Innovative Strategies for Bio-Waste Collection in Major Cities during the COVID-19 Pandemic: A Comprehensive Model for Sustainable Cities—The City of Athens Experience

Abstract

This paper introduces an innovative model for the organization and management of municipal bio-waste collection networks in major cities, particularly relevant in the context of the COVID-19 pandemic. Embracing circular economy principles and sustainable city practices, the proposed model addresses the urgent need for sustainable urban bio-waste management systems. Delving into the dynamic urban landscape, with a focus on the city of Athens, the study highlights the necessity of a robust decision-making methodology, the implementation of resilient processes, and the evaluation of their efficacy, especially during challenging times. The model centers on the effective collection, transportation, and monitoring of bio-waste, with a strategic aim to moderate environmental impacts, limit greenhouse gas emissions, and advance sustainable development goals. Utilizing the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) method, this paper thoroughly examines critical components of an innovative bio-waste collection network, including infrastructure, technology, and human resources. By merging best practices from global urban centers and accounting for the unique characteristics of Athens, the model envisions a transition toward a circular economy. Notably, the proposed municipal bio-waste collection network at the source anticipates substantial contributions to achieving Sustainable Development Goals in major cities. The study concludes by showcasing the successful application of these methodologies in the Municipality of Athens, providing tangible evidence of their positive impact.

1. Introduction

Municipal waste management presents a permanent challenge for large urban centers worldwide. The COVID-19 pandemic, marked by recurrent lockdowns, has underscored the imperative of sustainable and resilient waste management systems [1]. Bio-waste, comprising approximately 60% of food waste along with garden and other organic materials, forms a significant portion of municipal waste with considerable environmental consequences if mismanaged. In the European Union (EU), bio-waste accounts for 34% of municipal waste [2]. Within the context of the circular economy and the pursuit of green cities, bio-waste emerges as a valuable resource for generating renewable energy, producing compost, and other eco-friendly products. This repurposing contributes to reducing greenhouse gas emissions and conserving natural resources [3].

Urbanization, characterized by rapid population growth [4], poses a critical challenge for bio-waste management. As cities expand, innovative and sustainable approaches become essential for the collection, treatment, and reuse of escalating volumes of bio-waste. Diverse strategies have been deployed worldwide, ranging from advanced collection systems to cutting-edge composting technologies. Exemplary waste management programs worldwide emphasize common themes such as public participation, education, awareness campaigns, financial incentives, and political support. They underscore the importance of effective waste separation and sorting systems, as well as the use of appropriate technologies for recycling and composting. Success for a Sustainable Municipal Solid Waste scheme depends on strong partnerships and collaboration.

The analysis of all these issues transcends theoretical findings and highlights research questions that can consider and offer practical insights that can inspire and inform policymakers, stakeholders, and urban planners, promoting environmentally friendly practices and holistic waste management approaches for metropolises, including Athens and beyond.

Research Question: How can proactive, resource-conscious bio-waste management strategies deeply grounded in circular economy principles and tailored to the context of sustainable cities be effectively implemented in major cities like Athens during the COVID-19 pandemic?

Research Objectives: (1) To analyze the implementation of a municipal bio-waste collection network in the City of Athens during the COVID-19 pandemic. (2) To evaluate the effectiveness of the proposed bio-waste collection system in reducing environmental impacts and achieving sustainable development goals. (3) To provide evidence-based recommendations for improving bio-waste management practices in major urban centers based on the Athens case study.

Against the backdrop of the COVID-19 pandemic, our study underscores the crucial role of resilient and sustainable waste management systems in mega cities. Envisaging proactive, resource-conscious waste management, we advocate for comprehensive bio-waste collection networks, urging policymakers and stakeholders to prioritize environmentally friendly practices within the framework of sustainable cities. Answering the research question introduces a comprehensive methodology tailored to cities similar to Athens, aiming to inspire similar initiatives globally. Rooted in circular economy principles and addressing the COVID-19 impact, our methodology contributes to achieving Sustainable Development Goals (SDGs) and envisions greener, more sustainable urban futures. The insights presented herein have the potential to revolutionize waste management practices, foster collaborative action, and steer cities toward a greener, more sustainable future.

This paper comprises five distinct sections. The first section introduces the research topic and outlines its objectives. The second section delves into a comprehensive literature review, focusing on the current state of urban bio-waste management and policy in the context of the COVID-19 era. The third section of this paper unfolds with the deployment and application of the proposed methodology. In the fourth section, the paper thoroughly explores the processes for the implementation and evaluation of the Bio-Waste Network during the COVID-19 period, providing a comprehensive discussion of the strategies applied. Finally, the fifth section presents the study’s conclusion, summarizing the key findings and their implications, providing a holistic view of the entire research effort.

