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, dissolved oxygen measurements were taken to assess the effectiveness of each of the components in the setup 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. (Why did you expect that the sand filter would do this? When you list something as a cause, unless it is common knowledge, please include a brief description of the mechanism by which the sand filter would work.)

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.

(It is unclear about why you are replicating these experiments? Is there any reason to believe that there would be inconsistencies between experiments? Elucidate these if possible.)

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. (I thought we were testing this because it was unclear whether the sand filter was effectively removing bubbles)

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 was significantly lower than the expected. The discrepancy between laboratory results and theoretical predictions indicated that the current system is not functioning effectively. The dissolved oxygen measurements taken throughout the system (recounted below) indicate problems with the current setup (or the ineffectiveness of the sand at removing dissolved air from water).

Table 1: Dissolved Oxygen Concentrations (DO) at Sampling Ports in the System.

Measurements indicate that the aerator is able to supersaturate the water with 15.5 ml/L of dissolved gas; however, this value is less than the expected 18 ml/L. Additionally, subsequent measurements of dissolved gas at that point reveal inconsistent levels of gas supersaturation. The inability to regulate the amount of dissolved gas in the influent water into the sand filter may have significant impact on the ability of the team to run controlled experiments. (This is true, how can we make it so this doesn't happen in the future? We can redesign how we do this.)

Dissolved oxygen measurements taken after the sand filter suggest a greater problem with the system. The effluent water from the sand filter appears to contain more dissolved gas than the influent water.

In order to correct these issue, the following design modifications have been advised:

1. Currently, the large head loss is occurring throughout the sand filter. (In final wiki, refer to results showing this) As a result, it is possible that the additional release of pressure will allow the tiny bubbles to form tiny bubbles that make their way to the sand filter , and thus are dissolving them into the solution because of the high pressures at the bottom of the sand filter. One solution would be to focus on where the maximum head loss 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.

2. 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 in finer bubbles.

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 (How could we retrofit the bubble collector to collect smaller bubbles?). 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.