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Water treatment

Filterability results

Filterability results seem to underline the relevance of this simplified parameter to provide a preliminary idea about the feasibility of treating a surface water source with MSF technology. It could be used also to detect problems in an existing MSF plant by identifying significant changes in filterability results in the last one or two filtration stages. In all MSF plants, pH, total alkalinity and total hardness remained practically the same through the different filtration stages, which mean that the buffer capacity of the water was not significantly altered and precipitation did not take place in the filter beds. Dissolved oxygen (DO) did not become a limiting factor in applying the technology. Effluents of CGF and SSF stages were above 4 and 3 mgl -1 respectively, even at Javeriana with 4 m of gravel bed in HGF units before the SSF stage. During the monitoring period, all plants produced water consistently within the guidelines for total iron and manganese (table 2.3, section 2.3.1) with mean removal efficiencies in the range 91 to 97 % and 33 to 95 % respectively. MSF plants produce effluents with turbidity levels below 1 NTU with a frequency between 65 (Marina) and 98% (Ceylan), and below 5 NTU in at least 98% (Marina) of the samples. Effluent faecal coliform levels were below 3 CFU/100 ml with a frequency between 74 (Shaloom) and 100% (Rivera), and below 25 CFU/100 ml in at least 98% (Retiro, Marina, and Colombo) of the samples. Effluent colour levels were below 15 PCU in at least 98% (Retiro) of the samples. Therefore, at least 98% of all effluent samples were within treatment objectives or guidelines recommended by WHO (1993, 1997). Consequently, with the water quality obtained with these treatment plants, it is expected that only a low chlorine dose should be required as terminal disinfection and safety barrier against waterborne diseases. This low chlorine dose and the removal of natural organic matter (as indicated by colour reductions) should also contribute to lessening possible chronic adverse health effects related to disinfection by-products Figures 4.13 through 4.15 show mean faecal coliform, turbidity, and colour values in raw water and effluents of main filtration stages of each MSF plant. Table 4.8 shows mean removal efficiencies for the same parameters at each treatment stage. They show how the different MSF plants adapted to the type of raw water and the level of contamination in it. The plants show higher removal efficiencies for more contaminated water. In Ceylan, the mean removal efficiencies are 2.6 log units for faecal coliforms, 86% for turbidity, and 40% for colour. In Colombo, which receives more contaminated water, the removal efficiencies are 4.8 log units for faecal coliforms, 90% for turbidity, and 79% for colour. However, the SSF stage seems to be overloaded in Colombo. According to the multiple barrier strategy (Section 1.2), the priority should be to reduce the contamination levels in the water source. Besides, based on the integrated water treatment concept (Section 1.2), a more robust second filtration stage seems to be needed to reduce the risk of depending so heavily on SSF or terminal disinfection to produce water with low sanitary risks. 163

100000 Faecal Coliforms (CFU/100 ml) 10000 1000 100 10 1 0.1 Ceylan M arina R estrepo Shaloom Rivera Retiro C añas Javeriana C olom bo Raw water DyGF CGF SSF Figure 4.13. Mean faecal coliforms (CFU/100 ml) in raw water and effluents of DyGF, CGF, and SSF stages of MSF plants in the Cauca Valley of Colombia. 100 Turbidity (NTU) 10 1 0.1 Ceylan Shaloom Rivera M arina R estrepo C añas C olom bo Retiro Javeriana Raw water DyGF CGF SSF Figure 4.14. Mean turbidity values (NTU) in raw water and effluents of DyGF, CGF, and SSF stages of MSF plants in the Cauca Valley of Colombia. 35 30 Colour (PCU) 25 20 15 10 5 0 Ceylan M arina R estrepo Shaloom Rivera C añas R etiro Colom bo Javeriana Raw water DyGF CGF SSF Figure 4.15 Mean Colour values (PCU) in raw water and effluents of DyGF, CGF, and SSF stages of MSF plants in the Cauca Valley of Colombia 164

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