Chapter 5. Results
5.2. Sensitivity to changes in values
5.2.2. Sensitivity to changes in VIP values
VIPs are a well established, robust systems used by rural households for hundreds of years. Under conditions where household solid waste is disposed of elsewhere they may not need emptying for the life of the system which is assumed in the MCDA to be 20 years. However, their performance is
very different under urban circumstances. User acceptance is lower where those close by have flush lavatories and the addition of solid waste to the pits shortens their useful life. If flooding occurs, pathogen risk to residents and the risk of contamination of water become greater. Two scenarios were envisaged where the value and/or the weightings for criteria might change and affect the overall rating of VIPs.
5.2.2.1. GK: Disposal of sludge to wastewater treatment works
In this scenario, some attempt is made to account for the energy required to process the sludge from pit latrines. While more environmentally friendly options such as the LaDePa machine processing sludge to fertiliser and the deep row entrenchment in forests are still under investigation, many municipalities are likely to process the sludge at their wastewater treatment works (WWTW). In this scenario, energy use would be increased significantly due to the need to transport the sludge and for other processes in the WWTW.
Figure 5-7. Ratings for environmental impact of VIPs with sludge treated at WWTW
The energy requirement for aeration would be reduced since the sludge would already have undergone most of its potential biodegradation in the on-site sanitation facility. The energy requirement was set at 15kWh/p/y. Overall, COD and VS removal was the same as for VIPs (75%
and 65% respectively) since the sludge is assumed to have stabilised. N and P removal would be the values for the WWTW. Using Çiçek, et al. (1999) as a guide, the values for removal were adjusted to 99% for nitrogen and 89% for phosphorus (Figure 5-7). Furthermore, the cost of operations and maintenance would increase to the maximum value. The resulting score for VIPs with weightings equal across all criteria was 3.04 (S %.
5.2.2.2. GL: Urban resistance to on-site systems
On-site systems are often better accepted in peri-urban or rural settings. In urban areas, there may be greater resistance to any alternative other than a flushing lavatory. Political opposition might also be brought to bear on the situation.
Table 5-1. Changes in Socio-cultural ratings for S
Criterion VIP UDDT
S S S S
Acceptability: user perceptions of fitness for purpose
3 0 2 0
Convenience: provision of sanitation
where users require it 3 3 3 3
Equity: fulfillment of requirements of all gender groups. equivalence of sanitation provision for different income groups
3 3 3 3
Legal /institutional: fit with legal requirements, institutional support for construction, O & M, political support
4 2 4 2
Facility for ongoing hygiene education:
commitment of local authority or national government to fund
4 4 4 4
Participation: facility for user involvement
in planning and execution of project 4 1 4 1
Food security: contribution of system to household based food production
0 0 0 0
Pathogen exposure: requirements of system for contact with faeces
4 4 1 1
The socio-cultural forces might be thought to carry more weight in a politicised situation.
S envisages the ratings changing as shown in Table 5-1 and the weightings changing to 20/20/60 for environment/finance/socio-cultural. The resultant rating for VIPs is 2.49 and for UDDTs 2.30.
5.2.3. BCM: An experienced municipal engineer’s perspective
An engineer and former head of eThekwini Water and Sanitation (EWS) was asked to provide weightings and values for the MCDA (Neil Macleod, 2015, email communication, 16 January.). For the main categories, he suggested that the environmental, financial/technological and socio-cultural criteria be weighted 20:50:30. The suggested socio-cultural weightings are shown in Figure 5-8.
Figure 5-8. Socio-cultural weightings suggested by Neil Macleod
This expert also suggested the environmental values be changed to those shown in Table 5-2.
Table 5-2. Value scores allocated for environmental indicators from the researcher (FS) compared with those suggested by Neil Macleod (NAM)
Criterion Indicator VIP UDDT
FS NAM FS NAM
efficiency of reduction in
VS % removal 65 70 45 50
efficiency of removal of
N % removal 75 70 95 90
efficiency of removal of P % removal 12 10 95 10
efficiency of reduction in
COD % removal 75 50 55 40
net energy required MJ/person/year 0 0 0 0
net water required m3/person/year 0 1 0 1
nutrients recovered kg N+P+K/p/y 0 0 0 0
organic material recovered
kg DM/p/y 0 0 0 0
While this expert agreed with the researcher as to the robustness scores of the two systems (5 for VIPs and 3 for UDDTs), he suggested that UDDTs might require six-monthly rather than annual intervention. He agreed that the lifespan of both systems was approximately 20 years. Cost estimates were higher for construction, at R7 000 for a VIP and R10 000 for a UDDT. Maintenance costs were lower, at R50/p/y for VIPs (based on R1 200 every 5 years for 5 people) and R5/p/y for UDDTs. Both systems would create 5 new jobs per 1000 households.
The socio-cultural ratings suggested by this expert were very different from the researcher’s. VIPs were considered to be totally unacceptable (score 0) while UDDTs were accepted with persuasion (score 2). Both systems were considered to be inconvenient and while VIPs do not provide for children, both women and children are disadvantaged by UDDTs. He also scored both systems 2 for legal/institutional fit, indicating less certainty about political support for these options.
The changes to inputs suggested by the expert resulted in a score of 2.71 for VIPs and 2.50 for UDDTs.