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GPS-X Technical Reference

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Sedimentation and Flotation Models 256<br />

The solids flux due to bulk movement of the liquid is a straightforward calculation based<br />

on the solids concentration times the liquid bulk velocity, which is up or down depending<br />

on its position relative to the feed layer.<br />

The solids flux due to sedimentation is specified by a double exponential settling<br />

function, applicable to both hindered sedimentation and flocculant sedimentation<br />

conditions. The settling function, described by Takács et al. (1991), is given by:<br />

Equation 8.1<br />

where:<br />

v sj<br />

v max<br />

rhin<br />

rfloc<br />

X j o<br />

= the settling velocity in layer j (m/d)<br />

= the maximum Vesilind settling velocity (m/d)<br />

= hindered zone settling parameter (m 3 /gTSS)<br />

= flocculant zone settling parameter (m 3 /gTSS)<br />

= X j – X min, where X min is the minimum attainable suspended solids<br />

concentration, X j is the suspended solids concentration in layer j<br />

The minimum attainable solids concentration in a layer, X min, is calculated as a fraction<br />

(non-settleable fraction or fns) of the influent solids concentration to the settler:<br />

Equation 8.2<br />

It is subject to a maximum value specified by the user; the maximum non-settleable<br />

solids or X minmax. The settling velocity is lower bounded to zero, so that if the user<br />

specifies parameter values that would result in settling velocities becoming negative, a<br />

warning message is printed in the simulation Log window. The settling velocity is also<br />

subject to a maximum value specified by the user; the maximum settling velocity or v bnd.<br />

<strong>GPS</strong>-X <strong>Technical</strong> <strong>Reference</strong>

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