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Water treatment

The Cauca River, the

The Cauca River, the main river in the valley, has a length of 1,200 kilometres and drains an area of 60,000 square kilometres, 3,600 of which are located above a dam called Salvajina. Cali is located approximately 143 kilometres below the dam, along the river watercourse. Forests only cover a small area of the mountains because of high slopes or deforestation. Between Salvajina and Cali the Cauca River receives discharges of untreated wastewater from different towns and drainage of agricultural fields. The dam, made to increase bas flow of the river in passing by Cali from 40 m 3 s -1 to 130 m 3 s -1 (average flow is 278 m 3 s -1 ), thus reducing the impact of contamination during dry periods. The impact of the dam, however, is less during the rainy season because several rivers discharge into the Cauca River below the dam thus contributing significantly to its overall discharge and water quality. Figure 3.2 Mean precipitation curves in the Cauca Valley Region 3.1.2 Preliminary studies with MSF pilot units An experimental system with bench scale pilot units was built in Cali in 1986 to establish the potential of coarse gravel filtration (CGF) to overcome water quality limitations of slow sand filtration (SSF) with a highly polluted river in a tropical and Andean environment. The system was fed by gravity with Cauca River water taken from the inlet structure of the main treatment plant of the city, built in a place called Puerto Mallarino. The experimental system comprised three multistage filtration (MSF) parallel lines; each made up of CGF and SSF units working in series. The CGF stage included downflow gravel filtration in series (DGFS), and upflow gravel filtration in layers (UGFL) and series (UGFS). The filtration rates were set at 0.7 mh -1 and 0.15 mh -1 at the CGF and SSF stages respectively. The line with DGFS is illustrated in figure 3.3. Similar gravel beds to those shown in figure 3.3 were used in the case of UGFS, and three layers of gravel with a total depth of 0.6 m were placed in a column 70

in the case of UGFL. Initially, it was planned to control and to monitor the pilot units twice a week. However, due to flow interruptions related to limited instrumentation facilities, small diameters of connecting pipes between the experimental units, and low control frequency, it became necessary to establish a daily control of the experimental system. Finally, during the dry period July-September 1986, it was possible to obtain some reliable data. Figure 3.3 Pilot scale system for DGFS line working in series with an SSF unit at Puerto Mallarino. 1986 Cumulative distributions of turbidity and apparent colour in raw water and CGF and SSF effluents are summarised in figure 3.4. They show that close to 90% of turbidity data were less than 80 NTU for raw water, 15 NTU for the effluents of UGFS and DGFS units, and 30 NTU for the effluent of UGFL. Faecal coliform removals by the experimental lines were in the range of 96% to 99.9%. However, the levels of CFU were still too high in the SSF effluents, in the range 49 to 94 CFU/100 ml for the line with UGFL, 220 to 540 for the line with UGFS and 110 to 170 for the line with DGFS. The impact of fast drainage on the 1 st modules of the UGFS and DGFS pilot units is illustrated in figure 3.5. Based on these and previous results, the following preliminary conclusions were drawn (Galvis et al, 1989). 71