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Water treatment

mgl -1 , and operating

mgl -1 , and operating at filtration velocities in the range of 9 to 1 mh -1 (Galvis and Fernandez, 1991). Although the DyGF is designed primarily to reduce the load of solids, some removal efficiencies are also reported in other parameters. Table 2.7 Preliminary guidelines for the design of dynamic gravel filters (Galvis and Fernandez, 1991; Wegelin, 1996) Main treatment objective Parameter Improve water quality Reduce the impact of peaks of s. solids Filtration velocity (mh -1 ) 0.5-2.0 >5 Filter bed layer: Upper (thickness in m and size in mm) Middle (thickness in m and size in mm) Lower (thickness in m and size in mm) 0.20, and 3-5 0.20, and 5-15 0.20, and 15-25 0.20-0.30, and 1.5-3 0.10, and 3-5 0.10, and 5-15 Surface operating velocity (ms -1 ) Nil or 0.1-0.3 Nil or

case of Dortmund (Germany), the HGF was constructed in the 1960s. The abstracted water is aerated and fed into the HGF. After coarse media filtration (CMF) the water falls over a cascade, percolates through a sand filter bed and finally reaches the aquifer (Kunntschick, 1976). The other European cities included in table 2.8 applied the experience of Dortmund, but used a modified and shorter version of filter beds. European rivers usually present low turbidity levels, and CMF operation is stopped during the short periods of high turbidity, when the continuous supply is guaranteed by the aquifer's storage capacity. Hydraulic cleaning facilities are not included in the HGF used in Europe for groundwater recharge. The total operating period for the long filters of Dortmund is about 5 years, after which the gravel has to be removed, cleaned and replaced. Table 2.8 Use of HGF in artificial groundwater recharge (Wegelin, et al, 1991) Plant Country River Dortmund Aesch Graz W. Germany Switzerland Austria Ruhr Birs Andritzbach Suspended solids (mgl -1 ) Filter Filtration Mean Max length (m) rate (mh -1 ) 8 7 5 20 40 20 50-70 15 10 10 5-8 4-14 Horizontal flow gravel filtration and SSF. In a HGF (figure 2.10), water flows in a horizontal direction through gravel media which decrease in size. The filter media are divided in three or four sections which reduce in length and comprise gravel reducing in size, with coarse gravel in the first section to fine in the last section. The advantage of this system is that the gravel layers can be extended without the need of a higher structure (1-1.5 m). In the search for alternatives to overcome the limitations of SSF to treat turbid surface waters different studies have been realised to assess the potential of HGF. At AIT, the Asian Institute of Technology in Thailand, surface water ranging from 20 to 120 NTU in turbidity was used to test HGF operating at 0.6 mh -1 and working in series with a SSF operating at 0.15 mh -1 . The pilot HGF 5-m length consisted of seven cells with gravel sizes ranging from 15 mm down to 3.4 mm. The coarse filtered water varied from 10 to 20 NTU, and the SSF processing this water developed a headloss of 57 cm after 55 days of continuous operation (Thanh and Ouano, 1977, quoted by Wegelin, 1989). Then, in a field experience reported by Thanh (1978, quoted by Shulz and Okun, 1992), a full scale HGF incorporated six gravel zones in a filter box with a cross section of 2x1 m and a filter length of 6 m (figure 2.20). The unit was designed to treat 0.9 ls -1 (3.2 m 3 h -1 ), at a filtration rate of 1.6 mh -1 . Turbidity removal efficiencies of 60 to 70% were reported with raw water values ranging from 30 to 100 NTU. Hydraulic cleaning facilities are not reported in this experience. Besides, the final coarse gravel cell, after the finest gravel pack, seems to be unreasonable under the current filtration theory revised in sections 2.5.1 and 2.8.2, and particularly to the gravel fraction packing criteria illustrated in figure 2.15. During the first half of the 1980s extensive filtration tests were conducted in Switzerland (Wegelin et al, 1986; Wegelin et al, 1991; Boller, 1993) using short filter cells and suspensions of kaolin clay (K-clay). The studies indicated fraction removal efficiencies per cm of flow length (λ) of 0.003 to 0.04 cm -1 , for filter grain sizes in the range of 15-25 to 1.5-2 53