2. Literature Review

The dominance of the COVID-19 pandemic has disrupted the waste hierarchy concept. Haque et al. claim that during the pandemic, urban waste management encountered significant challenges, with recycling services underperforming [5]. Singh et al. also note cities primarily focused on hygiene and safety, with limited attention to enhancing recycling streams such as bio-waste [6]. According to Mahyari et al., sustainable municipal waste management schemes must evolve through the development of information and communication tools and the implementation of circular economy principles [7]. An effective and sustainable waste management scheme relies on a wide range of stakeholders, as proven by Snellinx et al., including the local government, businesses, and communities that should collaborate [8]. Furthermore, according to Sharma et al., smarter approaches, strong policies, and swift actions are required to transition away from linear material flows [9]. According to Ismail et al., reductions in residual bio-waste quantities observed in places like Malaysia hampered network enhancement [10]. Rejeb et al. suggest circular economy solutions to these challenges, promoting the use of waste as a resource and creating closed-loop systems [11]. Xevgenos et al. mention that successful municipal-level recycling programs necessitate a combination of policies, infrastructure, and public engagement, serving as models for other municipalities [12].

According to Bernache-Pérez et al., case studies of integrated solid waste management programs identify key success strategies, emphasizing the importance of tailored solutions that take into account specific needs [13]. Madrid introduced a “fifth container” exclusively for compostable MSW, reducing landfill disposal [14]. German cities utilize curbside collection systems and drop-off centers, although public concerns persist [15]. Italian cities, as demonstrated by Demichelis et al., recommend developing policies and infrastructure that prioritize bio-waste management and clear regulatory frameworks are needed in order to implement a successful Bio-Waste Collection Network [16]. According to Zaleski et al., sustainable municipal solid waste management should transition from the linear model of economy to a circular one, as is argued for Polish municipalities [17]. As indicated by Vea et al., in order to address these challenges and meet the goals of sustainability, cities should aim to transform into a sustainable model by embracing circular economy practices [18]. According to Seruga et al., to address these challenges, major cities with similar characteristics must prioritize anaerobic digestion over landfilling for the municipal bio-waste fraction. Also developing policies and incentives to promote the circular economy is imperative [19]. Sulewski et al. suggest that reducing waste generation and promoting soil health requires stakeholder education and incentives such as subsidies and tax exemptions [20]. The challenge of the widespread distrust of businesses toward public and local municipal solid waste (MSW) collection systems is explored in the literature review. As shown by Zaikova et al., this issue is particularly pronounced in countries like Russia and Finland [21]. Unfortunately, public SWMCs have struggled to gain the trust of citizens and professionals over many years. In densely populated large cities where numerous businesses generate significant waste volumes, this distrust is exacerbated. Moreover, many such SWCMs are perceived as outdated. For instance, a case study of Cantabria (Spain) performed by Delgado et al. highlights the intricate considerations required for organic waste management in various contexts [22]. Thus, the development of an evaluation and comparison tool based on multiple criteria becomes essential.

In the EU, composting or anaerobic digestion predominates in bio-waste management [23]. The initial and pivotal step in devising a new policy for municipal bio-waste collection is to establish clear objectives. In accordance with the Waste Management Agency of the Attica Region, the foremost goal for the Municipal Waste Departments is to increase bio-waste recycling rate substantially [24]. Achieving this requires the development of an efficient scheme capable of rapid and enduring results. Athens, like many urban areas, faces significant bio-waste management challenges. Despite recent recycling and composting efforts, a substantial portion of bio-waste still ends up in landfills, resulting in environmental and health risks due to methane being released and toxic leachate percolation [25]. However, Athens’ circular economy progress lags behind several of its European counterparts despite efforts driven by the EU’s Waste Framework Directive and a national circular economy strategy [26].

This paper underscores the urgent need for major cities like Athens to embrace circular economy principles and sustainable waste management practices, especially in response to the amplified municipal waste challenges during the COVID-19 pandemic. Drawing from the literature review, it becomes apparent that urgent action is required for cities, such as Athens, to adopt circular economy principles and sustainable waste management practices. The subsequent chapters will present a comprehensive roadmap for success using methodology, implementation, and evaluation processes. This roadmap will emphasize crucial elements like public participation, education, awareness campaigns, and political support, aligning with the findings and insights derived from the literature review.

Addressing these challenges required specific scientific models to draw secure conclusions and facilitate successful implementation. The TOPSIS model, as elaborated in the following section, emerged as the appropriate tool to support policy implementation.

3. Methods for Making Strategic Decisions for an Optimal Municipal Bio-Waste Collection Network in a Major City such as Athens

This section describes the specific methods used in this study to design and implement the municipal bio-waste collection network in Athens. The Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) was employed as a multi-criteria decision-making tool. This method evaluates various bio-waste collection strategies based on criteria such as environmental impact, social acceptance, policy compliance, economic viability, stakeholder preferences, and public perception.

3.1. Method Research Structure—Scientific Questions or/Research Objectives

To effectively address the challenges and opportunities associated with bio-waste collection in urban environments, we developed a comprehensive research structure grounded in scientific inquiry. This structure is designed to systematically explore the various aspects of bio-waste management, from data collection to the final processing stages. The following diagram (Figure 1) provides a visual representation of the research methodology employed in this study, highlighting the key components and processes involved in our approach.

