MSF technology showed great flexibility in dealing with different faecal contamination levels. However, in harmony with the multiple barrier strategy, it is emphasised that the selection and protection of good water sources should be a better solution than having to rely on more costly MSF alternatives. Besides, SSF stage may become overloaded and terminal disinfection can be compromised as a safety barrier. 6.4.2 Examples to illustrate level of performance achieved by MSF plants Based on the results presented in table and figure 6.4, examples similar to that presented by Lloyd et al (1991) and WHO (1993), included in Chapter 2 (table 2.15), can be produced but considering more specific CGF alternatives, in harmony with raw water contamination levels. Table 6.5. Examples of performance objectives for removal of turbidity, faecal coliform bacteria, and colour with different raw water sources and MSF alternatives Stage and process DyGF (3) UGFL SSF Disinfection Distributed Water DyGF (3) UGFS SSF Disinfection Distributed Water Turbidity Faecal coliform bacteria Individual Average (2, 3) Individual Average Maximum (%); (1, 3) (NTU) (NTU) (log units) (Per 100 ml) (Per 100 ml) Removal 1 loading P 95 Removal loading loading Raw water type X 2 Y 2 ; MSF alternative DyGF 2.0 + UGFL 0.60 + SSF 0.15 30 50 >70 - - 30 72 >70 - - 10 7 3
Sims and Slezak (1991) identified about 71 SSF plants in the USA operating with around 0.9 m filter bed depths and filtration rates in the range 0.01 to 0.50 mh -1 (71% operating at ≤ 0.25 mh -1 ). They report turbidity removals of 70 to 90% in SSF plants operating with influent turbidity values in the range 1 to 10 NTU. They also report coliform removal efficiencies ≥ 99% in 20% of SSF facilities processing influent raw water with coliform level ≥ 100/100 ml. In Andean and Caribbean environments, Pardon (1989), Eudovique (1992), and the present study report turbidity and faecal coliform reductions in full-scale plants averaging 19 to 87% and 71 to 99.95% respectively. Similar dispersion is found in removal efficiency data with CGF alternatives, as considered already in section 188.8.131.52. Therefore, if there is no experience with the technology in a region, it is important to adopt conservative design parameters in the first MSF plants and preferably start with pilot plant experiments to review the functioning of MSF systems under the local conditions. This review is best done in jointlearning scenarios (Garcia, et al., 1997; Galvis et al, 1999) where sector professionals, sector authorities and communities can together evaluate the viability of the systems. 6.5 Sustainability of Development Projects Including MSF Alternatives Following a request from the General Assembly of the United Nations, the World Commission for Environment and Development discussed the implications of the Sustainable Development Concept in their 1987 report. This discussion has had considerable implications in all development sectors. The main thrust of the concept is that activities by the present generation should not compromise the resources or the environmental condition of future generations. The Development Assistance Committee (DAC) of the Organisation for Economic Cooperation and Development (OECD) in reaction to this orientation produced a definition of sustainable development from a donor agency perspective. It considers a development programme to be sustainable when it can provide an appropriate level of benefits, over an extensive period of time, after the financial, administrative or technical support of an external agency has ended (OECD/DAC, 1988, quoted by MDF, 1992). Warner (1990) presents the same concept, orientating it more to the community level by stating that “ the success or sustainability of a project is achieved when it meets its objectives and is maintained by its users over a significant period of time” . Both these definitions, however, do not make a clear reference to the safeguarding of the environment, which is becoming the bottleneck in many water supply and sanitation systems. Also, it may not be reasonable to expect, as Warner appears to suggest, that the users should be left entirely on their own in keeping up their system. Although governments are changing their role from providers to facilitators in the water sector, several tasks remain that can not be catered for at the community level. Consequently, some external input will be necessary to sustain the systems, which should also contribute to building capacity at local level. Considerations about sustainability concepts in relation to the MSF technology follows, taking into account experiences in promoting SSF technology in Colombia (Quiroga et al, 1997; Visscher et al, 1997). 206
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