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Water treatment

Table 6.3. Individual

Table 6.3. Individual (at each treatment stage) and cumulative (up to the SSF stage) mean removal efficiencies of basic water quality parameters in full-scale MSF plants. Filtration Filter Influent mean values Individual mean efficiencies Stage bed – Faecal Faecal Rates Treatment plant Turbidity Colour Turbidity Colour (mh -1 length Coliforms Coliforms ) (NTU) (PCU) (%) (%) (m) (CFU/100 ml) (log. units) DyGF stage 1.5 0.6 Shaloom (UGFL) 3.8 15 2,895 21 20 0.2 8.7 0.6 Cañas G. (UGFS(2)) 12 21 7,046 8 10 0.1 DyGF 1.4 0.6 Retiro (UGFL) 15 24 6,416 52 17 0.5 0.9 0.6 Colombo (UGFL) 15 25 51,916 57 24 0.7 1.6 0.3 Javeriana (HGF) 24 30 14,935 29 10 0.2 CGF stage 0.7 2.0 Ceylan (UGFS(2)) 2.8 5 330 71 20 0.8 0.6 1.5 Shaloom (UGFL) 3.0 12 1,680 37 17 0.9 0.7 2.0 Cañas G. (UGFS(2)) 11 19 5,334 54 16 1.1 CGF 0.7 7.0 Restrepo (HGF) 7.5 12 831 65 42 1.0 0.5 0.9 Retiro (UGFL) 7.2 20 2,152 42 20 0.9 0.5 1.2 Colombo (UGFL) 6.4 19 10,063 30 21 0.7 0.9 4.0 Javeriana (HGF) 17 27 9,092 71 41 1.0 SSF stage 0.13 1.2 Ceylan (UGFS(2)) 0.8 4 52 50 25 1.8 0.15 1.0 Shaloom (UGFL) 1.9 10 214 58 40 1.7 0.12 1.2 Cañas G. (UGFS(2)) 5.1 16 408 84 75 2.4 SSF 0.17 1.2 Restrepo (HGF) 2.6 7 77 77 57 2.0 0.10 1.2 Retiro (UGFL) 4.2 16 301 79 69 2.3 0.08 1.2 Colombo (UGFL) 4.5 15 2,008 87 73 3.3 0.10 1.2 Javeriana (HGF) 4.9 16 922 82 75 3.1 Treatment Plant Effluent mean values Cumulative mean efficiencies UGFS(2) + SSF Ceylan (UGFS(2)) 0.4 3 0.9 86 40 2.6 DyGF + UGFL + SSF Shaloom (UGFL) 0.8 6 4.3 79 60 2.8 DyGF + UGFS(2) + SSF Cañas G. (UGFS(2)) 0.8 4 1.5 93 81 3.7 HGF + SSF Restrepo (HGF) 0.6 3 0.7 92 75 3.1 DyGF + UGFL + SSF Retiro (UGFL) 0.9 5 1.4 94 79 3.7 DyGF + UGFL + SSF Colombo (UGFL) 0.6 4 0.9 96 84 4.7 DyGF + HGF + SSF Javeriana (HGF) 0.9 4 0.8 96 87 4.3 The high capacity of DyGF to reduce SS and turbidity values with relatively low initial capital and running costs is the strength of this treatment stage. During period III, for example, DyGF stage at Puerto Mallarino showed an individual SS removal efficiency of 72% meanwhile the overall DyGF + UGFS + SSF 1 treatment line showed a cumulative SS removal efficiency of 99.7%. During the same period this treatment line had a cumulative turbidity removal efficiency of 98% (table 6.1), with DyGF stage contributing 43%. CGF stage is more costly than DyGF stage but cheaper than SSF stage. The CGF stage also has an important contribution in reducing SS and turbidity levels. However, UGFL presents lower individual removal efficiencies than UGFS (table 6.1). Mean turbidity levels of UGFS effluents were usually below 10 NTU meanwhile UGFL effluents were above 10 NTU. During test period III Both SSF 1 (working in series with UGFS) and SSF 2 (working in series with UGFL) produced effluents with mean turbidity values below 5 NTU in 99% of the samples (table 3.21). Consequently, both MSF alternatives DyGF + UGFL + SSF 1 and DyGF + UGFS + SSF 2 fulfilled the multistage water treatment concept (gradually reduced raw water turbidity with similar cumulative mean removal efficiencies and effluent turbidity values ≤ 5 197

NTU). But treatment line 2 (DyGF + UGFL + SSF 2 ) did not properly follow the integrated water treatment concept. In this treatment line too much turbidity (solids) was reaching the SSF 2 unit reducing its filter run lengths (figure 3.24) and increasing its running cost. SSF is the most labour demanding stage, during partial or total cleaning activities, of all filtration stages included in MSF plants. As shown in table 6.2, most of the influent turbidity (as with SS) to the UGFS line was removed in the 1 st filtration step. During period III, for example, the 1 st , 2 nd , and 3 rd steps removed on average 38, 20, and 21% of influent turbidity respectively. In spite of the relatively low headlosses observed in the 2 nd and 3 rd filtration steps of UGFS line, they seem to play an important role in removing small particles, which are not very relevant for SS measurements but are for turbidity. During period III, for example, mean turbidity effluents of UGFS and UGFL were 6 and 13 NTU respectively. Excluding Ceylan (table 6.3), full-scale MSF plants had influents with mean turbidity values in the range 12 to 24 NTU and turbidity removal efficiencies in the range 73 to 96%. Pilot scale MSF systems had influents with mean turbidity values in the range 51 to 109 NTU and turbidity removal efficiencies in the range 95 to 98%. In contrast with the great flexibility observed for faecal coliform reductions, removal efficiency did not increase in the same way for turbidity in the MSF alternatives. Besides, CGF and SSF stages at Puerto Mallarino were set to work at constant filtration rates and did not benefit from the overall protection capacity of DyGF stage discussed in Section 3.2.2.3. High mean turbidity values may originate from a series of turbidity peaks or from constant base levels. In the case of turbidity peaks, it is essential that their impact can be reduced with the use of the DyGF and that there is adequate O&M to avoid the reduction in treatment efficiency and shortening of SSF runs. Even more, at least during working hours, the operator may include partially or totally closing the intake for short periods of time to avoid these peaks reaching the system. Up to certain limits, an increase in constant base level of turbidity implies the need for a more robust CGF stage. This may be obtained by using a lower filtration rate in a given set of filter beds or a longer retention time, by using a deeper/larger gravel bed with a given filtration rate. Another possibility is to include an additional treatment stage such as plain sedimentation. Colour removal efficiencies. Ceylan, Cañas G, Shaloom, and La Rivera full-scale MSF plants fulfilled proposed treatment objective (≤ 15 PCU) for this parameter, processing surface water sources with mean and maximum colour values in the range 5 to 20 PCU and 21 to 70 PCU respectively. During test period III all MSF alternatives at Puerto Mallarino produced water with colour levels consistently ≤ 15 PCU processing Cauca River water with mean and maximum colour values of 35 and 72 PCU respectively. In contrast, as presented in Section 3.2.4.1, pilot scale MSF alternatives showed some limitations in producing water with colour levels consistently ≤ 15 PCU processing Cauca River water with mean colour levels of 81, 54 and 57 PCU during test periods I, II, and IV respectively. 198

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