand, only requires a simple rinse to remove any food residue. Therefore, quantifying plastic cleaning is challenging due to the difficulty in achieving uniformity; thus, this questionnaire only asked respondents how often they clean their plastics to capture their daily practice of this rule. For the RSP and AEPA variables, we referenced previous research to list specific practices, such as using “My bag” and “My bottle,” as well as participation in environmental activities [55,59]. The willingness to pay indicator includes the willingness to pay for eco-friendly packaging (WTP 1) [58,59] and the willingness to pay for waste management services [35], which involves purchasing designated garbage bags when using the municipality’s recycling route (WTP 2). Finally, socio-demographic factors such as age, gender, income, and household composition are widely recognized as significant determinants of recycling behavior in previous studies [28,29,30,31,32,33,34].

3.3. Sample Characteristics

The final sample consisted of 758 respondents from the urban areas of Sendai (n = 248), Kawasaki (n = 258), and Kyoto (n = 252) in Japan. The gender distribution was relatively balanced, with males accounting for 52% and females 48%. The respondents ranged in age from 20 to 85 years old, with an average age of approximately 53. A majority were middle-aged and elderly individuals, particularly those aged 50 to 64, who made up the largest age group (38%). Although younger respondents (aged 20 to 29) comprised a smaller proportion of the sample (12%), it was consistent with the current population structure of the three cities. In terms of education, 67% of respondents had completed a university or vocational school education. Regarding occupation, the most significant proportion was company employees (38%), followed by a substantial number of unemployed individuals (34%), including housewives/househusbands, students, retirees, and those who were unemployed. Most respondents had a low annual income of less than 3 million yen (59%). This figure likely reflects the inclusion of respondents without personal income, as the survey asked about individual rather than household income. The majority lived in households of two to three people (60%) and in communal housing, such as apartments or condominiums (57%). Table 2 presents a detailed summary of the respondents’ socio-demographic characteristics by city. Values are percentages within each city, and the “Total” column shows proportions for the full sample (n = 758).

4. Results

4.1. Main Findings

Based on consumers’ socio-demographic characteristics and cooperative behaviors in the recycling system, three different clusters were identified. Cluster 1, labeled “Fully Engaged Consumers” (n = 219); Cluster 2, labeled “Knowledge-Driven Consumers” (n = 276); and Cluster 3, labeled “Passively Engaged Consumers” (n = 263). Figure 1 shows the cluster distribution of the synthesized key components, indicating that the samples are evenly distributed, and the clustering results are satisfactory. The average silhouette coefficient is 0.31; although not very high, it represents a reasonable clustering structure, and such values are common in behavioral and survey-based research [58,60]. Thus, the cluster result remains meaningful, as it captures interpretable heterogeneity in consumer cooperation behaviors.
To interpret the cluster characteristics, all variables were standardized using a z-score transformation (mean = 0). Values above 0 indicate above-average levels, and those below 0 indicate below-average levels. A threshold of ±0.5 was used to indicate meaningful deviation from the mean. Table 3 summarizes the final cluster centers, which serve as the basis for interpretation and are visualized in Figure 2.
Cluster 1, “Fully Engaged Consumers,” is characterized by being highly cooperative in the post-consumer plastic waste recycling system. They participate actively in both municipal and retailer recycling routes, with a preference for the latter (MR = +0.24, RR = +0.42). Their knowledge and attitude towards the New Plastic Law and plastic waste recycling methods are moderately above average (NPL = +0.16, PRM = +0.40). They consistently engage in pro-environmental practices, such as washing plastic waste (FWP = +0.45), reducing single-use plastic waste (RSP = +0.51), and participating in environmental protection activities (AEPA = +0.45). While their willingness to pay is modest, it remains slightly above average (WTP 1 = +0.12, WTP 2 = +0.15).
Cluster 2, “Knowledge-Driven Consumers,” shows a moderate level of cooperation but possesses the highest levels of knowledge and policy awareness regarding plastic recycling (NPL = +0.46; PRM = +0.49). Their participation in both recycling routes is slightly above average (MR = +0.12, RR = +0.18), but lower than that of Cluster 1. Although their pro-environmental practices are moderate (FWP = +0.37, RSP = +0.16, AEPA = +0.31), they exhibit the strongest willingness to pay for eco-friendly packaging and designated garbage bags (WTP1 = +0.34, WTP2 = +0.33).
Cluster 3, “Passively Engaged Consumers,” exhibits the lowest level of cooperation within the post-consumer plastic waste recycling system. They participate minimally in both municipal and retailer recycling routes (MR = −0.32, RR = −0.54) and exhibit low levels of knowledge and policy awareness (NPL = −0.62; PRM = −0.85). Their pro-environmental practices are the least active among the three groups, including low frequency in washing plastics (FWP = −0.76), reducing single-use plastics (RSP = −0.59), and other activities (AEPA = −0.70). They also display low willingness to pay for eco-friendly packaging and designated garbage bags (WTP1 = −0.46; WTP2 = −0.76).
From the perspective of socio-demographic characteristics, there is no significant difference in education level and household size among the three clusters. All of them hold a bachelor’s degree or a vocational school degree and generally come from small families of 2–3 people. However, there are significant differences among the three types of consumers in other socio-demographic characteristics, such as gender, age, and income.
“Fully Engaged Consumers” tend to be older than average (Age = +0.71), more likely to be female (Gender = +0.39), and many live in detached houses (HT = +0.21). They are less likely to be employed (Employment = −1.3) and have lower-than-average incomes (Income = −0.97). “Knowledge-Driven Consumers” are of average age (Age = −0.04), more likely to be male (Gender = −0.25), and generally have a balance between the two types of residence, with more living in communal houses (HT = −0.027). Most are employed (Employment = +0.72) and report higher-than-average incomes (Income = +0.66). “Passively Engaged Consumers” are the youngest group (Age = −0.5531), with a nearly balanced gender ratio (Gender = −0.07), and most live in communal housing (HT = −0.15). The majority are employed (Employment = +0.33) and have slightly above-average incomes (Income = +0.12).