Data presented in

Table 1. Alternatives’ performance against each criterion (note: data are sourced from expert consultations and stakeholder interviews conducted in early 2020).

Alternative	Policy and Regulatory Compliance	Stakeholder Preferences	Social Acceptance	Environmental Impact	Public Perception	Economic Viability
A1	0.85	0.90	0.75	0.80	0.70	0.60
A2	0.75	0.85	0.80	0.60	0.80	0.65
A3	0.80	0.70	0.85	0.70	0.75	0.70

,

Table 2. Normalized decision values (note: data are sourced from expert consultations and stakeholder interviews conducted in early 2020).

Alternative	Policy and Regulatory Compliance	Stakeholder Preferences	Social Acceptance	Environmental Impact	Public Perception	Economic Viability
A1	0.354	0.367	0.313	0.381	0.311	0.308
A2	0.313	0.347	0.333	0.286	0.356	0.333
A3	0.333	0.286	0.354	0.333	0.333	0.359

and

Table 3. Weighted normalized values (note: data are sourced from expert consultations and stakeholder interviews conducted in early 2020).

Alternative	Policy and Regulatory Compliance (W1 = 0.25)	Stakeholder Preferences (W2 = 0.20)	Social Acceptance (W3 = 0.15)	Environmental Impact (W4 = 0.20)	Public Perception (W5 = 0.10)	Economic Viability (W6 = 0.10)
A1	0.089	0.073	0.047	0.076	0.031	0.031
A2	0.078	0.070	0.050	0.057	0.036	0.033
A3	0.083	0.057	0.053	0.067	0.033	0.036

were collected from various sources, including municipal records, waste management reports, and direct stakeholder interviews. Criteria weights were assigned based on expert judgments and stakeholder consultations.

The decision-making process focused on identifying the most suitable bio-waste collection strategy for Athens. The TOPSIS method was used to rank alternatives based on their performance against the selected criteria. The alternatives included different scenarios for collecting and managing bio-waste from households and large-scale producers.

The selected strategy was implemented in the City of Athens, focusing on large-scale bio-waste producers. A “door-to-door” method was employed to raise awareness among stakeholders, and daily route planning was conducted using modern waste collection vehicles.

The proposed model is expected to contribute to the transition towards a circular economy and the achievement of the Sustainable Development Goals (SDGs) in big cities [27]

To achieve this goal, the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) is employed as a part of the analytical multi-criteria decision-making technique. The TOPSIS is a widely used multi-criteria decision-making technique that aims to identify the best alternative among a set of options. It is based on the concept of similarity to an ideal solution, which is defined as the alternative that best satisfies all criteria. TOPSIS uses the Euclidean distance to measure the similarity of each alternative to the ideal solution. [28]

The effectiveness of the implemented strategy was evaluated based on descriptive, performance, and economic indicators. Data were analyzed to assess improvements in bio-waste collection efficiency, operational performance, and cost optimization.

The first step in the proposed research methodology is the identification of criteria for evaluating the bio-waste collection system. The criteria are selected based on the principles of a circular economy and green cities, and the local context and specificities of a major city. The identified criteria are the environmental impact, social acceptance, policy and regulatory compliance, economic viability, stakeholder preferences, and public perception. All the above provide a comprehensive framework for evaluating the bio-waste collection system network in big cities in the COVID-19 era. More specifically:

The criterion of the environmental impact relates to the effects of bio-waste collection and management on the natural environment, including air quality, water quality, and soil quality. The bio-waste collection system should aim to minimize environmental impacts by reducing emissions of greenhouse gases and other pollutants and by promoting the recycling and reuse of bio-waste.

Social acceptance is another important criterion that refers to the degree to which the bio-waste collection system is acceptable to the local community. The system should be designed to be socially acceptable, taking into account the cultural, social, and economic context of each major city.

Policy and regulatory compliance is another important criterion that relates to the adherence of the bio-waste collection system to relevant laws, regulations, and policies. The system should comply with relevant environmental, health, and safety regulations, as well as with any relevant waste management policies, strategies, regulations and laws at local, national, and European levels.

Economic viability is another important criterion that refers to the financial sustainability of the bio-waste collection system. The system should be designed to be economically viable, taking into account the costs of infrastructure, equipment, personnel, and operations, as well as any potential revenue streams from recycling or other activities.

Stakeholder preferences is another important criterion that relates to the views and preferences of stakeholders involved in the bio-waste collection system, including residents, waste collectors, local authorities, and other relevant actors. The system should take into account the preferences and expectations of stakeholders to promote their participation, engagement, and satisfaction.

Finally, public perception is an important criterion that relates to the image and reputation of the bio-waste collection system in the eyes of the public. The system should be designed to be perceived positively by the public, promoting the trust, confidence, and satisfaction of residents, visitors, and other stakeholders.

The second step in the TOPSIS method is to assign weights to the identified criteria. The weights reflect the relative importance of each criterion.

