Chapter 5. Results
5.1. Sensitivity to weightings
Changing the weightings of the three main categories (environmental, financial/technological and socio-cultural) cannot be done independently as all weightings must always sum to 100%. This becomes even more complicated once the weightings of the sub-categories must be varied. Here the scenario approach is applied.
5.1.1.Sensitivity to main category weighting
Initially, all weightings were set to the same value. Under these circumstances, the overall values or scores for the VIP and UDDT were 3.06 and 2.96 respectively. This confirms the observation by Flores et al. (2008) that there is not a great deal of difference between the two systems in the eThekwini situation.
Sensitivity analysis was then carried out for the three main criteria while keeping the other two evenly balanced. For example, when the rating for the environmental category was increased from 40% to 60%, the weights of the other two dropped from 30% to 20%.
The results of these three analyses are shown in Figure 5-1 to Figure 5-3. Graphs are plotted showing the change in performance for each of the two sanitation options with the change in criterion weighting. As the environmental rating increases in importance, the performance of the UDDT improves gradually and that of the VIP drops. Both score the same when the environmental criterion is weighted at approximately 45%.
Figure 5-1. Sensitivity analysis for the environmental criterion weighting
Figure 5-2. Sensitivity analysis for the financial/technical criterion weighting
Figure 5-2 suggests that as financial and technical considerations become more important, the difference between the two options increases slightly. When it is the only criterion to be considered (weighting 100%) then VIPs score 3.38 overall and UDDTs score 3.20. This suggests that if only financial and technical issues were considered, these technologies would score considerably better than they do when other aspects of sustainability are taken into account.
Figure 5-3. Sensitivity analysis for the socio-cultural criterion weighting
If the Socio-cultural criterion is given no weight, UDDTs perform slightly better than VIPs. As socio-cultural considerations become more important, the score for VIPs increases slightly.
However, UDDTs perform progressively less well, so that if this were the only consideration, VIPs would score 3.13 and UDDTs 2.63. The breakeven point is a weighting of 15%.
5.1.2. Sensitivity to scenarios with different weightings
The different scenarios proposed in this section mimic the weighting allocations of possible stakeholders. If weightings are not mentioned specifically, then they have been distributed evenly among the remaining criteria.
5.1.2.1. BC: Major constraints energy and water
A situation was considered where the important issues for the environment were that water use and energy use were low. This might be where sanitation is needed at a fairly remote location where there is no electricity or piped water. These indicators were therefore given a weighting of 40%
each. Furthermore, recycling of nutrients was given no weight and the reduction of VS, COD, N and P were all given a low weighting of 5%.
Figure 5-4. Screenshot of environmental indicator weightings for S
The input screen for environmental sub-criteria would appear as it does in Figure 5-4. The resulting scores are 3.79 for VIPs and 3.52 for UDDTs. Since estimates of both COD reduction and VS reduction were known, both were included in the analysis. In a situation where information was only available on COD or VS, the known indicator would have been weighted appropriately and the unknown would have been given a weight of 0%. A weighting of 6% each for N and P removal and 8% for VS removal leaves changes the score for VIPs to 3.77 and leaves the UDDT score unchanged.
5.1.2.2. BDand BE: Ecosan imperative: recycling is crucial
If it is considered essential that nutrients are recycled, there is no need for the system to reduce the nutrient and volatile solids content of the excreta. However, VS reduction may be desirable to reduce the unpleasantness of handling the material recovered from the system. For the purposes of this scenario, the reduction indicators were given a weighting of 0%.
Since nutrient recycling would probably be desirable in a situation where water and energy might also need to be conserved (a rural context, for example), these and the recovery criteria were all weighted equally at 25%. In this scenario, it might also be surmised that financial considerations carry less weight (e.g. with external funding), so the MCDA was run with equal weightings for the three main criteria ( ) and then again with a 40/20/40 split for environment/finance/socio-cultural weightings ( ).
Not surprisingly, the two sanitation options considered under the eThekwini circumstances do not perform well with these constraints since nutrients are not recycled (see section 5.2.1.1). With equal weightings for the main criteria VIPs and UDDTs score 3.00 and 2.77 respectively. With a higher weighting for the environment and socio-cultural categories, this drops to 2.93 and 2.69.
5.1.2.3. BF: People over environment
Another scenario might be that the environment is not considered important (10%) and while financial and technological considerations cannot be ignored (30%) the socio-cultural dimension is the most crucial. With these weightings, the performance of VIPs is improved over the baseline equal weighting scenario, but only to a score of 3.16, while the overall rating for UDDTs drops to 2.84.
In the next section scenarios are considered where different values are allocated to the two systems.
This could be the result of a consultation process with stakeholders, or if the situation in eThekwini should change, or the MCDA be applied in a different municipality altogether.