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galvis

Water treatment

Mean faecal coliform

Mean faecal coliform removal efficiencies seem to be higher for the lower filtration rates. The statistical significance of these differences will be considered in the following section. At least in the case of highly contaminated water, it seems likely that DyGF could contribute to the multistage water treatment concept (section 1.2) in reducing acute risks associated with drinking water. Further consideration will be given to this possibility when dealing with less contaminated water sources in full scale projects. The highest removal efficiencies in DyGF are for SS (table 3.9). The removal efficiencies are higher for the lower filtration rates in each test period. However, there is not a clear explanation for the high efficiencies found during periods II and III which presented low mean SS concentration in raw water, at least the intermediate partial cleaning run lengths (3.6 days) during these periods had benefited SS removal during these periods. Another possibility that was not tested in this study is that particle size distribution during periods II and III had a lower proportion of smaller particles. A similar tendency to that observed for SS was also found for mean turbidity removal efficiencies but with lower removal values. These lower values could be explained by assuming that the DyGF units remove preferentially bigger particles from the raw water with a more major impact in the SS results than in the turbidity measurements. Table 3.8 Descriptive statistics for several parameters in raw water and effluents of DyGF units working with different mean filtration rates (1991 – 1993). Raw Raw PERIOD I PERIODS II and III PERIOD IV Statistics Raw DyGF DyGF DyGF water 0.9 mh -1 1.3 mh -1 1.4 mh -1 water 1.4 mh -1 1.9 mh -1 2.6 mh -1 water 1.7 mh -1 1.9 mh -1 2.8 mh -1 Faecal Coliforms (CFU/100ml) Mean 39,619 18,521 14,683 26,513 56,116 13,796 13,743 20,246 99,788 33,207 32,742 50,434 SD 31,744 32,519 12,534 39,482 56,896 27,172 26,140 289,79 99,076 43,845 44,938 77,706 Min 6,200 800 818 1,300 2,700 850 1,018 1,100 12,091 2,267 3,300 3,667 Max 117,000 158,000 49,000 204,000 300,000 218,000 231,000 240,000 500,000 254,000 270,000 410,000 N (data) 31 31 31 31 84 84 84 84 42 42 42 42 Turbidity (NTU) Mean 113 63 73 74 56 31 32 34 69 42 42 47 SD 107 61 72 74 55 35 37 40 76 53 53 59 Min 29 16 15 15 17 5.8 6.4 6.0 12 3.7 3.0 4.3 Max 750 400 450 500 500 320 320 360 900 600 650 700 N (data) 311 311 311 311 835 835 835 835 691 691 691 691 Suspended Solids (mgl -1 ) Mean 197 46 73 85 84 17 21 24 118 50 56 69 SD 192 55 99 123 97 24 34 41 182 101 118 142 Min 53 3.2 4.3 4.3 17 2.2 2.2 2.4 11 1.7 1.7 2.0 Max 978 279 522 625 696 160 214 270 886 432 558 656 N (data) 33 33 33 33 84 84 84 84 42 42 42 42 Colour (PCU) Mean 75 63 66 65 47 40 41 40 67 58 57 58 SD 54 46 47 47 25 21 20 19 48 44 44 45 Min 24 18 19 18 16 12 13 13 16 13 13 13 Max 230 240 220 220 190 160 140 140 190 184 188 188 N (data) 33 33 33 33 82 82 82 82 42 42 42 42 Total Iron (mgl -1 ) Mean 6.00 3.73 3.63 3.37 3.90 1.44 1.65 1.57 3.68 2.03 1.98 2.11 SD 2.65 2.39 1.70 1.90 1.21 0.72 0.80 0.83 2.85 2.58 2.20 2.29 Min 4.00 1.80 2.00 1.40 2.00 0.50 0.80 0.58 0.61 0.30 0.45 0.51 Max 9.00 6.40 5.40 5.20 6.00 2.90 3.40 3.20 10.10 8.70 7.60 7.90 N (data) 3 3 3 3 10 10 10 10 9 9 9 9 93

Table 3.9 Mean removal efficiencies of suspended solids, turbidity, colour and faecal coliforms in DyGF units with different mean filtration rates. Mar. 1991-Feb. 1993 Sampling point Suspended Solids Turbidity Colour Faecal Coliforms Mean Efficiency Mean Efficiency Mean Mean Efficiency (mgl -1 ) (%) (NTU) (%) (PCU) Period I Efficiency (%) (CFU/100 ml) (%) Log. units Raw Water 197.3 113.8 62.3 39,619 DyGF A (Vf =0.9 mh -1 ) 46.4 79.4 62.8 45.3 51.2 15.4 18,521 58.6 0.33 DyGF B (Vf =1.3 mh -1 ) 72.5 69.7 72.3 37.8 55.6 12.0 14,683 56.0 0.43 DyGF C (Vf =1.4 mh -1 ) 85.4 66.2 74.3 36.5 51.9 13.3 26,513 31.6 0.17 Period II and III Raw Water 83.9 55.6 46.5 56,116 DyGF B (Vf =1.4 mh -1 ) 16.9 81.0 30.6 47.3 40.4 12.4 13,796 69.4 0.61 DyGF A (Vf =1.9 mh -1 ) 21.0 78.3 32.3 44.7 40.9 11.2 13,743 72.6 0.61 DyGF C (Vf =2.6 mh -1 ) 23.9 75.9 34.1 41.8 40.4 11.6 20,246 55.1 0.44 Period IV Raw Water 118.1 68.6 66.7 99,788 DyGF B (Vf =1.7 mh -1 ) 49.9 67.3 42.3 43.7 58.0 14.8 33,207 65.1 0.48 DyGF A (Vf =1.9 mh -1 ) 55.8 67.2 42.2 43.4 57.0 16.4 32,742 65.4 0.48 DyGF C (Vf =2.8 mh -1 ) 69.1 59.0 46.9 37.7 58.5 12.8 50,434 51.9 0.30 3.2.2.2 Comparative analysis of filtration rates in DyGF units. The ANOVA methodology was used to decided on the following (null) hypothesis (H 0 ): there are not significant statistical differences between the mean removal efficiencies of SS, turbidity, faecal coliforms, and colour when the DyGF units worked with distinct filtration rates (treatment levels). The ANOVA technique and Tukey test applications are included in Annex 4 for the case of mean removal of SS during test period I. Tables 3.10 and 3.11 summarise the comparative analyses made for SS and faecal coliform removals respectively. The findings in table 3.10 show that mean SS removal efficiencies are affected (with 1% significance) by tested filtration rates in DyGF units. In harmony with filtration theory, the efficiencies are less when the filtration rates are higher. However, it seems to be that these mean filtration rates should differ by at least 0.3 mh -1 to have a significant impact. The application of ANOVA and Tukey test with the turbidity data, produced the same results as those presented in table 3.10, meaning the rejection of the null hypothesis and the same hierarchical output for the tested filtration rates. According to the findings in table 3.11, tested filtration rates also affect (with 1% significance) the mean faecal coliform removal efficiencies in the DyGF units. But in this case, the efficiencies are less sensitive to the filtration rates than those observed in the case of SS (table 3.10) or turbidity. In contrast with the previous findings with SS, turbidity, and faecal coliform removals, the application of the ANOVA technique shows that the tested filtration rates did not have a statistically significant (1%) impact on the mean colour removal efficiencies. Unsurprisingly, the DyGF units presented poor colour removal efficiencies for all the tested filtration rates. 94

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