Original Article

Prioritization of Sanitary Landfill Criteria Using a Modified Delphi Approach and Development of a Tool for Efficient Site Selection

Please cite this article as follows: Bahrami Asl F, Amini Rabati SE, Samadi MT, Hosseinpoor S, Poureshgh Y, Alipour E, et al. Prioritization of sanitary landfill criteria using a modified delphi approach and development of a tool for efficient site selection. Avicenna J Environ Health Eng. 2025;12(1):29-39. doi:10.34172/ajehe.5506

1. Introduction

The management of solid waste poses various political, socioeconomic, institutional, and environmental challenges. It adversely affects the development of urban areas in developing countries, particularly those with low income (1). The urban environment and human health are adversely affected by the inadequate management of solid waste, which in turn impairs productivity and economic growth (2). This issue stems from poor urban policies and regulations and neglect of private and community participation in urban development and management (2).

The growing amount of waste and its environmental impact have changed the waste disposal concept in urban planning. Modern waste disposal strategies aim to be more sustainable and eco-friendlier, by reducing waste generation, increasing the use of recycled and reusable materials, and enforcing strict rules for proper waste handling (3). Landfilling is one of the most appropriate principles of waste management for residuals that cannot be reused, recycled, or recovered (4). The landfill is a common and cost-effective disposal method, accounting for 70% of waste disposal (5). Sanitary landfills are facilities where municipal solid waste (MSW) is disposed of in a controlled manner. They are designed and operated according to strict regulations and standards to protect human health and the environment from potential contaminants, such as leachate and landfill gas (4). However, the selection of suitable sites is a major challenge in landfilling (6-9).

Site selection affects not only urban solid waste landfills but also hazardous waste and recycling facilities (10,11). Choosing a proper site for a landfill ensures that the waste disposal is effective and sanitary and complies with regulatory and environmental standards (6,12). An inappropriate site can have negative impacts on important environmental aspects, such as natural habitats, water and soil quality, soil fertility, and landscape (13). Therefore, one of the most critical steps in landfill planning is identifying and finding a suitable location for the landfill. This process is complicated and entails the consideration of diverse criteria, such as environmental, financial, social, and technical aspects, each carrying a particular significance and imposing certain constraints on the selection (6,8,14). This step is essential for sustainable waste management and urban development (15).

A considerable effort has been made to address the issues surrounding the location of the sanitary waste landfill and the methodology used to determine its location. The suitability of landfill sites in Kolkata, India, was analyzed by Ali and Ahmad (16) using fuzzy analytical hierarchy process (AHP) and GIS. They used a 1-5 scale to rate the criteria for landfill suitability. They used a weighted linear combination (WLC) of the criteria to model the candidate sites. Using GIS, fuzzy logic, AHP, and WLC, Zarin et al (17) selected suitable landfill sites in Islamabad, Pakistan, based on 13 criteria grouped into environmental and socioeconomic categories. The criteria were weighted by AHP and standardized by fuzzy set theory. The Delphi method is a systematic approach or method in research to extract opinions from a group of experts on a topic or question. In other words, the Delphi method is a request for professional judgments from heterogeneous and independent experts on a specific topic at a large geographic level using a series of questionnaire rounds until reaching a group consensus while maintaining the anonymity of the respondents and feedback to the panel members. The Delphi method can be used for predicting the future, making decisions, and increasing its effectiveness, judging, facilitating problem-solving, assessing needs, setting goals, helping to plan, setting priorities, organizing group communications, group information gathering, determining policies, allocating resources, and group consensus or agreement.

