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Water treatment

As shown in tables 6.1

As shown in tables 6.1 and 6.3, colour is poorly removed at the DyGF stage. Colour is mainly removed at the CGF and SSF stages. Depending on colour levels in the raw water sources and the type of CGF option, the contribution to overall colour removal efficiencies in MSF alternatives can be higher at the CGF or SSF stage. Iron and manganese removal efficiencies. Both pilot and full-scale MSF plants showed a great potential to reduce the iron and manganese levels found in the surface water sources included in this study. Most of the total iron in influent raw water at Puerto Mallarino is already oxidised (ferric) facilitating its removal by filtration (Grandjean, 1996). The reductions of these parameters in the CGF stages did not limit the application of SSF technology in the Andean Cauca valley. Guidelines for Iron and Manganese before SSF were summarised in table 2.6. Based on those guidelines it is recommended to have < 1 and 0.2 mgl -1 for iron and manganese respectively before the SSF stage. 6.3 Water Treatment Concepts and Treatment Objectives in MSF Alternatives In this study, MSF pilot alternatives, processing a heavily polluted surface water source, showed mean faecal coliform, turbidity and colour cumulative removal efficiencies in the ranges of 4 to 5.6 log units, 95 to 98%, and 80 to 94% respectively. Full scale plants processing surface water sources with different levels of pollution showed mean faecal coliform, turbidity and colour cumulative removal efficiencies in the ranges of 2.6 to 4.7 log units, 79 to 96%, and 48 to 86% respectively. However, these removal efficiencies are meaningless if they are not enough to achieve established water treatment objectives or if they are not gradually obtained in harmony with the strengths and weaknesses of each treatment stage in a given treatment plant. The selection examples will be focussed on three parameters: faecal coliform, turbidity, and colour. However, treatment objectives for iron and manganese are also included below. Based on previous considerations and following the multistage water treatment concept, the treatment objectives proposed for MSF effluents before terminal disinfection are: • Mean and maximum faecal coliform levels ≤ 3 and 10 CFU per 100 ml • Turbidity levels ≤ 5 NTU • Colour levels ≤ 15 PCU • Iron ≤ 0.3 mgl -1 • Manganese ≤ 0.1 mgl -1 Based on previous considerations and following the integrated water treatment concept, the following complementary treatment objectives are proposed for CGF effluents in MSF plants to reduce possible limitations of the SSF stage, to produce effluents with low sanitary risks and to have filtration run lengths above 30 days: • Faecal coliform levels ≤ 1000 CFU per 100 ml • Turbidity levels ≤ 10 NTU • Iron levels ≤ 1 mgl -1 • Manganese ≤ 0.2 mgl -1 199

6.4 Selection of MSF Alternatives The databases developed during the present study after monitoring both pilot and full scale MSF plants are used as a reference to produce an example of a selection guide of MSF alternatives. The following steps are followed to produce the example: Adjusted databases. Initially, databases are “adjusted” to include only those data sections (blocks) having CGF and SSF effluents fulfilling the established treatment objectives in Section 6.3. These adjusted databases are used to obtain descriptive statistics and frequencies of raw water pollution levels that could be treated with each MSF alternative. These statistics are used only to obtain a preliminary idea of raw water types that could be treated with the MSF alternatives included in this study, following the proposed treatment objectives. Summaries of these adjusted or “ virtual” descriptive statistics are included in Annex 7. MSF alternatives and raw water types. Preliminary raw water types are identified based on 95-percentile (P 95 ) of the adjusted raw water databases for turbidity and faecal coliform levels. These percentiles are used to plot a preliminary zoning diagram (figure 6.3). MSF alternatives are distributed in this diagram based on their respective raw water statistics for turbidity and faecal coliform levels. Raw water types. Taking only as a reference the results summarised in figure 6.3 and the adjusted databases, several surface (river) raw water types are defined by combining different turbidity (A) and faecal coliform (B) levels. Turbidity is characterised by mean, P 95 , and maximum values; faecal coliform and colour levels are characterised by mean and maximum values. Several contamination levels and different combinations of these parameters are possible depending on local conditions in a particular site. As an example, related to the author’s experience in the Andean Cauca Valley, a set of raw water types is included in table and figure 6.4. The poorest water quality conditions of Cauca River, identified as A 6 B 6 and A 7 B 6 in figure 6.3, were intentionally not recovered in figure 6.4. These high contamination levels seem to take MSF technology to extreme limits in which O&M may be too demanding, at least for small WS systems. Assumed removal efficiencies. Individual and cumulative MSF removal efficiencies must be sufficient to fulfil proposed treatment objectives, processing P 95 turbidity values. It is assumed that maximum turbidity (peak) values can only be processed by a combination of flow reductions at the DyGF stage (Protection capacity concept introduced in Section and improved cumulative removal efficiencies. MSF alternatives summarised in the example presented in figure 6.4 are expected to produce effluents with mean turbidity values ≤ 1 NTU when processing surface (river) water below the mean raw water turbidity values. However, if this (≤ 1 NTU) is not a treatment objective, MSF alternatives can still be selected as long as they can fulfil the treatment objective (≤ 10 and 5 NTU in the effluents of CGF and SSF stages respectively) when processing raw waters with P 95 and maximum turbidity values. 200

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