Slow Sand Filtration
Aslow sandﬁlter ismerely a bed of sand, conﬁnedwithin a box, with appurtenances to deliver and remove water. Figure 13.1 illustrates the main elements of the process and technology.
Slow sand consists of a ﬁlter box with appurtenances that measure and control ﬂows for the operation, for example, inﬂow, ﬁltered water drainage, backﬁlling after scraping, headwater drainage, etc. All of this is to support a biological depth-ﬁltration process, which is within the sand bed, coupled with a straining process, by a layer of material deposited, and perhaps with organism growth. The straining occurs on top of the sand bed, called the schmutzdecke. Slow sand ﬁltration may be characterized as a passive process in that it occurs without operator intervention.