Evaluation of site selections for solid waste landfill by using GIS in Phu Quoc island, Kien Giang province, Vietnam.

Selecting the location for domestic waste disposal and waste treatment plant is a crucial issue as it directs impacts on health-related, social and environmental factors. One of the effective tools for solving this problem is Geographic Information System (GIS) which employs Multi-Criteria Analysis (MCA). This research aims to identify the optimal locations to evaluate the existing solid waste landfill site and waste treatment plant for land use planning in Phu Quoc island in Kien Giang province for reducing the plastic waste leaking into the ocean. The island is categorized as the urban at small scale with population of 100,000 to 500,000; construction of waste treatment facilities (middle scale) with the capacity of 500 tons per day is now planned. These contents of this study are the new contributions to the methodology for data processing and data collection. The main research methods were applied to geo-spatial data collection derived from maps, satellite imagery, and existing local land use planning information. The criteria for site selections for solid waste landfill were chosen under Vietnamese construction standards and Ministry of Natural Resources and Environment, Ministry of Construction, international regulation, and the provincial policy. Then Analytic Hierarchy Process (AHP) and GIS processes were based on 9 main factors such as (1) distance to an urban area, (2) distance to a rural village, (3) distance to the water surface, (4) distance to roads, (5) distance to electrical supply stations, (6) distance to industrial zones, (7) distance to waste disposal, (8) compliance with the local land use planning, and (9) agreement for approval for site locations for solid waste landfill from the local community. National standard requirements for waste landfill site selection are adjusted to meet the local context. The updated land use data source is applied, and the recommendation for the suitability of the waste landfill sites is formulated. Study results showed that there are four potential sites located in Cua Can, Bai Thom, and Ham Ninh communes with a total area of 84.5 hectares. These areas are identified to be very suitable for solid waste landfill locations and location of waste treatment plants; and corresponding to the land use planning of the local government. The results of the study also recommend policies and solutions for environmental management in the coastal area of Phu Quoc island.


Introduction
According to statistics, Vietnam is among the 20 countries with the largest amount of plastic waste that is thus higher than the world average [1].By the end of 2021, an average of 1.8 million tons of domestic waste will be discharged into the environment each year [1].This number is increasing and causing extremely serious consequences not only for the environment but also for the health of the people.Increased domestic waste can become an environmental and ecological "crisis" and is 'a headache' for managers, especially in large urban areas .
The selection of an appropriate site for the disposal of solid waste is a critical decision for any municipality or region.Sites must be carefully selected to minimize the potential for environmental and health impacts, while at the same time ensuring that waste management needs are met [2].Factors that 1247 (2023) 012015 IOP Publishing doi:10.1088/1755-1315/1247/1/012015 2 should be taken into consideration during the selection process include geological characteristics, soil type, groundwater conditions, presence of endangered species, local land use regulations, and accessibility [3].In addition, waste generation and transport patterns must be evaluated to ensure that the chosen site can accommodate the volume and type of waste generated.The selection process must be conducted openly and transparently, with the public given the opportunity to provide input and comment on the selection criteria and potential sites [4].Ultimately, a comprehensive site selection process is essential for ensuring that the disposal of solid waste is conducted in an environmentally responsible and socially acceptable manner.
Site selection for waste landfill is one of the most significant missions of land use planning.It involves assessing environmental, economic, and social criteria in order to determine the most appropriate location to dispose of waste.Environmental factors to consider include topographic features, soil characteristics, proximity to water bodies, and climate, as well as existing land uses and regulations [5].Economic and social factors to consider include proximity to population centers, the cost of transportation, and the potential for air and water pollution.All of these factors must be carefully evaluated to determine the best location for a waste landfill.Additionally, the potential impacts of the landfill on local populations must be carefully considered, as the presence of a landfill can have major implications for the health and well-being of nearby residents [6].
Currently, there are many methods of treating domestic solid waste, in which landfill is a method widely applied in Vietnam and many countries around the world.The selection of waste treatment sites for urban areas has been integrated with construction planning for a long time.However, choice of the location of landfill sites in some urban areas where a scientific basis for such decisions is lacking still faces many difficulties with regards to implementation.In Vietnam, situations where landfills are planned and then located in inappropriate sites still occur, such as Vinh municipal landfill (Nghe An) [7] and Trang Cat landfill (Hai Phong) [8].

