PSS Summer 2009 Experiments Varying Alum Concentration

Overview of Methods

In these experiments, the alum concentration was varied to study how a change in alum dose affects effluent turbidity from the tube settler. The experimental set-up is the same as the one used in Spring 2009. From these results, the velocity gradient thresholds and effect of organic matter will be investigated with the intent to characterize the system in terms of tube settler efficiency.

Results and Discussion

Using the Spring 2009 team's 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 chemistry in the influent water during the summer 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. With these results, we then subjected this tube settler to varying alum dosage to investigate the dependency of the performance of the tube settler on the alum dosage. 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

Process Control Files

Conclusions

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 underdose produced better results than the ideal alum dose, so further experiments are being performed to collect data with a lower alum dose.