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Water treatment

deep bed filter (data

deep bed filter (data not included in figure 3.22), with uniform headlosses distribution along the whole filter bed. At the end of filtration run N°37 (filtration rate 0.75 mh -1 ), headlosses were high (close to 50 cm) but only 20% were taking place in the bottom and top gravel layers. Therefore PCA, based on surface shovelling and bottom drainage (table 3.2), were more or less successful until headlosses became concentrated in one the intermediary gravel layers (period IV), between piezometers P 2 and P 1 (see figure 3.9). Headloss (cm) A 1 7 6 5 4 3 2 1 0 0 RL UGFL Run 6 Vf = 0.30 mh -1 P1 (0.3) P2 (0.6) P3 (0.9) P4 (1.2) Piezometer No. (depth in m) RD1 RD7 RD15 RD21 RD26 RD30 P5 (1.55) Bottom Headloss (cm) B 1 60 50 40 30 20 10 0 0 RL UGFL Run 37 Vf = 0.75 mh -1 P1 (0.3) P2 (0.6) P3 (0.9) P4 (1.2) Piezometer No. (depth in m) RD1 RD6 RD7 P5 (1.55) Bottom Cumulative % of headloss 100 80 60 40 20 UGFL Run 6 Vf = 0.30 mh -1 Cumulative % of headloss 100 80 60 40 20 UGFL Run 37 Vf = 0.75 mh -1 A 2 0 0 RL P1 (0.3) P2 (0.6) P3 (0.9) P4 (1.2) Piezometer No. (depth in m) P5 (1.55) Bottom B 2 0 0 RL P1 (0.3) P2 (0.6) P3 (0.9) P4 (1.2) Piezometer No. (depth in m) P5 (1.55) Bottom Figure 3.22. UGFL headloss developments through gravel bed depth in different running days (RD) of filtration runs N°6 (A 1 ) and N°37 (B 1 ). Cumulative (%) headloss through gravel bed depth at the end of filtration runs N°6 (A 2 ) and N°37 (B 2 ) Figure 3.23 presents headloss data from the 1 st compartment (UGFS 1 ) of UGFS. Headlosses at the end of filtration run N°6 (filtration rate 0.3 mh -1 ) were very low (

concentrated in the intermediate gravel layers. Surface and bottom cleaning activities used in this study allowed some of the previously identified limitations of DGF to be overcome, originating from the necessity of transporting stored sludge in the upper part of the gravel bed to the drainage system during hydraulic cleaning (e.g. Fox, 1990; Collins et al, 1994). Headloss (cm) A 1 1 0.8 0.6 0.4 0.2 0 0 RL UGFS 1 Run 6 Vf = 0.30 mh -1 P1 (0.3) P2 (0.6) P3 (0.9) P4 (1.2) Piezometer No. (depth in m.) RD1 RD7 RD15 RD24 RD30 P5 (1.55) Bottom A 2 Piezometer No. (depth in m.) Bottom B 2 B 1 100 UGFS 1 80 Run 6 Vf = 0.30 mh -1 60 40 20 0 0 P1 P2 P3 P4 P5 RL (0.3) (0.6) (0.9) (1.2) (1.55) Figure 3.23. Cumulative % of headloss UGFS 1 Run 27 Vf = 0.75 mh -1 Headloss development through gravel bed depth in the first compartment of UGFS (UGFS 1 ) in different running days (RD) of filtration runs N°6 (A 1 ) and N°27 (B 1 ). Cumulative (%) headloss through the gravel bed depth at the end of filtration runs N°6 (A 2 ) and N°27 (B 2 ) In summary, headloss development in vertical flow CGF units seems to have a pattern. Removed solids initially accumulate mainly at the bottom (in UGF) or top (in DGF) gravel layers. Gradually (with higher filtration rates during the present experience) removed solids (headlosses) become more evenly distributed throughout the gravel bed. Finally headlosses tend to concentrate in intermediary gravel layers where the PCA, used in this study, is less effective. Comparing all vertical flow CGF options included in this study, this headloss development pattern is fastest in UGFL and slowest in UGFS. More frequent and efficient application of the PCA should defer the need for total cleaning, which nevertheless becomes unavoidable after some years of continuous operation of CGF units. Initial drainage velocities during partial cleaning activities of CGF units. During this study initial drainage velocities were in the range 15 to 20 mh -1 , combined with sequential opening and closures of drainage valves to induce changes in the flow pattern inside the lower part of the gravel beds. Pardón (1989) reports higher initial drainage velocities, in the range of 60 to 90 mh -1 . In full-scale studies in Peru with DGFS and HGF, he reported cleaning efficiency of stored solids of 21% in DGFS and 40% in HGF. After socio-economic Headloss (cm) Cumulative % of headloss 50 40 30 20 10 0 100 80 60 40 20 0 RL 0 0 RL P1 (0.3) P2 (0.6) P3 (0.9) P4 (1.2) Piezometer No. (depth in m.) RD1 RD4 RD9 RD19 RD24 RD30 UGFS 1 Run 27 Vf = 0.75 mh -1 P1 (0.3) P2 (0.6) P3 (0.9) P4 (1.2) Piezometer No. (depth in m) P5 (1.55) Bottom P5 (1.55) Bottom 120

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