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Water treatment

Grain size (d c ) and

Grain size (d c ) and filter length (L). The influence of d c on λ=f(σ) was tested in several experiments with short layers of HGF using kaolin clay (K-clay) suspensions (Wegelin, 1986; Boller, 1993). The results (figure 2.15A) reveal that the removal efficiencies per unit of filter length are greater in the finer collectors of filter media, as predicted by the trajectory analysis. However, since the hydraulic gradients (headlosses per unit of filter length) are smaller in the coarser collectors of filter media, the design of succeeding layers from coarse to fine media is practised in the design of full scale CMF, as shown in figure 2.15B. This offers the possibility to remove the larger or heavier solids in the coarse media with low head loss, and restrict the use of the efficient finer collectors for the smaller or lighter particles. Thus, the values of solids load (σ) should decrease with the grain size, along the flow direction, and the headlosses could be kept acceptably low along the filter bed during the whole filter run. The required overall filter length is also reduced with the use of graded successive layers, as illustrated in figure 2.15B (Perez et al, 1985; Boller, 1993). Figure 2.15 A. Filter coefficients λ=f(σ) for different filter grain sizes at a filtration rate of 1 mh -1 using K-clay suspensions (Wegelin, 1986; Boller, 1993). B: Conceptual solids reduction along a three layer coarse filter (adapted from Perez et al, 1985). The filtering media should have a large surface area to enhance particle removal and a high porosity to allow the accumulation of the separated solids. Filtration tests with K-clay suspensions revealed that neither the roughness nor the shape of the filter material had a great influence on filter efficiency (Wegelin, 1986). Any inert, clean and insoluble material meeting the previous criteria could be used as filtering media. Gravel is the commonly used material, but broken bricks, palm fibre, and plastic material have also been reported in different experiences. In a review of CMF performance with different filter media Wegelin (1996) found that the filter filled with palm fibre presented a better turbidity removal compared to a gravel filter. Greater porosity (92% versus 37%), producing a lower effective velocity, was considered to be the explanation for this observation. However, since the use of palm fibre caused a considerable drop of dissolved oxygen, odour and taste problems were considered as possible limitations of this filtering medium alternative. Using plastic material may require some technical solution to the uplift forces of the water. 46

Operation and maintenance (O & M). The operation of CMF units requires a frequent (at least daily) control of the influent and effluent flow and the quality of the raw and filtered water. The maintenance is associated mainly with the cleaning process, which tries to restore the initial headloss. To facilitate maintenance a minimum of two units should be constructed in parallel. Frequent cleaning of the CMF units is recommended to limit headloss development and to avoid operational or maintenance difficulties due to solids consolidation or organics decomposition inside the filter media (Pardón, 1989). CMF units are cleaned both manually and hydraulically. Manual cleaning involves media removal, washing and replacement, which is time consuming and labour intensive. Then hydraulic cleaning facilities for in-place media flushing become a key component of the units to ensure a long-term sustainability of this treatment technology. Only surface raking was initially applied to clean hydraulically dynamic gravel filters (DyGF) and then it was combined with filter bed drainage. Only manual cleaning was initially used to clean HGF and gradually fast drainage of the filter bed compartments has been incorporated in its application. Fast or Moderate drainage velocities, combined in some cases with some surface raking, are being applied to maintain DGF and UGF. The area and the height of the filter boxes should be limited to facilitate both frequent hydraulic cleaning and eventual manual cleaning. The drainage system. In the case of DyGF, HGF, and DGF, the drainage system collects and provides an outlet for filtered water during normal operation, as well as for washing water during the hydraulic cleaning by fast drainage. In the case of UGF the drainage system distributes the water to be filtered, and collects and provides an outlet for washing water during hydraulic cleaning. The system may consist of a small trough (Pardón, 1989), a false filter bottom (Eudovique, 1992; CEHE, 1999), or perforated pipes or manifolds (Galvis et al, 1989). One small trough would have limitations to produce an even flow distribution across the entire filter bed compartment. A good false bottom would ensure an even water collection or distribution but imply additional hydraulic structures. A properly designed manifold should have a good hydraulic efficiency (Hudson, 1981, Castilla and Galvis, 1985) with lower construction costs, although requires an additional gravel layer to embed the pipes. Basic concepts and design criteria about manifolds are included in Annex 2. The decision between false bottom and manifolds should be taken after analysing local conditions. 2.8.3 Dynamic gravel filters (DyGF) Antecedents. The preliminary ideas of the author of this thesis to start developing the DyGF were originated after reviewing two experiences. The first was with a special type of SSF, called dynamic SSF (DySSF), initially developed in Russia and used in some Latin American countries such as Argentina and Ecuador (Arboleda, 1973; Shulz and Okun, 1992). The DySSF consists of a channel about 1 m in depth, with a sand bed and drainage system similar to those used in a conventional SSF unit (figure 2.16A). The raw water flows at 0.2 to 0.3 mh -1 over the bed surface in a thin fluid layer, about 1 to 3 cm deep, and then over a weir into an overflow channel to waste. Part of the flow (only 10% of the influent) percolates through the sand bed (d 10 = 0.25 to 1 mm, and U C = 2 to 3) at about the same filtration rate as in a conventional SSF. The advantages of DySSF are claimed to be low construction cost; 47