Overview of Methods
The critical velocity is when the velocity on the outer edge of the floc particle is equal to the floc settling velocity. Any velocity exceeding the critical velocity is when floc roll up begins.
In order to experimentally determine the critical velocity at which floc roll up begins, flow rates through the tube settler were increased incrementally utilizing a ramp state function in process control software.
By incrementally increasing the flow rate through the tube settler, we can compare the effluent turbidity performance over time. A critical velocity could be identified based upon effluent performance and compared to our theoretical model.
Using the same experimental apparatus as was used in Summer 2009 and Spring 2009, and the ramp state process controller function, we hope to understand if our theoretical model of floc roll up behavior describes system behavior. Ultimately, we hope to minimize the floc roll up in the plate settlers and further reduce the effluent turbidity. Also, we want to potentially understand how to create flocs that will experience less roll-up and have better performance.
Results and Discussion
By running the ramp state function on different tube settler diameters, we hope to further develop the relationship between plate settler size and floc roll up.
Experiment 1: Ramp State with 9.5 mm Plate Settler Tube Diameter
This experiment starts with a flow rate of 6 mL/min and over the course of 24 hours, gradually increases to a flow rate of 50 mL/min. This flow rate range corresponds to a capture velocity range of approximately 11 m/day to 91 m/day.
Figure 1: Effluent Turbidity vs. Flow Rate
Experiment 1 Conclusions
The very clear spike in the data represents the point at which the floc particles began to roll up the tube settler, which was confirmed visually in the experimental apparatus. The velocity represents the critical velocity. At a certain velocity, the turbidity stabilizes, and stops increasing. This is because at a certain point, the number of flocs rolling up in the settler cannot increase anymore, therefore the turbidity cannot increase anymore. Given the current data that has been collected, we are not sure if this curve accurately represents how the turbidity should change during the ramp state function. Futher experiments are being run to confirm this.
Experiment 2: Ramp State Function with 15.3mm Tube Settler Diameter
This experiment starts with a flow rate of 6 mL/min and gradually increases to a flow rate of 140 mL/min over the course of 24 hours. This flow rate range corresponds to a capture velocity range of approximately 11 m/day to 256 m/day.
Figure 1: Effluent Turbidity vs. Flow Rate
Experiment 2 Conclusions
Unlike the results for the 9.5mm tube, there isn't a sharp increase in the turbidity. Although the turbidity of the effluent water increases, the increase in the turbidity is minimal compared to the 9.5 mm tube. Thus, there is no clear evidence of floc roll up.