1. Introduction

2. Materials and Methods

Figure 1. Flowchart of the investigation.

Figure 1. Flowchart of the investigation.

3. Results

3.1. Comparative analysis between generated waste and GDP by EKC

The relationship between waste generated and GDP can be visualised in Figure 2. We can easily trace an increase between the mentioned indicators, with a stepwise progression at the beginning, but up to a certain level of approximately 45,000 euros. After this value, the increase is already smooth, and the curve describing it gradually flattens its slope, tending to reach the turning point. Following the EKC dynamics, the curve is expected to enter the downward part of its development after getting the turning point.

Figure 2. Municipal waste generated and GDP per capita in the EU-27 2021 (Bulgaria, Ireland, Greece, Italy and Austria 2020 values).

Figure 2. Municipal waste generated and GDP per capita in the EU-27 2021 (Bulgaria, Ireland, Greece, Italy and Austria 2020 values).

Figure 2 shows that the overwhelming number of EU countries are concentrated in the lower upward part of the curve. This implies that any further increase in economic growth will still be associated with increased waste. For some countries, this increase is more gradual. That’s why they are below the curve. Countries at the two opposite ends are also of interest. On the one hand, these countries are Luxembourg and Ireland. Compared to countries on the same horizontal axis, such as Denmark, Belgium and Austria, Luxemburg’s economic growth is nearly double, despite the similar levels of waste generated. The same trend is observed when comparing Ireland with Germany, Finland, Cyprus and Malta. The gradual decline of the curve describing the correlation between these indicators proves the same conclusion. Hence, as economic growth increases, the increase in waste generation starts to decrease gradually. At the opposite end are the countries with the lowest economic growth - Bulgaria and Romania. A parallel between Bulgaria and the other countries on the same horizontal line can be drawn - Hungary, Sweden and Estonia. Despite the significant reduction of waste generated in Bulgaria in the last decades, the waste generation figures are still too high against the background of the insignificant GDP, which is two or more times lower compared to countries with similar waste generation figures. Romania has the least waste generation and recorded higher economic growth than Bulgaria.

3.2. Comparative analysis of the relationship Average GDP euro/capita - Average municipal waste kg/capita

The variables average GDP per capita ($\overline{g}$) and average municipal waste per capita ($\overline{w}$) are analysed for the 27 EU countries studied. The columns in Figure 2 represent the average GDP values for the period (shown on the primary vertical axis). In contrast, the line represents the average waste generated (shown on the secondary vertical axis). Note that the variable ($\overline{w}$) for Denmark is the highest. In addition, the variable ($\overline{g}$) has a significantly higher value for Luxembourg than those for the other 26 countries considered.

Figure 3. Graphical analysis of average GDP euro/capita and average municipal waste kg/capita in the EU-27 in the period 2000-2021.

Figure 3. Graphical analysis of average GDP euro/capita and average municipal waste kg/capita in the EU-27 in the period 2000-2021.

Countries can be divided into categories according to the average GDP per capita level and look at the average values of municipal waste generated per person for each type. In this case, we will use the following categories:

• High GDP per capita: Luxembourg, Denmark, Ireland, Sweden, Belgium, Netherlands, Austria, Finland, Germany;
• Medium GDP per capita: France, Italy, Cyprus, Spain, Greece, Portugal;
• Low GDP per capita: Bulgaria, Czech Republic, Estonia, Croatia, Hungary, Latvia, Lithuania, Malta, Poland, Romania, Slovenia, Slovakia.

Here are some conclusions we can draw from the comparative analysis:

• Luxembourg has an extremely high average GDP per capita and correspondingly high levels of average household waste per capita. This can be linked to the specific economic conditions and the country’s significant financial sector presence;
• Countries with high average GDP per capita are associated with higher average waste generated per capita levels than countries with medium and low GDP. This can be explained by more significant economic activity and higher consumption levels;
• Countries with medium GDP per capita are in an average position regarding average municipal waste per capita. Although the average GDP per capita is lower than high GDP per capita countries, the average waste generated per capita is not significantly lower;
• Low GDP per capita countries have various values for average municipal waste generated per capita. Although some low-GDP countries generate less waste (e.g. Romania and Poland), other countries such as the Czech Republic, Estonia, and Latvia have higher waste generated;
• Although Bulgaria has a low GDP per capita, its municipal waste generated per capita is relatively high. Consumption habits, the efficiency of the waste management system and other socioeconomic factors are among the factors that can influence this result.