4.2. Other Findings

Although the variables shown in Table 4 were not included in the clustering algorithm, we conducted a supplementary profiling of the three consumer segments by examining their average age and responses to additional behavioral and attitudinal questions. The percentages in the table represent the proportion of respondents within each cluster who selected the respective option. This post hoc analysis provides further insights into the differences among clusters and supports the behavioral trends identified in the clustering results.
As shown in Table 4, the average chronological ages of Clusters 1, 2, and 3 are approximately 64, 52, and 44, respectively. This trend is consistent with the z-score pattern of the Age variable reported in Table 3, where Cluster 1 showed an above-average level and Cluster 3 a below-average level. Regarding reasons for using the retailer recycling route, the convenience of space and time is the dominant motivator, particularly among Clusters 1 and 2. Cluster 1 shows the highest proportions selecting “close to home” (27%) and “throw waste in anytime” (26%), and also had the strongest association with environmental concern (17%). In contrast, Cluster 2 appeared more motivated by economic savings (13%). In terms of reasons for not using the retailer recycling route, Cluster 3 respondents cited “far from home” (49%) and the limited types of plastic accepted (26%) as key reasons. Additionally, this group had the highest share of individuals unwilling to wash plastic waste (17%). Preferences for system improvement also varied: Cluster 1 favored policy convenience and physical access (e.g., more recycling stations), while Cluster 2 emphasized both convenience and economic incentives. Cluster 3 placed the most outstanding value on technological improvements to the recycling system.
These findings, while not central to the clustering model, help deepen the interpretation of each cluster’s behavioral profile and offer practical implications for targeted policy design.

