Introduction

Decreasing available land for assimilation of increasing Municipal Solid Waste (MSW) as a cause of rapid urbanization and economic growth and increasing negative impacts on public health and environment due to uncontrolled dumping of MSW are major concerns governing landfill site selection process. Landfill site selection has two important steps: the identification of potential sites through preliminary screening and the evaluation of their suitability based on environmental, engineering, socio-economic and legal criteria [1]. This makes landfill siting a difficult, complex, tedious, and protracted process. Since many siting factors and criteria should be carefully organized and analyzed during landfill site selection hence Multi-criteria Decision Making (MCDM) tools are required to be developed to solve the landfill site selection problem by evaluating the landfill suitability based on these criteria for the study region.

Numerous landfill siting techniques have been developed which includes Site Sensitivity Index (SSI), developed by Central Pollution Control Board (CPCB) with National Environmental Engineering Research Institute (NEERI), Nagpur, India [2,3,4]. Although it considers 32 attributes related to accessibility, public receptor, environment, geology etc., relevant to Indian conditions, yet no stress is given on economy which is essential for a developing country like India. The researchers have [5] used SSI along with economic viability test to select landfill site for disposal of hazardous materials in Tamil Nadu, India.

AHP, combined with Geographic Information System (GIS), is another important technique used by many researchers for landfill siting based on different evaluating criteria [6,7,8,9]. Some of the investigators have [7, 9] have considered both environmental and economic criteria relevant to Beijing, China and Konya, Turkey respectively, two growing cities of developing countries. The result of these techniques is the evaluation of the suitability for the entire study region based on a suitability index, which is useful in order to make an initial ranking of the most suitable areas [6].

KMC has proposed three candidate sites for construction of sanitary landfill at Gangajoara, Natagachi and Kharamba. Thus it has been necessary to check the suitability of the sites and choose the best site for landfilling among the three. In order to select the best landfill site, ranking of the three sites considering environmental sustainability, technological feasibility, social and legal acceptability and economic viability should be done. This paper thus contributes a useful and prompt decision making tool in the present Indian Municipal Solid Waste Management (MSWM) scenario where site is acquired not only on the basis of environmental sustainability and social accessibility but also on the basis of land area availability and economic viability as well.

A MCDM tool has been developed based on AHP in this study to check suitability of landfill site in terms of Landfill Site Sensitivity Index (LSSI) and Economic Viability Index (EVI). Hence the evaluation criteria related to the selection of the landfill sites have been first identified and a hierarchical structure has been formed to define the inter-relationship between the goal, criteria and sub-criteria by AHP. The pairwise comparison data and data regarding qualitative/quantitative magnitude of those criteria and sub-criteria have been collected by primary survey by designing relevant questionnaire and site investigations, secondary survey and literature review. The data collected by questionnaire survey has been analyzed by AHP to calculate the weights of the criteria/sub-criteria. The landfill site suitability of the candidate sites has been evaluated by both LSSI and EVI methods and they have been ranked accordingly.

The development of AHP based MCDM tool to evaluate the suitability of the landfill sites in terms of LSSI and EVI is also described in Sect. 2 followed by methodology of calculating useful life of landfill site. Results obtained and discussions are included in Sect. 3. Finally the paper is ended with some concluding remarks in Sect. 4.

Materials and Methods

Study Area

The metropolitan city of Kolkata is located in eastern India on the east bank of River Hooghly and centered on latitude 22°34′ North and longitude 88°24′ East and is the capital of West Bengal. Kolkata Municipal Corporation (KMC) area covers 187.33 km2 [10]. The city has an elevation around 1.5–9 m above mean sea level. The whole area is in the Ganges Delta and is monotonously plain with alluvial deposits. According to the Bureau of Indian Standard (IS: 1893-1984), on a scale ranging from I to V in order of increasing susceptibility to earthquakes, the city lies in seismic zone III [11]. Kolkata has an annual mean temperature of 26.8 °C. Annual rainfall is around 160 cm. Relative humidity varies between 85% in August and 68% in March. It has a total population of about 8 million [10].

