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Water treatment

than in sand samples

than in sand samples from other SSF units. This tendency is similar in the sand samples taken after scraping the SSF units although less intense. Silt test values obtained at the end of SSF runs in this study were all >20%, the value reported by Lloyd (1974) for filter skins before scraping procedures. Only SSF 1 was around this value during test period II. All SSF units in this study, except SSF 4 (after HGF), had silt test results at the beginning of SSF runs ≤5%. During test period IV SSF 4 was the only SSF unit showing silt test values ≤5%. Table 3.28. Silt test (silt volume/sand volume) results (%) in samples collected from sand bed surface of SSF units at the end (before scraping) and the beginning (after scraping) of filtration runs. Period Mean ± SD before scraping Mean ± SD after scraping SSF 1 SSF 2 SSF 3 SSF 4 SSF 5 SSF 1 SSF 2 SSF 3 SSF 4 SSF 5 II 21 ± 12 23 ± 6 24 ± 12 40 ± 41 ± 12 3 ± 1 5 ± 2 3 ± 1 6 ± 1 3 ± 1 III 34 ± 13 45 ± 17 52 ± 22 39 ± 5 43 ± 27 5 ± 2 6 ± 2 6 ± 2 6 ± 2 6 ± 2 IV 33 ± 7 37 ± 8 36 ± 10 31 ± 8 33 ± 8 6 ± 3 7 ± 2 7 ± 4 5 ± 2 8 ± 4 These results, like those found when comparing mean turbidity removal efficiencies and headloss developments in CGF units, suggest that UGFS develops headloss and becomes more stable faster than HGF and DGFS. But on medium or long-term basis HGF and (less clearly) DGFS seem to become more efficient, protecting their SSF units better. However, both “ conventional” HGF and DGFS seem to require more frequent total cleaning cycles. 3.2.5. Specific studies on CGF lines working in series with SSF units. Based on the previous results more specific studies were made to improve our understanding and the design criteria for CGF working in series with SSF units, during the 2 nd phase of this study. Parts were carried out with the participation of M.Sc. students. Those studies aimed to compare HGF with UGFS are included in this section. In general, during the 1 st phase, HGF did not perform better than UGFS, despite having longer theoretical hydraulic retention time (T 0 ). This suggests the possibility of improving its hydraulic behaviour and removal efficiencies by dividing its main compartment into smaller units operating in series (HGFS), following the theory of hydraulic behaviour of reactors (Hudson, 1981; Galvis and Perez, 1985). The present comparison includes aspects of treatment efficiency, hydraulic performance and operation and maintenance requirements. Changes were made in the MSF pilot system to carry out these more specific studies. They are shown in figure 3.26. Three horizontal flow compartments hydraulically independent and working in series (HGFS) replaced the MHGF line. Gravel bed changes were also made aiming to test the CGF lines under higher filtration rates or contamination levels than those tested in the previous phase of this study. Total gravel bed lengths were kept similar to those used in previous phase but thicker gravel bed layers in the smaller size ranges were packed into the CGF units. The new HGFS line was packed with gravel bed layers having gravel size in the same ranges as those used in UGFS. Table 3.29 summarises gravel size ranges and layer lengths used in the CGF lines included in these additional studies. 123

Figure 3.26. Schematic view of MSF pilot system used to compare UGFS, HGFS, and HGF. The HGF line was optimised by including a drainage system consisting of manifolds and fast opening drainage valves in each compartment, similar to drainage facilities included in the other CGF units. Whereas other water quality parameters were measured following the sampling frequency and analytical procedures described in section 3.1 these studies concentrated on suspended solids, turbidity, and faecal coliform removal. Table 3.29. Gravel bed specifications for CGF lines during specific studies. 1994-1996. Parameter UGFS (1) HGFS (2) HGF (2) DGFS (1) UGFS 1 UGFS 2 UGFS 3 HGFS 1 HGFS 2 HGFS 3 HGF 1 HGF 2 HGF 3 DGFS 1 DGFS 2 DGFS 3 Gravel size ranges Gravel layers length (m) 19 - 25 0.30 0.15 (3) 0.10 (3) 0.30 0.15 (3) 0.10 (3) 13 - 19 1.25 0.15 0.10 2.1 3.27 1.25 0.15 0.10 6 - 13 0.625 0.10 1.15 0.1 2.32 0.625 0.10 3 - 6 0.625 0.10 0.6 1.0 0.625 0.10 1.6 - 3 1.15 0.55 0.55 1.15 Useful bed length (m) 1.55 1.40 1.45 2.1 1.15 1.15 3.27 2.32 1.55 1.55 1.40 1.45 Total useful bed 4.40 4.40 7.14 4.40 length (m) (1) Both UGFS and DGFS had 3.14 m 2 cross sectional area (2) Both HGFS and HGF had 1.57 m 2 cross sectional area (3) The main role of this layer, placed before finer gravel layers in the flow direction, is to support the useful gravel layers. The water volumes used for optimised partial cleaning activities (OPCA) were measured. The water volumes for cleaning were established on the basis of a weekly hydraulic cleaning and one more intense monthly cleaning. Weekly cleaning consisted of opening and closing the fast drainage valve ten times and then leaving it open until effluent water became clear. In the monthly cleaning, this procedure was repeated twice and also the top gravel layers of UGFS units were moved with a shovel until the upflow wash water was clarified. Wash water volumes used during OPCA were expressed in m 3 per m 2 of gravel bed (m 3 .m -2 ). 124

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