5. Discussion

5.1. Consumer Segmentation and Behavioral Characteristics

The clustering results indicate significant differences in the cooperative behavior of Japanese urban consumers within the post-consumer plastic waste recycling system, particularly in terms of the retailer recycling route. We divide them into three groups: “Fully Engaged Consumers,” “Knowledge-Driven Consumers,” and “Passively Engaged Consumers,” which represent a continuum from substantial behavioral investment to minimal participation in the post-consumer plastic waste recycling system. Similar segmentation has been reported in other contexts, such as food waste behaviors in Europe, identified groups ranging from highly committed to weakly engaged households [43,44], green food consumption in Indonesia confirmed the persistence of consumer heterogeneity and the role of contextual barriers [42], and pro-environmental household practices in Canada and China highlighted the coexistence of pro-environmental clusters alongside less active ones [47,48]. These parallels suggest that consumer heterogeneity is a common feature across regions, reinforcing the need for targeted strategies within circular economy and sustainability transitions. Against this background, we now discuss the three Japanese clusters in more detail.
Cluster 1, “Fully Engaged Consumers,” showed the highest engagement in all variables, especially in the retailer recycling route and pro-environmental activities. Despite low income and employment levels, their active cooperation is likely to stem from deep-rooted internalized environmental values or habitual civic practices. This group is mainly composed of the elderly and females, which is consistent with previous research findings that the elderly and females are more active in waste sorting and recycling and participating in environmental activities [33,61,62,63]. This finding also aligns with a more recent study in urban Japan, which found that the elderly were associated with a higher household plastic waste recycling rate [34]. However, their willingness to pay for eco-friendly packaging and designated garbage bags is limited. As the group with the lowest income among the three clusters, their financial capacity constrains their ability to pay, rather than a lack of willingness [35]. This lower ability to pay also explains their preference for using recycling boxes, which reduces the need to purchase designated garbage bags.
Cluster 2 is the Knowledge-Driven Consumer, who showed the highest level of awareness and knowledge about the New Plastic Law and plastic recycling methods. They also had the highest levels of education and income. This suggests that individuals with higher education levels possess a greater understanding of the knowledge and legal aspects related to plastic issues [3,33]. However, their behavioral compliance and participation are significantly lower than those in Cluster 1. This finding reflects the gap between attitude and behavior identified in previous studies [3,64,65,66]. Many studies have shown that even if people show positive attitudes towards environmental issues or recycling, they do not necessarily take corresponding actions [30]. Members of this group tend to be younger, have stable jobs, and have higher incomes. For them, structural barriers such as time constraints, lack of incentives, and system accessibility may prevent them from taking action, even if they are willing to do so [67]. Notably, they have the highest willingness to pay for eco-friendly packaging and designated garbage bags, which is consistent with previous studies indicating that people with higher education levels and income have a greater ability to pay and a higher willingness to pay [35,68].
Cluster 3, “Passively Engaged Consumers,” exhibits the lowest cooperation among the three clusters. Despite being young and employed, many members of this group have limited motivation to participate in recycling activities. According to Table 4, this group is most likely to cite inconvenience as a barrier; 49% report that recycling boxes are far from home, and 17% are reluctant to wash plastic waste. This aligns with research findings that perceived inconvenience significantly hinders recycling participation among individuals with low motivation [69]. They also showed low support for incentives (28%) or educational measures (8%) and a preference for technological solutions (39%), indicating a more passive attitude toward recycling. Literature suggests that individuals’ motivation to recycle decreases when they perceive policy measures as ineffective or overly burdensome [70]. These findings underscore the importance of addressing practical barriers, enhancing perceived policy effectiveness, and promoting environmental awareness.
Among all 16 variables analyzed, the retailer recycling route (RR) emerged as the most discriminative factor among clusters. As a resource channel that requires consumer voluntary participation, it not only reflects consumers’ behavioral compliance but also their active engagement in environmental issues. The qualitative response results in Table 4 further illustrate the pattern. For Clusters 1 and 2, time and location convenience are key motivating factors. Cluster 1 also emphasizes that the use of recycling boxes is environmentally friendly, while Cluster 2 emphasizes economic benefits. In contrast, the long distance, limited collection types, and unwillingness to wash plastic waste are the main barriers to using recycling boxes in Cluster 3. These findings indicate that system design and logistics factors have a crucial impact on consumer behavior in recycling systems [23,27].