MSW generation in Kolkata city as on January, 2013 is approximately 4000 ton per day (tpd) [12]. After collecting un-segregated MSW in KMC area, 95% is openly dumped at Dhapa landfill site which is located at the eastern fringe of the city at an average distance of 10 km from the collection points and the rest at the Garden Reach disposal site.

With the near exhaustion of the Dhapa landfill, KMC has identified three sites on the outskirts of Kolkata city in South-24 Parganas for landfilling. The three sites are at Gangajoara (22.46°N and 88.44°E), Natagachi (22.43°N and 88.48°E) and Kharamba (22.50°N and 88.54°E). Gangajoara is located at South-East of KMC near Sonarpur-Bantala road at a distance of8 km from Eastern Metropolitan (E.M.) Bypass at Garia and 18 km from the centre of KMC area at Rabindra Sadan metro station with available landfill area of about 93 ha. Natagachi has an available area of about 76 ha and is located at South-East of KMC area near Sonarpur-Chakbaria road (a local road) at a distance of 12 km from E.M. Bypass at Garia and 23 km from Rabindra Sadan. Kharamba is located at South-East of KMC near Tardah-Bhojerhat road (a local road) at a distance of 18 km from Ambedkar Bridge on E.M. Bypass, about 14 km east from present landfill site and 23 km from the centre of KMC area at Rabindra Sadan metro station with an area of about 97 ha.

Methodology

In the present study, suitability of landfill site is checked by developing an AHP based MCDM tool in terms of Landfill Site Sensitivity Index (LSSI) and Economic Viability Index (EVI).

Method to Calculate LSSI

The method to calculate LSSI has been broadly divided into three steps:

  • Step 1 Selection of the evaluation criteria (both qualitative and quantitative) and related sub-criteria.

  • Step 2 Formation of the hierarchical structure of the multiple criteria and calculation of the relative importance weights of the criteria and related sub-criteria using AHP.

  • Step 3 Estimation of the Landfill Site Sensitivity Index of the sites and ranking accordingly.

Five evaluating criteria have been selected, namely-accessibility, receptor, environment, public acceptability and geology. These criteria were further classified into thirteen sub-criteria to form a hierarchy as shown in Fig. 1.

Fig. 1
figure 1

Hierarchical structure of the decision problem for identification of best suitable landfill in terms of LSSI

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The AHP method is composed of three main stages:

  • The first stage is the decomposition of the problem into a hierarchical structure. Based on the selected criteria and sub-criteria the problem is constructed into a hierarchical structure for efficient analysis (Fig. 1). The hierarchical structure of the problem consists of three levels. The first level represents the ultimate goal of the hierarchy (suitability for landfill siting), the second level represents main evaluation criteria utilized in this work for ranking and the third level represents the sub-criteria related to the main criteria.

  • The second stage is the designing of a survey instrument in form of a questionnaire which contains decision tables at each level of the hierarchical decomposition to facilitate all of the possible pair-wise comparisons among the criteria/sub-criteria. The questionnaire was used to interview experts from different fields of with adequate knowledge of SWM. The experts were supplemented with the Fig. 1 showing the hierarchical structure of the problem for providing primary information about the problem in the questionnaire. By comparing pairs of criteria at a time and using a 9-point scale (Table 1), experts have quantified their opinions about the criteria’s magnitude in a questionnaire. This pair-wise comparison allows for an independent evaluation of the contribution of each factor, thereby simplifying the decision making process [13]. An example of the filled questionnaire designed and used for the purpose of this survey is demonstrated in Fig. 2.

    Table 1 The definition and explanation of the AHP 9-point scale
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    Fig. 2
    figure 2

    Sample questionnaire for comparison of main criteria

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After the questionnaires have been administered to the experts, a matrix of importance ratios is developed from the data obtained from the decision table which is used to describe the results of the pair-wise comparisons. The matrix is a symmetrical and reciprocal matrix for the pair-wise comparisons, known as Pairwise Comparison Matrix (PCM).

The next step is the calculation of the criteria’s relative importance weights implied by the previous comparisons. The normalized principal Eigen vector or priority vector, which is the relative importance weight of a criterion, can be obtained by averaging across the rows. Since it is normalized, the sum of all elements of priority vector is 1.