3.3. Correlation analysis applied to the survey data on GDP euro/capita and municipal waste kg/capita

A correlation analysis is performed for the above data on gross domestic product and municipal waste generated for the 27 EU member states mentioned. The calculated correlation coefficients are visualised in Figure 4.

The results allow us to distinguish several separate groups - countries with very strong, strong, moderate and weak correlations. No correlation is observed between the indicators considered for some countries examined.

Figure 4. Correlation coefficients in the EU-27 2000-2021.

Figure 4. Correlation coefficients in the EU-27 2000-2021.

The correlation coefficients for these two indicators for Lithuania, Latvia, Slovakia and Croatia are 0.941-0.850, indicating a very strong positive relationship. There is an increase of 2.8 and 2.4 times in the values of one of the indicators (GDP per capita) at the end of the period for the first and second-listed countries. The third and fourth country’s increase is about 2 and 1.7 times. The other variable examined (municipal waste generated per person) shows similar results. The most significant increase of about 1.9 times is calculated for Slovakia. For Latvia and Croatia, it is about 1.7 times; for Lithuania, it is over 1.3 times. It should be noted that there are some years where a decrease is found for the above variables. The observed increase in GDP per capita and municipal waste generated per capita in Lithuania, Latvia, Slovakia and Croatia can be explained by the significant economic growth these countries have achieved. The increase in the economic activity and income of the population has probably encouraged consumption and consumption of resources, which ultimately leads to a higher amount of municipal waste generated.

The results show that this relationship is inherent in all four countries studied, but there are some differences in the intensity of the relationship. For example, the increase in GDP per capita and household waste is more pronounced for Slovakia and Croatia than for Latvia and Lithuania. This may be due to differences in economic models, consumption habits and waste management infrastructure in these countries. A problem for Lithuania, Latvia and Croatia is a strong reliance on landfilling and limited levels of separately collected recyclables (especially bio-waste, plastics and metals), and for Slovakia, fragmented management of municipal waste and a lack of sufficient capacity to treat bio-waste [1]. A good practice in Latvia is developing a website that provides information on the location of sorting facilities and associated waste separation practices. In Lithuania, home composters are provided to encourage composting. In Croatia, an online platform allows companies to publish data on the waste they generate and others to locate and operate them.

The correlation coefficients for GDP and waste generated for six listed countries ranged from 0.786 to 0.650, indicating a strong positive relationship. These countries are Italy, Czechia, France, Portugal, Finland and Denmark (Figure 4). A more significant increase in the indicators is found for Czechia at the end of the study period. The correlation between higher GDP per capita and higher amounts of municipal waste generated per capita is evident in this group of countries. However, some countries report significant progress in municipal waste management and have increased recycling rates [1]. Denmark has set up a sound system for managing its municipal waste with precise responsibilities for the actors involved - government, citizens, and local authorities. The online knowledge-sharing platform is an excellent practice to help municipalities exchange information on successful municipal waste recycling practices. In France, extended producer responsibility; introducing a repairability index, which allows consumers to choose electrical and electronic appliances that are easier to repair; labelling for recycled content, etc., have produced good results.

Finland has been successful with high recycling rates for municipal waste (41.6% in 2020). Despite high incineration rates (59.7% in 2020), the deposit-return scheme for beverage packaging and the system for preparing wooden pallets for reuse are performing well. The combined door-to-door separate collection and pay-as-you-throw scheme is a good practice in Italy. It increased the separate collection rate from 48.5% to 72% and reduced with 15% the waste generated in just four years. The Czech Republic has also expanded its recycling rate of municipal waste (45.5% in 2020) and reports the establishment of a Reuse Centre as good practice. This centre prevented 77 tonnes of municipal waste from entering landfills for a year. It is also used for educational purposes. Another part of this group of countries has additional challenges in dealing with municipal waste [1]. Portugal faces challenges such as insufficient infrastructure for separate collection and treatment of biowaste and packaging, a strong reliance on landfilling (47.5% in 2020) and low levels of separately collected recyclables.

