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Although the total volume of aggregate used for filters is relatively small, filters nevertheless play important and diverse roles in many projects. Table 13.1 shows some of the main uses of filter aggregates and the first four parts of this chapter discuss some of the requirements and properties of these materials. The final three parts of the chapter describe the functions and specification of filter aggregates in their three main applications, namely water filtration, effluent treatment and as drainage filters for earthworks and other civil engineering structures. These will normally be subject to different design criteria and may also call for filter materials with distinct sets of physical and mechanical properties.

Filter aggregates generally consist of sand, gravel or crushed rock. Manufactured aggregates are also occasionally used and these often include blastfurnace slags.

Although filter materials for water and effluent treatment works are often used in relatively small quantities, the high quality of aggregate normally required is not always readily available from commercial production processing, which may be designed to yield a satisfactory general purpose aggregate at least cost. On the other hand, drainage layers in major civil engineering works, such as embankment dams, are usually designed to make the best use of the available natural materials with the minimum of processing.

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