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Water treatment

Flow direction reduces

Flow direction reduces interference due to temperature or density differences. This, together with the possibility of having units in series, should contribute to improve the hydraulic behaviour of the vertical flow coarse filtration systems (DGF and UGF) resulting in a more homogeneous retention time and thus a better treatment process (Hudson, 1981; Galvis and Pérez., 1985). UGFS pilot units operating at 0.7 mh -1 and processing source water turbidities under 100 NTU produced effluent turbidities below 20 NTU. Removal efficiencies between 50 and 70% were reported for raw water turbidities above 100 NTU. Apparent colour removal efficiency ranged from 50 to 80%, and faecal coliform removals were around 90%. An UGFL prototype was built in El Retiro, Cali, Colombia. In this prototype turbidity removal averaged 52% on a relatively low turbidity source, resulting in effluent turbidities < 5 NTU for 90% of samples taken. Faecal coliform reduction was 89%, total iron was 62%, and true colour was 45% (Galvis et al, 1989; Wolters et al, 1989). Collins et al (1994) made hydraulic cleaning studies in a pilot UGFL column of 15-cm diameter after 90 days of continuous operation with a mixture of natural water and K-clay suspension. The column was packed with 60 cm of gravel average size of 5.55 mm on top and 60 cm of gravel of 2.68 mm at the bottom. The column was 1 st predrained once (one bed volume at 37.4 mh -1 ), 2 nd flushed at 10 mh -1 for six bed volumes, and 3 rd flushed at 25 mh -1 for another five bed volumes. Overall stored solids removal efficiency was 32% in this three steps cleaning procedure, 7% in the 1 st step, 25% in the 2 nd , and 7% in the 3 rd . In all the steps, a decrease in cleaning efficiency over time was noted, despite the amount of solids still retained in the filter. Collins et al (1994) considered that a change in flow rate will induce different flow dynamics through the media (degree of turbulence, rerouting) and will continue to dislodge deposits until new flow channels are formed. This consideration would support the current CMF cleaning practice promoted by Cinara in Colombia. This is based on quite low initial flushing velocities (

in spite of the low removal efficiencies initially found by Pardón (1989). Besides, hydraulic cleaning with moderate flushing velocities, together with quick changes of the flush flow pattern (Galvis et al, 1989; Collins et al, 1994) need to be studied as an option to reduce the frequency of manual cleaning or the practice of high initial flushing velocities (Pardón, 1989; Wegelin, 1996). Due to their relative short filter bed length, the vertical flow CMF systems are expected to have lower buffer capacity than HGF to deal with abrupt water quality changes. As far as suspended solids is concerned, it is expected that the innovative combination of this treatment step with DyGF may contribute in overcoming this limitation, in a practical application of the multistage and integrated water treatment concepts. To overcome possible problems with algae, Galvis et al (1989), Di Bernardo (1993), and Wegelin (1996) have recommended covering the upper filtering medium layer with an additional coarse gravel layer to avoid solar radiation on the supernatant. However, this additional layer could limit maintenance if some type of surface cleaning procedure is required to remove settled or strained material at this level. Significant headlosses have been reported after several pilot plant studies (e.g. Perez et al, 1985; Collins et al, 1994). However, Pardón (1989) did not observe significant headlosses in the DGF system of Azpitia. Then if it this happens in other prototypes, the head available above the top gravel layer would be determined mainly by water storage requirements to ensure an appropriate hydraulic cleaning procedure, and not by headloss development along the filter run. 2.9 An Innovative Multistage Filtration Water Treatment Approach The Andean Cauca Valley (ACV) is representative of the high surface water use in the WS systems in Colombia, other valleys in the Andean Region, and possibly of other mountainous areas around the tropics. Two main types of rivers, highland and lowland rivers can be distinguished in the Cauca Valley. The highland rivers receive water from relatively small catchment areas, many of which are facing erosion problems. As a result, the water shows low to moderate levels of turbidity but high turbidity peaks during the rainy periods. Water in these rivers also shows a wide range of microbial contamination because of inappropriate management of human and animal wastes. The lowland rivers receive water from both the highland rivers and untreated sewage from small and large settlements. Thus, the water in lowland rivers has usually higher levels of microbiological contamination, and moderate to high levels of turbidity, still with some abrupt changes during the rainy periods. Figure 2.24 summarises intensive monitoring of three Andean rivers made during the development of the present research work. One of them, the Cauca River, is a lowland river. The curves in Figure 2.24 illustrate the levels of contamination observed in surface waters in the Andean region and the abrupt water quality changes that may occur even in larger lowland rivers during the rainy periods. 63

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