The correlation coefficient between the indicators considered for the two countries (Poland and Malta) is 0.591 and 0.521, indicating a moderate positive relationship. Again, the analysis proves the hypothesis that higher economic development (as measured by GDP) is associated with higher amounts of municipal waste generated. GDP in Poland increased continuously in the analysed years except for 2020. Economic factors such as investment growth, increased business competitiveness, political stability and other measures to stimulate the economy are possible explanations for the mentioned GDP growth in Poland. Household waste per person in Poland grew at a low rate, from 320 kg per person in 2000 to 362 kilograms per person in 2020. Poland faces problems such as a low capture rate of biowaste, limited capacity to treat separately collected biowaste and a heavy reliance on landfilling, resulting in limited resource reuse [1]. Identifying individual waste-producing households through labelling garbage containers and bags that improve separate waste collection and promoting composting through a garbage fee exemption are good practices in Poland.

GDP per capita in Malta increased from €13,750 thousand in 2000 to €22,760 thousand in 2020. Despite the fluctuations in municipal waste per capita figures in the last years (2018-2020), the values have remained lower than the previous years. This may signify an improvement in the efficiency of waste management systems and the implementation of sustainable practices in Malta [38]. However, low household waste recycling rates (10.5%) and limited waste recycling infrastructure are characteristic of Malta [1]. The high tourist activity, around 2.5 million people annually, also hurts recyclable waste levels. By producing comic books, Malta is committed to raising public awareness of waste generation, collection, separation and disposal, including school children.

In addition, the 12 correlation coefficients discussed above are statistically significant at α=0.05 significance level.

As shown in Figure 4, the estimates of the coefficients of correlation between variables in the study for all four countries ranging from 0.396 to 0.292, indicate a weak positive relationship. These examined countries are Luxembourg, Cyprus, Germany and Slovenia. The correlation coefficient found between GDP and waste generated for Belgium is 0.106. In this case, there is no relationship between the two mentioned indicators. The five correlation coefficients listed here are not statistically significant. All five countries are in the rich country category according to the World Bank classification. They are most likely in the downward part of the EKC, where increased economic growth leads to less waste generation. These results are thanks to government investment and a high degree of responsibility from higher-income societies. The disparity in economic and social conditions may cause the effects. Each country studied has unique economic and social conditions that may influence the relationship between GDP and household waste. Differences in industrial structure, consumption habits, waste management policies and other factors may lead to a weak correlation between these indicators.

Other factors influence the amount of municipal waste generated in a particular country, regardless of its GDP. Such factors include a developed waste management infrastructure, environmental awareness and education of the population, recycling and waste sorting policies, urban planning and regulation, etc. Luxembourg had a municipal waste recycling rate of 52.8% in 2020 and a meagre landfill rate of 3.8% in 2020 [1]. The country boasts an excellent waste management system covering the disposal and recycling of almost all waste materials, including the disposal, treatment and recycling of electrical and electronic equipment. Germany is one of the leading countries in Europe in terms of waste recycling. In 2020, around 69% of the country’s total waste is recycled or composted [1]. The country’s landfill rate is below 1%. Germany’s national deposit refund scheme is one of the first in Europe.

Slovenia is also known for its high recycling rates. In 2020, around 59.3% of the country’s municipal waste volume is recycled [1]. According to Eurostat, Belgium boasts all European Union countries’ highest recovery and recycling rates. It recycled 79.2% of all waste in 2020, well above the EU average of 53% [39]. Regarding packaging waste, Belgium shows a recycling rate of 94.9% in 2020. The last country in this group, Cyprus, has a high dependency on landfilling (67% in 2020), a low recycling rate (16.8% in 2020), and insufficient infrastructure and systems for separate collection and treatment of bio-waste [1]. Data show that Cyprus achieved economic growth from 2000 to 2020, with GDP per capita increasing. Municipal waste per capita has remained stable without significant fluctuations. However, the country needs to improve the efficiency of its waste management system [40].