5.2. Policy Implications

Overall, this study identified three types of consumers in Japan’s household post-consumer plastic waste recycling system through cluster analysis. Based on these consumer characteristics, we suggest the formulation of differentiated policy recommendations as follows:
  • For “Fully Engaged Consumers,” local municipal policies should focus on how to maintain their enthusiasm. For example, through community publicity and recognition, increase their social recognition. Alternatively, let them become environmental advocates, influencing and guiding those around them to participate actively and practice plastic-related environmental protection activities, as well as use recycling boxes effectively.
  • For “Knowledge-Driven Consumers,” policies should focus on filling the gap between attitudes and behavior. According to their responses in Table 4, more direct incentives should be taken to increase their participation. For example, improving the accessibility of recycling boxes and offering more direct incentives (such as points, coupon rewards).
  • For “Passively Engaged Consumers,” interventions should prioritize strengthening environmental education to enhance their environmental awareness and understanding of plastic pollution and related laws and policies. The improvement of knowledge and awareness does not necessarily mean that behavior can be changed directly [71]. However, environmental education can still promote more environmentally friendly attitudes and further reduce plastic pollution, making it crucial [72,73].
It is worth noting that all three groups expressed a high degree of approval for municipalities to introduce more convenient sorting policies, provide incentives for consumers, and for supermarkets and recycling companies to establish more accessible recycling boxes. This shows that although consumers have different characteristics and motivations, an optimized recycling system and a supporting incentive mechanism can benefit all consumer groups. Previous studies have shown that the convenience or cost of recycling, such as the need to wash plastic waste [74], as well as contextual factors like the availability of recycling boxes [75], are essential factors influencing recycling behavior. Therefore, local municipalities should make plastic waste sorting policies more convenient in the future. For example, many cities have responded to the New Plastic Law by expanding the recycling categories of plastics (including not only plastic containers and packaging, but also 100% plastic products), making it easier for citizens to separate and discard most of the household plastic waste, and mobilizing citizens’ enthusiasm for recycling. We suggest that supermarkets and recycling companies establish more recycling boxes and add categories for recycled plastics to facilitate the use of nearby citizens. Additionally, supermarkets and recycling companies can reward citizens who use recycling boxes with corresponding points and coupons to further boost their enthusiasm.
Beyond Japan, these findings also have broader implications for the theory of circular economy. Japan represents a high-consumption society that has achieved one of the highest global plastic utilization rates, but mainly through thermal recovery [16], which contrasts with the material-oriented hierarchy emphasized in the EU Waste Framework Directive [17,18]. Our results suggest that promoting material and chemical recycling requires not only technological innovation but also differentiated consumer engagement strategies. International evidence indicates similar challenges: in Europe and North America, perceptions, attitudes, and behavioral gaps often constrain recycling outcomes despite policy support [2,8]; in Southeast Asia, consumer awareness is growing but voluntary practices remain inconsistent [1,10]; and in China, stricter policy measures, such as waste import bans and mandatory sorting, have reshaped the recycling landscape but also exposed systemic barriers [11,12,13]. In this broader context, Japan can be positioned as a case study for other high-consumption nations, demonstrating that the transition toward a circular economy and sustainability depends not only on regulation and infrastructure but also on mobilizing heterogeneous consumer groups to achieve high-quality recycling pathways [24,34].

6. Conclusions and Future Task

This study applied the KAP framework and K-means clustering to investigate consumer cooperative behaviors in the household post-consumer plastic waste recycling system in urban Japan, with a specific focus on the voluntary retailer recycling route. Based on data from 758 respondents in Sendai, Kawasaki, and Kyoto, we identified three consumer segments: Fully Engaged Consumers (28.9%), Knowledge-Driven Consumers (36.4%), and Passively Engaged Consumers (34.7%). These groups showed significant differences in plastic waste recycling practices, environmental knowledge and attitudes, and socio-demographic characteristics. Notably, the use of the retailer recycling route was a key differentiator of cooperation levels.
Our findings provide empirical evidence of behavioral heterogeneity, reinforcing the need for targeted policy interventions. Fully Engaged Consumers are typically older, female, and motivated by internal values. Knowledge-Driven Consumers, despite strong awareness, often fail to act due to structural constraints. Passively Engaged Consumers, who are younger and less aware, require targeted interventions to boost engagement. These insights contribute to the limited of research on voluntary recycling systems in Japan, offering valuable guidance for optimizing policy design and system implementation.
Several limitations should be noted. The online survey may limit the generalizability beyond urban areas, and self-reported data can introduce bias. Additionally, while K-means cluster analysis reveals behavioral patterns, it does not identify causal relationships. Future research could incorporate regression models or explore retailers’ operational data to validate and expand these findings. The exclusion of respondents under 20 may limit the generalizability of the results. Future research could target younger individuals, such as high school or university students, to capture intergenerational perspectives.
In light of Japan’s policy shift toward sustainability and the circular economy for plastics, understanding consumer segmentation is crucial. This study highlights how behavioral and demographic diversity shapes cooperation in voluntary systems, providing a foundation for more inclusive and effective recycling strategies. Beyond Japan, the findings have broader implications for the theory of a sustainable and circular economy. They show that even in high-consumption societies with advanced waste management systems, technological and institutional measures alone are insufficient without consumer engagement. Japan’s case demonstrates both the achievements and limitations of relying on thermal recycling, underscoring the need to mobilize consumers to support higher-quality recycling pathways. As such, Japan can serve as a case study for other high-consumption nations, highlighting that transitioning to a sustainable and circular economy requires integrating infrastructure, policy, and diverse consumer participation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17177939/s1, S1: Questionnaire Survey (English Version).

