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3.2.4.3. Filtration run

3.2.4.3. Filtration run lengths and headlosses in CGF units CGF run lengths. Cleaning cycle lengths are summarised in table 3.26. As described in table 3.2, CGF lines were subject to total cleaning procedures when their maximum available hydraulic heads were reached and they could not be recovered by partial cleaning procedures. HGF (without hydraulic cleaning facilities) completed its total cleaning cycle after 769 days (2.1 years) of continuous operation due to headloss development close to 15 cm. Total cleaning cycles in all other CGF lines included two periods (table 3.26). During the 1 st period partial cleaning activities (PCA) were implemented once a month, as initially planned. During the 2 nd period cleaning frequency was higher than once a month. With integrated water having mean SS concentration of 40 mgl -1 (table 3.15) feeding CGF lines in this study, CGF units showed total cycle lengths in the range 2.1 (for HGF) to 2.3 years (for UGFS 1 ). Higher total cycle lengths could be possible with vertical flow CGF lines but having more frequent PCA, at least once a week for UGFL and DGFS 3 . Table 3.26. Cleaning cycle lengths in CGF units. Puerto Mallarino, January 1991, April, 1993 Cleaning Cycle UGFS DGFS UGFL MHGF HGF (1) Lengths (days) 1 2 3 1 2 3 Total 848 837 799 827 With planned PCC (2) 704 616 703 769 704 With PCC < 30 days 144 221 96 123 132 • Mean PCC 22 25 25 8 25 24 7 (1) HGF was the first CGF line to reach its maximum hydraulic head available (15 cms) (2) Planned partial cleaning cycle (PCC) frequency was a month (see table 3.2). Headlosses in CGF units. Piezometers were used to measure available hydraulic head at different points of filter bed depths (see figure 3.9) and to calculate headlosses between these points. An overview of minimum and maximum headlosses in CGF units during all test periods is presented in table 3.27. They were measured while the units were undergoing PCA once a month. Clearly, both initial (minimum) and final (maximum) headlosses were becoming greater in time due to higher filtration rates and accumulated solid loads. PCA were sufficient to recover headlosses until the end of test period III when all CGF units show maximum headlosses < 10 cm except UGFL which reached close to 30 cm. Table 3.27. Headlosses in CGF pilot units while partial cleaning activities were implemented once a month. Period Headlosses (cm) (Filtration UGFS DGFS rate) UGFL MHGF HGF UGFS 1 UGFS 2 UGFS 3 DGFS 1 DGFS 2 DGFS 3 I Minimum 0.2 0.2 0.1 0.1 0.3 0.6 0.1 0.1 0.1 (0.30 mh -1 ) Maximum 3.0 0.8 1.3 7.5 1.9 1.9 1.3 1.1 1.2 II Minimum 0.7 0.3 0.4 2.5 0.6 0.8 0.2 0.6 1.0 (0.45 mh -1 ) Maximum 5.7 2.2 2.9 8.6 6.0 2.7 3.9 1.5 3.7 III Minimum 0.8 0.5 1.3 3.3 0.8 2.7 0.6 1.2 1.4 (0.60 mh -1 ) Maximum 11.4 6.2 9.9 29.9 5.9 9.6 5.7 2.0 7.3 IV Minimum 5.5 1.2 2.6 6.0 1.1 7.2 1.5 2.2 4.2 (0.75 mh -1 ) Maximum 26.4 7.9 14.8 46.1 7.0 10.1 25.7 3.9 10.5 117

Headlosses in HGF were 14 cm at the end of its total cycle length. Wegelin and Mbwette (1989) recommend final headlosses < 30 cm, but considering that values in the range of 10 to 20 cm should be more frequent. They advise also that filter material should be filled to approx. 30 to 40 cm above effluent’s weir level. Based on three experiences with HGF in Peru (table 2.9), Pardón (1989) advised available hydraulic heads< 10 cm allowing savings in filtration material and structures. Collins et al (1994) reported lower headlosses in CGF units processing K-clay water than those processing K-clay + algae water. In using “conventional” HGF and based on the present experience, the writer would recommend to fill filter material to 20 cm above effluent’s weir level. However, practical decisions should take into account factors such as raw water characteristics, filtration velocities, and the feasibility of including hydraulic cleaning facilities in planning the HGF units. Initial headlosses in vertical-flow CGF units filtering at 0.3 mh -1 were very low in this study. Pardón (1989) did not detect measurable headlosses in DGFS units filtering at 0.3 mh -1 and processing settled river water with turbidities in the range 20 to >300 NTU. During 1 st and 2 nd test periods I and II of the present study, headlosses were practically the same in 1 st and 3 rd compartments of DGFS. During test period III headlosses became highest in the 3 rd compartment (DGFS 3 ) with respect to DGFS 1 and DGFS 2 , showing at the same time the highest individual removal efficiencies in turbidity, faecal coliforms and colour. However, during test period IV this 3 rd compartment required earlier more frequent PCA than the other two compartments (see table 3.26). This evidence suggests that appropriate application of water treatment concepts (section 1.2) should look not only for a good balance in sharing required removal efficiencies between treatment units (or stages) but also in obtaining a good balance in O&M requirements between them. Both UGFS 2 and DGFS 2 had consistently the lowest headlosses and mean removal efficiencies for SS, turbidity, and faecal coliforms in their respective CGF lines during all tested periods. These findings indicate that better design criteria are necessary in packing the gravel beds of 2 nd and 3 rd compartments of UGFS and DGFS to obtain a more balanced distribution between removal efficiencies and O&M requirements in these CGF lines. Final headlosses (before PCA) in all CGF units became gradually higher and more difficult to recover during test period IV, making it necessary to increase PCA’s frequency. However, PCA was not so effective in some of the units at the end of this period as they were in the previous ones in dislodging all the units, this may be due to a deeper penetration of solids load into gravel bed at the end of test period IV. Headloss developments in CGF units. Figure 3.22 presents headloss data from the last filtration run observed in the UGFL during test period I (N°6), with PCA once a month and filtration run N°37 in UGFL from test period IV, PCA had frequency of once a week. Headlosses along gravel beds are presented for different running days (RD) during each filtration run. Figure 3.23 also includes cumulative (%) headlosses along gravel beds at the end of each filtration run. By the end of filtration run N°6 (filtration rate 0.3 mh -1 ), headlosses were small (close to 6 cm) and concentrated mainly (80%) at the bottom (40%) and top (40%) gravel layers. During run N°19 (filtration rate 0.6 mh -1 ) UGFL became almost an “ ideal” 118

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