As shown in Figure 4, the calculated correlation coefficients for Bulgaria and Netherlands are -0.863 and -0.752, indicating a very strong and strong negative relationship, respectively. In this case, an inversely proportional dependence is observed between the studied indicators. GDP per capita increase of more than 2.1 times is considered over the 21 years in Bulgaria. An insignificant decline is recorded for only two studied years - 2009 and 2020. It should be noted that for the 25 countries listed, there is also some decline in GDP for 2020. The values of the other indicator (generated waste) decrease about 0.6 times for Bulgaria at the end of the time interval considered. The situation is similar for the Netherlands, but here the increase in GDP per capita is approximately 1.19 times, while the decrease in the second variable (generated waste) is more than 0.8 times. The same conclusions could be drawn for Romania and Hungary.

GDP shows some growth, while the second variable shows some decline at the end of the period. The correlation coefficients for the three studied countries, Romania, Hungary and Greece, are -0.588, -0.585 and -0.44, respectively, indicating a moderate negative relationship. Therefore, an inversely proportional dependence is obtained between the studied indicators. It should also be noted that the correlation coefficients obtained are statistically significant at α=0.05 for Bulgaria, Netherlands, Romania, Hungary and Greece for the data considered on the two indicators studied.

The inverse relationship between GDP per capita and municipal waste per capita is a positive fact that can be explained by several reasons. First, higher GDP per person can be associated with more advanced technology and more efficient use of resources, which reduces the amount of waste generated. Second, in more developed economies, industry has shifted away from industry towards the service and information technology sectors. These sectors typically generate less municipal waste than industry. Third, people’s consumption behaviour changes as GDP and living standards rise. Wealthier societies focus more on longer-lasting products and recyclable materials, reducing waste generation. Fourth, more developed countries have more efficient waste management systems, including separate collection, recycling and treatment of waste. These activities reduce the volume of untreatable waste and contribute to lower waste generation. Finally, high public awareness of the importance of environmental protection and sustainable development can lead to a change in people’s behaviour and to a more responsible attitude towards waste. However, the above applies to the Netherlands, which has one of the highest GDP per capita of the 24 countries surveyed. The Netherlands invests in developing recycling infrastructure, including recycling centres and waste treatment facilities. These facilities permit efficient separation, sorting and recycling of the different waste fractions. The recycling rate of municipal waste in the country is 56.8% in 2020, and only 1.4% of municipal waste is landfilled [1]. The Netherlands applies a ’pay-as-you-go’ system for waste collection. The citizens pay a fee depending on the amount of waste they produce. This system encourages citizens to reduce waste and actively participate in separate collection and recycling.

The other three countries (Romania, Hungary and Bulgaria) have Europe’s lowest GDP per capita. In this case, the inverse relationship between GDP per capita and household waste per capita is due to different reasons. All three countries are economies in transition. Studies prove that transition countries improve their environment faster due to Rising energy prices and penalising energy-intensive activities [41,42]. In addition, countries with low GDP per capita usually go through early industrialisation and economic development stages. Production processes and resource consumption are typically less efficient in these stages and create more waste. Countries with lower economic growth may face challenges in establishing an effective waste management infrastructure. A lack of separate waste collection, recycling and treatment results in higher waste generation. All three countries have low recycling rates (13.7% for Romania, 32% for Hungary, 31.5% for Bulgaria in 2020) and high landfill rates (74.3% for Romania, 54% for Hungary, and 61% for Bulgaria in 2020 [1]. Economic development and increasing incomes are a priority in these countries rather than environmental sustainability and waste management.

