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, 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. While no theoretical model of gas removal by the sand filter was developed, the team believed that the extra surface area provided by the sand would remove gas by increasing the nucleation sites for bubbles and also providing sites to which bubbles could adhere and grow.

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 were performed to assess the functionality of the new system in terms of its ability to collect good, consistent data and to ensure that previous sand grain experiments results were replicable using the new system. The control experiment with no sand was performed after it was observed that measured gas removal from the sand filter fell short of the 18 mL/L that was theoretically possible to be removed. When it was found that gas removal was less than expected, dissolved oxygen measurements were taken in order to assess the component of the system that was not functioning effectively.

[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.
• 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.
• Control Experiment with no sand was performed to test the effectiveness of the setup in the absence of sand in the sand filter.

Dissolved Oxygen Measurements

• When it was confirmed that the measured gas removal values did not match theoretical values, concentrations of dissolved oxygen were measured at various points in the experimental apparatus. It was found that the measurements taken after the sand filter indicated dissolved oxygen concentrations that were higher than the influent water going into the sand filter suggesting a problem with the setup or the fluidized bed mechanism. It was confirmed with the control experiment that the sand filter was not a suitable mechanism for gas removal.

General Conclusions

The experimental results for gas removal rate using Sand 40 and Sand 30 gave results of about 5.09 mL/L and 2.01 mL/L, while the control experiment indicated a gas removal of 7.47 mL/L. These results suggest that the sand seemed to inhibit rather than facilitate gas removal. In addition to the gas removal rates, the dissolved oxygen concentrations (reiterated below) measured at the four ports in the system support the notion that the sand filter did not provide a suitable mechanism for gas removal since the measurements indicated higher oxygen concentrations in the water after the sand filter.

Articles found concerning bubble formation and behavior indicate that rough, hydrophobic surfaces are most suitable for bubble formation (See "Fundamentals of Bubble Formation during Coagulation and Sedimentation Processes" by P. Scardina and M. Edwards on the Floating Floc Annotated Bibliography page). The believed cause of the sand filter failure is thought to be the lack of hydrophobicity exhibited by the sand used.

Table 1: Dissolved Oxygen Concentrations (DO) at sampling ports in the experimental setup with a sand bed.

Additionally, measurements indicate that the aerator is able to supersaturate the water with 15.5 mg/L of dissolved oxygen; however, this value is less than the expected 18 mg/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. In order to address this issue, the team has decided to alter the pressurized aerator by replacing the single aeration stone with a junction of four cylindrical aeration stone that would displace gas into the water in finer bubbles that would be more easily incorporated into solution.

It is also thought that the bubble collector used to measure gas removal may not be effective at capturing small bubbles. However, the source of the small bubbles traveling through the system is thought be the pressurized aerator and not the sand filter. At this point, it is unclear whether small bubbles formed in the sand filter itself. Before the team redesigns the bubble collector, the team plans to modify the flow accumulator so that small bubbles coming from the aerator will be collected and the water going through the sand filter will be supersaturated without small bubbles. If small bubbles are formed in the filter and it is deemed necessary to redesign the bubble collector in order to capture these bubbles, a new design for the bubble collector will be developed.