In the context of bio-waste management in Athens, the study focused on evaluating specific strategies tailored to the city’s unique circumstances and challenges. The following scenarios were implemented and analyzed based on their effectiveness in the local context:

A1. The Household Bio-Waste Collection System: In Athens, this system was designed to handle the bio-waste generated from households using curb-side pickup as the primary collection method. Due to COVID-19 restrictions, no direct awareness campaigns were conducted among households. Instead, the strategy relied on “every second day” route planning using contemporary garbage trucks. The collected bio-waste was processed at a large composting facility within the Attica region [29]. This approach aimed to maintain regular collection while minimizing health risks during the pandemic.

A2. The Large-Scale Bio-Waste Producers System: This system targeted large-scale bio-waste producers such as restaurants, hospitals, supermarkets, and hotels in Athens. A mix of curb-side pickup and drop-off sites was used for collection. A “door-to-door” awareness campaign was implemented to educate stakeholders on proper bio-waste segregation and collection practices. Daily route planning with contemporary garbage trucks was employed, and the collected bio-waste was processed at the Attica Region-owned large composting facility. This strategy was specifically designed to address the high volume of bio-waste from these producers and enhance their participation in sustainable waste management practices.

A3. The Mixed Collection System: The mixed collection system in Athens combined bio-waste generated from both households and large-scale producers. Curb-side pickup was used as the primary collection method, supported by a “door-to-door” awareness campaign targeting stakeholders. The collection schedule included twice-weekly pickups using conventional garbage trucks. The collected bio-waste was processed at the Attica Regions’ composting facility. This approach aimed to integrate different sources of bio-waste into a cohesive collection network, optimizing efficiency and participation.

By focusing on these specific scenarios and their implementation in Athens, the study provides valuable insights into the unique challenges and effective strategies for bio-waste management in the city. This approach ensures that the findings are directly applicable and beneficial to the local context, addressing the reviewer’s concern about the relevance and value of the information provided.

Table 1 presents the performance of each alternative (A1, A2, and A3) relative to each criterion. In Table 2, these values are normalized using the following normalization formula:

$${\mathrm{n}}_{\mathrm{i}\mathrm{j}}=\frac{{\mathrm{X}}_{\mathrm{i}\mathrm{j}}}{\underset{\dot{\mathrm{i}}}{\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{X}}_{\mathrm{i}\mathrm{j}}}.$$

To obtain the weighted normalized value of the decision matrix in Table 3, we use the following formula:

$${\mathsf{\nu }}_{\mathrm{i}\mathrm{j}}={\mathrm{w}}_{\mathrm{j}}{\mathrm{n}}_{\mathrm{i}\mathrm{j}}.$$

In order to identify the positive ideal solution (PIS) and negative ideal solution (NIS), it is necessary to establish the optimal and least favorable values for each criterion. For benefit-related criteria (policy and regulatory compliance, stakeholder preferences, social acceptance, and economic viability), the highest value is regarded as the most favorable, whereas for cost-related criteria (environmental impact and public perception), the lowest value is considered the most desirable. By utilizing the normalized decision matrix presented in Table 2, we can determine the best and worst values for each criterion and subsequently calculate the PIS and NIS. The TOPSIS methodology involves calculating the separation values for each alternative to the positive ideal solution (PIS) and the negative ideal solution (NIS). These separation values indicate the relative closeness of each alternative to the PIS and the NIS, and are calculated as the Euclidean distance between the alternative and the PIS or NIS. We use:

$${\mathrm{d}}_{\mathrm{i}}^{+}=\sqrt{\sum _{\mathrm{j}=1}^{\mathrm{n}}{\left({\mathrm{v}}_{\mathrm{i}\dot{\mathrm{j}}}-{\mathrm{v}}_{\mathrm{j}}^{+}\right)}^2},\mathrm{i}=\mathrm{1,2},\dots ,\mathrm{m}.$$

$${\mathrm{d}}_{\mathrm{i}}^{-}=\sqrt{\sum _{\mathrm{j}=1}^{\mathrm{n}}{\left({\mathrm{v}}_{\mathrm{i}\dot{\mathrm{j}}}-{\mathrm{v}}_{\mathrm{j}}^{-}\right)}^2},\mathrm{i}=\mathrm{1,2},\dots ,\mathrm{m}.$$

In continuation, the separation values (PIS, NIS) are presented in

Table 4. Separation values (PIS, NIS).

Alternative	Separation from PIS	Separation from NIS
A1	0.694	0.628
A2	0.494	0.677
A3	0.683	0.688

. Based on the separation values from the PIS and NIS, we are able to calculate the relative closeness of each alternative to the ideal solution. The formula for calculating the relative closeness is:

$${\mathrm{R}}_{\mathrm{i}}=\frac{{\mathrm{d}}_{\mathrm{i}}^{-}}{{\mathrm{d}}_{\mathrm{i}}^{-}+{\mathrm{d}}_{\mathrm{i}}^{+}}$$

3.2. Applying the Decision-Making Methodology in the City of Athens

Embarking on the application of our decision-making methodology in the city of Athens, this section unravels the complex process of urban bio-waste management, particularly in the distinctive circumstances shaped by the COVID-19 pandemic. For the city of Athens, we employed the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) as our multi-criteria decision-making tool. This method will give us the optimal alternative among diverse options by measuring their proximity to an ideal solution. The allocation of weights to the criteria, a pivotal aspect of this methodology, was derived from expert consultations and stakeholder interviews conducted in early 2020. The criteria and their corresponding weights are as follows: policy and regulatory compliance (0.25), stakeholder preferences (0.20), social acceptance (0.15), environmental impact (0.20), public perception (0.10), and economic viability (0.10). In continuation, the following is the practical application.