The calculated correlation coefficients for the following three countries - Ireland, Sweden, and Spain are -0.292, -0.24 and -0.212, respectively, indicating a weak negative relationship. Here, an inversely proportional dependence is also found between these two variables. This means that a higher GDP per person is usually associated with less waste generated per person and vice versa. A more efficient use of resources and a lower dependence on materials leading to waste explain such a result. Ireland has a household waste recycling rate of 40.4% in 2020 and a landfill rate of 22.5% in 2020 [1]. Ireland is progressing in its municipal waste management by relying on producer responsibility and public awareness activities despite low waste separation rates and insufficient infrastructure for the separate collection and treatment of bio-waste. Sweden is known for its use of waste as a source of energy. Much of the waste generates heat and electricity in highly efficient waste-to-energy facilities. This helps reduce the use of fossil fuels and the country’s carbon footprint [43]. Sweden has a landfill rate of less than 1% due to National Landfill Bans for certain types of waste [1]. Spain has a high landfill rate for its municipal waste (52% in 2020) and an insufficient population covered by separate waste collection. Nevertheless, highly efficient recycling schemes and the pay-as-you-throw system are good practices in the country, but these practices are only implemented in some cities.

The calculated correlation coefficients for Estonia and Austria are -0.144 and -0.037. There is no relationship between these two indicators in this case. Hence, other factors or influences are vital in determining the waste generated in these two countries. The five correlation coefficients listed above are not statistically significant in this case. Austria is known for its high waste management standards. It is one of the leading recycling nations in Europe [44]. The country uses extensive separate waste collection systems, with citizens separating the waste into different fractions. Austria has achieved high recycling rates for packaging, paper, cardboard and glass (62.2% in 2020). It has also developed programmes for composting bio-waste and recovering energy from waste. The landfill rate of municipal waste is 1.8% [1]. Creating incentives for Austrian citizens to give electronic devices for repair instead of throwing them away through vouchers for half of the repair costs is a good practice in the country. Estonia is also a country with good waste management. It is actively working to reduce waste and increase recycling [45]. The country offers incentives and rewards for citizens actively participating in recycling and waste reduction. Estonia emphasises the development of innovative technologies and recycling centres, such as the Real-Time Data Collection System. The country still has low levels of separate collection and recycling of household bio-waste (28.9% in 2020) but a high recycling rate for packaging waste - 71.4% in 2020 [1].

We can draw the following conclusions comparing the data on average GDP per capita and average household waste per capita with the resulting correlation coefficients from Figure 4:

• The countries with a high correlation (close to 1) between GDP per capita and municipal waste generated per capita are Lithuania, Latvia, Slovakia and Croatia. These countries have relatively low average GDP per capita compared to other countries, with average municipal waste per capita in the middle range;
• Countries with a low correlation (close to 0) between GDP per capita and municipal waste generated per capita are Germany, Slovenia, Belgium, Ireland, Sweden, Spain, Estonia and Austria. These countries have a higher average GDP per capita than others, with average municipal waste per capita in the middle or lower range;
• The countries with a negative correlation (close to -1) between GDP per capita and municipal waste generated per capita are Bulgaria, the Netherlands, Hungary, Romania, Greece and Poland. These countries have a lower average GDP per capita than others, with average municipal waste per capita in the middle or higher range.

In summary, countries with high correlation have lower average GDP per capita but relatively high levels of municipal waste generated per capita. Countries with low and negative correlation have higher average GDP per capita but different levels of municipal waste generated per capita.

3.4. Correlation between per capita income and waste generation

Differences can also be observed depending on the level of income received. This indicator is closely linked to the economic growth and GDP of the country. The World Bank divides countries into Lower income, Lower middle income, Upper middle income and High income. None of the European countries is in the category of the first group. Bulgaria is the only country in the second group. Lithuania, Latvia, Poland and Romania are the four countries in the third group, and the remaining European countries belong to the last group. The waste generated by these three groups can be seen in Figure 5:

Figure 5. Waste generation by income group EU-27 in euro per capita.

Figure 5. Waste generation by income group EU-27 in euro per capita.