While various multi-criteria decision-making methods, such as the AHP, Technique for Order Preference by Similarity to Ideal Solution (TOPSIS), and GIS-based approaches, have been extensively applied for landfill site selection and criterion weighting (18-24), these methods often rely on predefined criteria sets or can be susceptible to biases inherent in direct group discussions. The unique challenge in landfill siting is the need to integrate diverse expert knowledge across environmental, social, and economic domains to form a robust consensus-driven set of priorities. This study proposes a novel application of a modified Delphi approach to systematically prioritize a comprehensive set of sanitary landfill criteria. The innovation of this research lies in leveraging the iterative, anonymous expert feedback mechanism of the Delphi method to minimize bias and achieve a robust consensus on critical site selection factors, a distinct advantage over single-round surveys or direct group interaction methods. Furthermore, this research develops a practical tool based on these expert-derived priorities, offering a systematic and efficient way to evaluate potential landfill sites. This approach not only ensures a more reliable and context-specific prioritization of criteria but also addresses the inherent subjectivity in expert judgment through a structured consensus-building process. The most important conditions required for the application of Delphi are the need for experts’ judgment and opinions of a broad group, group agreement on achieving results, the existence of complex, large, and interdisciplinary problems and lack of agreement or insufficient knowledge, availability of experienced specialists, geographical dispersion, the need for anonymity in data collection, no time limit, and lack of another cost-effective method (25). This study aims to collect and prepare a list of all effective criteria in selecting the appropriate location of sanitary landfills, modifying, finalizing, and prioritizing them using the Delphi method so that finally an index can be presented according to the majority of effective criteria in selecting the location of sanitary landfills to select the suitable location for these landfills.

2. Materials and Methods

This study followed a three-round Delphi design. The Delphi method involves a sequence of surveys that are designed to facilitate agreement among a panel of specialists on a certain issue. It has been employed in domains such as forecasting, decision-making, problem-solving, and consensus formation (26-30). The main steps of the Delphi method are: (a) selecting the experts, (b) designing and distributing the questionnaire, (c) analyzing and summarizing the responses, (d) modifying and redistributing the questionnaire, and (e) repeating the process until a convergence of opinions is achieved (26).

2.1. Participants

A team of researchers was formed to implement and monitor the Delphi process. While there is no universally fixed number for Delphi panels, a panel of 10-15 experts is commonly recommended, even for heterogeneous groups, to ensure sufficient statistical power and diverse perspectives while maintaining manageability. Our primary recruitment criterion was the proven expertise and extensive practical and academic experience in fields directly related to landfill site selection and solid waste management. To minimize selection bias and ensure comprehensive coverage of the multifaceted aspects of landfill siting (environmental, geological, hydrological, social, economic, and engineering), the team selected a multidisciplinary panel of 15 experts from relevant academic or professional backgrounds, including environmental engineering, waste management, urban planning, and hydrogeology. The experts also had sufficient knowledge and experience in landfill site selection and were familiar with the literature on the topic. The inclusion criteria for selecting the experts were as follows: (a) having at least a master's degree or equivalent qualification in a related field, (b) having at least five years of work experience in a related field, (c) having published at least one paper or report on landfill site selection or related topics, and (d) being willing to participate in all three rounds of the questionnaire. The experts were asked to participate in all three rounds of the questionnaire and were assured of anonymity and confidentiality.

2.2. Questionnaire

The main material used in this study was a questionnaire that consisted of 31 questions. The questions were derived from a comprehensive literature review on landfill site selection criteria (14,31-33). The questions covered various aspects of landfill site selection (Table 1). The criteria were grouped into three levels: first level (general and important criteria), second level (a subset of first level criteria), and third level (a subset of second level criteria). The questionnaire was designed using a scale ranging from 1 (less important) to 10 (very important). The experts were asked to rate each criterion according to their level of agreement or importance. They were also encouraged to provide comments or suggestions for improving or modifying the questions.

Table 1. Studied Criteria, their Weights, and Averaged and Weighted Scores after Three Rounds of the Delphi Survey to Select a Suitable Landfill Site

Close .

The table is based on highlighted numbers and the colors are provided here for visual comparison

The selection of a 1-10 Likert scale for expert ratings was a deliberate choice based on its advantages for our Delphi methodology. This scale offered an optimal balance between sufficient granularity and cognitive ease for experts, ensuring consistent and thoughtful responses. Its intuitive understanding and ample discriminatory power allowed for clear distinction between varying degrees of importance without forcing artificial precision. This facilitated the iterative consensus-building process essential for robust criteria prioritization. While a formal sensitivity analysis comparing the 1-10 scale to other scales was not performed due to practical constraints, the robustness of our results was ensured by several methodological aspects. The iterative Delphi rounds and predefined consensus thresholds validated the ability of the scale to capture meaningful distinctions. Furthermore, the detailed calculation of sub-level scores provided the necessary granularity for practical application, ensuring that the final prioritized list and derived tool reliably could reflect expert judgment. The questionnaire was validated by a group of experts in the relevant fields before being distributed to the panel. The reliability was assessed using the test-retest method with a sample of 10 experts who completed the questionnaire twice at a two-week interval. The reliability was measured by calculating Pearson's correlation coefficient between the two sets of data. A high correlation coefficient indicated that the questionnaire was reliable.