The study area
Phu Quoc city produces about 180 tons of domestic solid waste and the demand for waste treatment in Phu Quoc by 2025 is estimated at 400-650 tons/day [9].Currently, the daily amount of garbage on Phu Quoc island is still being dumped at a temporary landfill in Dong Cay Sao, Cua Duong commune.The 'mountain' of garbage has increased and the stench from garbage and sewage as well as the presence of rats and flies etc. [10] in this area has also become more concentrated (Figure 1).Phu Quoc National Park is a large area with a protected zone covering most of the island that is not compliant with the regulation for waste landfill location as in the National standard requirements.For this urban at the small scale , this research finds new methods for adjusting the National standard requirements for waste landfill site selection with new (2020) land use data sources from European Space Agency (ESA).This data presents land use land cover types with medium spatial resolution of 10m x 10m.Moreover, infrastructure data (roads, streams, and market locations) is supplied from OpenStreetMap.Phu Quoc is located in the Gulf of Thailand, in Kien Giang province, located at the extreme southwest of Vietnam's territorial waters.Phu Quoc is an island with both forests and mountains, with a population of about 177,560 people (according to statistics from the provincial People's Committee in 2022) [11].The island is the urban grade 3 with population 100,000 to 500,000 and construction of middle scale waste treatments facilities (with a capacity of 500 tons per day) is proposed here.Located 45 km west of Ha Tien, and 15 km south of Cambodia, Phu Quoc possesses the most beautiful beaches and the best seafood in Vietnam.The whole island has a total area of 589 km² or 58,900 hectares as according to land statistics of the Province People's Committee in 2011.The main reasons for selection for the study area are: In 2020 Phu Quoc island was approved by the Prime Minister to become a special administrative and economic zone directly under the Central Government.This is a favorable condition for Phu Quoc to continue developing the key tourism industry.
Kien Giang province adopted a strategy for the development of tourism on Phu Quoc island by 2020, intending to receive 1-1.2 million visitors in 2015, of which foreign visitors accounted for 35%, generating a total revenue of 209 million USD.In 2020, the island welcomed 2-3 million visitors, of which foreign visitors accounted for 40% bringing a total revenue of 771 million USD as reported by Province People Committee in 2021.
Phu Quoc island relies on the domestic tourist market, focusing on large cities and the Mekong River Delta, but also expanding to international markets, especially focusing on high-paying markets such as Northeast Asia, North America, Western Europe, and ASEAN.
The Phu Quoc Nature Reserve, Kien Giang Province, has been changed to Phu Quoc National Park since 2001 with a total area of 31,422 ha (Figure 2) by the Decision No. 91/2001/QD-TTg of the Prime Minister of Vietnam.This area makes it difficult to meet the National standard requirements for waste landfill site selection as a the area excluded from possible location of a waste landfill site now covers most of the island.Moreover, high mountains with steep slopes also preclude location of the site according to the National standard requirements.

Method and materials
The selection of suitable locations for solid waste treatment is a complex spatial analysis problem to be solved for the purposes of land use planning and urban planning.It requires the assessment of many different natural, economic, social, and environmental criteria.To solve this problem, methodologically, multi-criteria analysis is the most appropriate approach, and Geographic Information Systems (GIS) is the most effective decision support tool.The utilization of GIS has been widely investigated in the context of landfill site selection due to its efficacy in efficiently managing large volumes of spatially distributed data from multiple sources.This digital database management system is advantageous in this context, as it is able to store, retrieve, analyze, and display information according to user-defined specifications.As a result, GIS has been identified as a viable option to reduce the cost of the landfill site selection process [8].These processes are used to evaluate the locating suitability for Phu Quoc island based on a suitability index.

The Analytic Hierarchy Process (AHP) and GIS processes
The Analytical Hierarchy Process (AHP) is a technique utilized for decision-making that is especially suitable for situations involving multiple and competing objectives.This technique involves breaking down a complex problem into a series of simpler problems organized in the form of a decision hierarchy [12].Through the use of a pairwise comparison matrix, stakeholders can assess the relative importance of each element within each level, while taking into account the levels both above and below it.This enables the user to mathematically link the entire system together with the perspectives on choosing the location of the solid waste treatment area including: -Plans for the construction of solid waste treatment zones must be consistent with the regional construction plans and solid waste management master plans already approved by competent authorities.-Solid waste treatment areas must ensure to treat 100% of solid waste that is generated within the service area of each facility.-Solid waste treatment areas must ensure centralized, inter-urban, and inter-regional treatment, limit scattered treatment and cause environmental pollution.-Solid waste treatment areas must ensure transport convenience, cannot obstruct general transport activities, and cannot adversely affect the environment and urban landscape.AHP is commonly used in the evaluation of a limited number of alternatives regarding an overarching goal.The landfill siting decision hierarchy model presented in Figure 3  Giang province to 2025.with the environmental criteria narrowing down the geographic area for the analysis and the economic factors helping to identify the most suitable option.

