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The hypothesis that absorbed air would be released in the apparatus was qualitatively observed by bubble formation. The adverse effects of bubble formation on floc blanket formation and effluent turbidity were supported qualitatively by the cloudiness of the liquid exiting the sedimentation tank through the tube settler. It is possible that these air bubbles broke up larger flocs resulting in smaller floc particles than expected. The velocity gradient controls the transport of flocs that enter the tube settler to the effluent for turbidity measurement. Floc roll-up is characterized based on a force balance. Due to the fact that smaller floc particles are entering the velocity gradient in the tube settler, the foce balance reveals that these particles are more likely to escape into the effluent, increasing effluent turbidity.
Quantitatively, data collected over twenty four hours showed an increase in effluent turbidity when comparing the experimental run with saturated air to the control experiment experiment. We ran this experiment on both high and low floc blanket levels. In the high floc blanket formation state the floc blanket level is above the plate settlers. In the low floc blanket formation the floc blanket formation level is below the plate settlers. The presence of air bubbles could break up some floc particles and force floc particles up into the clarified effluent.
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It was expected that the release of saturated air as bubbles in the sedementation column would prevent effective floc blanket formation. The bubbles were expected to break up larger flocs and, create smaller, lighter flocs that would leave through the plate settler (due to a force balance velocity gradient analysis) and increase overall effluent turbidity. An increase in effluent turbidity and the appearance of bubbles in the apparatus supported this hypothesis in the experiment on low floc blanket formation. However, for the experiment on high floc blanket formation, the data was not consistent with this prediction. If AguaClara is considering designing additional plants undergoing elevation drops, results from this experiment should be considered.
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