10 months ago


Water treatment

the capacity to deal

the capacity to deal regularly with turbidities up to 50 NTU and higher turbidities during short periods; and cleaning simplicity, basically daily surface raking (about 5 cm) to prevent excessive clogging. Removal and replacement of the filter media is usually accomplished every 6 to 12 months. The main mentioned disadvantages are that the source of supply must have a capacity >10 times the treated water; and that the DySSF efficiency is lower than the conventional SSF (Arboleda, 1973), although there have been no conclusive studies supporting this assumption (Shulz and Okun, 1992). During performance studies (Rodriguez, 1977, quoted by Shulz and Okun, 1992) rates of filtration at the beginning of the filter runs were above 0.4 mh -1 , dropping to 0.25 and 0.1 mh -1 after 60 and 80 hours of operation. The second experience was with a type of river dam filtration system, or dynamic filtration intakes (DyFI), used to improve surface water quality in small communities of the mountainous region of the Cauca Department of Colombia (Salazar, 1980). The system consists of gravel bed in a triangular shape at a slope around 3:1, supported on a small concrete or masonry dam (figure 2.16B). Periodic raking of the surface of the gravel bed is necessary to maintain filtration capacity. Removal efficiencies for DyFI have been reported as high as 98% from rivers with turbidity levels in the range of 48 to 200 NTU (Salazar, 1980). However, performance during periods of high discharges in the rivers was poor because frequent cleaning (sometimes every three days) was needed, and during such periods the upper part of the filter beds was washed away (Wolters, 1988). In a field evaluation the turbidity removal efficiencies were found to be around 20% (Cinara and IRCWD, 1989). This was may be due to some type of preferential flow channels or by-passes originating difficulties to implement the periodic cleaning or reposition of the clogged filtering material during the rainy season. Since construction of this type of DyFI is likely to be costly in rural areas, and because of the maintenance difficulties, this design (figure 2.16B) was recommended only for construction on existing dams, and treating waters of low to moderate turbidity levels (Smet et al, 1989). Figure 2.16 Schematics views of (A) a DySSF, dynamic slow sand filter (Arboleda, 1973), and (B). DyFI, Dynamic (or river dam) filtration intake (Salazar, 1980; Smet et al, 1989). The previous experiences were useful to the author in designing a modified version of a DyFI. This new version aimed at overcoming the problems associated with high turbidities of a few hours duration in an upland river called "Chorro de Plata", which presents low levels of contamination. The coarse intake filter was built in a regulated derivation channel of this stream (photo 2.1 and figure 2.17) and represents the only pretreatment stage of two SSF units working 48

in parallel (Galvis, 1983; Smet et al, 1989). Under normal operation conditions, 5 ls -1 are treated in the SSF units. After short periods of high turbidity levels, the abstracted flow is quickly reduced making it necessary to clean the coarse filter. To do this the inlet control valve is closed; then the filter surface is raked, starting in the upstream side and following the flow direction; and finally, the inlet control valve is opened. All these activities usually take around 30 minutes. Then the filtrate can be run to waste for some time, if the quality of the abstracted water make it necessary. The experience of DyFI in "Chorro de Plata" has been successful, and is still working after around 15 years of continuous operation. Photo 2.1 Modified version of a dynamic filtration intake (DyFI) at the "Chorro de Plata" water treatment plant. Cali, Colombia. Figure 2.17 Layout of the Dynamic filtration intake (DyFI) at "Chorro de Plata" (Galvis, 1983). 49