Floe zone

In some applieations, where the solid particles are too fine to separate on their own, it is necessary to use a flocculating aid. The Hocculant can be added upstream of the decanter, but there are many circumstances where, for best efficiency, it is admitted in the bowl. On these occasions there will be an extra chamber, the floe zone", built into the hub of the conveyor. Where necessary this floe chamber can be used as a rinse chamber instead, to admit rinse liquor onto the scrolling cake,... 

From the floe zone, channels or tubes are provided to lead the flocculant into the area of feed discharge to ensure an intimate and economic mix at the appropriate point. Flocculant can be relatively expensive, and on an effluent application contributes a large percentage to the total treatment costs. It is therefore very important to ensure that just the right amount of flocculant is used, and that no extra is required due to bypassing. 

The large number of conveyor variants arise from the permutation of the different hubs, flight design, feed and floe zone types, and the different types and degrees of hard surfacing. Devices added to the conveyor, and the type of flow, two-phase, three-phase (see Section 2.4.1.2). co-current or countercurrent, add to the permutations. 

The discharge from the floe zone is either simply through holes in the conveyor hub or via tubes or channels fabricated in it. 

In gravity thickening, large and relatively fragile floes are needed to aUow high settling rates and fast coUapse in the compression zone. 

As in boundary-layer flows, smaller reference floe diameters are found with gas sparging than with the same average power input in a baffled stirred tank 127] or [44,45]. This can be explained if it is assumed that the floes come into close contact with the gas phase and find their way into the zones of higher stress. 

Organic matter deposited onto the suspended sediment has a large influence on the specific surface area. On one hand, it appears that organic matter blocks some sites available for physical adsorption of inert gas (BET adsorption) and, on the other hand, it probably partly causes flocculation and agglomeration of particles in the upper estuary, such floes and agregates being destroyed downstream in the salinity intrusion zone. 

As settling continues, the depths of zones D and A increase. The depth of zone C remains nearly constant, and that of zone B decreases. This is shown in Fig. 30.32c. Eventually zone B disappears and all the solids are in zones C and D (see Fig. 30.32ri). Meanwhile, the gradual accumulation of solid puts stress on the material at the bottom, which compresses solids in layer D. Compression breaks down the structure of the floes or aggregates, and liquid is... 

Equations 20 and 21 apply only to free, particulate separation not affected by flocculation or aggregation and for all particles having the same density. Under gravity field, few shear forces exist and have little effect on floes. Flocculation may be strong to change behavior of settling suspension to that of zone settling (i.e., all particles in slurry settle en masse at the same velocity irrespective of their size). 

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