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galvis

Water treatment

Photo 4.24 Drainage

Photo 4.24 Drainage facilities in upflow gravel filters built during the 1980s, including locally made fast drainage valves Photo 4.25 Drainage facilities in upflow gravel filters built during the 1990s, including commercially available fast drainage valves 171

• Initial SSF plants in Colombia did not include adequate facilities for flow measurement and control (e.g. photo 4.1). A better hydraulic inlet structure was adapted, allowing for energy dissipation, flow measurement, an overflow of excess water (photo 4.26). However, this structure still requires development to improve flow control possibilities in small WS systems. • The drainage system of SSF units used to be rather deep (40 to 50 cm). After reviewing experiences with manifolds (e.g. Hudson, 1981), a commercially available corrugated PVC manifold, currently used in land drainage, was included in the first SSF units introduced in the Cauca Valley Department (Galvis, 1983). This type of drainage system is used in all MSF plants monitored during the present study (figure 4.18). • The long time required for supernatant drainage became a limiting factor to ensure that PCA (scraping) could be accomplished in less than a working day (8 hours). This limitation was considered important not only for social and economic reasons, but to reduce the impact of dewatering SSF units on microbial organisms active in the sand bed (Lloyd, 1974; Hendricks and Bellamy, 1991). To overcome this limitation a movable drainage device (popularly called gooseneck in the Cauca Valley) was introduced (photos 4.27 and Annex 1). This device is now being utilised to (i) function as an overflow weir when the SSF unit has reached its maximum headloss; (ii) drain the supernatant water, if maintenance is required; and (iii) extract floating materials (leaves, algae) from the water surface. Development, evaluation and transferring MSF technology in the Colombian Andean Context has been a gradual learning process, which is still going on. During the 1980s and early 1990s, it usually took more than a year to introduce a demonstration project, including community participation and promotion of a well-informed water committee or users association. Now MSF technology is becoming more accepted at community, institutional, and private sector levels. Mondomo is a MSF plant recently built in a rural area of Cauca Department of Colombia. Working in a project learning environment, in less than a year the plant was built in 1997 and a users association was established, which is now managing the WS system including the plant. It has DyGF units combining constant (at the beginning of filtration runs) and declining filtration rates, UGFL, and SSF units working in series. 4.4.3 General discussion 4.4.3.1 Raw water sources to MSF plants All raw water sources show short but relatively high turbidity peaks when it rains. These short peaks were not always detected during the monitoring programme due to the frequency of sampling. In spite of that limitation, collected data still provided evidence of the general behaviour of water sources included in this study. Maximum suspended solids (SS) and turbidity levels were in the range 34 (Ceylan) to 1,040 mgl -1 (Javeriana) and 55 (Ceylan) to 300 NTU (Javeriana) respectively. Considering 95 percentile values the ranges for SS and turbidity levels become 6.9 (Ceylan) to 64 mgl -1 (Javeriana) and 4.3 (Ceylan) to 57 NTU (Javeriana) respectively. In Ceylan, 95 percentile values of SS and turbidity correspond to 20 and 8% of maximum values respectively. In Javeriana they correspond to 6 and 19% respectively. Even more importantly, SS or turbidity peaks are short (usually no more than a few hours) and they are not concentrated in a short period of the year but are more or less distributed throughout the two annual rainy periods of 172

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