8 months ago


Water treatment

Based on the author's

Based on the author's experience in "Chorro de Plata", another DyFI was designed and built in the bed of a channel fed with water from the Pance River. This DyFI was used to abstract the water to be treated for the Colegio Colombo-Británico (British-Colombian College). This college is located in the suburban southern part of Cali, Colombia. Because in this later case, now called the Colombo project, the flow in the channel could not be properly regulated, the filter bed was washed away several times during the rainy periods. Then, in the absence of properly regulated water streams or small channels, DyFI are not recommended any more (Galvis and Fernandez, 1991; Wegelin, 1996). To avoid the previous O&M difficulties, the dynamic filtering beds were distanced from the unregulated water streams, either close to a more conventional water intake, or located at the main treatment plant site. The first case is illustrated in figure 2.18, and corresponds to the actual situation of the Colombo project, where the DyFI is now attached to the water intake and around 50 m away from the main treatment plant site. In this location the dynamic filter protects the total WS system, including the transmission main. If however, the intake site is not easily accessible to facilitate O&M activities in small WS systems then it is important to locate the dynamic filter at the main treatment plant site. Figure 2.18 Illustration of dynamic gravel filter placed close to the intake site (Galvis and Fernandez, 1991). Dynamic gravel filtration in the 1990s. DyGF is being developed as the first filtration step in a multistage water treatment plant. Under the integrated water treatment concept, it should contribute to protect the subsequent treatment steps from high solid loads and contribute in improving the overall water quality. The DyGF performance is expected to be as described by its adjective, dynamic. With low or moderate levels of suspended solids in the water source, it should gradually clog. If quick changes in water quality occur, the clogging should happen faster. Eventually, with high concentration of suspended solids, the gravel bed should be blocked and the influent would just flow over the clogged surface area. Reducing or stopping the flow at the DyGF will protect the following treatment units. The DyGF should be easier and less costly to maintain than the protected treatment units. To look for its dynamic behaviour, and keep its frequent 50

cleaning simple, DyGF should behave basically as a surface filter, and not as a deep bed filter. Then, contrary to the criteria behind figure 2.15B, DyGF would require that the influent water be first in contact with the finest fraction of the media in the filter bed. A DyGF consists of two or more parallel units packed with gravel of different sizes ranging from coarse at the bottom to fine at the surface (Figure 2.19). The flow (Q f ) percolates through the gravel bed from top to bottom, reaching the drainage system from where it flows to the next treatment unit. Due to the relative coarse gravel that is being used, the headloss over the filter bed is very small (of the order of 0.01 m). However, If the outlet valve controlling Q f is not periodically opened to compensate for the headlosses in the gravel bed, the flow through the filter will decline making cleaning necessary to recover the filtration capacity. On the surface, opposite the inlet zone, an overflow weir is installed with its crest either at the same level of the gravel bed or at around 0.03 to 0.05 m above the gravel bed (Guzman, 1997). In the first case Q f will decline from the beginning of the filter run. In the second case, the filter operates initially under constant rate conditions. Gradually the head loss increases until the supernatant water reaches the level of the overflow weir. The filter then starts to operate as a declining rate filter, distributing the influent flow between the flow that filters through the gravel (Qf) and the flow that pass on the overflow weir (Qe). Figure 2.19 Layout of a dynamic gravel filter (DyGF). During the early 1990s, it was considered possible to distinguish between two main types of water treatment objectives in this filtration step (Galvis and Fernandez, 1991). In the first type DyGF was oriented at treating water from sources usually transporting low levels of suspended solids (< 5 to 15 mgl -1 ), but occasionally showing sharp peak loads of short duration. In this case the design was aimed to obtain quick reductions of the filtered effluent, and the units were expected to behave as "automatic" or "plug" valves during periods of highsuspended solids. The second type was oriented to treat water from sources usually transporting moderate to high levels of suspended solids. In this case the units were expected to contribute more integrally to the water treatment, particularly in the reduction of suspended solids. The preliminary design criteria of DyGF to fulfil these two water treatment objectives are summarised in the table 2.7. Suspended solids removal efficiencies have been reported in the range of 23 to 77% for DyGF units processing natural raw water with suspended solids in the range of 7.7 to 928 51