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This chapter is excerpt(__''om Small and Decentralized Wdstewater Manal"�4ntstems(Crites and <br /> Tchobanaglous, 1998, M Zraw-Hill)and has been reproduced by permission-t f the author. <br /> " s <br /> f <br /> - s � <br /> CHAPTER 11 <br /> Intermittent and . Recirculating <br /> " a <br /> Packed-Bed Filters <br /> Packed-bed filters are biological and physical treatment units with a long history <br /> of use in wastewater management. Packed-bed filters used for the treatment of <br /> wastewater include: (1)conventional and high-rate trickling filters, discussed previ- <br /> ously in Chap.7; (2)high-rate granular and porous medium filters used for effluent <br /> filtration; and (3) low-rate granular and porous medium filters. High-rate filters <br /> used for effluent filtration from centralized treatment facilities are considered in <br /> Chap. 12, which deals with effluent repurification and .reuse. The focus of this <br /> chapter is on low-rate filters used for the treatment of wastewater from individual " <br /> r homes and other small decentralized facilities. The most commonly used low-rate <br /> packed-bed filters are;(1)intermittent(single-pass)sand filters and(2)recirculating <br /> (multipass) granular"medium filters.Both types of filters are described in this chap- <br /> ter, along with other types of packed-bed filters. Before considering the individual <br /> types of packed-bed filters, it will be useful to consider the functional elements that <br /> make up these units and a description of the operative removal mechanisms. <br /> 11-1 FUNCTIONAL FEATURES OF PACKED-BED FILTERS <br /> The"most commonly used packed-bed treatment units comprise the following basic " <br /> elements: (1) a container with a liner for holding the medium, (2) an underdrain <br /> system for removing the treated liquid, (3) the filtering medium, (4) a distribution <br /> and dosing system for applying the.liquid to be treated onto the filtering medium, <br /> and(5) supporting appurtenances. Each of these features is considered briefly in the <br /> following discussion. A cutaway view of a packed-bed filter, without the container, <br /> is shown in Fig. 11-1. <br /> 703 <br />