8 months ago


Water treatment

In spite that headloss

In spite that headloss development and water quality deterioration were not detected, two cleaning operations by fast drainage were conducted in the period March-April 1985. Design drainage velocities of about 60 to 90 mh -1 were reached by using a specially designed fast opening gate of 400 cm 2 (annex 1) connected to an underdrain in the bottom of each gravel bed. However, the best solids removal efficiency obtained was 21%. Then, Pardón (1989) considers that scour velocity alone may be not sufficient for an efficient removal of deposited solids. However, by 1989 the DGF system of Azpitia had completed 5 years of continuous operation with this regularly performed hydraulic cleaning procedure. Using up and downflow short filter cells (60 cm) run in parallel and loaded with K-clay solids, Collins et al (1994) found that drainage velocity was the main variable in determining solids removal by drainage. In general, the highest removal rates (82 to 98%) were observed in cells drained at 50 mh -1 . Drainage velocities of 10 and 30 mh -1 provided average removals of 64 and 67% respectively. Collins et al (1994) also made hydraulic cleaning studies in a pilot DGF column of 20-cm diameter after 90 days of continuous operation with a mixture of natural water and K-clay suspension. The column was packed with 90 cm of gravel average size of 7.94 mm on top, 30 cm of gravel of 5.55 mm in the middle, and 30 cm of gravel of 2.68 mm. The column was predrained three times (one bed volume each time at 24.5, 16, and 13 mh -1 respectively), allowed to dry during 28 days, then predrained twice, and flushed at 10 mh -1 for five bed volumes, and then at 20 mh -1 for another five bed volumes. Overall cleaning efficiency was only 6.4%. The configuration should have contributed to this low cleaning efficiency, since the filter was constructed of three media ranging coarse to fine but in one column. During flushing, solids removed from the coarser fraction were collected at the top of the finer adjacent fraction hindering overall solids removal (Collins et al, 1994). In a typical DGF system each main gravel fraction composes a separate filter bed. 2.8.6 Upflow gravel filters in layers (UGFL) and series (UGFS) Antecedents. The apparatus developed for J. Peacock (section 2.1.1) for water filtration, with upflow to clarify the water and reverse flow to clean the filter medium, is one of the first documented antecedents of an UGFL. During the 1800s upflow filters, some as special types of upflow SSF, were built in England, France, Scotland and the USA (Baker, 1981). Upflow rapid filters (URF) have been used extensively in Russia for clarification of water with low levels of turbidity and organic matter (Azevedo, 1987). URF was introduced in Brazil during the 1960s, and Colombia during the 1970s. These experiences with URF contributed to stimulate the initial studies of the author on upflow coarse filtration in combination with SSF at the Cinara Institute of the Universidad del Valle in Colombia. Upflow Gravel filtration and SSF. Pilot and full scale studies with UGFL and UGFS were developed in Colombia, during the 1980s (Visscher and Galvis, 1987; Galvis and Visscher, 1987). Pilot studies with UGFL were later developed in Brazil (Di Bernardo et al, 1988; Di Bernardo, 1993). During the 1990s the studies continued at Cinara (Galvis et al, 1993; Wegelin et al, 1997; Galvis et al, 1998). Based on the experience with UGFL in Colombia, 60

the HGF units of Aesch (see table 2.8) were replaced by UGFL because the UGFL (1 m length) showed higher performance in solids removal and hydraulic cleaning (Wegelin et al, 1991; Wegelin, 1996). The experience in Colombia also stimulated the construction of UGFS prototypes in Pakistan (Latorre and Manzoor, 1995 and 1996) and Saint Lucia (Eudovique, 1992), as well as the beginning of pilot studies with UGFS in Brazil (Di Bernardo et al, 1999). Upflow Gravel filtration (UGF) systems consist of one or more units. In systems of one unit (UGFL), the gravel is placed in layers of different grain sizes ranging from coarse at the bottom to fine at the surface (Figures 2.10 and 2.23). In systems in series (UGFS), each module is filled mainly with a gravel size, starting with coarse grains in the first and fine in the last (Figures 2.10). Figure 2.23 Layout of upflow gravel filter in layers (UGFL). The height of the filter unit depends on the height of the gravel bed, the height of the support material, the level of supernatant water, the height of additional water to improve hydraulic washing, and the freeboard. The total heights of the UGF units are usually 2.5 >2.5 Initial fast drainage velocity for cleaning (mh -1 ) >10 >10 Filter bed area per unit (m 2 )