Author Contributions

Conceptualization, Z.Y., X.L. and J.Y.; methodology, Z.Y., X.L. and J.Y.; formal analysis, Z.Y., X.L. and J.Y.; investigation, Z.Y., X.L. and J.Y.; resources, Z.Y., X.L. and J.Y.; Z.Y., X.L. and J.Y.; writing—original draft preparation, Z.Y., X.L. and J.Y.; writing—review and editing, Z.Y., X.L., J.Y., M.S. and G.M.; visualization, Z.Y., X.L. and J.Y.; supervision, J.Y.; project administration, J.Y., X.L., M.S. and G.M.; funding acquisition, J.Y. and X.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by JST SPRING, grant number JPMJSP2114, Hirose Foundation, and The APC was funded by the Tohoku University Support Program.

Institutional Review Board Statement

The study was conducted in accordance with the Declaration of Helsinki, and approved by the Research Ethics Committee of the Graduate School of International Cultural Studies, Tohoku University (2024–22, 12 March 2025).

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Acknowledgments

The authors would like to thank all persons who directly or indirectly contributed to the completion of this manuscript. The authors also gratefully acknowledge the support provided by the Tohoku University Support Program for covering the APC.

Conflicts of Interest

The authors declare no conflicts of interest.

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Sustainability 17 07939 g001
Figure 1. Cluster plot of the synthesized key components (k = 3).
Figure 1. Cluster plot of the synthesized key components (k = 3).
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Figure 2. Z-score distribution by cluster. All values are standardized (z-scores); 0 indicates the sample mean. Positive values indicate above-average levels, and negative values indicate below-average levels.
Figure 2. Z-score distribution by cluster. All values are standardized (z-scores); 0 indicates the sample mean. Positive values indicate above-average levels, and negative values indicate below-average levels.
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Table 1. Variables in K-means cluster analysis.
Table 1. Variables in K-means cluster analysis.
CategoriesVariablesCodeDescription
Recycling behaviorMunicipality recycling routeMR1 = Use, 0 = Not use
Retailer recycling route (use of recycling boxes)RR1 = Use, 0 = Not use
Knowledge and attitudesKnowledge and attitudes towards the New Plastic LawNPL1 = I know it, and I think it is good
2 = I know it, but I do not care about it
3 = I know it, but I think it is not good
4 = I do not know it
Knowledge and attitudes towards different plastic recycling methodsPRM1 = I am familiar with various plastic recycling methods.
0 = I do not know or care about the plastic recycling methods
Pro-environmental practicesFrequency of washing plastic wasteFWP1 = Always
2 = Often
3 = Sometimes
4 = Rarely
5 = Never
Practices of reducing single-use plasticsRSPWe listed six ways to reduce single-use plastics, and respondents will earn 1 score for each practice they checked.
Attitudes and practices towards environmental protection activitiesAEPA1 = I have never participated, and I am not interested
2 = I have never participated, but I am interested
3 = I have participated
Willingness to payWillingness to pay for the premium of eco-friendly packagingWTP 11 = Don’t want to pay
2 = Less than 5% of the product price
3 = 5–10% of the product price
4 = 11–20% of the product price
5 = 21–25% of the product price
6 = More than 26% of the product price
Willingness to pay for designated garbage bagsWTP 21 = Free
2 = under 15 yen per bag
3 = 15–30 yen per bag
4 = 31–45 yen per bag
5 = 46–60 yen per bag
6 = Over 60 yen per bag
Socio-demographic characteristicsGenderGender1 = Male, 2 = Female
AgeAgeAge of respondents (consecutive integer)
Education levelEducation1 = Elementary school or below
2 = Middle school
3 = High school
4 = University/vocational school
5 = Graduate school or above
Personal annual incomeIncome1 = 3 million yen or less
2 = 3 million−5 million yen
3 = 5 million−7 million yen
4 = 7 million−10 million yen
5 = 10 million yen or more
Household sizeHSNumber of family members of the respondent (consecutive integer)
Housing typeHT1 = Detached house, 0 = Communal house
Employment statusEmployment1 = Employed, 0 = Unemployed
Table 2. Socio-demographic characteristics of the respondents by city.
Table 2. Socio-demographic characteristics of the respondents by city.
Socio-Demographic CharacteristicsSendai
(n = 248)		Kawasaki
(n = 258)		Kyoto
(n = 252)		Total
(n = 758)
Age20 to 2912%12%12%12%
30 to 3912%12%12%12%
40 to 4913%14%13%13%
50 to 6438%39%38%38%
65 and above25%24%26%25%
GenderMale49%48%47%52%
Female51%52%53%48%
Education levelElementary school or below0%0%0%0%
Middle school1%2%2%1%
High school33%20%25%26%
University/vocational school59%74%67%67%
Graduate school or above7%4%6%6%
OccupationSelf-employed3%4%12%6%
Professional (lawyer, doctor, teacher, etc.)3%2%6%4%
Civil servant/organization employee7%4%4%5%
Company employee35%41%31%36%
Part-time/casual worker13%14%15%14%
Housewife/househusband13%17%13%14%
Student2%1%1%1%
Retired8%7%6%7%
Unemployed13%10%12%12%
Other2%1%1%1%
Individual annual income3 million yen or less60%57%60%59%
3 million−5 million yen21%16%22%20%
5 million−7 million yen9%14%10%11%
7 million−10 million yen7%9%5%7%
10 million yen or more4%5%3%4%
Household size1 person24%24%25%25%
2–3 people60%61%60%60%
4–6 people16%14%15%15%
Housing typeDetached house44%31%50%42%
Communal house55%68%48%57%
Other1%1%2%1%
Table 3. Final cluster centers.
Table 3. Final cluster centers.
Cluster 1Cluster 2Cluster 3
MR+0.24+0.12−0.32
RR+0.42+0.18−0.54
NPL+0.16+0.46−0.62
PRM+0.40+0.49−0.85
FWP+0.45+0.37−0.76
RSP+0.51+0.16−0.59
AEPA+0.45+0.31−0.70
WTP1+0.12+0.34−0.46
WTP2+0.15+0.33−0.76
Gender+0.39−0.25−0.07
Age+0.71−0.04−0.55
Education−0.06+0.17−0.13
Income−0.97+0.66+0.12
HS−0.13+0.11+0.00
HT+0.21−0.03−0.15
Employment−1.30+0.72+0.33
Table 4. Average age and responses to selected survey questions across the three clusters (not used in the K-means cluster analysis).
Table 4. Average age and responses to selected survey questions across the three clusters (not used in the K-means cluster analysis).
Questions and ResponsesCluster 1Cluster 2Cluster 3
AgeAverage age645244
Reasons for using the retailer recycling routeClose to home.27%19%2%
Throw waste anytime.26%17%2%
Environmentally friendly.17%12%2%
Save money.16%13%1%
Reduce municipal waste disposal costs.9%8%0%
Special benefits (points, coupons).8%6%0%
Reasons for not using the retailer recycling routeFar from home.30%31%49%
Strict separation rules.6%9%13%
Do not want to wash plastic waste.4%9%17%
Fewer types of plastic waste can be collected.11%16%26%
Preference for improvement measuresMunicipalities introduce more convenient separation policies.58%51%38%
Offer incentives to consumers, such as points awarded for using recycling boxes.45%51%28%
Governments and universities conduct education and awareness-raising activities on plastic issues.15%17%8%
Supermarkets and recycling companies set up more recycling stations/boxes.54%41%17%
Research and development of recycling technology.47%33%39%
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MDPI and ACS Style

