ANC CONTROL

EXPERIMENT 2: Testing Carbonates Hypothesis

Introduction

In order to test the calcium carbonate hypothesis developed by the Fall 2009 team, the Spring 2010 team ran a second experiment to compare the performance of the reactors using distilled water versus tap water. There should be no carbonates present in the distilled water, so if the formation of calcium carbonate precipitate inhibiting dissolution is what is limiting the reactor's performance, the use of distilled water should produce better results. The two runs in this experiment were identical except for the water type so that that variable could be isolated.

Procedure

The team used the same procedure described on materials and methods. The lime was added as dry powder. For the up-flow velocity of 3.6 mm/sec, it was calculated that 20g of lime were required for the effluent to be saturated for 12 hours. The amount of lime was calculated as explained in Experiment 1., proportions were kept.

Results

Figure 1. shows effluent pH over time. The green line represents the 2.5cm diameter tube settler, and the 5cm diameter tube settler is represented by yellow and orange lines. We can see that none of the runs were anywhere close to reaching the goal of twelve hours with a saturated effluent. Both tap water and distilled water runs with the 5cm diameter tube settler failed within an hour and a half, while the run with the 2.5 cm diameter tube settler never produced any saturated effluent. There is certainly no performance improvement running the reactor with distilled water.

                                                                  Figure 1. Effluent pH vs time in hours. Comparison of performance between tap water and distilled water.

Figure 2. shows the fraction of saturated hydroxide concentration versus the fraction of time the run is theoretically calculated to last. This is simply a clearer way to display the reactor's performance over the course of the experiment relative to our goal, which would be to have a straight line at an OH fraction of one. In this case, Figure 2. illustrates what are likely measurement errors: a hydroxide concentration nearly double the saturation value in one run and a failure to reach saturation in what should be an identical trial.

                                                                     Figure 2. Fraction of saturated hydroxide vs Fraction of time

Discussion

The similarity in performance between the 5 cm diameter tube settler run with tap water and the same apparatus run with distilled water indicates that calcium carbonate precipitate may not be the main issue. Failure with distilled water suggests that there must be another issue. See the hypotheses page for further information.
Even with the results, one experiment is not enough to reject the carbonates hypothesis. Other factors could affect such as coating of the pH probe with lime particles. Further experiments are required to compare results.