A well-visible trend is that high incomes are associated with more waste generated. The exception to this rule is the Lower Middle-Income group. Bulgaria is the only representative in this category, and all conclusions are related only to it. Waste per capita was significantly above the European average despite the low-income level in the country in 2000 when the country was still outside the EU. Bulgaria’s accession in 2007 and the adoption of several European waste management standards led to a gradual reduction in waste. It is evident already in 2010 but even more noticeable in 2021, when the figures fall below those of the High-income countries but are still above the Upper middle-income group. Low values of waste generated, influenced mainly by Romania and Poland, are typical for the Upper middle-income group. These are the countries with the least waste within the EU. However, in the last decade, there has been a more marked increase, more significant even compared to the high-income countries. There has also been an increase but at a slower pace. This verifies the thesis that as incomes rise and get closer to the turning point, the waste increase gradually begins to slow down. Upon reaching it, the waste generated will decline following its downward tendency.

3.5. Hierarchical cluster analysis according to average GDP euro/capita and average municipal waste kg/capita

The grouping of the countries according to average GDP euro/capita and average municipal waste kg/capita is presented in the dendrogram of Figure 6:

Figure 6. Grouping of the countries according to average GDP euro/capita and average municipal waste kg/capita.

Figure 6. Grouping of the countries according to average GDP euro/capita and average municipal waste kg/capita.

The resulting clusters are four:

The first cluster includes nine countries - Poland, Latvia, Lithuania, Hungary, Croatia, Slovakia, Estonia, Romania and Bulgaria. These countries are characterised by lower average GDP per capita and higher average waste generated per capita compared to the other countries. These are countries lagging behind economically - mainly Eastern European and Baltic countries.

The following cluster contains two sub-clusters. The first includes five countries - Slovenia, Malta, Portugal, Greece and the Czech Republic. These countries with average GDP per capita and average waste generated per capita are higher than the first cluster but lower than the third cluster. The second sub-cluster contains three countries - Cyprus, Spain and Italy. These countries have an average GDP per capita and average waste generated per capita that are higher than those in the third cluster but lower than those in the first cluster. The second cluster highlights the need to improve waste management systems and introduce more efficient recycling and waste reduction practices.

Two sub-clusters are represented in the third cluster. The first sub-cluster comprises four countries (Ireland, Denmark, Sweden and the Netherlands) with high average GDP per capita and relatively low average waste generated per capita. The second sub-cluster includes the following five countries - Austria, Finland, France, Belgium and Germany. These countries have high average GDP per capita and average waste generated per capita compared to the other clusters. This cluster includes countries with more developed economies and efficient waste management systems. However, they must continue to invest in sustainable practices and infrastructure to maintain the high level of waste management achieved. Countries from Western Europe and Scandinavia are represented in this group.

Luxembourg forms a separate cluster as it has an extremely high average GDP per capita and high average waste generated per capita compared to the other countries in the analysis. This could point to the direction of economic resources and focus on the growth of industries that can generate more waste.

3.6. Multivariate regression analysis applied to the indicators studied for Bulgaria

The relationship between GDP and municipal waste in Bulgaria is of particular interest due to the following factors:

• The fact that Bulgaria has the lowest GDP per capita among the countries analysed highlights the country’s economic challenges and constraints. This may affect consumption habits and the ability of the population to avoid unnecessary waste, as well as investment in modern waste management technologies;
• Above-average municipal waste per capita. Despite Bulgaria’s low GDP per capita, municipal waste is above the average. Hence, the population generates significant waste despite the limited economic opportunities. Consumption habits, the approach to recycling and waste management are the factors that can cause this result.

A multivariate regression analysis is performed to analyse the relationship between municipal waste and GDP in Bulgaria covering the period 2000 - 2020. Municipal waste in a million tonnes is the dependent variable in this analysis, while indicators related to economic growth, social development and population are included as independent variables.

