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Water treatment

der Waals forces, and

der Waals forces, and Brownian diffusion, with different particle sizes (0.1, 1, and 10 µm). Based on these results, Boller (1993), underlines that gravity is the dominant factor for coarse collectors (larger grain diameters) and particle sizes >1 µm, while diffusion dominates only for smaller particles. Besides, van der Waals and chemical double-layer forces seems to grow with decreasing d c , and increasing d p , and become negligible when d c > 5 mm in CMF. "The obvious conclusion from this would be to consider roughing [coarse] filters as sedimentation reactors with large inner surfaces" (Boller 1993). Although the RF filtration theory contributes to rationalise the design of CMF, it does not consider other possible relevant factors such as biological activity. Figure 2.12 Fundamental collision probability for spherical particles and collectors of different diameters, as a result of Brownian diffusion, van der Waals forces, and gravity (Boller, 1993). Biological activity. Evidence is available about the biological activity in the coarse filtration units when they are processing natural waters and synthetic waters with organic matter or nutrients (Trueb, 1982; Pardón, 1989; Wolters et al, 1989a; Collins et al, 1994; CEHE, 1999). It has been assumed that through mechanisms similar to those described for SSF, bacteria and other microorganisms may form sticky layers in some areas of the filter media or produce exocellular polymers that contribute to particle destabilisation and attachment. Macro-biological creatures inhabiting the coarse filters are thought to contribute to the sloughing off of stored material or biofilm (CEHE, 1999). Pardón (1989) observed evidence of organic matter decomposition during cleaning procedures of full scale HGF in Peru, calling for frequent maintenance activities of units susceptible to high biological activity. Flow conditions and coarse filtration efficiency. The working group at the University of Dar es Salaam (Tanzania) carried out tests with vertical flow filter columns of 1 m depth filled with gravel varying from 1 to 64 mm in size and filtration rates from 0.5 to 8 mh -1 . The turbidity of the raw water mixture was maintained at around 60 NTU. Good turbidity reductions were obtained at filtration rates < 2 mh -1 . This experience shows that significant solids removal efficiency is only achieved under laminar flow conditions (see figure 2.13). Due to the small silt storage capacity of the vertical CMF, Wegelin and Mbwette (1989) decided to concentrate the activities of the working group on horizontal CMF. Then, an open channel of 15-m length was filled with three gravel fractions of 32-16, 16-8, 8-4 mm in size, whereas turbidity of the raw water was maintained at 60 NTU. Under these given conditions, the CMF produced effluent turbidities

Figure 2.13 Influence of flow conditions on coarse filtration efficiency (Wegelin and Mbwette, 1989; Wegelin et al, 1991) Grain size, filtration rate, and treatment efficiency. The separation efficiency per unit filter length, λ (the filter coefficient in the phenomenological approach to the filtration theory), can be estimated based on theoretical considerations in the trajectory analysis to calculate ∂C/C (the basic collision probability). Based on equation 2.3, ∂C C = Basic collision probability = λ∂L ⇒ λ ≅ ∂C C 1 ∂L (2.15) in which the values of λ for d p =1 µm, and for different values of collector size, d c , were calculated by Boller (1993). The results (figure 2.14A), show that efficiencies per unit filter length >0.001 cm -1 would be possible only when filtration rates of

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