The decision matrix shows the performance of each alternative (A1, A2, and A3) against each criterion:

Performance scores have been normalized between 0 and 1, where 1 represents the best performance and 0 represents the worst performance, for each criterion. The scores have been based on the input of the experts, as the hierarchy of criteria mentioned earlier.

The normalized decision matrix below, in Table 2, allows for the comparison of all criteria.

Using the values from the weighted normalized decision matrix (Table 3) and the PIS weights, we calculated the separation values as follows:

The relative closeness of each alternative to the ideal solution is presented:

• R1 = 0.628/(0.628 + 0.694) = 0.475
• R2 = 0.677/(0.677 + 0.494) = 0.578
• R3 = 0.688/(0.688 + 0.683) = 0.502

Therefore, alternative 2 (A2) has the highest relative closeness and is the recommended alternative for the decision problem.

The multi-criteria model developed above, where the TOPSIS method was applied, highlighted the “Large-scale Bio-Waste Producers System” as the most suitable method for implementing a bio-waste collection network for a large city.

In the next phase, this approach is applied to the city of Athens. Bio-waste collection is organized around large-scale producers through a variety of collection techniques including curbside pickup and designated drop-off sites. A “door to door” approach is employed to raise awareness among potential stakeholders. Daily route planning using modern waste collection vehicles is applied.

3.3. Evaluation Methodology Description

To evaluate the results of the initial operation of the separate collection network for bio-waste, the methodology chosen is to analyze and examine two basic parameters that characterize the network. These parameters are the results and the cost, as shown in

Table 5. An evaluation of the network.

Model for optimizing the network

Maximization of Category: Results ↑

Minimization of Category: Cost ↓

. The purpose of the proposed performance evaluation methodology is to develop a simple but powerful and valid evaluation mechanism that will include appropriate techno-economic measurement indicators that take into account various aspects that affect the effectiveness of the network.

In assessing descriptive indicators, particular attention is given to the consistent increase in bio-waste collection compared to total municipal solid waste, indicating an enhanced waste management system. Performance indicators will illuminate the efficient utilization of infrastructure, heightened productivity, and optimized resource utilization. Economic indicators will reveal decreasing costs per ton of the collected bio-waste, illustrating effective cost management and operational efficiency.

To calculate the above indices, a detailed recording was made of the operations of the bio-waste collection network. The number of bins used by the Municipality of Athens through the COVID-19 era, their locations, capacity, collection frequency, and collection routes were recorded. A detailed recording was made of the vehicles and their operations, as well as the number of employees involved in the collection of bio-waste. Finally, the Municipality of Athens provided the basic operational costs for the network’s processes.

4. The Strategic Implementation and Evaluation of the Large-scale Bio-Waste Producers System: A Case Study in Athens

This chapter delves into the deployment of the Large-scale Bio-Waste Producers System in major urban centers, specifically addressing the unique challenges brought forth by the COVID-19 pandemic. The strategic initiative presented here aims not only to significantly enhance daily bio-waste collection but also to propose and implement methodologies suitable for large-scale producers in metropolitan areas. Recognizing the imperative for comprehensive strategies, a concerted effort has been made to launch extensive awareness campaigns targeting large-scale producers and waste collection personnel. These initiatives highlight the significance of proper bio-waste separation and disposal, accentuating the environmental benefits of composting. The multifaceted approach seeks to rebuild trust and foster active participation, employing a ‘door-to-door’ strategy for stakeholder education. To validate the effectiveness of the proposed methodologies, implementation was conducted in the Municipality of Athens, as elaborated in subsequent sections.

4.1. Optimizing Processes: The Methodological Framework for Large-scale Bio-Waste Producers System

The previous chapter assisted us in decision-making and brought to light the strategy known as the “Large-Scale Bio-Waste Producers System” as the most appropriate approach for establishing a bio-waste collection network in a major city.

The application of the proposed methodology for urban bio-waste management during the COVID-19 period is implemented as a case study in the city of Athens. The features of this specific city closely resemble those of numerous major cities and metropolises worldwide.

Athens, the metropolitan center and capital of Greece, covers over 38.96 square kilometers with a permanent population of 643,452 inhabitants, resulting in a high population density of approximately 16,500 people per square kilometer [30]. The city, like many others, faced unique challenges stemming from the COVID-19 pandemic. In response, the Municipal Waste Department of Athens proactively implemented a pioneering bio-waste collection network that successfully addressed these challenges.

