Flocculator Design and Construction (Summer 2007)

The picture below shows the mixing column, flocculator, and sedimentation tank of the Summer 2007 Demo Plant.

Figure 1. Original demo plant

Design Problems

Horizontal Length of Flocculator

This design featured a flocculator with a horizontal length of 48 cm. It was determined that this length was too long. It unnecessarily increased the residence time of the plant. This resulted in an increased degree of mixing, so that the shear at the end of the flocculator was too high, and caused large flocs to break up prior to entering the sedimentation tank. This length was originally chosen to ensure enough shear to form flocs, but was overestimated, since the maximum floc size was observed at approximately 28 cm into the flocculator. Thus, the length after reaching the maximum floc size became detrimental to maintaining these flocs.

We fixed this in the next design by decreasing the horizontal length of the flocculator to approximately 30cm to ensure flocs reach their maximum diameters, and are maintained at this size until they reach the sedimentation tank.

Early Sedimentation Buildup

Due to the increase in valve diameter at the connection between the flocculator and sedimentation tank, the velocity of flocs moving through this connection drastically decreases. This leads to the build-up of sediment at the connection between the flocculator and the sedimentation tank, which is undesirable.

In the flocculator, it was also observed that after running the plant for a few minutes, sedimentation and settling begins in the last ten channels. This may be happening due to the fact that maximum floc size has already been reached, so gravity begins to pull the larger clumps down early on. Also, since our channels are rectangular, there are corners in the 180° bends between up and down channels. This results in a lower flow rate in the corners, allowing the settled flocs to remain there, and not be swept up by the faster flow of water in the centers of the channels. However, this is a constraint of the corrugated plastic material that we are using. Thus far, there are no plans to rectify this, since the benefits of using corrugated plastic greatly outweigh the advantages of using expensive, fragile acrylic.