Determining Flocculation Efficiency

Problem with Previous Efficiency Factor

The efficiency value assumed at the beginning of Spring 2008 was 0.5. This means that we assumed it would take only two collisions for a particle of clay to stick to a particle of aluminum sulfate. Using the MathCAD program, we determined that this efficiency value was inaccurately high by inputting the dimensions of the Fall 2007 Demo Plant, and comparing the MathCAD output for horizontal length of maximum floc size with the actual horizontal length of maximum floc size we observed. We found that assuming an efficiency of 0.5, the program reported an optimal horizontal length about 2 times smaller than what we observed.

This issue was addressed this semester in the experiment described below.

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 Spring 2008 Flocculator Design and Construction 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.