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galvis

Water treatment

Pardon (1989) reports

Pardon (1989) reports similar evidence in the Andean region. Three MSF plants located in the highlands of Peru showed influent turbidity levels below 30 NTU in 90% of samples taken but turbidity peaks in the range 300 to 1,200 NTU (see table 2.9, Section 2.8.4). These peaks were also short, lasting less than a day. However, in another plant (Azpitia) located in a low land area, close to the Pacific-coast of Peru, mean turbidity values were > 70 NTU during the period January-March 1986. The distribution pattern of solids levels in surface water sources is needed to take decisions about introducing MSF technology in a new region. Turbidity and SS raw water data in this study indicates that a highland river in the Andean Cauca Valley would usually presents low to moderate levels of solids, together with turbidity peaks of short duration. However, this may not be the case with other highland water sources. Information about levels of solids in surface water sources must be gathered before introducing treatment technology into a zone. Experimental projects should be developed before going to a wider application of MSF. All raw water sources monitored in this study need to be filtered before terminal disinfection due to the high levels of faecal contamination. With the possible exception of Ceylan’s water source, all other sources require pre-treatment before SSF. The faecal pollution level in some water sources could be the limiting factor to select the best MSF alternative rather than the level of solids, as usually considered in the literature. Besides, catchment protection should be a priority to reduce the high levels of faecal contamination reaching treatment plants like Colombo. Considering all raw water sources, mean and maximum colour levels were in the ranges 5 to 30 PCU and 21 to 200 PCU respectively. Colour reductions was not considered a priority in introducing MSF technology in Cauca Valley but was monitored considering its potential impact on users acceptance of drinking water as well as on terminal disinfection. River monitoring data are essential for the selection and design of treatment plant alternatives. This is particularly relevant for MSF plants, but is often lacking in the rural areas and small municipalities of the Andean Region. A strategy based on sanitary inspections of catchment areas, combined with some basic water quality analyses, as recommended by Lloyd and Helmer (1991) and WHO (1997), should contribute in overcoming this limitation. 4.4.3.2 Removal efficiencies in MSF plants MSF alternatives monitored during the present study were adapted to the different contamination levels in the raw water sources. At least 98% of turbidity, faecal coliform, and colour levels measured in samples taken from effluents of MSF plants were within the treatment objectives or guidelines recommended by WHO (1993, 1997) before terminal disinfection. However, having now the results of the present study, it seems that some plants are over designed while others seem to be overloaded. The SSF stage in Ceylan is demonstrating relatively low removal efficiencies. This plant could have just a DyGF stage instead of UGFS2 (upflow gravel filtration with two units working in series). In contrast, SSF stage in Colombo is demonstrating relatively high 175

emoval efficiencies. This plant should have a more robust second gravel filtration stage instead of the actual UGFL (Upflow gravel filtration in layers). This type of discussion implies risk management, capital investment, and running cost considerations. Final decisions should take into consideration local conditions. Based on the experience in the Cauca valley an example of a selection matrix for MSF alternatives will be considered in Chapter 6. Five plants have DyGF. Two of them do not show the tendency of increasing efficiency with increasing contamination levels in the water source. Cañasgordas with the highest filtration rate (8.7 mh -1 ) and a short filter bed depth (0.3 m) and Javeriana having a similar bed depth to Cañasgordas. These results at Cañasgordas and Javeriana are in harmony with the results obtained during pilot plant studies at Puerto Mallarino, meaning that gravel bed depth and filtration rates at DyGF stage should be 0.6 m and < 4 mh -1 respectively. Turbidity removal efficiencies at DyGF stage at Retiro (52%) and Colombo (57%) are higher than those observed with a more polluted lowland river at Puerto Mallarino. A possible explanation could be related to higher capacity of transporting bigger or heavier particles of highland rivers than lowland rivers. Due to the influence of contamination levels and load sharing characteristic between treatment stages, it seems to be impractical to make comparative considerations about second or third filtration stages between treatment plants. 4.4.3.3. Operation and Maintenance (O&M) of MSF plants All WS systems including MSF plants are being operated and maintained by local workers with low level of schooling (< 5 years). They have the support of local administrators or members of community based organisations. The local level running the system has little support from water related institutions. All running costs are covered at local level. Capital costs are also covered in the systems located in the southwest part of Cali. These facts, together with the water quality data included in this study indicate that MSF filtration technology is under the operation, management and financial control of community based organisations or local institutions in the Andean Cauca Valley. However, a more systematic database about O&M activities and labour requirements is needed to improve management capacity of local organisations and cost analyses at local level and between different WS systems including MSF plants. 176

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