Consistency Index (CI) is used to express the results’ degree of consistency given by Eq. 1 [14]:

$$CI = \frac{{\lambda_{ \hbox{max} } - n}}{n - 1}$$
(1)

where λmax is the principal Eigen value of PCM and n is the number of criteria. Principal Eigen value is obtained from the summation of products between each element of Eigen vector and the sum of columns of the PCM.

Consistency Ratio (CR) is given by Eq. 2 [14]:

$${\text{CR}} = {\text{CI}}/{\text{RI}}$$
(2)

where the Random Index (RI) is presented in Table 2.

Table 2 Random Index (RI)
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If the value of the Consistency Ratio (CR) is less than or equal to 0.1, the response is considered acceptable. If the CR is greater than 0.1, the response is not acceptable [14].

Example of PCM developed from the sample questionnaire (Fig. 2) and calculation of relative importance weights and CR is demonstrated in Table 3.

Table 3 Sample of calculated PCM and relative importance weights of the main criteria
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The consideration of several respondents was used to avoid some bias in the computation of respondent judgments [15]. The responses which have passed the consistency test, were combined to obtain the final values of relative importance weights of the criteria/sub-criteria. The final PCM has been developed by taking the Geometric Mean (GM) of PCMs from the consistent responses. From the final PCM the weights of the evaluation criteria and sub-criteria have been calculated and then the consistency has been checked.

  • For the third and final stage, the importance weight of each sub-criterion is multiplied to the importance weight of its related criterion to get the final weight which is used in the calculation of the LSSI score.

    The data collected for quantitative measurement of each sub-criterion of individual sites was rated in terms of a Sensitivity Index (SI) on scale of 0–1 to facilitate computation of aggregate scores called LSSI. While “0” indicated no or very less potential hazard (preferable), “1” indicated the highest potential hazard (undesirable). Thus, for each criterion a four level sensitivity scale (0–0.25, 0.25–0.50, 0.50–0.75 and 0.75–1.00) has been considered. Allotment of sensitivity indices for the selected parameters was made following earlier studies [2,3,4]. Since the weights of the criteria calculated by AHP sum up to 1, the weight of individual criterion is multiplied by 1000 (conversion to a larger scale) so that the aggregate of the weights become 1000. The final weightage of the sub-criteria have to be calculated by multiplying weights of sub-criteria with final weightage of related criteria. Thus the summation of the SI value multiplied by the corresponding weightage of all the sub-criteria would give the LSSI for an individual site out of 1000. This score is compared with the similar scores of the other available sites and all the sites are ranked on the basis of LSSI with the lowest scoring site given top ranking.

    The LSSI for a site is calculated using Eq. 3:

    $${\text{LSSI}} = \mathop \sum \limits_{{{\text{i}} = 1}}^{\text{n}} {\text{WiSi}}$$
    (3)

    where LSSI = Total Landfill Site Sensitivity Index score, Wi = Weight of the ith sub-criterion ranging from 0 to 1000, Si = SI of the ith sub-criterion ranging from 0 to 1, n = Number of sub-criteria for calculating LSSI = 13.

To assess the suitability of the sites the LSSI is classified into three categories: Highly sensitive (total score >750), moderately sensitive (total score ranges between 300 and 750) and low sensitive (total score <300).

Method to Calculate EVI

The method followed three steps. First, three evaluating criteria were selected, namely—quantity of waste transferred per day, average transportation cost and distance from collection point to disposal site.

In the second step the AHP has been used to form the two-level hierarchical structure (Fig. 3) of the multi-criteria problem and calculate the weights of the criteria. A questionnaire has been designed containing decision tables at each level of the hierarchical decomposition to facilitate all of the possible pair-wise comparisons among the criteria. The questionnaire was used to interview experts from February to March 2013. The generation of PCMs from the data collected by questionnaire survey, the calculation of weights of the criteria and the consistency check has been done in a similar fashion as in the second step of calculating LSSI.

