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close this bookSurface Water Treatment by Roughing Filters - A Design, Construction and Operation Manual (SKAT, 1996)
View the document(introduction...)
View the documentPublishers
View the documentForeword
View the documentPreface
View the documentExecutive summary
close this folderPart 1: General aspects of roughing filter application
View the document1. Historical development and experience with water treatment
View the document2. Water treatment concept
close this folder3. Raw water quality
View the document3.1 Raw water characterisation
View the document3.2 Catchment area
View the document3.3 Water quality analysis
close this folder4. Solid matter separation
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View the document4.1 Sedimentation
View the document4.2 Roughing filtration
close this folder5. Bacteriological water quality improvement
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View the document5.1 Slow sand filtration
View the document5.2 Chlorination
close this folder6. Layout of a water supply scheme
View the document6.1 General considerations
View the document6.2 Hydraulic profile
View the document6.3 Treatment steps
View the document6.4 Water distribution
close this folder7. Roughing filtration application
View the document7.1 Historic use
View the document7.2 Development of roughing filters
close this folderPart 2: Design, construction and operation of roughing filters
View the document8. Classification of roughing filters
close this folder9. General aspects of roughing filter design
View the document9.1 Main features
View the document9.2 Basic filtration theory
View the document9.3 Design variables and guidelines
View the document9.4 Flow and headloss control
View the document9.5 Filter drainage system
View the document9.6 General design aspects
close this folder10. Detailed filter design
View the document10.1 Intake Filters
View the document10.2 Dynamic filters
View the document10.3 Vertical-flow roughing filters
View the document10.4 Horizontal-flow roughing filters
close this folder11. Roughing filter efficiency
View the document11.1 Practical experience
View the document11.2 Pilot plant tests
close this folder12. Selection criteria for roughing filters
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View the document12.1 Raw water quality as selection criteria
View the document12.2 Layout and operational aspects as selection criteria
close this folder13. Construction of roughing filters
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View the document13.1 Filter box
View the document13.2 Filter material
View the document13.3 Inlet and outlet structures
View the document13.4 Drainage system
View the document13.5 Gravel and sand washing facilities
close this folder14. Operation and maintenance of roughing filters
View the document(introduction...)
View the document14.1 Caretaker training
View the document14.2 Treatment plant commissioning
View the document14.3 Flow control
View the document14.4 Water quality control
View the document14.5 Filter cleaning
View the document14.6 Filter maintenance
close this folder15. Economic aspects
View the document(introduction...)
View the document15.1 Construction costs
View the document15.2 Operating and maintenance costs
View the document15.3 Overall costs of water supply schemes
close this folder16. Design examples
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View the document16.1. Treatment of an upland river
View the document16.2 Treatment of a lowland stream
View the document16.3 Treatment of reservoir water
View the document16.4 Rehabilitation of a slow sand filter plant
View the document16.5 Standard designs for compact water treatment plants
close this folder17. Final remarks
View the document(introduction...)
View the documentReferences
View the documentAbbreviations
close this folderAnnexes
View the documentSimple methods for water quality analysis
View the documentSimple methods for discharge measurements
View the documentSalient data and features of slow sand filters
View the documentRoughing filter theory
View the documentPilot plant design examples
View the documentRoughing filter design examples
View the documentOutline for caretaker training
View the documentMonitoring of filter operation
View the documentAcknowledgements and credits

13.1 Filter box

Filter structures can generally be located below or above ground, as illustrated in Fig. 45. The respective choice depends on soil characteristics, available construction material and hydraulic profile. On a flat surface, gravity flow often requires the structures to be placed below ground. This, however, might cause some problems or additional costs for adequate drainage of the washwater. A partially buried filter will require less excavation work and provide support to the sidewalls by the back-filled soil.

Fig. 45 Location and Materials of Roughing Filter Boxes

Roughing filters are usually shallow structures of about 0.6 m (intake and dynamic filters) to 1 - 2 m (roughing filters). The size of the filter box is dependent on hydraulic capacity (see also Section 9.4) and filter length. The filter boxes should not be too big (maximum filter area for vertical-flow roughing filters should amount to 25 30 m², maximum cross section area for horizontal-flow roughing filters about 4 - 6 m²), to avoid problems with high washwater discharge rates. Furthermore, the filter box should also not be too high (preferably around 1 m) to allow easy removal of the filter material during manual cleaning.

A trench excavated in impervious soil, such as clay, silt or lateritic ground, presents a low-cost solution for a filter box. The trench has sloping sidewalls which do not exceed the slope stability of the water-saturated soil (slope less than 1:1). Lining of the base and sidewalls prevents clean filter material from mixing with the surrounding ground. A layer of sand, prefabricated slabs, in-situ applied coatings (concrete lining, ferro-cement, lime mortar) or in exceptional circumstances (e.g. refugee camps), prefabricated plastic linings or the use of geotextiles, are the most appropriate materials to use.

A watertight box has to be constructed in permeable ground or if the filter is installed above ground. In such cases, vertical sidewalls are recommended. Burnt clay bricks with cement mortar lining, concrete bricks or reinforced concrete should be used for such filter boxes. The foundation and floors of the box need special attention to avoid cracks caused by uneven soil settlement. Finally, watertight expansion joints will probably have to be made for long filter boxes constructed for horizontal-flow roughing filters. Alternatively, long filter boxes resting on difficult ground can be split into two or more separate units with flexible pipes interconnecting the compartments. U-shaped units can also reduce the total length of filter boxes. In such a layout, inlet and outlet are to be placed on the same filter side, and the box divided into two equal parts by a longitudinal separation wall.

The filter box should be tested for watertight-ness, preferably before it is filled with filter material. Leaks can be detected and repaired more easily in an empty structure. Special attention must be paid to the joints at the floor-wall interface or the inlet and outlet boxes fixed to the filter box. Watertight joints require water stoppers made of PVC or rubber. Other weak points in the structure include the pipe sealings which possibly need additional reinforcement to prevent cracking of the walls, and seep rings to prevent leaks.