Determining Flocculation Efficiency

Current Efficiency Factor

The efficiency factor previously being used in the flocculation model for plant design had been the best estimate of an unknown parameter; it was not necessarily an accurate representation of flocculation efficiency in the plant. When the parameters of the existing plant were used to create a design in MathCAD, the location of the maximum floc size as predicted by the model did not match the observed location. We therefore determined that the efficiency factor had been incorrectly estimated, and that models being created with the design program were not practical. In order to determine a more realistic model for the new plant design, we set out to determine the actual efficiency factor.

Procedure

We observed flocculation in the demo plant in order to determine at what point in the length of the flocculator the flocs reached a maximum size before beginning to break up. These flocs were also measured in order to be able to more accurately represent in the flocculation model the maximum floc size that it is possible to achieve in the plant. Using the location of the largest flocs as an estimate of the optimum plant length, we set the maximum floc size to the size of the measured flocs and changed the value of the efficiency factor in the flocculation model until the design length of the plant model created by the Demo Plant MathCAD Design program was consistent with the observed occurrence of maximum floc size.

Results

The maximum flocs were observed at a distance of 28 cm from the flocculator inlet. After that point in the plant, they ceased to get any bigger, and eventually began to break up from too much shear. The size of the largest flocs was about 0.7 mm, so that value was set as the maximum floc size in the flocculation model. When various efficiencies were tried in the model, the plant design program returned a design length of approximately 28 cm for a value of 0.2, and so we have determined that the flocculation efficiency is equal to this value.