chapter  14
32 Pages

Cake Filtration

Figure 14.1 encapsulates the key aspects of the cake filtration process. As illustrated, a ‘‘cake’’ filter (also called a ‘‘pre-coat’’ filter) has a media, usually diatomaceous earth, deposited on a ‘‘septum,’’ comprising a stainless steel fabric. The mesh size of the fabric must be small enough to retain the media. The circulation of a specified mass of slurry through the septum transfers the media to a ‘‘pre-coat’’ deposit on the septum, illustrated as the portion of the filter cake just to the left of the septum in Figure 14.1. After the pre-coat forms, the filtration process can begin. The raw water is introduced, along with a ‘‘body feed’’ injected into the raw-water flow to give a specified concentration, usually of the same material as the pre-coat. At this point the filter run begins. The body-feed deposits on the pre-coat, building up the filter ‘‘cake,’’ illustrated in Figure 14.1 as the portion of the filter cake just to the left of the pre-coat. When the headloss reaches a specified design limit, or when the cake thickness reaches a limit as defined by the spacing between septum leaves, whichever occurs first, then the cake is removed and the cycle is repeated. As also illustrated in Figure 14.1, the suspended matter deposits within the matrix of the cake that has been deposited. Without the body feed, the suspended matter would deposit as a layer, forming its own ‘‘cake’’ on the precoat, resulting in a rapid rate of headloss increase. Not shown in Figure 14.1 is the structural support for the septum that must be provided in order to withstand the pressure differential between the two sides of the filter cake.