2.3. Procedure

The research team conducted three rounds of Delphi using email as the communication medium. In each round, the questionnaire was sent to the experts, and they were given a specified time frame to complete and return it. After each round, the researchers analyzed the data and prepared a summary report that showed the descriptive statistics, comments, and feedback from the previous round. The summary report was then sent to the experts along with the next round of questionnaires. The experts were encouraged to revise their ratings considering the summary report, if needed, or maintain their original ratings with justification. The process was terminated after three rounds when a satisfactory level of consensus was achieved, and no significant changes were needed and observed in the ratings. We used the coefficient of variation (CV) to assess the degree of consensus among experts. A lower CV indicated a higher degree of consensus. We established a pre-defined threshold for the CV (CV < 15%) to define when consensus was reached for a given criterion. Experts were also asked to assign numerical weights, ranging from 1 to 5, to each of the three levels of the criteria to indicate their relative importance. The criteria were organized hierarchically; in other words, the first level was more significant than the second, and the second was more significant than the third.

2.4. Data Analysis

The data collected from the expert questionnaires were analyzed using SPSS version 16.0. For each Delphi round, descriptive statistics were calculated, including means, standard deviations, and CVs for the expert ratings of each criterion. The CV was specifically used to assess the convergence of expert opinions, with a predefined threshold (CV < 15%) indicating consensus. Criteria that did not reach consensus after a round were re-evaluated in subsequent rounds. The final round of the Delphi process resulted in a prioritized list of criteria for landfill site selection. Each criterion studied had 5 sub-levels. Following the calculation of the total score of each criterion, we distributed it among its sub-levels using a unique methodology. This method ensured that the sub-level scores directly reflected the overall hierarchical importance established by the experts. Specifically, after all main criteria were ranked based on their final weighted scores, the numerical difference between any two consecutively ranked criteria was calculated. This difference was then evenly divided by the 5 sub-levels to yield an incremental value. Starting from the weighted score of the higher-ranked criterion, this incremental value was successively subtracted to determine the scores for its corresponding 5 sub-levels. This granular and consistent scoring system was crucial for evaluating different quality or condition levels within each criterion, directly linking it to the overall expert-derived importance. Tables 2 to 4 show the scores of all sub-levels of the studied criteria at three different levels. The results of the Delphi study were analyzed using Excel software and the final score of the proposed landfill was calculated using equation 1.

Where TSL is the total score of landfills, Wi is the weight of i level, and Si is the total score of i level.

3. Results and Discussion

According to the study design and the obtained results, the first-level criteria had the highest weight (4.8), the second-level criteria had a moderate weight (3.6), and the third-level criteria had the lowest weight (2.5). Table 1 summarizes the results of the Delphi method, showing the weight, average score, and weighted score for each criterion. Groundwater is widely recognized as one of the most critical criteria for selecting a landfill site, as it reflects the potential impact of leachate leakage on the quality of groundwater resources, which are essential for human and ecological use (14,34-40). Groundwater pollution from landfill leachate poses a persistent and serious risk to human health and environmental quality. To mitigate this risk, a buffer zone must be established between the groundwater table and the produced leachate, as recommended by previous studies (14,37,41). Additionally, monitoring wells must be installed both downstream and upstream of the landfill site to detect any anomalies and incidents (14).

In our study, groundwater was ranked as the most important criterion with an average score of 9.4 and a weighted score of 45.12 (Tables 1 and 2), which is in agreement with previous studies (38,39,40,42,43). Other studies ranked groundwater as the second most important factor (13), however it was less influential than the population factor, and the third most important factor (9), less influential than the surface water and aquifer type, but still gave it a high weight or score.