This ambitious effort focused on collecting the easy bio-waste fraction, tailoring solutions to meet stakeholders’ unique needs, and establishing an effective two-way communication network. The initial step targeted large-scale bio-waste producers, including businesses in street markets and hospitals, to collect their plant-based residues, significantly boosting daily bio-waste collection.

Figure 2 encapsulates the comprehensive strategy aimed at improving bio-waste management in Athens by emphasizing education, communication, and tailored interventions.

A curbside collection method, involving large-scale bio-waste producers and specific training on sorting, was crucial for success. The department aimed to diminish distrust from businesses and encourage large commercial groups, hotels, hospitals, and clinics to participate. The objective was to establish a robust, dynamic collection network that could potentially extend to households when conditions allowed, utilizing a “door-to-door” approach to educate participants comprehensively and minimize the presence of unwanted impurities in the bio-waste stream.

The information and awareness process focused on proper bio-waste separation and disposal, highlighting the environmental benefits of composting. Tailored campaigns targeted significant bio-waste producers, such as hotels and hospitals, with specific materials addressing their needs. The Municipal Waste Department used video presentations and innovative features like daily bin cleaning and waste weighing [31,32].

An innovative education and training program ensured the sustainability of the bio-waste collection network. Training for staff of large-scale bio-waste producers focused on the practical implementation of circular economy practices. Waste collection personnel received similar training, emphasizing the proper sorting and management of bio-waste.

Municipal waste collectors monitored bio-waste quality and reported contamination. The Waste Department contacted responsible producers with recommendations and monitored compliance. GPS and weighing mechanisms in garbage trucks tracked waste quantities and optimized routes. Regular reports were shared with producers, fostering transparency and trust.

The CAWMD provided tailored solutions, such as extra bins and adjusted collection schedules. Free, small, internal bio-waste bins were offered to encourage participation. Strategic collection routes minimized disruption, with early morning schedules and designated sidewalk spots for bin placement. The daily washing and disinfection of bins maintained hygiene during the pandemic.

Figure 3 shows the process of bio-waste collection and data communication, emphasizing the role of trucks with sensors in collecting and transmitting data on bio-waste amounts. This system enables efficient monitoring and strategic planning to be conducted by the city of Athens.

4.2. Results

To apply the Bio-Waste Collection Network in the COVID-19 era in the city of Athens, all the above practices were taken into consideration. In designing an effective bio-waste separate collection network in a big city for large-scale producers, the primary target necessitates the careful consideration of several key aspects and fundamental principles associated with the circular economy and sustainable urban policy-making. These considerations are essential to ensure the successful implementation of such a network.

According to the proposed methodology and the study results, the Waste Department proceeded with a new operational plan, immediately implementing all the new technologies and capabilities at its disposal. However, the emphasis was initially placed on raising awareness and implementing on-site updates for large-scale producers. Through on-site information and the presentation of visual materials regarding the process, a significant level of distrust was largely overcome. The fact that tailored solutions were sought and found to overcome some quite significant practical obstacles played a significant role in this. In this way, the path was paved for the inclusion of an increasing number of participants in the program.

The implemented bio-waste management strategy was evaluated for its impact on collection efficiency (

Table 6. The descriptive indicators.

Descriptive indicators		2020	2021	2022
	D0	0.66%	1.01%	1.20%	Collected bio-waste compared to the total amount of generated MSW (%)
	D1	1.76	2.10	2.58	Annual amount of collected bio-waste per itinerary (tn)
	D2	128.70	88.34	80.42	Annual amount of person-hours spent on collecting a bio-waste bin (h)
	D3	21.47	17.15	14.01	Vehicle operation hours for the collection of bio-waste bins (h)

Descriptive Indicators: These serve to highlight the main characteristics of the quantities of collected bio-waste. They are calculated based on the network’s annual performance, so as to include the seasonal factor. Index D0 (%): This is the percentage of collected bio-waste compared to the total amount of generated MSW and is calculated as D0 = (the quantity of collected waste (tons per year)/quantity of urban solid waste generated (tons per year)) × 100. Index D1 (tons/year): This is the annual amount of collected bio-waste per itinerary. D1 = this is the quantity of collected waste (tn per year)/total itineraries number. Index D2 (hours/year): This is the annual number of person-hours spent on collecting a bio-waste bin. D2 = The person-hours (hours/yearly)/total bin number. Index D3 (hours/year): This is the number of hours that the waste collection vehicles operate for the collection of bio-waste bins. D3 = The vehicle operating hours (hours/year)/total bin number.

), operational performance (

Table 7. The performance indicators.

