GPS-X Technical Reference

Sedimentation and Flotation Models 260
Influent Flow Distribution
The sedimentation models will account for hydraulic effects caused by an increase in
influent flow. Flow conditions are considered normal when the influent flow divided by
the surface area is less than the quiescent zone maximum upflow velocity specified by
the user. The load to the settler under normal flow conditions enters the settler at the feed
layer. However, as the influent flow increases, the load to the settler is distributed to the
layers below the feed point. When the upflow velocity in the settler surpasses the
complete mix maximum upflow velocity specified by the user, the entire load enters the
bottom of the settler.
When the upflow velocity is between the quiescent zone maximum upflow velocity and
complete mix maximum upflow velocity, a smooth transition of feed distribution
between the low loading case and the high loading case is generated. An average
(medium) loading condition is initially calculated where the upflow velocity (vuavg) is
the average of the quiescent zone maximum upflow velocity (vumin) and the complete
mix maximum upflow velocity (vumax). At this hydraulic loading, the input distribution
is equal to the feed layer and all layers below.
Smooth distribution is achieved by two linear interpolations. The first interpolation is
between vumin and vuavg. At vumin, the influent fraction to the feed layer is 1.0 (all flow
enters the feed layer), while at vuavg the influent fraction to the feed layer is 1.0 divided
by the number of layers below the feed layer (including the feed layer itself). Once the
influent fraction to the feed layer is calculated, the remaining flow is equally distributed
to the layers below.
A similar algorithm will ensure smooth flow distribution above vuavg. The algorithm
first calculates the feed fraction to the bottom layer. If vu is higher than vuavg, then the
algorithm distributes the rest evenly.
This procedure of flow distribution is modelling the feed distribution to the settler by the
influent baffle. The feed distribution is shown in Figure 8-4.
During higher flows, the momentum of the incoming flow tends to carry the load further
past the bottom edge of the influent baffle, effectively changing the feed point in the
settler. The flow distribution aspect of the model captures this phenomenon.
GPS-X Technical Reference