Cocurrent filters

Washing by successive dilution is used when the soHds are separated into a slurry, such as in filter thickeners. The soHds, thickened into a small amount of mother Hquor, are diluted into a wash Hquid and then separated again, diluted, separated, etc until clean of mother Hquor. The consumption of the wash Hquid can be reduced in countercurrent washing systems, sometimes referred to as countercurrent decantation. Cocurrent dilution washing, however, can be built into some dynamic filter-thickeners such as the Escher-Wyss filter. 

Figure 1. Direction of gravity force action and filtrate motion in filters A-cocurrent B-countercurrent C-crosscurrent solid arrow-direction of gravity force action dashed arrow- direction of filtrate motion 1-filter plate 2-cake 3-sludge 4-filtrate 5-clear liquid.

A liquid may be introduced into a tower as fine drops through a nozzle. This device is known as a spray chamber. The flow could be cocurrent or countercurrent. A countercurrent spray chamber is shown in Fig. 8. These towers are considerably more resistant to plugging when solid particulates are present in the inlet gas. However, difficulties with plugging in spray towers and erosion can be troublesome when the spray liquid is recycled. Particle settling followed by fine strainers or even coarse filters is beneficial to eliminate... 

FIGURE 43.4 Typical sludge dewatering equipments (a) vacuum filter (b) belt press filter (c) press-filter (d) rotary press (e) rotary drum press combined with EOD (f) cocurrent centrifuge (g) countercurrent centrifuge. 

Cocurrent or top-feed filters employ flat and cylindrical filtering media. In flat designs, the angle between the directions of gravity force and filtrate motion is 0°, but... 

The combination of hi pressure (300 psi) and thin cakes produces high rates. Washing is accomplished either cocurrently or countercurrently. Separate filters in series can be employed in a manner similar to conventional thickeners. Washing may also be performed within a single unit, whereby, an initial portion of the filter must be used to remove liquid. In this case, the final stages are used for concentration. Clean wash liquid may be injected after the initial filtering at one or several intermediate stations. Injection wash tends to increase the overall filtrate rate but decreases the cake ouqtut rate. 

Upflow filters with cocurrent blowered air flow. 

The added oxygen required for aerobic development is provided by maintaining an air cocurrent jointly with the rising water current through tire filter. The filter is generally open and can be built of concrete on a larger scale than pressurized filters. The facilities must nevertheless be made up of several units because excess velocities when a unit is being washed are even less acceptable than in pressurized facilities. 

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