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Water treatment

Table 3.24 Comparative

Table 3.24 Comparative analyses of mean faecal coliforms removal efficiencies in CGF. (a) F. coliforms (CFU/100 ml) in raw water (mean ± SD) 24,758 ± 31,500 8,843 ± 8,668 16,823 ± 22,374 26,226 ± 34,647 CGF lines Descriptive statistics Decision on Ho (a) Hierarchical levels Removal efficiencies (%) Based on F-Test Based on Tukey Data Standard and ANOVA Mean Test (α=1%) (N) deviation (SD) technique (α=1%) Period I (0.3 mh -1 ) UGFS 3 99.39 1.2 HGF 3 (1) DGFS 3 98.76 1.9 DGFS 3 (1) MHGF 3 93.36 6.2 MHGF 3 (2) HGF 3 31 98.51 1.8 Ho is rejected UGFS 3 (1) UGFL 97.96 1.7 UGFL (1) Period II (0.45 mh -1 ) DGFS 3 99.43 0.5 DGFS 3 (1) UGFS 3 99.34 0.9 UGFS 3 (1) MHGF 3 83.07 4.6 MHGF 3 (3) HGF 3 53 98.76 1.5 Ho is rejected HGF 3 (1) UGFL 94.67 3.8 UGFL (2) Period III (0.6 mh -1 ) DGFS 3 99.38 0.5 DGFS 3 (1) UGFS 3 99.36 0.7 UGFS 3 (1) MHGF 3 93.92 3.8 MHGF 3 (2) HGF 3 30 98.71 1.5 Ho is rejected HGF 3 (1) UGFL 94.73 3.2 UGFL (2) Period IV (0.75 mh -1 ) UGFS 3 99.21 0.8 UGFS 3 (1) DGFS 3 99.20 0.8 DGFS 3 (1) MHGF 3 89.87 10.9 MHGF 3 (2) HGF 3 29 99.16 0.8 Ho is rejected HGF 3 (1) UGFL 92.53 6.3 UGFL (2) Ho: The mean faecal coliforms removal efficiencies are statistically the same for all coarse gravel filtration alternatives. Table 3.25 Comparative analyses of mean colour removal efficiencies in CGF lines. (a) Colour (PCU) in raw water (mean ± SD) 72 ± 54 46 ± 23 30 ± 11 48 ± 32 CGF lines Descriptive statistics Decision on Ho (a) Hierarchical levels Removal efficiencies (%) Based on F-Test Based on Tukey Test Data Standard and ANOVA Mean (α=1%) (N) deviation (SD) technique (α=1%) Period I (0.3 mh -1 ) UGFS 3 66.63 15 UGFS 3 (1) HGF 3 63.38 16 HGF 3 (1) DGFS 3 34 55.93 17 Ho is rejected DGFS 3 (2) MHGF 3 47.45 18 MHGF 3 (3) UGFL 46.31 18 UGFL (3) Period II (0.45mh -1 ) UGFS 3 53.49 14 UGFS 3 (1) HGF 3 50.10 13 HGF 3 (1)(2) DGFS 3 51 47.20 17 Ho is rejected DGFS 3 (2) MHGF 3 37.08 13 MHGF 3 (3) UGFL 27.79 10 UGFL (4) Period III (0.6 mh -1 ) UGFS 3 55.34 13 UGFS 3 (1) HGF 3 54.23 11 HGF 3 (1) DGFS 3 30 48.42 10 Ho is rejected DGFS 3 (2) MHGF 3 41.26 123 MHGF 3 (3) UGFL 33.64 11 UGFL (4) Period IV (0.75 mh -1 ) HGF 3 67.82 16 HGF 3 (1) UGFS 3 63.53 17 UGFS 3 (1-2) DGFS 3 30 59.17 18 Ho is rejected UGFS 3 (2) UGFL 3 45.16 22 UGFL (3) MHGF 44.73 19 MHGF 3 (3) Ho: The mean colour removal efficiencies in are all statistically the same for all CGF alternatives. 115

The results shown in table 3.22 to table 3.25 could be due to shorter actual hydraulic retention times presented in MHGF alternative when compared to UGFS and DGFS alternatives, since it was observed that short peak loads of turbidity show up in MHGF effluent before they do in the effluents of the other two alternatives. This seems to be consistent with the fact that UGFS and DGFS have similar reactor volumes (V) compared with the MHGF option, but distributed in three compartments hydraulically independent, working in series, each one having a volume of V/3 (Hudson, 1981). This hypothesis was validated during the phase of specific studies to be discussed later. Comparative analysis included in table 3.23 shows that statistically UGFS had the best mean turbidity removal efficiency during test period I when all the CGF lines had the lowest tested filtration rate (0.3 mh -1 ) and their gravel beds started to be gradually clogged. As pointed out by Fox (1990), the drift of previously removed material to the bottom of HGF and possibly of DGF units may leave clear passages for the water through the upper media layers affecting their removal efficiencies of finer particles more relevant for turbidity than SS measurements. In UGFS the filter gradually builds up sediment over its entire cross sectional area, i.e. it develops more rapid filter resistance than HGF and DGF, but their treatment efficiency becomes greater. During test periods II and III HGF presented statistically similar mean removal efficiencies compared with UGFS. Finally, during testing period IV, HGF presented the best removal efficiencies. This gradual improvement of HGF line could be explained considering that while UGFS and DGFS were exposed to partial cleaning activities every month, HGF was not. Because HGF was not regularly washed its gravel bed could become gradually more “ mature” and more efficient for removing colloidal particles compared with those that had been partially cleaned periodically. MHGF presented the poorest mean faecal coliform removal efficiencies of all CGF lines during the first two test periods, but it became similar in this respect to UGFL during the last two test periods (table 3.24). In spite of its longer gravel filter bed, HGF shows statistically the same removal capacity as UGFS and DGFS during all tested periods. A similar explanation to that given before in the case of turbidity is also considered feasible in this case. The explanation could be related again to poor hydraulic behaviour of HGF becoming more relevant in this case (faecal coliform removal) than the role of a more “ mature” gravel bed considered before in discussing mean turbidity removal efficiencies. During the first three test periods (table 3.25), DGFS mean colour removal efficiencies were statistically lower than those found in UGFS. In contrast, during the last two test periods DGFS became gradually statistically similar to UGFS in mean colour removal efficiencies but second to HGF. A similar explanation to that suggested before, related to mean turbidity removals, is proposed here in the case of colour removal efficiencies. 116

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