Among the other criteria in the present study, the distance from protected areas of sanitary water supply sources holds paramount importance, adhering to stringent guidelines and standards. This criterion received a weighted score of 41.76, indicating its high relevance for landfill site selection. However, this criterion was not explicitly addressed in most previous studies under this name. Nevertheless, groundwater and surface water, which were commonly used as criteria in other studies, can also be regarded as sources of drinking water. Hence, the literature review reveals the significance of distance from drinking water for landfill site selection (28,34,35). For example, Ghaed Rahmat et al (39) ranked groundwater and surface water as the top two criteria among 10. Pasalari et al (40) gave more weight to environmental than socio-economic criteria, and within the environmental ones, groundwater, surface water, and distance to wells had the highest weights. Compared to these studies, the distance from the wells and surface water in our study, with weighted scores of 29.88 and 29.52 respectively, were assigned a lower level of importance. The possible reasons for the discrepancy could result from using greater criteria in this research, which is more than most other studies, and also from the diverse perspectives of the experts involved.

The hydrogeological attributes of the site were the third most important criterion, with a weighted score of 39.84. This criterion assesses how the geological and hydrological features of the site, such as soil type, permeability, porosity, and hydraulic conductivity, influence the generation, movement, and natural degradation of leachate. Therefore, the site should have favorable hydrogeological conditions that minimize leachate production and facilitate its natural attenuation. Previous studies have also highlighted the importance of hydrogeology for landfill site selection (28,32,37). For example, Gorsevski et al (37) assigned the highest rank of importance to this criterion. Leachates may leak out of geologic faults and cause environmental damage by flowing preferentially along the fault lines (44). Reflecting its significance, the criterion of geotectonic features was a first-level criterion according to the experts in our study (Tables 1 and 2), which agrees with the study conducted by Donevska et al (20).

Our findings indicate that the proximity of the landfill to populated areas is a moderately significant criterion in level one, with a weighted score of 33.12. This criterion is also commonly used and valued in several previous studies that evaluate the environmental and social impacts of landfill siting (13,19,32,40). Studies have recognized the potential effects of noise, odor, dust, traffic, or visual degradation on the nearby communities, which may impair their well-being and property values and cause social conflicts and opposition (22,39). Therefore, they recommend selecting a site that is sufficiently distant from residential areas and implementing measures to mitigate or prevent the negative consequences of landfill operation. However, the relative importance of this criterion varies across different studies, depending on the context and methodology adopted. For instance, Khorsandi et al (42) prioritized this criterion after the factors related to water resources protection. Ghaed Rahmat et al (39) ranked this criterion as the fourth most important one, after ground and surface waters, and sensitive ecosystems. In contrast, Pasalari et al (40) considered residential areas at the top of the socio-economic criteria list. Kapilan and Elangovan (13) assigned the highest weight to the population factor, compared to other factors such as depth to the water table and land use/landcover.

The second-level criteria comprise 12 factors that reflect various aspects of landfill siting, such as the geotechnical properties, terrain preparation, landscape features, infrastructure, and so on (Tables 1 and 3). Some of these factors, such as land use, distance to waste production/generation centers, distance to faults/seismic risk, and distance to archaeological sites/cultural areas, have been widely used by other researchers for landfill siting, as shown by Donevska et al (32), who reviewed 89 papers on this topic. However, infrastructure, which is a moderately important factor in our study, was found to be less significant in some other studies (38,40). Among the third-level criteria (Tables 1 and 4), some of the least influential criteria were the temperature, access road, and airstream, with average scores of 4.40, 5.10, and 6.00, respectively, and weighted scores of 11.00, 12.75, and 15.00, respectively. Our results are in line with some previous studies that have also considered these criteria as relevant but not decisive for landfill siting. For example, Rahimi et al (21) included average annual temperature as a factor affecting construction operations, disease spreading, and contamination rate, but it was ranked higher than the distance from urban and rural areas and had more priority compared to our study. Aksoy and San (18) studied 12 criteria including temperature, which had the lowest weight in their list, consistent with our results. Fallahpour et al (38) included the secondary road with the lowest weight as a factor affecting accessibility. Pasalari et al (40) included the access road and wind direction as sub-criteria of socio-economic criteria to select the optimal location for a landfill.

