Evaluation of previous experiments with the new system

Objective

This set of experiments attempted to replicate the grain size research performed with the previous experimental setup. Additionally, the effectiveness of each of the components in the setup was assessed with respect to theoretical expectations. Theoretically, it was expected that the aerator under 2 atm of pressure would be able to supersaturate the water with 18 ml/L of dissolved gas by this [theoretical model|^Dissolved atmospheric gases.xmcd], which predicts the bubble formation potential to be around 18mL/L for water that has been previously exposed to 1 atm gage pressure at temperature of 25 C. The model calculates the theoretical bubble formation potential as a function of the air pressure that the water equilibrated with prior to returning to atmospheric pressure. With the current aerator, a major assumption made was that the dissolved gas concentration would equilibrate with the pressure in the aerator, resulting in 18 ml/L of dissolved gas in the influent water through sand filter. We expected that the sand filter would be able to remove all or a significant portion of the excess gas in solution, and that all the gas removed by the sand filter in the form of bubbles would be collected in the bubble collector.

General Procedure

For the two experiments listed below, the same procedure was used with varying sand grain sizes. Sand 40 (0.49 mm - 0.57 mm) and Sand 30 (0.59 mm - 0.84 mm) were used for experiments one and two, respectively. The details of the procedure are available here.

Results and Discussion

[Experiments 1 and 2 - Replicates of the Previous Fluidized Bed Experiments]

• Experiment 1 was performed shortly after the installation of the new setup. Sand 40 was used with the purpose of replicating previous grain size results and assessing the overall functionality of the setup.
• Experiment 2 was performed after making modifications to the system to account for the issues found in the Experiment 1. Sand 30 was used with the purpose of testing the functionality again and replicating previous results.

Dissolved Oxygen Measurements

• When it was confirmed that the new aerator was not saturating water as much as possible, concentrations of dissolved oxygen were measured at various points in the experimental apparatus.

General Conclusions

The experimental results for gas removal rate using Sand 40 and Sand 30 gave consistent results of 5.09 mL/L and 2.01 mL/L, respectively. While this data suggests that the sand filter is removing gas, the gas removal rate recorded is significantly lower than the expected. The discrepancy between laboratory results and theoretical predictions indicates that the current system is not functioning effectively. there could be an array of possibilities where some of the system components could be redesigned to achieve better performance in producing air saturated water and consequently removing released air using the suspended matter. For additional analysis, we measured the dissolved oxygen concentration at the effluents from the aerator, sand filter, and bubble collector. Both the model and DO measurements have been instrumental in pointing out several improvements in design of the current apparatus and optimizing the process conditions.

Based on further collaboration with the TA, the following design modifications have been advised to be made:
1. It has been estimated that the aerator is producing the water that is not aerated enough. This could be remedied by inserting multiple aeration stones with different volume so that aerator would displace more air into the water.
2. Currently, the large headloss is occurring throughout the sand filter. As a result, it is possible that the additional release of pressure will allow the tiny bubbles to form, and thus dissolving them into the solution. One solution would be to focus on where the maximum headloss occurs and redesign the sand filter so it will effectively act as if it is open to the atmosphere. Alternatively, if the situation allows, it might be possible to optimize the parameters at which the minimum headloss takes place.

It is also possible that there are a few other factors affecting the collection and removal of dissolved air. An example might be the case when the bubble collector is collecting the large bubbles but not the tiny ones. It is hoped that some of the modifications will help make the system more efficient, thereby yielding the results which would eventually converge to the theoretical predictions.