PSS Summer 2009 Experiments Varying Alum Concentration

Overview of Methods

In these experiments the alum dosage supplied to the flocculation system was varied in order to study how floc and floc blanket formation affect the effluent turbidity produced by the tube settler.  The experimental set-up is identical to the one used in Spring 2009, and from our results we hope to analyze velocity gradient thresholds and possibly investigate how changing influent water chemistry affects the setter's efficiency.

Results and Discussion

Using the Spring 2009 team's process controller methods, we subjected an ideal geometry to non-ideal conditions. Though the Spring 2009 team had success with a 9.5 mm diameter tube, due to a change in influent water chemistry over the summer, (ineffective air bubble traps in the flocculator,) or the addition of a flow accumulator to the method, we experienced failure with this geometry. We achieved an acceptable effluent turbidity (less than 1 NTU) with a 15.1 mm diameter tube that had a length of 30.5 mm. With the ideal results, we then subjected this tube settler to varying alum dosage to investigate the dependency of the performance of the tube settler on this parameter. At each alum dosage, the tube settler was tested at a variety of capture velocities and at two different floc blanket levels.

Experiment 1: Alum Dose = 45 mg/L

Experiment 2: Alum Dose = 35 mg/L

Experiment 3: Alum Dose = 65 mg/L

Experiment 4: Alum Dose = 15 mg/L

Experiment 5: Alum Dose = 105 mg/L

Process Controller Files

Conclusions

Figure 1: Capture Velocity vs. Average Effluent Turbidity shown for each alum dose at low floc blanket level.


Figure 2: Capture Velocity vs. Average Effluent Turbidity shown for each alum dosage at high floc blanket level.

The above table shows the average effluent turbidities for each alum dosage, floc blanket state and capture velocity.

Overall, the system performed well and most of the average effluent turbidities were below 1 NTU. The overdose of alum did cause the effluent turbidity to be slightly higher than the ideal dose, however it was still within the range of ideal effluent turbidity. It was expected that the 35 mg/L alum dose would perform poorly. However, this dosage produced better results than the ideal alum dose, so further experiments are being performed to collect data with a lower alum stock concentration. (Include your results from 25 mg/L. Relate your graphs and results to your conclusions. What are these results indicating? Is alum dose affecting performance? Is it fair to indicate that these doses are over and under dosed? If not, do we need higher and lower doses? What does this mean for how we are dosing alum? Are the water chemistry conditions we have in lab applicable to Honduras? What would be different in Honduras about dosing that we do not see here.)