10 months ago


Water treatment

Parts of the suburban

Parts of the suburban settlements of Cali are not being supplied with drinking water from the centralised network system of the city. These settlements include communities and institutions established in a hilly area in the south-west of the city. They use surface water from highland rivers/streams to feed, by gravity, their independent small WS systems. During the early 1980s, some of these systems included conventional rapid filtration treatment plants performing poorly due to design and operational limitations. A pilot plant was built in the premises of one of these systems called Retiro (photo 4.6). Initially, it was planned to control and to monitor the pilot systems twice a week. However due to interruptions in filtered water flow and low control frequency, it became necessary to establish a daily control of the experimental system. In spite of the support obtained from local organisations to control the systems built in the coffee region, it was not possible to obtain reliable data with them. Finally, during the dry period July – September 1986, with the support of undergraduate students in sanitary engineering, it was possible to obtain some reliable data with the pilot plant located in El Retiro. Photo 4.6 Pilot scale system built in Retiro, Cali, Colombia, 1986. The results obtained in Retiro show that close to 90% of turbidity data were below 8 NTU for raw water, 2.2 NTU for effluents of UGFS and DGFS lines, and 1.2 NTU for effluents of SSF units. Faecal coliform removals by the experimental CGF stage (UGFS or DGFS) were in the range 70 to 99.0 %. In spite of the limitations of this experimental experience, the results obtained with the pilot units were better than those produced by the poorly designed and operated full scale RF filtration plant. The community decided to invest in modifying the conventional water treatment in Retiro to convert to a MSF having plain sedimentation, UGFL as CGF stage, and SSF. The first phase of the plant was built by January 1987, keeping the option of introducing more robust pre-treatment alternatives, depending on raw water quality 139

during the rainy periods. Being near to the main base of Cinara, the plant was monitored intensively during the period April-June 1987, in spite of budget limitations (figures 4.1, 4.2, and photo 4.7). Figure 4.1 Inlet flow and headloss developments in an UGFL stage at the Retiro water treatment plant. Cali, Colombia. April-May, 1987. The results obtained are summarised below, based on Galvis et al (1989). • The first phase of the plant was designed to treat 9 ls -1 . However, the plant was initially overloaded due to problems in the distribution system and the water consumption pattern in the community. These problems were gradually solved, including the introduction of water meters. • Three UGF units operated in parallel during this phase. Each designed to process 3 ls -1 at 0.64 mh -1 . Gravel bed depth in each unit was 0.7 m, having layers with gravel bed size in the range of 25 to 3 mm. Maximum permissible headloss before partial cleaning activities (PCA) was 0.3 m (figure 4.1). • PCA were performed every week approx. Effluent valves were closed and supernatant water was allowed to rise until there was only 0.05 m of freeboard. Hydraulic cleaning was performed with successive closures of the fast drainage valve. Initial drainage velocity was around 20 mh -1 . Upflow was then introduced again into the units. Then, the top gravel layers were vigorously mixed with a shovel while the influent upflow was being eliminated through plugged holes, previously opened. Afterwards the units were put back into operation. These PCA take 1.5 h approx. • Two SSF units were included in this phase. Each having a mean surface area of 110-m 2 approx., and designed to process 4.5 ls -1 at 0.15 mh -1 . Each unit is out of operation approx. 6-h during PCA (scraping activities), including supernatant drainage, scraping and back-filling of the unit. This was undertaken by four men, two doing the scraping and two transporting the scraped sand to the sand washing facility. 140