Yu, Z.; Liu, X.; Yu, J.; Sujauddin, M.; Manago, G. A Cluster Analysis of Cooperative Recycling Behaviors for Post-Consumer Plastic Waste in Urban Areas: A Case Study on Sendai, Kawasaki, and Kyoto City in Japan. Sustainability 2025, 17, 7939. https://doi.org/10.3390/su17177939

AMA Style

Yu Z, Liu X, Yu J, Sujauddin M, Manago G. A Cluster Analysis of Cooperative Recycling Behaviors for Post-Consumer Plastic Waste in Urban Areas: A Case Study on Sendai, Kawasaki, and Kyoto City in Japan. Sustainability. 2025; 17(17):7939. https://doi.org/10.3390/su17177939

Chicago/Turabian Style

Yu, Zhuojiao, Xiaoyue Liu, Jeongsoo Yu, Mohammad Sujauddin, and Gaku Manago. 2025. "A Cluster Analysis of Cooperative Recycling Behaviors for Post-Consumer Plastic Waste in Urban Areas: A Case Study on Sendai, Kawasaki, and Kyoto City in Japan" Sustainability 17, no. 17: 7939. https://doi.org/10.3390/su17177939

APA Style

Yu, Z., Liu, X., Yu, J., Sujauddin, M., & Manago, G. (2025). A Cluster Analysis of Cooperative Recycling Behaviors for Post-Consumer Plastic Waste in Urban Areas: A Case Study on Sendai, Kawasaki, and Kyoto City in Japan. Sustainability, 17(17), 7939. https://doi.org/10.3390/su17177939

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