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Water treatment

1000.0 100.0 ) Turbidity

1000.0 100.0 ) Turbidity (NTU) 10.0 1.0 0.1 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec A Cauca River (Pilot Units) Pance River (Colombo Project) La Elvira (Ceylan Project) 10000000 B Faecal Coliforms (CFU/100 ml) 1000000 100000 10000 1000 100 10 1 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Cauca River (Pilot Units) Pance River (Colombo Project) La Elvira (Ceylan Project) 3500 3000 2500 Units 2000 1500 C 1000 500 0 1 24 47 70 Hours -1 Suspended Solids (mgl ) Turbidity (NTU) Colour(CU) Figure 2.24 Water quality changes in three rivers of the Andean Cauca Valley, Colombia. 1990. A. Turbidity levels. B. Faecal coliform levels. C. Hourly changes in the Cauca River at Puerto Mallarino, Cali, March 26, 27 and 28 of 1990. In the process of adapting the SSF technology to the conditions of the Cauca Valley during the 1980s, it became clear that this technology alone (section 2.6 and tables 2.5 and 2.6) could not continuously cope with the raw water quality of most of the rivers in the region. In 64

order to overcome the water quality limitations of the SSF, the author and his working group at the Cinara institute of the Universidad del Valle, in collaboration with community based organisations and national and international institutions, gradually began to develop an innovative multistage filtration (MSF) technology (figure 2.25). In this technology, one stage of CMF (DyGF) or two stages of CMF (DyGF and CGF) precede SSF to enhance its efficiency without compromising its operational simplicity. Figure 2.25 General layout of a multistage filtration water treatment plant There are several options for the MSF technology based on the possible combinations of alternatives for the different treatment stages. These combinations need to be properly specified to fulfil established water quality guidelines and treatment objectives, including those related to the basic parameters for community water supply reviewed in section 2.3 and summarised in table 2.3. However, in harmony with the multiple barriers strategy (section 1.2), here it is good to reiterate that selecting and protecting the best available water source is far more economical and effective than allowing the catchment areas to deteriorate and then having to rely on the more complex and costly options of the MSF technology. If the raw surface water frequently transport large or heavy particles it should be initially screened or settled before the multistage filtration plant. DyGF should be the 1 st CMF stage because of the abrupt changes in suspended solids or turbidity in most Andean rivers. In well-protected surface water sources the 2 nd CMF stage could be omitted. However, if necessary a selection should be made among the several options already reviewed from the literature (sections 2.8.4, 2.8.5, and 2.8.6), depending on the level of contamination of the water source and the O&M requirements of this 2 nd treatment stage. SSF should be the final filtration stage. In harmony with the basic concepts of water treatment (section 1.2) these filtration stages should be selected and integrated in such a way that they progressively remove the contaminants from the raw water and consistently produce treated water with low sanitary risk. Ideally, terminal chemical disinfection with a fairly constant low-dose should be sufficient to form an efficient and reliable final safety barrier. 65