Waste Management: Landfill Selection Using Gis and Analytical Hierarchy Process

The widespread use of landfilling as a convenient waste management method has become unsustainable due to the rapid development causing a surge in waste generation, threatening the lifespan of existing landfills. The closure of landfills presents waste management challenges, necessitating more extensive garbage disposal facilities to accommodate the growing urban population. However, poor site selection methods can have a negative impact on both the environment and human health, which goes against the objectives of sustainable development goal (SDG). Methods like Geographic Information System (GIS) and Multi-Criteria Decision Analysis (MCDA) facilitate decision-making processes. This integration reduces analysis time and costs enhances accuracy and minimizes errors. The Analytical Hierarchy Process (AHP) provides a framework to validate judgments and derive measurable values in decision-making involving theories and concepts. In this study, several criteria were used to create a final suitability map which is the distance from the residential area, distance from the plantation area, distance from the main road, distance from waterways, type of soil, slope, and land cover. The results indicate that distance from residential has the highest weightage of importance, which is 27%. It also found that the highly suitable new landfill area is located at Batu in Kuala Langat, Pulau Carey, and Bukit Rotan in Kuala Selangor.


Introduction
For years, landfilling has been the most widely used method in waste management as it is convenient and more consumer friendly.In recent years, the rate of development across the nation has caused an increase in waste material and affects the longevity of existing landfills.This will be caused landfills to be closed, bringing waste management problems.Given the tendency of urban living, the requirement for a larger garbage disposal facility has become vital, and the existing approach needed to be improved [1].However, if the site selection method is applied inappropriately, it will cause problems for human health and the environment, contradicting the sustainable development goals (SGDs) [2].Using cuttingedge technologies to reduce the adverse effects caused by various wastes is an essential and absolute necessity for developing countries [3].
Malaysia's population is greatly expanding over the past 30 years.The quarter of 2023 was estimated at 33.2 million compared to 19.1 million in 1993 [4].The growth rate shows a decrease in percentage from 2.82% in 1993 to 1.09% in the first quartile of 2023, but the population is increasing yearly.As the population expands, the waste problem will become an issue.Typical domestic waste management uses the landfilling method.The problem would lead to more spaces being converted into landfills, leaving limited space for the people [5].In recent years, many landfills have been closed.Several criteria must be fulfilled to create new landfills, such as geographic, environmental, and social.
Depending on how it is classified, solid waste has various properties.It can be classified as hazardous, biodegradable and non-biodegradable, combustible, and non-combustible.Hazardous wastes can react when exposed to specific things, such as gasses, and can be extremely harmful to people, animals, and plants [6].They can also be caustic, highly combustible, or explosive.Contrary to non-biodegradable waste (textiles, chemicals, rubber, and plastics), which cannot be broken down over time, processed, or repurposed into valuable by-products like biogas and compost, biodegradable waste contains organic materials can fulfil such claim.Combustible and Non-combustible materials such as rubber lather and paper have a high heat value that will burn quickly and easily spread when combusted.This paper aims to determine the new landfill area using a Geographic Information System (GIS) and factors that affect the area selection.GIS connects data to a map, integrates location data with descriptive problem statements, and can manage large volumes of spatial data.It also helps to comprehensively visualise the best location of the new landfill by considering the spatial criteria within the study area.Past similar research is reviewed, and the results are hierarchically processed to facilitate the criteria of the selected landfill location through Multi-Criteria Decision Analysis (MCDA).A similar approach using Multi-Criteria Decision Making (MCDM) has been conducted to determine the landslide hazard area [7].MCDA is used to aggregate the criteria and grade or rank the options after decision-makers have analysed potential courses of action based on various non-comparable characteristics [8][9].This combination of GIS and MCDA creates beneficial ways to reduce analysis time and cost, increase the accuracy, and reduce decision-making errors.

Methodology
The conceptual framework of this investigation is summarised in Figure 1.The issue statement and scope of the investigation were identified in Phase 1. Comprehensive studies have been conducted to establish the research gaps, relevant methodologies, and criteria that must be addressed.

Study Area and Criteria for Establishing New Landfill Site
The selected studied area is one of Malaysia's states, Selangor (Figure 2).It is located on the west coast of Peninsular Malaysia.It covers the federal territories of Kuala Lumpur and Putrajaya, both of which were formerly under the territorial authority of Selangor.Many factors have been considered in determining the selection of the land site.These include waterways, distance from the residential area, the main road, slopes, etc. Besides, the main criteria of the selected location should also minimize potential environmental damage and offer a solid foundation for efficient management of environmental protection and public health issues.

Data Acquisition
The primary objective of this study is to evaluate potential new landfill sites.This study is carried out with a reference from past literature, existing legislation, and the questionnaire findings from the local experts [10].These elements included several primary criteria and a wide variety of sub-criteria and their respective features.These data were obtained from both primary sources and secondary sources.

Data Processing
Data processing utilising Geographic Information Systems (GIS) and the Analytic Hierarchy Process (AHP) involves integrating spatial data collected from various sources into a GIS platform.The data is then organised into feature layers with geospatial references.AHP is applied to define decision criteria and sub-criteria, and pairwise comparisons determine the relative weights of these criteria.

Survey
Judgements of importance for each criterion is essential to start the AHP method.So, to increase the study's validity, the experts' survey (pair-wise comparison) is conducted based on the seven main criteria shown in Figure 3.The experts who took part in the survey are from Selangor.The survey is done online and face-to-face.