Performance indicators		2020	2021	2022
	P1	10.95	10.42	10.46	Quantity of collected bio-waste relative to that available
	P2	0.05	0.07	0.07	Quantity of collected bio-waste per working hour (tn/h)
	P3	0.29	0.35	0.43	Collected bio-waste per hour of collection vehicle operation (tn/h)

Performance Indicators: Their purpose is to present the efficiency of the network by analyzing the adopted strategies and the equipment used for the collection of MSW for the fraction of collected bio-waste. Index P1 (tn/m³) concerns the quantity of collected bio-waste relative to the available capacity of the collection means (bins). P1 = the quantity of collected waste (tons per year)/total volume of bins. Index P2 (tons/hour): The quantity of collected bio-waste per working hour. P2 = the quantity of collected waste (tons per year)/person-hours (hours/yearly). Index P3 (tons/hour): This is the quantity of collected bio-waste per hour of operation of the collection vehicles. P3 = the quantity of collected waste (tons per year)/vehicle operating hours (hours/year).

), and economic viability (

Table 8. The economic indicators.

Economic indicators		2020	2021	2022
	E1	0.00	0.00	0.00	Cost of the collection means per total collected bio-waste (EUR/ton)
	E2	173.16	124.84	113.29	Cost of personnel per total collected bio-waste (EUR/ton)
	E3	17.87	14.99	12.21	Cost of vehicle operating per total collected bio-waste (EUR/ton)
	E4	191.03	139.84	125.50	Total cost (EUR/ton)

Economic Indicators: These provide an initial estimate of the cost of the waste collection network processes. Index E1 (EUR/ton): The cost of the collection means per total collected bio-waste. E1 = the cost of the means of collection (EUR)/quantity of collected waste (tons per year). Index E2 (EUR/ton): The cost of personnel per total collected fraction of waste. E2 = the personnel cost (EUR)/quantity of collected waste (tons per year). Index E3 (EUR/ton): The cost of operating the bio-waste collection vehicles (fuel, maintenance, etc.) per the total collected bio-waste. E3 = the operating cost (EUR)/quantity of collected waste (tn per year). Index E4 (EUR/ton): The sum of the above cost indicators. Ε4 = Ε1 + Ε2 + Ε3.

). The results showed that the strategy significantly improved these metrics, demonstrating its effectiveness in the context of Athens.

The percentage of collected bio-waste compared to the total amount of generated municipal solid waste (MSW) showed a steady increase over the years of COVID-19 era, indicating an improvement in waste management practices. The annual amount of collected bio-waste per itinerary has been consistently increasing, suggesting more efficient collection processes and increased awareness among stakeholders. The number of person-hours spent collecting a bio-waste bin has decreased, indicating improved operational efficiency and potential cost savings. The vehicle operation hours for the collection of bio-waste bins have decreased, implying optimized routes and improved time management due to continuous monitoring and route scheduling.

The quantity of collected bio-waste relative to the available capacity remained relatively stable over the 3-year period, indicating an efficient utilization of collection infrastructure. The quantity of collected bio-waste per working hour has shown a slight increase, suggesting improved productivity and the effective utilization of labor resources. The collected bio-waste per hour of collection vehicle operation has increased, indicating improved operational efficiency and potentially reduced environmental impacts.

The positive outcomes of this methodology underscore the effectiveness of these recommendations. Incidents of unwanted impurities in bio-waste from producers who had received guidance became exceedingly rare. Only a few cases necessitated additional recommendations. Similarly, instances of program interruption or the withdrawal of bio-waste bins due to improper waste management became nearly non-existent. As a result, the CAWMD’s comprehensive program monitoring model successfully achieved its intended objectives. This approach not only improved bio-waste collection and management but also reduced contamination, leading to higher-quality compost production.

These successful developments were not isolated instances but part of a broader trend. Since 2020, there has been a noticeable improvement in several key areas related to bio-waste collection and management. Quantities of bio-waste collected have shown a consistent increase, signifying heightened public awareness and participation. The person-hours required for the program’s operation have become more streamlined and efficient. This enhanced efficiency has not only resulted in a more cost-effective process but also facilitated better resource allocation. As a consequence, the CAWMD has experienced significant cost reductions in its bio-waste management operations while simultaneously enhancing the program’s environmental sustainability. These positive trends, observed since 2020, reflect the program’s growing success and the potential for continued improvements in bio-waste management practices in major cities.

The cost of the collection means per total collected amount of bio-waste and the cost of personnel per total collected amount of bio-waste have shown a decreasing trend, indicating potential cost optimization and efficient resource allocation. The cost of vehicle operation per the total collected bio-waste amount has also decreased, suggesting improved fuel efficiency and maintenance practices. The total cost per ton of collected bio-waste has decreased over the years, indicating overall cost optimization and potential savings.

4.3. Discussion

In comparison to other studies, our findings show both similarities and unique contributions. For example, similar to studies by Xevgenos et al. [12] and Demichelis et al. [16], our results indicate that a well-implemented bio-waste collection network can significantly reduce landfill waste and greenhouse gas emissions. The adoption of a “door-to-door” awareness campaign was instrumental in achieving high stakeholder participation, aligning with the recommendations of Snellinx et al. [8] and Sulewski et al. [20], who emphasized the importance of stakeholder education and engagement. Despite these improvements, challenges similar to those highlighted by Haque et al. [5] and Ismail et al. [10] were encountered, such as initial resistance from stakeholders and logistical issues during the COVID-19 pandemic. The economic viability, while improved, still faces challenges due to high initial implementation costs, which is consistent with findings from studies conducted in other major cities like Madrid. Our study complements the work of Rejeb et al. [11] by incorporating IoT and AI technologies for real-time monitoring and route optimization, enhancing the overall efficiency of the bio-waste collection system. The strategic focus on large-scale producers aligns with the findings of Sharma et al. [9], who highlighted the importance of targeting specific waste streams to maximize the impact of waste management initiatives.

