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Water treatment

3.2.2 Dynamic gravel

3.2.2 Dynamic gravel filtration (DyGF) units Observed filtration rates and filter runs. During the test period I the initial planned filtration rates were in the range 1 to 2 mh -1 in the DyGF units (table 3.1) with a weekly partial cleaning frequency (table 3.2), when the CGF lines were operating at low filtration rates (0.3 mh -1 ). Gradually, with greater flow requirements in the subsequent treatment stages, higher filtration rates and more frequent cleaning procedures became necessary in this 1 st filtration stage. Table 3.7 presents descriptive statistics based on the observed filtration rates and filter runs values in the DyGF units. Table 3.7 Observed filtration rates and filtration runs at the DyGF units of the MSF pilot system. Cinara’s Research Station, Cali, Colombia. Descriptive statistics Run length (1) (days) DyGF A DyGF B DyGF C Filtration Run Filtration Run rate length (1) rate length (1) (mh -1 ) (days) (mh -1 ) (days) Filtration rate (mh -1 ) Period I (March 3 – July 14 1991) Mean 6.1 0.88 5.9 1.32 5.6 1.36 Standard Deviation 1.5 0.15 1.4 0.24 1.6 0.63 Minimum 4 0.02 4 0.49 2 0.01 Maximum (2) 9 1.12 8 1.6 8 2.45 Data (N) 25 153 26 153 28 156 Period II – III (July 15 1991 – July 12 1992) Mean 3.69 1.86 3.6 1.44 3.6 2.55 Standard Deviation 0.9 0.24 0.9 0.1 0.9 0.6 Minimum 2 0.22 2 0.45 2 0.23 Maximum (2) 7 3.67 7 1.93 7 3.74 Data (N) 140 493 141 499 140 496 Period IV (July 13 1992 – April 19 1993) Mean 3.02 1.88 3.1 1.70 3.04 2.78 Standard Deviation 0.26 0.21 0.7 0.20 0.3 0.76 Minimum 2 0.7 3 0.11 3 0.62 Maximum (2) 5 2.77 8 2.49 6 5.74 (3) Data (N) 135 385 126 379 134 392 (1) The last day of a filter run was also considered to be the first day of the following filter run (2) Some of the maximum values coincide with O&M activities in others DyGF units (3) DyGF units A and B were having total cleaning procedures and DyGF C was overcharged during one day (January 23 1993). Small changes (≤8.3%) were observed between the mean filter run lengths in each test period in spite of the higher changes (≤35.3%) between the mean filtration rates (table 3.7). This was due to the pre-programmed cleaning activities of the DyGF units, altered only some times for abrupt water quality changes or flow requirements by the subsequent treatment stage. The mean filtration rates during each test period will be used in the following sections to identify the DyGF units. Results in table 3.7 also show that mean filtration rates are all above 51% of the maximum (initial) filtration rates. Furthermore, these percentages were in the range of 68 to 83 % when initial filtration rates were in the range of 1.1 to 2.4 mh -1 . Therefore, lower filtration rates (≤2.4 mh -1 in the case of Cauca River water) should produce less than 32% overflow water (Q e in figure 2.19) during a filter run cycle due to filtration rate’s decline. 91

3.2.2.1. Water quality changes in DyGF units. During the 1 st test period (March-July 1991) the filterability test in the raw water produced a mean filtered water volume of 32 ml with a SD of 17 ml, meanwhile these values were increased to 132±83 and 68±53 after DyGF-A (0.9 mh -1 ) and DyGF-C (1.4 mh -1 ) respectively. These values represent on average an increase of 112 to 312% in filterability as measured by the test. The pH, total alkalinity and total hardness remained practically the same after this 1 st filtration stage. Dissolved oxygen levels in the effluents of the DyGF units were in the range of 3.6 to 5.6 mgl -1 of O 2 , thanks to some aeration taking place after the inlet flow weirs and due to the low biological activity that seems to take place at this treatment stage. High mean removal values were observed for total iron (50 to 54%) and manganese (41 to 45%) considering the integrated effluent of all DyGF units along the four testing periods. These removal values are associated mainly with the filtration of oxidised forms of iron and manganese. Low mean removal values were observed for volatile solids (in the range of 6 to 15%), and colour (11 to 16%), which, together with the higher removal values for iron and manganese, suggest that colour may be mainly associated with natural organic matter (NOM). The faecal coliform colony forming units (CFU per 100 ml) were removed in the range 32 to 73% (0.2 to 0.6 logarithmic units (log units)) meanwhile the chemical oxygen demand (COD) was reduced in the range of 31 to 57%. These efficiencies could be explained by the CFU and COD being associated with suspended solids (SS), which are efficiently removed in this 1 st filtration stage. Descriptive statistics for five of the monitored parameters are included in table 3.8 and mean removal efficiencies for four of them are reported in table 3.9 Photo 3.5 Professor Lloyd at the inlet side of a DyGF unit during a visit to the MSF pilot system at Puerto Mallarino. 92