Fig. 3
figure 3

Hierarchical structure of the decision problem for identification of best suitable landfill on the basis of economic viability

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The final step was the estimation of the Economic Viability Index (EVI) for each of the candidate sites at Gangajoara, Natagachi and Kharamba. The data collected for quantitative measurement of each criterion of individual sites was rated in terms of a Viability Index (VI) on scale of 0–1 to facilitate computation of aggregate scores called EVI. While “0” indicated lowest economic viability (undesirable), “1” indicated the highest economic viability (preferable). Thus, for each criterion a four level viability scale (0–0.25, 0.25–0.50, 0.50–0.75 and 0.75–1.00) has been considered. Allotment of VI for the selected parameters was made following earlier studies [2,3,4]. Since the weights of the criteria calculated by AHP sum up to 1, the weight of individual criterion is multiplied by 1000 so that the aggregate of the weights become 1000. Thus the summation of the VI multiplied by the corresponding weightage of all the criteria would give the EVI for an individual site out of 1000. This score is compared with the similar scores of the other available sites and all the sites were ranked on the basis of EVI with the highest scoring site given the top ranking.

The EVI for a site was calculated using Eq. 4:

$${\text{EVI}} = \sum\limits_{{{\text{i}} = 1}}^{\text{n}} {\text{WiVi}}$$
(4)

where EVI = Total EVI score, Wi = Weight of the ith criterion ranging from 0 to 1000, Vi = VI of the ith criterion ranging from 0 to 1, n = Number of criteria for calculating EVI = 3.

To assess the economic viability of the sites the EVI is classified into three categories: Highly viable (total score >750), moderately viable (total score ranges between 300 and 750) and low viable (total score <300).

Useful life time of landfilling area is another important criterion to check the suitability of a landfill site since more the available life time more is economically viable. To calculate the useful life time of the proposed landfill sites present MSW generation of KMC area was considered along with rate of increase @ 4.25% per annum [16]. The landfill area required to dispose the waste was calculated with available data and suitable assumptions following the method described in CPHEEO Manual for MSW Management and Handling [17].

Results and Discussion

The criteria related data or Criteria Measurement (CM) of the three proposed sites (Gangajoara, Natagachi and Kharamba) have been collected by literature survey, primary survey, secondary survey and questionnaire survey. Based on the CM, the SI was assigned and the Criteria Score (CS) was calculated by multiplying SI with the respective weight of that attribute. The results are presented in Table 4.

Table 4 LSSI calculation for the sites of Gangajoara, Natagachi and Kharamba
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From Table 4 it is evident that the sites at Gangajoara, Natagachi and Kharamba with the LSSI of 509.43, 494.61 and 570.77 respectively are moderately sensitive sites for landfilling, where Natagachi ranks first followed by Gangajoara and Kharamba.

The CM and VI of all three proposed sites are presented in Table 5 and EVI of all the three proposed sites was calculated by aggregating the CS of each criterion.

Table 5 EVI calculation for the sites of Gangajoara, Natagachi and Kharamba
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The calculated EVI for Gangajoara, Natagachi and Kharamba are 467.5, 377.5 and 387.5 respectively. Thus Gangajoara, Natagachi and Kharamba are all moderate economically viable sites. From EVI it can be remarked that Gangajoara ranks first followed by Kharamba and Natagachi.

The actual useful life of the three candidate landfill sites at Gangajoara, Natagachi and Kharamba were obtained 6, 5 and 6 years respectively.

Conclusion

The study presented a MCDM tool to evaluate and rank three available sites at Gangajoara, Natagachi and Kharamba, proposed by KMC for sanitary landfilling of MSW generated by Kolkata. Only five main criteria (both qualitative and quantitative) and 13 sub-criteria were considered for evaluation of a landfill site in terms of LSSI, the relative important weights of which were obtained by AHP technique. Lesser number of evaluating criteria saves time on data collection and computation compared to other popular methods. EVI on the other hand helps the decision making process by highlighting the financial aspect directly involved with landfilling. The LSSI and EVI analysis reveal that all the proposed sites are moderately suitable for landfilling (total scores ranges between 300 and 750). Natagachi is the best in terms of LSSI (494.61) where Gangajoara is the best in terms of EVI (467.5). The useful life of Gangajoara (6 years) is more than Natagachi (5 years). Hence, given importance to economy to a growing metropolis in a developing country like India and useful life, Gangajoara may be the best choice as landfill site. Though case specific; the proposed MCDM tool will be very useful for landfill site selection for developing countries.