...
As can be seen from figure 1 below, the effluent pH remained at saturation for approximately six hours after lime was added each time. The second half of the second high-pH segment must be ignored due to problems with the experimental set-up. Because excess lime was not removed from the reactor before adding the next 200 grams, a total of 600 grams of lime had been added to the reactor by the beginning of the third segment seen in the graph. Despite the fact that the reactor was overloaded, it seemed to perform no better with the new addition of lime than it did with the first, when there was far less solid lime in suspension. This suggests that only the "new" lime is effective in saturating the effluent.
Possible failure modes could be:
- Because the smallest particles dissolve fastest and are most prone to being carried out with the effluent due to their low settling velocities, they do not persist in the reactor even though it appears to be filled with solids. Since they have the highest surface area to volume ratio, the small particles are the ones that dissolve most effectively, whereas the larger particles, which may remain for a very long time, do not dissolve well enough to saturate the effluent within the residence time of the reactor. See hypotheses.
- Preferential flow paths form over time in the densely-packed reactor, so that eventually clean water is allowed to pass through with very little contact time with solid lime.
- Although the results from experiment 2 suggest otherwise, chemical/physical interference by calcium carbonate precipitate may still be part of the problem. See hypotheses.
Fig 1. Experiment 3
...