Analytical Hierarchy Process
Multiple Criteria Decision Analysis (MCDA) is a category under which the Analytical Hierarchy Process (AHP) falls.By using procedures for calculating the weights and scores received by alternatives based on pairwise comparisons between criteria and options, respectively, AHP creates a linear additive model in its standard format.The hierarchy must reflect the key elements of the decision problem in order to comply with the decomposition principle.The pairwise comparisons of items at one level of the hierarchy with their parent at a higher level are evaluated in accordance with the comparative judgement

Attribute
After creating the hierarchy, an all-inclusive pairwise comparison matrix (PCM) is constructed.The weight (Wi) for each level is then determined by solving the set of linear simultaneous equations.
The consistency ratio (C.R) will be determined using the following equation to assess the consistency of decisions and highlight the potential need for judgement revisions: where C.I represents the consistency index, n is the element being compared, and R.I represents the random consistency value based on the size of the matrix.C.R should be approximately 10% (0.1) or less to be considered acceptable.

Results and Analysis
Analysing the data from questionnaires collected from Sabak Bernam District Council (SBDC) and the Department of Social Welfare (DSW), the Analytic Hierarchy Process (AHP) is employed to provide valuable insights and aid the decisions.

Overall Priority Ranking
From the AHP findings for SBDC, the distance from residential areas is the highest ranked for the new landfill site (Table 3).Logically, a landfill site should be located far away from residential areas, which may affect their health and well-being.Besides, the new landfill sites should also avoid near the waterways area, which could harm aquatic ecosystems and taint drinking water supplies.
Compared to the SBDC, the AHP results for DSW (Table 4) shows the distance from the waterways area indicated the highest rank at 29%.This is followed by the distance from residential areas at 25%, the slope at 17%, and the distance from plantation areas and land cover both at 9%.These weightage values provide a quantitative understanding of the relative importance of each criterion in selecting suitable new landfill site locations.The aggregate of AHP (Table 5) shows the three main criteria of new landfill site selection, i.e., the distance from the residential, waterways, and slope area.

Site Map for Landfill
The weights of the criteria are obtained from the AHP method.The decision-makers and experts often need help to express consistent preferences.To determine the overall consistency of the Pairwise Consistency Matrix (PCM), the Consistency Ratio (CR) is derived by dividing the Consistency Index (CI) by the Randomized Index (RI).The contradiction is acceptable if the value of CR is 10% or below.
Based Consistency Ratio (CR) between the two surveys in this study was less than 0.10(-0.06),indicating good consistency of the comparison-based judgment.The areas under the study were divided into four classes which are: 1) very low suitability; 2) low suitability; 3) moderate suitability; 4) high suitability, and 5) very high suitability (Figure 11).

Overlap Existing Landfill Site
The area of the suitability map has the currently used landfill sites overlaid on it.The locations considered least suitable for landfilling are found in 'very low' and 'low-suitability' regions.These locations are frequently near sensitive regions like waterways, homes, or other sensitive areas.
Landfilling is feasible for the sites that are situated in the 'moderate suitability' area.These locations may have some restrictions, such as poor soil conditions or steep slopes, even if they are often far from sensitive areas.The locations determined to be the most ideal for landfilling are found in the 'high' and 'very high' suitability regions.The majority of these sites are situated distant from sensitive areas.Within the suitability map area generated by the Analytical Hierarchy Process (AHP), certain existing landfill sites coincide with different suitability categories (Table 7).The existing landfill sites to befall under the 'very low', 'low suitability', 'moderate suitability', 'high suitability', and 'very high suitability' area indicated 0%, 12.5%, 50%, 12.5%, and 25%, respectively.

Conclusion
Disposing municipal solid waste to open dumps leads to many environmental and public health concerns as the number of closing existing landfills and the yearly amount of waste keep increasing yearly in Selangor.Therefore, a new landfill site in Selangor within the next few years is crucial.The use of GIS and AHP in this study aided the decision-making process.Besides, the expert survey is conducted to determine the importance level of selected criteria based on previous studies.Different data from various parameters were obtained and prepared in a GIS environment.Next, the AHP will determine the relative importance of each criterion.The results show that both the 'distance from residential areas' and 'distance from Waterways' is highly ranked and the least important criterion is 'distance from the main road'.As a result, 7.57% of the whole region in this study is suitable for landfilling.On the other hand, 'high suitability' and "moderate suitability' areas constitute more than 50%.Thus, three areas were selected to be the preferred area for the new landfill site, which are Batu, Pulau Carey, and Bukit Rotan.The outcome of this study provides enough information to help policymakers, urban planners, developers, municipalities, and other authorities to build or relocate landfill sites.This study might be as starting point for further research in other interests associated with landfill site selection.

Figure 1 .
Figure 1.Conceptual framework IOP Publishing doi:10.1088/1755-1315/1316/1/0120034 principle.The synthesis principle uses each local priority's resulting ratio-scale at different levels of the hierarchy.It creates a composite set of priorities for the components at the bottom of the hierarchy.

Table 1 .
Data Acquisition

Table 5 .
Aggregate of AHP

Table 6 .
The Percentage Area of each Suitability (%)

Table 7 .
The Percentage of Overlapped Existing Landfills (%)