GDP per capita (thousand euros) is the first independent variable. This indicator can serve as a measure of a country’s economic growth and well-being. Economically developed countries usually have a higher waste generated due to a higher level of consumption, but also more efficient waste management systems [46,47,48,49]

The Human Development Index is an indicator of a country’s standard of living that gives information on the population’s economic, educational and health indicators. A higher index usually means a higher standard of living. Therefore, it can be linked to specific consumption patterns of the population that influence municipal waste generation. On the other hand, the Human Development Index can provide insight into the level of awareness of the population on the importance of environmental protection as well as the policies adopted by the government for waste management. This consideration can also influence the municipal waste generated in a country [50,51,52,53,54].

Population density persons/km² is a measure of population density. A larger population usually produces more waste due to a higher level of consumption. At the same time, higher population density is associated with more efficient waste management systems [28,55,56,57,58].

Analysing the relationship between GDP and municipal waste in Bulgaria can help us understand two main things.

First, the multivariate regression analysis, including the GDP per capita, Human Development Index, and Population Density, can give us a more detailed insight into the impact of economic development on waste generation. This can help us identify relationships and dependencies between these factors and build a more comprehensive understanding of Bulgaria’s waste management trends and challenges. For example, we can understand whether higher GDP generates more waste or whether the human development index impacts preferences and habits for sustainable waste management. Also, we can explore how population density can influence the waste generated.

Second, multivariate regression analysis allows us to examine the existing relationships between factor variables and the municipal waste generated while controlling for the influence of other factors. This enables us to identify and analyse the significant factors explaining municipal waste in Bulgaria. Such an analysis can help develop sustainable waste management strategies and policies, focusing efforts on areas with the most important potential to reduce waste and improve the country’s sustainability.

Examining the relationship between GDP, Human Development Index, Population Density, and municipal waste can help us understand and justify the interaction between economic, social and environmental factors in sustainable development. This will provide us with valuable guidance to improve waste management in Bulgaria and achieve a more sustainable and environmentally responsible future.

In general, the regression model has the following form:

y=a₀+a₁x₁+a₂x₂+a₃x₃

(4)

where a₀, a₁, a₂, a₃ - coefficients of the equation, y - municipal waste (million tons), х₁ - GDP per capita (thousand euros), х₂ - Human Development Index, х₃ - Population density persons/km2.

Finding these coefficients, the following equation is obtained:

y = -18.95+0.93x₁-8.91x₂+0.4x₃

(5)

The calculated correlation coefficient is 0.9777, meaning a strong correlation exists between the independent and dependent variables. The coefficient of determination shows that the variation in the independent variables can explain 95.58% of the variation in the dependent variable. This is a high percentage and proves that the model is good at predicting municipal waste in Bulgaria. The regression model presented is statistically significant as Significance F is 0.00, less than α = 0.05.

Based on the results of the multiple regression analysis, we can draw the following conclusions about the influence of different factors on the municipal waste generated in Bulgaria:

Influence of GDP per capita: The regression coefficient for GDP per capita is positive (0.925), indicating that a higher GDP per capita in the country leads to an increase in municipal waste. With every rise in GDP per capita by 1 thousand, municipal waste increases by 0.9252 million tonnes when other factors are constant. This result can be explained by the fact that higher living standards and economic growth encourage waste consumption and production.

Impact of HDI (Human Development Index): the regression coefficient for HDI is negative (-8.912). This indicates that higher levels of human development associated with better education, health and social services can contribute to more effective waste management and a reduction in municipal waste generated. Municipal waste is reduced by 8.91 million tonnes for every unit of HDI growth.

Impact of population density: the regression coefficient for population density is positive (0.401), indicating that a higher population density in a given area leads to increased municipal waste. When the population per square kilometre grows by a single person, municipal waste increases by 0.4012 million tonnes when other factors are constant. This can be explained by the higher concentration of people in a small area, which imposes a greater need for waste services and management.

Figure 7. Correlation between municipal waste and GDP per capita, Human Development Index and population density over the period 2000-2020 found in a univariate regression analysis.

Figure 7. Correlation between municipal waste and GDP per capita, Human Development Index and population density over the period 2000-2020 found in a univariate regression analysis.

