Tube Flocculator Construction History

Original Construction

The tube flocculator was constructed in the summer of 2008. To prevent flocs from settling in the tube flocculator is arranged in a vertical figure-eight. The flocculator has the same configuration and dimensions as the lab flocculator. FReTA will be constructed at the plant in Fall of 2008. FReTA measures the rate of change in turbidity as opposed to just measuring the turbidity at a specific point in the flocculator. This is advantageous because the the change in turbidity rate is comparable to the settling velocity.

The current flocculator is made of soft flexible clear tubing. The tubing is pinched as it coils along the flocculator. Additionally, when water is pumped through the tubing it compresses. Air leaks into the tube at each of the bulkhead fittings. This air prevents flow through the flocculator. A possible solution to this problem would be to reconstruct the flocculator out of more rigid tubing. Disadvantages to using rigid tubing are that it is opaque and may not solve the problem of air entering the flocculator through the fittings. Air entering the flocculator is not a problem in the lab flocculator. It is unclear what difference between the two flocculator set up is causing this problem.

Settling Velocity in the Tube Flocculator

Introduction

The tube flocculator at the Pilot plant will be used to test the effects of raw water on floc formation and settling velocity. Similar experiments have been conducted in the laboratory. The tube flocculator is a long narrow tube coiled as in a figure eight formation. Alum is added as a coagulant in the flocculator. The positively charged alum particles promote floc formation. Flocs collide because of shear,G, in the long narrow tubing. The amount of mixing, Gtheta, that flocs are subjected to is a function of the shear and residence time in the flocculator. Large dense flocs settle out faster in sedimentation. Research has been done in the lab to determine G and Gtheta values that produce the largest settling velocities.

The previous lab experiments have examined flocculation with mixtures of water and clay. The chemistry of raw water is much more complex than this. Results from the lab experiments may not accurately represent flocculation involving raw water. For example, raw water contains humic acids dissolved from soils. Humic acids are large negatively charged organic particles. They bind to positively charged alum particles and may absorb more alum than the lab flocculator. The goal of this semesters research is to compare data from the Pilot Plant to lab experiments. The data should show the adequacy of the lab data to model the behavior of raw water.

Troubleshooting

A vaccuum pump was used to siphon water out of the tube flocculator in the summer of 2008. The siphon pressure created flow in the flocculator.

The same procedure has been attempted this semester with little success. Air in the flocculator has prevented flow. Furthermore, air has been leaking into the flocculator through the bulkhead fittings. For additional information on this problem see Flocculator Maintenance