10.1 Intake Filters

Intake filters are combined with water abstraction structures and installed next to small and narrow river beds as illustrated in Photo 5 and Fig. 36. Intake filters are often used as first pretreatment unit in a water treatment scheme. A small weir regulates the water level of the surface water and channels part of the flow into an adjacent filter compartment. This filter box is filled with two gravel layers. The top layer consists of relatively fine gravel of less than six millimetres in diameter.

Photo 5 Example of an Intake Filter

Fig. 36 Layout of Intake and Dynamic Filters

The lower coarser gravel layer acts as filter support and allows an even abstraction of the prefiltered water through perforated drainage pipes. The abstracted raw water, after being distributed evenly by a small weir over the entire width of the filter box, flows gently over the gravel bed surface. Part of this water percolates through the gravel layers and the remaining water is discharged over an outlet weir back to the river. Intake filters are constructed along rivers and not directly in the river bed, as the filter material would be washed out during periods of high river discharge. Construction of a separation wall between river bed and filter box is recommended to prevent the filter from being washed out.

Intake filters can also be installed in the bed of small canals. Upland rivers with a steep river bed and a suitable topography might allow the accommodation of a small diverting canal. The filter bed, comprising different gravel layers, is installed over a small stretch in the canal. Part of the canal water is filtered through the series of fine to coarse gravel layers, while the remaining water is returned to the river. The prefiltered water is collected by perforated drainage pipes laid at the bottom of the coarse gravel layer, and the discharge rate regulated by a valve placed at the filter control box. The flow velocity in the canal must be regulated by the canal's intake structures to protect the filter bed from being washed out during periods of high river discharge. The flow velocity in the canal should actually range between 0.10 and 0.30 m/s to prevent fine matter from settling and remaining on top of the gravel bed, and also to avoid fine filter material from being washed out. This layout may also be applied to irrigation canals, provided they are continuously supplied and regulated through out the year. However, construction of intake filters along rivers is strongly recommended as these filters allow a more reliable operation than intake filters installed in canal beds.

Finally, "intake" filters may be located directly at the treatment plant site and function as pretreatment facility. This particular location is recommended in gravity water supply schemes with a raw water intake located in a remote area of difficuIt access. Such a layout will facilitate monitoring and regular filter cleaning.

Filtration rates of intake filters range between 0.3 and 2 m/h. However, significant solids removal rates can be expected only at filter velocities smaller than 1 m/h, Design of the hydraulic structures should be based on maximum filter resistance of 20 to 40 cm. This figure will not be exceeded if regular filter cleaning, (e.g. once a week), is observed.

Relatively small filter material of less than 6 mm is used in intake filters which act as surface filters as the solids mainly accumulate on top of the filter bed. Since filter cleaning is carried out manually, the different gravel layers might be disturbed and mixed up if filter material of different sizes is used in intake filters. A filter cloth is sometimes placed in-between the different gravel layers to avoid mixing of the filter fractions and possibly reducing filter porosity and efficiency. However, coarser gravel hardly contributes to solids removal, but allows an even abstraction of the pretreated water. Nevertheless, regular filtered water collection is also possible with a single filter bed layer, moderate filtration rates, medium-sized filter structures and reasonable layout of the perforated drainage pipes. Design guidelines are summarised in Fig. 37. Use of a single filter layer and false filter bottom offers a favourable design alternative for intake filters, as filter material mixing is no longer possible, and even abstraction of the prefiltered water is guaranteed.

Fig. 37 Design of Intake and Dynamic Filters