The implementation of the bio-waste management strategy in Athens led to a 35% increase in bio-waste collection volumes, a 34.5% reduction in costs, and a 32% decrease in greenhouse gas emissions. These results indicate that the strategy not only improved waste collection efficiency but also had positive environmental and economic impacts. The daily route planning and the use of modern waste collection vehicles were crucial in achieving these outcomes. Additionally, the door-to-door awareness campaigns played a significant role in increasing stakeholder participation and compliance.

The study successfully met its research objectives. We analyzed the implementation of a municipal bio-waste collection network in Athens during the COVID-19 pandemic. The study provided a detailed analysis of the strategy’s implementation, including the challenges and outcomes. The effectiveness of the proposed bio-waste collection system in reducing environmental impacts and achieving sustainable development goals was evaluated. The results showed significant improvements in environmental and operational metrics. Recommendations were grounded in evidence from the Athens case study for enhancing bio-waste management practices in major urban centers. The study offered practical insights and recommendations that can be applied to other cities.

This study has several limitations that must be considered when interpreting the results. The study focuses exclusively on Athens, and the findings may not be fully generalizable to other cities with different socio-economic, cultural, and environmental conditions. Each urban context has unique characteristics that can influence the success of bio-waste management strategies. The evaluation period was limited to the first three years of implementation. While the initial results are promising, longer-term studies are necessary to assess the sustainability and continued effectiveness of the bio-waste collection system. Factors such as long-term stakeholder engagement need to be explored in future research. Data collection relied on municipal records and stakeholder interviews. While these sources provided valuable insights, there is a potential for reporting bias and data inaccuracies. Future studies should incorporate more robust data collection methods, such as direct measurements and longitudinal studies, to enhance the reliability of the findings.

Other factors like politics and economic conditions can significantly influence the effectiveness of waste management strategies and should be considered in future research to develop more resilient and adaptable systems.

5. Conclusions

The COVID-19 pandemic has posed unprecedented challenges to urban waste management systems worldwide. In Athens, the pandemic necessitated swift adaptations to bio-waste collection practices to ensure both public safety and continued efficiency. This study highlights the resilience and adaptability of Athens’ bio-waste management strategy in the face of such challenges, providing valuable lessons for future crises.

The COVID-19 pandemic significantly impacted waste management practices, necessitating rapid adaptations to maintain efficiency and public health safety. In Athens, several measures were implemented to address these challenges:

The implementation of the municipal bio-waste collection network in Athens has demonstrated significant improvements in urban waste management practices. By targeting large-scale producers and optimizing collection processes, the city achieved notable enhancements in collection efficiency, environmental impact, and economic viability. Utilizing the TOPSIS method for decision-making provided a systematic and objective framework for evaluating and selecting the most suitable bio-waste collection strategies. These findings underscore the importance of tailored strategies and robust stakeholder engagement in developing sustainable waste management systems.

To further enhance bio-waste management practices, it is recommended to expand awareness campaigns to include more community groups and utilize diverse communication channels, integrate advanced technologies such as the IoT and AI for real-time monitoring, route optimization, and predictive analytics, develop and enforce policies that incentivize waste reduction at the source through tax rebates or subsidies for businesses, and foster stronger collaboration between municipal authorities, private companies, and the community to ensure a cohesive approach to waste management.

Further research and implementation of the proposed model will be essential to evaluate its effectiveness, efficiency, and scalability in different urban contexts. Additionally, continuous monitoring and evaluation of the bio-waste collection network will enable ongoing improvements and adaptation to meet the evolving needs and challenges of a circular economy and sustainable cities. In this paradigm, urban centers across the world are invited to adopt these groundbreaking strategies to effectively manage the increasing volumes of bio-waste. By ensuring proper separation and disposal, promoting the environmental advantages of composting, and providing clear guidelines for various sectors, major cities can significantly enhance their bio-waste management practices. This innovative methodology represents a crucial turning point in sustainable waste management and invites urban centers to embark on a journey towards a more eco-conscious future.

The COVID-19 pandemic posed unprecedented challenges to urban waste management systems worldwide. In Athens, the pandemic necessitated swift adaptations to bio-waste collection practices to ensure both public safety and continued efficiency. The study highlights the resilience and adaptability of Athens’ bio-waste management strategy in the face of such challenges, providing valuable lessons for future crises. Specific measures included adjusted collection schedules, targeted public awareness campaigns, and operational adjustments, all of which contributed to the continued effectiveness of the bio-waste management system during the pandemic These measures highlight the adaptability of Athens’ waste management system and provide a framework for other cities to develop resilient waste management strategies capable of handling future public health crises.