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Water treatment

Table 4.8 Mean removal

Table 4.8 Mean removal efficiencies of turbidity, faecal coliform, and colour in MSF plants. Cauca Valley, Colombia. Raw Filtration stages DyGF CGF SSF Treatment water DyGF CGF SSF Individual Individual Cumulative Individual Cumulative plant Mean values Removal efficiencies Turbidity (NTU) Turbidity removal (%) Javeriana 24 17 4.9 0.9 29 71 80 82 96 El Retiro 15 7.2 4.2 0.9 52 42 72 79 94 Colombo 15 6.4 4.5 0.6 57 30 70 87 96 Cañasgordas 12 11 5.1 0.8 8 54 58 84 93 Restrepo 7.5 - 2.6 0.6 65 77 92 La Marina 6.0 - 3.1 1.1 48 65 82 La Rivera 5.9 3.9 * 1.7 0.6 34 56 71 65 90 Shaloom 3.8 3.0 1.9 0.8 21 37 50 58 79 Ceylan 2.8 - 0.8 0.4 71 50 86 Faecal coliforms (CFU/100ml) Faecal coliforms removal (log units) Javeriana 14,935 9,092 922 0.8 0.2 1.0 1.2 3.1 4.3 El Retiro 6,416 2,152 301 1.4 0.5 0.9 1.3 2.3 3.7 Colombo 51,916 10,063 2,008 0.9 0.7 0.7 1.4 3.3 4.8 Cañasgordas 7,046 5,334 408 1.5 0.1 1.1 1.2 2.4 3.7 Restrepo 831 - 77 0.7 1.0 2.0 3.1 La Marina 803 - 139 1.8 0.8 1.9 2.6 La Rivera 3,600 1,174* 69 0.5 0.5 1.2 1.7 2.1 3.9 Shaloom 2,895 1,680 214 4.3 0.2 0.9 1.1 1.7 2.8 Ceylan 330 - 52 0.9 0.8 1.8 2.6 Colour (PCU) Colour removal (%) Javeriana 30 27 16 4 10 41 47 75 87 El Retiro 24 20 16 5 17 20 33 69 79 Colombo 25 19 15 4 24 21 40 73 84 Cañasgordas 21 19 16 4 10 16 24 75 81 Restrepo 12 - 7 3 42 57 75 La Marina 7 - 6 3 14 50 57 La Rivera 15 14* 8 3 7 43 47 63 80 Shaloom 15 12 10 6 20 17 33 40 60 Ceylan 5 - 4 3 20 25 40 4.4.2.3 Run lengths and productivity in SSF units Maximum available hydraulic head in SSF units determined run lengths between partial cleaning activities (scraping) in SSF units. This head varied between 0.3 m in Colombo, a low value due to hydraulic constraints determined by existing hydraulic structures utilised to construct the system, up to 0.7 m in most MSF plants. During the monitoring period SSF units were operating at filtration rates (see table 4.2) in the range 0.07 (Rivera) to 0.17 mh -1 (Restrepo). The influent to SSF units had mean suspended solid values in the range 0.2 (Shaloom) to 2.7 mgl -1 (Cañasgordas). SSF units showed mean run lengths in the range 46 (for La Marina) to 190 days (Javeriana). Figure 4.16 shows the relationship between suspended solids, turbidity and run lengths. Filtration rates are included in table 4.2 and should be considered to analyse the results summarised in this figure. SSF units at Marina treatment plant were operating at 0.16 mh -1 and mean influent SS of 1.6 mgl -1 . This MSF plant demonstrated periodic algal growth, which may be the main reason for 165

its short filter runs. In fact, this plant had a minimum filter run length of 15 days meanwhile SSF units in other plants had minimum values above 1.5 or 2 months. SSF units at Javeriana operate at 0.1 mh -1 with mean influent SS of 1.7 mgl -1 . Besides, the SSF units in this plant have a plastic cover to control leaves from the nearby trees falling into the SSF units. This could have also contributed to longer mean filter runs. Maximum SSF run lengths in Ceylan and Javeriana were 233 and 265 days respectively. 1000 100 Units 10 1 0.1 La Marina Shaloom Cañas G. La Rivera Colombo El Retiro Restrepo Ceylan Javeriana -1 Suspended solids (mgl ) Turbidity (NTU) Run lenght (days) Coarse gravel filtration (CGF) effluent Parameter Statistic Marina Shaloom Cañas Rivera Colombo Retiro Restrepo Ceylan Javeriana SS (mgl -1 ) Turbidity (NTU) Operational Vf (mh -1 ) Run Length Figure 4.16 Mean ± (SD) Mean ± (SD) 1.6 ± 4.8 3.1 ± 6.8 0.2 ± 0.3 1.9 ± 1.4 2.7 ± 6.8 5.1 ± 4.9 0.3 ± 0.3 1.7 ± 3.0 1.4 ± 3.1 4.5 ± 7.2 1.3 ± 2.2 4.2 ± 3.8 0.8 ± 2.4 2.6 ± 3.0 Slow sand filtration (SSF). Run lengths 0.3 ± 0.7 0.8 ± 0.9 1.7 ± 3.5 4.9 ± 4.9 Mean 0.16 0.15 0.12 0.07 0.08 0.13 0.17 0.13 0.10 Mean 46 82 85 90 92 92 178 178 203 Min 14 51 44 90 71 45 177 95 141 Max 76 130 120 143 159 165 187 233 265 Run lengths (days) of SSF units in MSF plants. Cauca Valley, Colombia. The lowest productivity of SSF units between scraping activities is shown by Marina MSF plant (101 m 3 per m 2 of filtering surface area) while Javeriana and Ceylan reached the highest productivity, 1,212 m 3 .m -2 and 1,502 m 3 .m -2 , respectively. These productivity values are higher than those reported for SSF units in the USA by Letterman (1991), being 112 to 650 m 3 /m 2 . 4.4.2.4 Ripening period after resanding a SSF unit in Retiro MSF plant During July 1999, SSF 1 unit (see figure 4.2) of Retiro MSF plant was taken out of operation, its sand bed was excavated, washed and placed back into the unit. SSF 1 went back into operation on July 31, 1999. Faecal coliforms, turbidity, and colour levels in the effluent of this unit were monitored until the levels reached acceptable values before terminal disinfection, meaning less than 5 NTU and 25 CFU / 100 ml, according to WHO (1993, 1997). After about a month of ripening, SSF 1 reached the expected levels, as shown in figure 4.17. 166

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