4 months ago


Water treatment

plain sedimentation and

plain sedimentation and because of its ability to maintain treatment simplicity comparable to that of SSF. CMF units are easier to operate and maintain than long term storage reservoirs and are not dependant on the hydraulic behaviour of water streams or rivers, as are riverbank and riverbed filtration, particularly during the rainy seasons in tropical countries. 2.8.1 Classification of coarse gravel filters Different CMF alternatives, using gravel (G) as filter media, are schematically illustrated in figure 2.10. Galvis and Visscher (1987), and Wegelin (1988) have classified them according to the main application purpose and the flow direction as shown in figure 2.10. Figure 2.10 Schematic view of coarse gravel filtration alternatives (Galvis and Visscher, 1987) 2.8.2 General considerations Effluent water quality. Coarse gravel filters (CGF) have been normally specified to produce an effluent with turbidity

Headloss and flow control. Final headloss in CMF units is small, usually a few centimetres, with maximum value around 0.30 m. Because of these low values CMF units usually have inlet flow control. The inlet structure should include facilities for energy dissipation, flow control, flow measurement, and overflow water. A well-designed inlet box facilitates the operation and control of the system. A weir or a raised and aerated effluent pipe maintains the water above the filter bed level. Flow measurement devices are recommended at the inlet and outlet sides to control the operation and to verify that the filter boxes are watertight. Since the CMF units in small WS systems deal with low flow and low pressure values, some simplified valves, gates, and weirs can be used together with more commercial hydraulic devices (Annex 1). Design criteria and filter run time. The main criteria for CMF design have been removal efficiency and head loss related to particle retention in the filtering bed. Process variables, such as particle nature and size distribution, collector size (d c ), filtration rate (V), and filter length (L), determine the filter run time up to the breakthrough, related to a maximum concentration value (C m ) in the effluent, or up to a clogging point, related to a maximum headloss (H m ). A qualitative illustration of the impact of some process variables on breakthrough and clogging in RF and CMF is illustrated in figure 2.11. As predicted by the trajectory approach in the filtration theory (section 2.5.1), the removal efficiencies in coarse filtration will be smaller due to its greater collector size. This limitation is partly overcome by lower filtration rates and longer filtering beds in CMF. Figure 2.11 Effects of some process variables on the breakthrough and clogging points in rapid (RF), and coarse media gravel (GF) filters (CMF). (Adapted from Boller, 1993) Particle transport and removal mechanisms. A comparison between rapid (RF) and coarse media filters (CMF) regarding particle collector efficiency, was made by Boller (1993). This comparison was based on the trajectory analyses and previews work of Kim and Rajagopalan (1982) on convective diffusion transport, and Rajagopalan and Tien (1976) on transport efficiency influenced by all the flow forces and moments including hydrodynamic wall effects, gravity, and retarded van der Waals forces. The semi-empirical correlation resulting from these studies were applied for vertical flow as a function of collector size and filtration rate. Figure 2.12 shows the results calculated for particle collision probability, based on the terms of gravity, van 43

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