The secondary data sources
The materials used for the purpose of this study include: -current land use map of Phu Quoc island (2020) at 1:50,000 scale in digital form, topographic map at scale 1: 25000 scale in digital form, geological map and minerals in Kien Giang province at scale 1:200,000 in digital form.
-Documents such as the planning of urban construction planning in the province, and the explanation of new rural construction planning in districts on Phu Quoc island.-Data on distance to the water surface (C1), roads C3), electric supply station (C4), and waste disposal sites (C5) extracted from OpenStreetMap data using QGIS software for data analysis.-Data on distance to industrial zones (C2), existing land use (C6), distance to urban areas (C7), and distance to rural settlement (C8) are identified from the Land use land cover 2020 data (from ESA) as the new data set that has been produced from the Sentinel 2B satellite with the spatial resolution of 10m x 10m (pixel size) [14].-Approval on site selections from the local community (C9) is reported from Decision No.
1418/QD-UBND approving the solid waste management planning project in Kien Giang province to 2025 [15].Geo-spatial features was extracted by QGIS software.Various criteria are applied to obtain GIS data sets of the buffer distance and zone.The criteria were chosen under Vietnamese construction standards of MOSTE, Ministry of Construction, international regulation, and the provincial policy.Buffer distance and zones were generated and classified into five grades according to the relevant criteria.Each grade was assigned a different score from 1 to 5 as the higher the score is, the more suitable the area is for landfill site selection as displayed in Table 1.
The evaluated criteria AHP weights were chosen based on the relevant literature reviews [16,17].The relative importance of elements within a hierarchy was determined through a pairwise comparison of the associated attribute values.An evaluation of the contribution of paired elements to the hierarchy above was conducted, followed by a pairwise comparison of all values for the given attribute.The weights (W) of each factor in each hierarchy were calculated based on the structural models presented in Table 2 and Table 3  The calculation of the criteria weight (Wi), presented in Table 2, was derived from the weight (W) of each factor.Wi, being the criteria weight, represents the corresponding weight of each factor in Figure 3 to the goal hierarchy (A).The consistency ratio (CR) values for all comparisons were found to be lower than 0.10, verifying the satisfactory utilization of the weights [18].

Results and discussion
This case study shows the use of spatial information technologies and AHP (Analytic Hierarchy Process) in selecting landfill candidate sites in Phu Quoc, Vietnam.The study proposes a hierarchy model that considers both environmental and economic factors, providing a reference for landfill site selection in rapidly developing cities in developing countries.
The buffer zone of 100m was created on each side of the road as an unsuitable zone (Figure 4).A 100m buffer reaches the National Park, and for other, larger zones 200m and 300m buffers of unusable area are designated.Following the National standard for waste management, a 500m buffer zone should be reserved around important surface water and canals (Figure 5).This unsuitable zone also can be seen in some parts of the National Park and residential areas.The land use cover for 2020 was downloaded from the ESA website (Figure 6) and the urban area was identified.The urban buffer was assumed as a 500 m distance from the unsuitable areas (Figure 7).In the southern and the northwest parts of the island, the urban 500m buffer covers all types of spaces.Buffer to industrial and other important locations could be created as 500m radius zone (Figure 8).
One critical issue in the factor standardization process is setting a different value for each criterion.The study used a classification scheme for criteria, such as proximity to waste production centers, which were assigned a higher score if they were located within 500 m of waste production centers, due to high pollution impact.The results identify three suitable zones with four sites for waste landfill sites as in (Figure 8).The four suitable sites for waste landfill locations were also identified with geographical coordinates with commune names and their areas (Table 4).Figure 9 displays the suitable levels for waste landfill site potential as the results of the data analysis.The three existing waste landfill sites named Dong Cay Sao, Bai Bon, and Khu Pho 7 where all of those sites lie inside the unsuitable 500m distance from urban areas.That is the reason why local news report complaints about foul smell that is felt in the residential areas nearby.This research recommends four optional sites for replacing the existing waste landfill sites.The recommended suitable sites are described in detail in Figure 10.However, proposed small site 4 would be located on a mountain slope so that the recommendation is somewhat tentative..
According to the Government's Decree No. 08/2005/ND-CP dated January 24, 2005 on construction planning, and according to the Circular No. 07/2008/TT-BXD dated April 7, 2008, of the Ministry of Construction guiding the formulation, appraisal, approval, and management of construction planning, the medium size for waste landfill plants for Phu Quoc island must be sufficiently large that is 10 -30 hectares.Therefore, sites 1, 2, and 3 are strongly recommended.
According to the Decision No. 145/QD-UBND issued on 11 January 2022 by the People's Committee of the Kien Giang province on approval of the project "Dumping solid waste management in Kien Giang province for the period of 2022 -2023, direction to 2030", domestic solid waste in Phu Quoc city is classified, collected and transported to the Phu Quoc Waste Treatment Plant in Ham Ninh commune and treated by incineration.The proposed dumped site by the local Government is located near site 4 in this research results.
The approach used in this study can help decision-makers to identify optimal and backup sites for potential waste landfill sites.By considering both environmental and economic factors, the model can help to ensure that the selected sites are sustainable and their operation cost-effective.This can be especially important in rapidly developing cities where the demand for waste disposal facilities is increasing rapidly.

