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Water treatment

Operation and

Operation and maintenance criteria. All the DyGF units were planned to filter around 40 to 80% of the influent flow at the beginning of each filter run. DyGF runs were initially programmed to last 8 days (d) until July 1992, then 5 d until March 1992 and finally 3 d until April 1993. However, run lengths were shortened when water production in this 1 st filtration water treatment stage was insufficient to ensure the established filtration velocities in CGF lines included in the 2 nd stage. The DyGF units were cleaned by surface raking combined with sequential drainage of the units to stimulate flow pattern changes inside the clogged filter media. During the raking procedure the surface flowing velocity over the filter was kept below 0.2 ms -1 to avoid hydraulic transportation of the finer gravel layer. Filter run lengths of CGF units (2 nd stage of treatment) were planned to last 30 days. However, the filter runs were shortened when head losses were close to 0.5 m in the 1 st filtration steps in the case of vertical flow alternatives or 0.15 m in the case of HGF and MHGF options. All these units, except HGF, were cleaned by fast drainage. In the case of the vertical flow CGF units, this drainage was complemented with upflow to waste and movements of the surface gravel with a shovel. Excavation and manual cleaning procedures of the filter media was the only option to recover filtration capacity of the "conventional" HGF line. The SSF run lengths were determined by the 0.8 m of hydraulic energy available in each SSF unit. Table 3.2 summarises maintenance criteria and cleaning procedures for the MSF stages. Flow control. The influents to the DyGF units and CGF lines were measured with 30º V-shaped weirs that were calibrated volumetrically. The effluents from the DyGF units and the influents to the SSF units were measured volumetrically. These flow measurement points are identified in figure 3.8. Because declining flow values was taking place at the 1 st treatment stage and filtration rates were higher in CGF lines than in SSF units, overflow facilities were provided after DyGF units and CGF lines. Flow control points and frequencies are summarised in table 3.3. Hydraulic headlosses. Piezometers were located along the filtration beds for head measurements and to quantify head losses between different points (See figure 3.9). They included piezometer taps to facilitate samples collection wherever necessary. 3.1.6 Sampling and frequency. Sampling points are identified in figure 3.8. Samples were collected from the raw (01) and integrated (05) water constant head tanks, and at the end of each filtration unit. Sampling frequencies planned for periods I (January- July 1991), and II (July 1991-January 1992) are summarised in table 3.3. These frequencies had some changes in the following periods (III and IV) according to the parameter fluctuation, behaviour of the MSF system, and time and cost limitations. Footnotes in table 3.3 summarise these frequency changes. The sampling points 3.1, 3.2, 4.1, and 4.2 in the "conventional" HGF and MHGF lines were eliminated after period I due to difficulties in obtaining representative samples of the flow across the units. 80