Data Analysis
Population sampling
The study area is made up of 35 wards. Because of time constraint, 18 wards were sampled in order to engage the public in the siting process as public participation is necessary in landfill siting. The selected wards are those where there is high level of indiscriminate and illegal dumping. Quite a large number of respondents agreed that waste disposal is the major problem and the main reason why illegal dumping is a commonplace.
Allocation of transfer stations
Allocation of transfer stations involves giving skips to selected points where illegal dumping of refuse is common. The allocation of waste collection centres was done based on the responses obtained from the questionnaires that were distributed. Of the 720 questionnaires distributed, 538 were returned. Out of the 538, 380 (70.6%) chose proximity to streets in skip allocation, 60 (11.15%) indicated proximity to road, 67 (12.5%) indicated proximity to landfill site while 31 (5.76%) indicated other factors as factors to be considered in deciding which points to use as transfer stations for onward waster transfer to the landfill site (Figure 4.1). One skip each was allocated to Oje, Danaduru, Cultural centre and Gbenla because these areas house the biggest markets where a lot of wastes are produced on daily basis.
Spatial distribution of skips
In examining the spatial pattern of skips in the study area, GPS points of various locations where skips are allocated were collected. ArcView nearest neighbour index was utilised to calculate the R-value from which the pattern of spread was determined. Based on the analysis of the 14 transfer stations that were created, the R-value result was 0.19521 (Figure 4.4). This indicates a clustered pattern which reveals that the skip allocations in transfer stations are not evenly distributed (4.3). Some areas are adequately served while others are not. As station allocation was based on areas with high illegal dumping, the concentration of skips in this part may be due to the fact that majority of the market centres in the study area are around this zone.
Selection of suitable sites
Several factors were considered in arriving at the suitable sites for landfill siting. This is of great importance considering the nature of what is to be sited.
The inhabitants of the area must not be disturbed by the odour which could be very dangerous to their health. To achieve this proper selection, the roads and rail map were buffered at a distance of 100m (Javaheri et al. 2006) to ensure sites are not located close to roads and rails (Figure 4.5). Street map was buffered at a distance of 30m (Figure 4.6).
The available land-use map was queried to get open lands which are not currently in use (Figure 4.7). The open lands and land use map were then overlaid to produce available sites (Figure 4.8).
Elevation overlay
Deciding the suitable elevation for siting was based on judgement from planners in charge of waste management. Out of the 10 responses, 9 were filled. 6 respondents indicated that elevation of the suitable area should be between 200m and 300m, 3 indicated that elevation should be between 100m and 200m (Figure 4.9).
Network analysis of transfer stations
Refuse lorry drivers in Ibadan North were given questionnaires to specifically determine the number of kilometres they are willing to travel when transporting wastes from the transfer stations. 50 questionnaires were circulated among them for opinion sample and 39 responses were obtained. Most respondents indicated that the distance between the disposal site and the farthest collection point should be between 1km and 10km. Based on this, the shortest and farthest points were borne in mind when determining the points for waste collection from transfer stations to the disposal site (Figure 4.13).
Feedback on selected sites
Soliciting feedback on selected area for facility siting is an essential part of any siting process especially when it involves siting an obnoxious facility. This is because facilities which are obnoxious to potential neighbours may have some characteristics that are unique in nature relative to other location decisions (Erkut and Moran 1991). Informing the general public about a potential landfill site sometimes creates controversy among people. As a result of this, many developers/planners out of fear do not let the people of the potential areas know until a final decision is made. One of the major reasons for this is the fear of increase in the price of land (Walsh and O'Leary 2002).
The suitable sites in this study fall within areas of natural barriers (trees) with low concentration of people compared to other areas. People living close to the sites are found between 2km-3km away. Due to large number of people, it is impossible to survey every ward within the time frame to get feed back on the sites that were selected. However, 55 questionnaires were circulated among experts (planners) and members of the public which were part of the initial samples that have been selected to be part of this survey.
Feedback obtained indicates that majority of the respondents supported siting of facility in Yemetu area. The population sampled for this were those living in Yemetu and Oniyanrin (the candidate sites). Of the 55 questionnaires that were distributed, 51 were returned, fully completed. While 29 respondents said that siting should be done in Yemetu, 22 were of the opinion that the location should be Oniyanrin. 17 of the responses that supported siting in Yemetu gave the reason for their choice as nearness to Yemetu transfer station while 12 gave different reasons that range from large size of the area compared to the second alternative site, presence of alternative roads under all weather condition (Banar et al. 2007) to having two close wards that have the tendency to generate huge amount of wastes. Of the 22 people that indicated Oniyarin as the best site, 17 said they chose it because of its location (very close to the border). Other 5 did not give specific reasons. When asked about the impact of the siting on its potential neighbours, 35 respondents agreed that there may be some negative impacts (e.g. unpleasant odour). 15 indicated that there would be no impact and 10 were not sure if there would be any negative impact.
AHP analysis
The next step is to come out with the best alternative after the GIS procedure has revealed the two potential sites. In order to achieve this, the Super Decision software is used to connect the goal, criteria and alternative together in order to achieve a desired result.
In arriving at the criteria to include in the AHP process, expert opinions were sought. Having revealed the suitable sites on a map, it was decided that rail should be excluded among the criteria for AHP analysis, as the only rail line that passes through the study areas is very far from the selected sites (Figure 4.11).
Figure 4.17(a) shows the goal, the criteria to be evaluated for their importance to the goal and the available alternatives that are evaluated for how preferred they are with respect to each criterion. A line connects the main goal to each of the criteria for pairwise comparison for their importance with respect to the overall goal (Saaty 2001). In the same vein, a line connects each criterion to the two available alternatives (Yemetu and Oniyanrin). This implies that the alternatives would also be pairwise compared as the more preferred alternative for the criteria.
Assessments/Pairwise Comparisons Overview
One of the major strengths of the AHP is the use of pairwise comparisons to derive ratio scale priorities. This is better than the traditional method of assigning weights which is sometimes difficult to justify (Saaty 2003). Pairwise comparison is the process of comparing the relative importance, preference, or likelihood of two elements with respect to another element.
The 9-point scale as indicated in the above table ranges from 1 (indifference or equal importance) to 9 (extreme preference or absolute importance). This pairwise comparison assist decision makers to evaluate the contribution of each factor to the objective independently, hence simplifying the decision-making process (Rezaei-Moghaddam and Karami 2008)
Steps in calculating the priority vector (total weight of criteria)
The priority vector shows relative weights among criteria and alternatives. This result shows that Yemetu would be the best choice for siting the landfill. Yemetu is 52.5% while Oniyanrin is 47.5%. This reflects the preference of the public in giving preference to factors for siting landfill. The Normals column represents the result in the form of priorities (Figure 4.17e). This is a way to report results. The Ideals column is obtained from the Normal column by dividing each of its entries by the largest value in the column. The Raw column is obtained from the Limit Supermatrix. The Raw column and the Normals column are the same in any hierarchical model.