In conclusion, the model illustrates the relationship between municipal waste and the independent variables used. However, it is still important to remember that the results may be influenced by other factors that are not included. The model’s predictive ability may be limited because it is based on data from 2000 to 2020, and new factors may emerge to influence the generated municipal waste. The model may need regular updating and revision of the coefficient values of the independent variables to be accurate and up-to-date for predictions in the future.

Based on the multivariable regression analysis results and the conclusions about municipal waste management in Bulgaria, we can propose specific measures for municipal waste management in the country. These measures are based on analysing the factors that influence the amount of municipal waste. A high GDP per person seems to increase municipal waste, which higher consumption and production can explain.

In this case, measures such as promoting sustainable consumption patterns, stimulating recycling and reducing unnecessary single-use items can help reduce municipal waste generated. Also, a high human development index (HDI) level is linked to more effective waste management. Investing in education, public services and health can facilitate more effective management and reduction of household waste. Population density also has an impact on the municipal waste generated. A larger population in a given area has a greater need for waste services and management. Here, separate waste collection and efficient waste storage and treatment systems can be encouraged. All these measures must be supported by cooperation between different sectors - government, business, public institutions and civil society.

4. Conclusions and recommendations

• A study of the relationship between economic growth and the waste generated in EU countries shows a progressive relationship. In the beginning, the progression is steep, but as incomes rise, it gradually decreases and flattens out, tending to reach its turning point. This tendency is observed when similar levels of waste generated grow two or more times higher.
• The comparative analysis demonstrates a relationship between average GDP per capita and average waste generated per capita in most countries analysed. Specifically, higher GDP is generally associated with higher amounts of waste, while lower GDP is associated with less waste generated. This relationship can be explained in several ways. Countries with a higher GDP per capita have more advanced and intensive economies, leading to a higher production and consumption volume. This results in increased waste generated by industry, households and services. On the other hand, richer countries are starting to invest more in environmental protection. In this way, they create a better-developed infrastructure for waste collection, treatment and management, leading to more efficient waste management and reducing its negative environmental impact. Countries with lower GDP per capita have limited resources and weaker waste management infrastructure. They encounter challenges such as a lack of waste collection and treatment, inefficient use of resources and environmental problems.
• Some countries have no significant correlation between income and waste. These are primarily high-income countries in the lower part of the EKC or countries in transition. As a representative case, Luxembourg shows an economic growth rate twice as high as countries with comparable levels of waste generation, with no correlation between these indicators being observed.
• The results of the correlation analysis suggest that, whilst GDP per capita and municipal waste per capita in most countries in the EU have a positive relationship, the strength and direction of this relationship differ between countries. Still, there are exceptions to this rule, with certain countries with low GDP per capita producing less waste and countries with high GDP per capita producing lower levels of waste, highlighting the importance of effective waste management and environmental protection policies.
• The cluster analysis shows regional differences in average GDP and average waste generated per person. Eastern European and Baltic countries have lower GDP per capita and higher waste generated per capita. Western European and Scandinavian countries have higher GDP and lower waste per person. Various economic factors and development levels explain those differences between the countries. Higher GDP in Western Europe and Scandinavia is linked with more advanced industrialisation, service development and higher living standards. At the same time, although Eastern European and Baltic countries have made economic progress, they still struggle with financial challenges and a lower level of development. Understanding these differences is essential for formulating policies and strategies for sustainable development and waste management. Regions with lower GDPs and higher waste can focus on improving economic infrastructure, energy efficiency and increasing recycling. In comparison, regions with higher GDPs can strive to achieve more sustainable economic models and waste management.
• The multivariable regression analysis focusing on Bulgaria shows that economic development and socio-demographic factors must be considered to achieve more sustainable and efficient municipal waste management in the country. It is essential to concentrate on the financial aspects of progress, including investment in waste infrastructure, promotion of recycling industries and innovation in sustainable resource management, not forgetting social and demographic factors, such as education and awareness of citizens.

References

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