Conclusion
The issue of municipal solid waste generation in Phu Quoc islands has emerged as a significant challenge confronting government authorities.In response to this challenge, the current study aims to mitigate the deleterious effects of landfill sites on the environment, public health, and the economy by developing a model integrating GIS and the Analytical Hierarchy Process (AHP) for evaluating site suitability for landfills.The assessment process considers both environmental and economic factors and involves the computation of nine criteria grouped into two categories as environmental and socio-economic factors.
The research improved that the three existing sites are not suitable and the complaints of the local people are reasonable because they locate inside the 500m buffer of urban zones.The three strongly recommended sites with appropriate large areas meet the requirement of the National Standard for waste landfill site selections.These sites are identified on the map with geographically coordinated positions.The Phu Quoc National Park spans a considerable land area and is designated as a restricted zone that encompasses a significant portion of the island.It is incompatible with the criteria set forth by the National standard requirements for the siting of waste disposal facilities.In light of this situation, the present study proposes novel techniques for adapting the National standard requirements for waste disposal site selection to the specific needs of urban grade 3.This is achieved through the integration of newly available land use data sources from the European Space Agency (ESA), infrastructure data, including information on roads, streams, and market locations, obtained from OpenStreetMap.
Intermediate suitability maps were generated for each criterion and subsequently combined to produce a composite suitability map.The use of AHP provided an objective weight assignment process, and the set of weights granted flexibility in the aggregation procedure.The study provides scientific evidence for the study area and highlights the importance of economic factors in solid waste landfill selection, especially in developing countries and districts where urban sprawl occupies the best landfill areas.The study's siting process offers a practical and valuable tool for waste disposal site selection in fast-growing regions and supports decision-makers in solving waste management problems.Ultimately, the research aims to deepen the understanding of environmental decision-making processes.

Figure 1 .
Figure 1.Dong Cay Sao temporary landfill is rapidly expanding, while the problem of handling all of Phu Quoc's daily waste has not yet been solved (source: Youth news)

Figure 2 .
Figure 2. Location of Phu Quoc islands (source: OpenStreetMap 2020) was established to aid the selection process of a suitable landfill site.This model was based on nine criteria, which were divided into two main groups: (1) environmental criteria, (2) economic, and (3) social factors.The criteria were chosen based on: -Ministry of Construction.Vietnamese construction standards QCVN 01: 2021/BXD.Solid waste landfill -Design standards.Construction Publishing House, Hanoi, 2021.-Ministry of Natural Resources and Environment (MONRE), Ministry of Construction by the Circular No. 01/2019/TT-BXD issued by the Ministry of Construction dated 28 June 2019.-A guideline and regulation approved for land fill sites WASTE -ECON issued by the Government of Canada [13] -Decision No. 1418/QD-UBND approving the solid waste management planning project in Kien

Table 1 .
. Waste landfill suitable levels as score from to 1 to 5

Table 2 .
The pairwise comparison matrix for the three main

Table 3 .
The pairwise comparison matrix criteria weights of all factors.

Table 4 .
The four sites suitable for waste landfill location.