The findings of our study carry significant practical implications for the sustainable management of MSW, particularly concerning the critical process of landfill site selection. The systematically prioritized criteria, derived from a robust modified Delphi approach involving expert consensus, provide a valuable framework for decision-makers, environmental agencies, urban planners, and local authorities. First, the hierarchical importance assigned to criteria (e.g., the high priority of groundwater quality and hydrogeological characteristics) offers clear guidance, enabling policymakers to focus resources and regulatory efforts on the most critical environmental and public health safeguards during the planning phase. This reduces the subjectivity often associated with such complex decisions and promotes a more objective evidence-based approach. Second, by providing a transparent and replicable methodology for criteria prioritization, this research empowers local authorities and stakeholders to engage in more informed discussions regarding landfill development. It facilitates a common understanding of the critical factors, which can lead to greater public acceptance and more effective implementation of waste management strategies. Ultimately, the results of this study directly contribute to building more sustainable waste infrastructure by guiding the selection of sites that mitigate risks, protect natural resources, and align with long-term environmental protection goals.

3.1. Challenges and Solutions

The Delphi method, despite its advantages, faced some challenges and limitations that could affect the validity and reliability of the findings. These challenges included the difficulty of finding and retaining qualified and willing experts, the subjectivity and bias of some experts, the complexity and ambiguity of some criteria, and the lack of sufficient literature or data on some criteria. To address these issues, the researchers adopted several strategies, such as conducting a thorough search for potential experts, sending reminders and follow-up messages to the experts, and ensuring the anonymity of participants (30). We also provided clear instructions and definitions for each criterion and question, used multiple rounds of questionnaires, and verified and cross-validated the findings with other studies or methods. These strategies improved the validity and applicability of the criteria and the consensus among the experts. Additionally, the authors assessed the reliability of the questionnaire by correlating the responses of 10 experts who completed it twice with a two-week interval, and the high correlation indicated high reliability.

4. Conclusion

This study used a modified Delphi method, a reliable technique for gathering and synthesizing expert opinions, to identify and prioritize the criteria for choosing a suitable and sustainable landfill site. The study involved 15 experts who rated 31 criteria on a 10-point scale in three rounds of questionnaires. The criteria were grouped into three levels of importance, based on their weight and role in ensuring the feasibility and performance of the landfill site. The results revealed that the most important criteria for landfill site selection were groundwater, distance from protected areas of sanitary water supply sources, hydrogeological attributes of the site, and geotectonic features. These criteria reflect the potential impacts of landfill leachate on the quality and quantity of water resources, which are essential for human and ecological use. The second- and third-level criteria encompassed various environmental, social, economic, and technical aspects of landfill siting, such as land use, infrastructure, engineering-geological properties, temperature, and wind direction. These criteria indicate the multiple and often conflicting factors that influence the decision-making process. The proposed method provides a flexible and adaptable framework for evaluating and comparing potential landfill sites, taking into account the local conditions and preferences. The study acknowledges the limitations and challenges of the Delphi method, such as the difficulty of selecting and recruiting experts, the potential for bias or influence among panel members, and the requirement for multiple rounds of questionnaires and feedback. However, the method can contribute to the sustainable management of solid waste and the protection of human health and the environment, by facilitating the selection of the most suitable and acceptable sites for landfilling.

Acknowledgments

The Authors would like to thank the Student Research Committee, Urmia University of Medical Sciences, Urmia, Iran, for supporting this project (Grant No. 10669).

Competing Interests

The authors declare that they have no conflict of interests.

Ethical Approval

The protocol and procedures of this study were approved by the Ethics Committee of Urmia University of Medical Sciences (IR.UMSU.REC.1400.076).

Funding

This work was supported by the Urmia University of Medical Sciences (Grant No.: 10669). Farshad Bahrami Asl received research support from Urmia University of Medical Sciences.

Using Artificial Intelligence Chatbots

We utilized AI-powered tools (Gemini) to check grammar and enhance the academic quality of the text, which was primarily written by the authors.

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