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Water treatment

guideline for colour is

guideline for colour is < 15 PCU (WHO, 1993). Therefore, to improve community acceptance and to reduce potential precursors of OBPs, it is considered desirable to have ≤ 15 PCU as treatment objective for colour in the effluents of MSF alternatives. This desirable treatment objective is considered a second priority to those proposed for faecal coliforms and turbidity. Iron and Manganese levels. Taste and possibility of staining clothes can cause community rejection. The treatment objectives proposed for iron and manganese are < 0.3 and 0.1 mgl -1 , following WHO (1993) guidelines. 6.2 Treatment Efficiencies of Multistage Filtration Alternatives Two main types of rivers, highland and lowland Andean rivers have been included in this study. Although the sanitary risks in the highland rivers are lower than in the lowland river, they all need to be treated before water graded A or B can be consistently supplied. An overview of water quality in four of the surface water sources used in this study is shown in figure 6.2. Treatment efficiencies observed in MSF alternatives processing these types of surface water sources are summarised in tables 6.1 and 6.3. Considerations about the water treatment concepts introduced in Section 1.2 are presented below to underline some criteria to be taken into account in the process of developing an example of a selection guide for multistage filtration (MSF) alternatives, before terminal disinfection. Faecal coliform removal efficiencies. During the 1980s, water surveillance strategies and tools were developed and transferred to the Andean Region (Lloyd and Helmer, 1991; WHO, 1997). However, the information available about the surface water quality in most areas of the region is not clear due to the limited implementation of water surveillance activities. Nevertheless, based on some project and institutional reports quoted in chapter 1, it seems that the great majority of surface water sources in the region supply water graded C, D, or E, according to the classification proposed by Lloyd and Helmer (1991). The pilot and full-scale MSF plants monitored during the present research work have processed water with a wide range of microbial contamination levels but all inside the D and E grades (figure 6.2). Both pilot and full-scale MSF alternatives included in this study showed great potential to treat this wide range of microbial contamination levels, as summarised in table 6.1. Most of the time all MSF alternatives followed the multistage water treatment concept. They gradually removed faecal contamination and consistently produced effluents low in sanitary risk prior to terminal safety disinfection. Under the integrated water treatment concept, DyGF played a minor role in reducing faecal coliform levels. At pilot scale DyGF contributed 0.6 to 0.8 log units of the overall removal of 4.1 to 5.6 log units obtained in the MSF system at Puerto Mallarino during test periods II, III and IV. However, the combination of DyGF and CGF played a very important role in this respect. For example, during test period III at Puerto Mallarino, the combination of DyGF and CGF stages contributed 2.2 to 3.2 of the overall 4.8 to 5.3 log-units reduction. As shown in table 6.1, load sharing occurs between filtration stages. SSF units with the highest efficiency usually follow the CGF lines with lowest efficiency. A similar observation is reported by CEHE (1999) for turbidity removal. 193

A Cumulative frequency (%) 100 80 60 40 20 0 10 100 1000 10000 100000 1000000 Faecal Coliforms (CFU/100ml) 100 B Cumulative frequency (%) 80 60 40 20 0 0 1 10 100 1000 Turbidity (NTU) C Cumulative frequency (%) 100 80 60 40 20 0 1 10 100 1000 Colour (PCU) Ceylan Cañas Gordas Colombo Cauca Figure 6.2. Cumulative (%) frequency distribution of faecal coliform (A), turbidity (B), and colour (C) levels in surface (river) water sources feeding MSF pilot plant system (Cauca river) and three full scale plants placed on the hillsides of the Andean Cauca Valley of Colombia. 194