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Water treatment

mm, and up to a filter

mm, and up to a filter deposit (σ) of about 10 gl -1 , as illustrated in figure 2.15A. Based on this type of study, a filtration model was developed and used as a design tool for the evaluation of different filter layouts, as illustrated in figure 2.15B. The possibility of filter cleaning by fast drainage at 60 to 90 mh -1 was also observed during the filtration tests. Field tests were carried out in Tanzania with one unit built on site (Cross section 1.6x1.0 m, filter length 10 m) and PVC models of 250-mm diameter. The use of HGF contributed to improving the capacity of SSF to treat turbid surface water sources at three different sites. Filter loads (σ) of 35 to 50 gl -1 were recorded, confirming the high silt storage capacity of HGF (Wegelin and Mbwette, 1989). Figure 2.20 Horizontal gravel filter used ahead of SSF in Jedee-Thong, Thailand (Thanh, 1978) Full scale demonstrations of HGF were carried out in several countries in the 1980s, including China, Peru, and Sudan. In China, one plant of 2.8 ls -1 (10 m 3 h -1 ) has two lines, each consisting of a tilted plate settling tank, a HGF, and SSF unit. During the evaluation period raw water turbidities were in the range of 20 to 190 NTU; the HGF units operated at a filtration rate of 1.7 mh -1 , and were cleaned by drainage every 40 days. After 5 months of operation, head loss in the SSF units were about 0.3 m (Wegelin et al, 1991). In Sudan, one plant, built as part as the Blue Nile Health Project, included an HGF 4.4 m length with three gravel fractions: 2.7 m with gravel of 50 to 25 mm; 0.85 m with 20 to 15mm, and 0.85 m with 10 to 5 mm. The HGF treating 0.06 ls -1 (0.21 m 3 h -1 ), and operating at a filtration rate of 0.30 mh -1 , reduced the water turbidity from the range of 40 to 400 NTU, to the range of 5 to 50 NTU. The E. coli counts were reduced from levels > 300 CFU, to levels

Table 2.9 Data about three experiences with HGF in Peru (adapted from Pardón, 1989) Parameters Communities Cocharcas La Cuesta Compin Communities Altitude (m) 3,280 1,800 1,370 Temperature (ºC) 12 (0-25) 22 (10-25) 24 (10-30) Population (N o ) 624 385 1,500 Design criteria Turbidity (NTU) 10-500 10-500 10-500 Flow (m 3 d -1 ) 103 50.4 180 Filtration rate (mh -1 ) 0.6 0.5 0.75 Gravel fraction length (m), and (gravel size range in mm) 3 (38-25); 2 (25-13); and 1 (13-6) 2 (38-25); 1 (25-19); 1 (19-13); and 1 (13-6) 3 (38-25); 2 (25-19); 2 (19-13); and 1 (13-6) Turbidity leaving the HGF units (NTU)

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